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                                 AURORÆ:
                      THEIR CHARACTERS AND SPECTRA.

                                   BY
                        J. RAND CAPRON, F.R.A.S.

    “And now the Northern Lights begin to burn, faintly at first,
    like sunbeams playing in the waters of the blue sea. Then a
    soft crimson glow tinges the heavens. There is a blush on
    the cheek of night. The colours come and go; and change from
    crimson to gold, from gold to crimson. The snow is stained with
    rosy light. Twofold from the zenith, east and west, flames
    a fiery sword; and a broad band passes athwart the heavens,
    like a summer sunset. Soft purple clouds come sailing over the
    sky, and through their vapoury folds the winking stars shine
    white as silver. With such pomp as this is Merry Christmas
    ushered in, though only a single star heralded the first
    Christmas.”—LONGFELLOW.

                                 LONDON:
                   E. & F. N. SPON, 46 CHARING CROSS.
                                NEW YORK:
                            446 BROOM STREET.
                                  1879.

                             [Illustration]

                     PRINTED BY TAYLOR AND FRANCIS,
                      RED LION COURT, FLEET STREET.




                                   TO
                  PROF. CHARLES PIAZZI SMYTH, F.R.S.E.,
                     ASTRONOMER ROYAL FOR SCOTLAND,
               ONE OF THE EARLIEST SPECTROSCOPIC OBSERVERS
                                   OF
                     THE AURORA AND ZODIACAL LIGHT,
                               THIS VOLUME
                                   IS
                         RESPECTFULLY DEDICATED
                                   BY
                               THE AUTHOR.




PREFACE.


Probably few of the phenomena of Nature so entirely charm and interest
scientific and non-scientific observers alike as the Aurora Borealis, or
“Northern Lights” as it is popularly called. Whether contemplated as the
long low quiescent arc of silver light illuminating the landscape with
a tender radiance, as broken clouds and columns of glowing ruddy light,
or as sheaves of golden rays, aptly compared by old writers to aerial
spears, such a spectacle cannot fail at all times to be a subject of
admiration, in some cases even of awe.

Hence it is no wonder that the Aurora has always received a considerable
amount of attention at the hands of scientific men. Early explorers of
the Arctic Regions made constant and important observations of it and its
character; and the list of references to works given in the Appendix will
show how often it formed the subject of monographs and communications to
learned Societies. The early contributions seem relatively more numerous
than those of a later date; and the substance of them will be found well
summed up in Dr. Brewster’s ‘Edinburgh Encyclopædia’ (1830), article
“Aurora.” A most complete and able epitome of our more recent experience
and knowledge of the Aurora and its spectrum has been contributed by
my friend Mr. Henry R. Procter to the present (9th) edition of the
‘Encyclopædia Britannica,’ article “Aurora Polaris.” It is, however,
a drawback to Encyclopædic articles that their matter is of necessity
condensed, and that they rarely have the very desirable aid of drawings
and engravings to illustrate their subjects. In spite, therefore, of the
exhaustive way, both as to fact and theory, in which the contributor to
the ‘Encyclopædia Britannica’ has realized his task, it seemed to me
there was still room left for a popular treatise, having for its object
the description of Auroræ, their characters and spectra. The question
of the Aurora spectrum seems the more worthy of extended discussion in
that it still remains an unsolved problem. In spite of the observations
and researches of Ångström, Lemström, and Vogel abroad, and of Piazzi
Smyth, Herschel, Procter, Backhouse, and others at home, the goal is
not yet reached; for while the faint and more refrangible lines are but
doubtfully referred to air, the bright and sharp red and green lines,
which mainly characterize the spectrum, are as yet unassociated with any
known analogue.

With these views, and to incite to further and closer observations, I
have been induced to publish the present volume as a sort of Auroral
Guide. For much of the history of the Aurora I am indebted to, and
quote from former articles and records, including the two excellent
Encyclopædic ones before referred to. Mr. Procter, Mr. Backhouse, and my
friend Mr. W. H. Olley have each kindly furnished me with much in the way
of information and suggestion. Dr. Schuster has lent me tubes showing the
true oxygen spectrum; while Herr Carl Bock, the Norwegian naturalist, has
enabled me to reproduce a veritable curiosity, viz. a picture in oil
painted by the light of a Lapland Aurora. The experiments detailed in
Part III. were suggested by the earlier ones of De la Rive, Varley, and
others, and demonstrate the effect of the magnet on electric discharges.
For assistance in these I am indebted to my friend Mr. E. Dowlen.

The illustrations are mainly from original drawings of my own. Those from
other sources are acknowledged. Messrs. Mintern have well reproduced
in chromo-lithography the coloured drawings illustrating the Auroræ,
moon-patches, &c.




TABLE OF CONTENTS.


                                 PART I.

                     THE AURORA AND ITS CHARACTERS.

                               CHAPTER I.

  The Aurora as known to the Ancients: pp. 1-5.

                               CHAPTER II.

  Some general Descriptions of Auroræ: pp. 6-15.

      By Sir John Franklin: pp. 6, 7.

      By Rev. James Farquharson: pp. 7, 8.

      By Mons. Lottin (Lardner): pp. 8-11.

      By Lieut. Weyprecht (Payer): pp. 11-14.

      Article in ‘Edinburgh Encyclopædia:’ pp. 14, 15.

                              CHAPTER III.

  Some specific Descriptions of Auroræ: pp. 16-30.

      Capt. Sabine’s Auroræ: p. 16.

      Aurora seen at Sunderland, February 8th, 1817: p. 16.

      Dr. Hayes’s Aurora, 6th January, 1861: pp. 16, 17.

      Prof. Lemström’s Aurora, 1st September, 1868: pp. 17, 18.

      Mr. J. R. Capron’s Aurora, October 24th, 1870: pp. 18, 19.

      Mr. Barker’s red and white Auroræ, 9th November (1870?): p. 19.

      Mr. J. R. Capron’s Aurora, February 4th, 1872: pp. 19, 20.

      Cardiff, Aurora seen at: p. 21.

      Mr. J. R. Capron’s Aurora, February 4th, 1874: pp. 21, 22.

      Mr. Herbert Ingall’s Aurora, July 18th, 1874: pp. 22, 23.

      Mr. J. R. Capron’s white Aurora, September 11th, 1874: pp. 23, 24.

      Dr. Allnatt’s Aurora, June 9th, 1876: p. 24.

      Herr Carl Bock’s Lapland Aurora, October 3rd, 1877: p. 25.

      Rev. T. W. Webb’s Aurora: pp. 25, 26.

      English Arctic Expedition, 1875-76: p. 26.

      Aurora Australis: pp. 26-29.

      Prof. Piazzi Smyth’s typical Auroræ: pp. 29, 30.

                               CHAPTER IV.

  Phenomena simulating Auroræ: pp. 31-32.

      Auroric Lights (Kinahan): pp. 31, 32.

      Luminous Arch: p. 32.

                               CHAPTER V.

  Some qualities of the Aurora: pp. 33-51.

      Noises attending Auroræ: pp. 33, 34.

      Colours of Aurora: pp. 35-37.

      Height of Aurora: pp. 37-40.

      Phosphorescence attending Aurora: pp. 41-44.

      Aurora and Ozone: pp. 44, 45.

      Polarization of Aurora light: pp. 45, 46.

      Number of Auroræ: p. 46.

      Duration of Aurora: p. 47.

      Travelling of Auroræ (Donati): pp. 47, 48.

      Geographical distribution of Auroræ (Fritz and Loomis): pp. 48, 49.

      Extent and principal zone of the Aurora: pp. 50, 51.

                               CHAPTER VI.

  Aurora in connexion with other Phenomena: pp. 52-69.

      Auroræ and Clouds: pp. 52-54.

      Aurora and Thunder-storms: pp. 54, 55.

      Aurora and Magnetic Needle: pp. 55-58.

      Auroræ, Magnetic Disturbances, and Sun-spots: pp. 58-62.

      Aurora and Electricity: pp. 62-64.

      Aurora and Meteoric Dust: pp. 64, 65.

      Aurora and Planets Venus and Jupiter: pp. 66, 67.

      Aurora and Zodiacal Light: pp. 67-69.

                              CHAPTER VII.

  Aurora-like patches on the partially-eclipsed Moon: pp. 70-77.

                              CHAPTER VIII.

  Aurora and Solar Corona: pp. 78-81.

                               CHAPTER IX.

  Supposed Causes of the Aurora: pp. 82-87.

      Prof. Lemström’s Theory: pp. 87, 88.

      Theories of MM. Becquerel and De la Rive: p. 88.

      M. Planté’s electric experiments: pp. 89, 90.


                                PART II.

                       THE SPECTRUM OF THE AURORA.

                               CHAPTER X.

  Spectroscope adapted for the Aurora: pp. 91-93.

      Spectrum of Aurora described: pp. 94-100.

      Flickering of the green line: p. 100.

      Mr. Backhouse’s graphical spectra of four Auroræ: p. 101.

      Lord Lindsay’s Aurora-spectrum, October 21, 1870: p. 102.

      Spectrum of the Aurora Australis: pp. 103, 104.

      Professor Piazzi Smyth’s Aurora-spectra: p. 104.

      Author’s Catalogue of the Auroral lines: pp. 104-106.

      Theories in relation to the Aurora and its spectrum: pp. 106, 107.

                               CHAPTER XI.

  The comparison of some Tube and other Spectra with the Spectrum of
        the Aurora: pp. 108-120.

      Hydrogen-tube: pp. 108-110.

      Carbon- and oxygen-tubes: pp. 110-114.

      Geissler mercury-tube and barometer mercurial vacuum: pp. 114, 115.

      Air-tubes: pp. 115, 116.

      Phosphorescent tube: pp. 116, 117.

      Spark in air: p. 117.

      Spark over water: p. 117.

      Phosphoretted-hydrogen flame: pp. 117, 118.

      Iron-spectrum: p. 118.

      Spectrum of mercury: p. 119.

      Table of coincidences: p. 119.

                              CHAPTER XII.

  Notes on Professor Ångström’s Theory of the Aurora-spectrum: pp. 121-127.

                              CHAPTER XIII.

  The Oxygen-spectrum in relation to the Aurora (Procter and Schuster):
       pp. 128-131.


                                PART III.

       MAGNETO-ELECTRIC EXPERIMENTS IN CONNEXION WITH THE AURORA.

                       INTRODUCTION: pp. 133-135.

                              CHAPTER XIV.

  Examination of Geissler-tubes under action of the Magnet: pp. 136-146.

      Nitrogen-tubes: pp. 136, 137.

      Oxygen-tubes: p. 138.

      Hydrogen-tubes: pp. 138, 139.

      Water-gas tube: p. 139.

      Ammonia-tube: pp. 139, 140.

      Carbonic-acid tube: p. 140.

      Chlorine-tubes: pp. 140, 141.

      Iodine-tubes: pp. 141-143.

      Bromine-tubes: pp. 143, 144.

      Silicic fluoride-tubes: p. 144.

      Sulphuric-acid tubes: pp. 144, 145.

      Sulphur-tube: pp. 145, 146.

                               CHAPTER XV.

  Effect of Magnet on a capillary Glass Tube: pp. 147, 148.

      Action of magnet on a bar of heavy glass: pp. 147, 148.

                              CHAPTER XVI.

  Effect of Magnet on wide Air (Aurora) tube: pp. 149-153.

      Stratification (note on): pp. 149, 150.

      Effect of magnet on Plücker (air-) tube: pp. 150-152.

      Effect of magnet on Plücker tube (tin chloride): pp. 152, 153.

      Effect of magnet on tin-chloride Geissler tube: p. 153.

                              CHAPTER XVII.

  Effect of Magnet on bulbed Phosphorescent Tube: pp. 154-156.

      Effect of magnet on small phosphorescent (powder) tubes: pp. 156,
         157.

      Lighting-up tubes with one wire only (Marquis of Salisbury’s
         observations): pp. 157, 158.

                             CHAPTER XVIII.

  Action of the Magnet on the Electric Spark: pp. 159, 160.

                              CHAPTER XIX.

  The Discharge _in vacuo_ in Larger Vessels, and Magnetic Effects
       thereon: pp. 161-165.

      Some of Baron Reichenbach’s magnetic researches tested: pp. 165, 166.

  Summary of the foregoing Experiments and their Results: p. 167.

                               CHAPTER XX.

  Some concluding Remarks: pp. 168-171.

                               APPENDICES.

  A. References to some Works and Essays on the Aurora: pp. 173, 174.

  B. Extracts from the Manual and Instructions for the (English) Arctic
       Expedition, 1875: pp. 175-181.

  C. Extracts from Parliamentary Blue-book, containing the “Results
       derived from the Arctic Expedition, 1875-76:” pp. 182-188.

  D. Aurora and Ozone: pp. 189-193.

  E. Dr. Vogel’s Inquiries into the Spectrum of the Aurora: pp. 194-207.




LIST OF PLATES.


  Plate.

      I. The Aurora during the Ice-pressure              _To face page_ 14

     II. Aurora seen by Dr. Hayes, 6th January, 1861          ”    ”    16

    III. Aurora, Guildford, Oct. 24, 1870                     ”    ”    18

     IV. Aurora, Guildford, Feb. 4, 1872; Eclipsed Moon,
           Aug. 23, 24, 1877                                  ”    ”    20

      V. Corona, Graphical Auroræ, Zodiacal Light, &c.        ”    ”    21

     VI. Aurora, Guildford, Feb. 4, 1874; Spectrum des
           Nordlichts (Vogel)                                 ”    ”    22

    VII. Aurora, Kyle Akin, Isle of Skye, Sept. 11, 1874      ”    ”    24

   VIII. Herr Carl Bock’s Lapland Aurora, Oct. 3, 1877        ”    ”    25

     IX. Compared Aurora and other Spectra. Loomis’s
           curves of Auroras, Magnetic Declination, and
           Solar Spots                                        ”    ”    59

      X. Spectroscope, Micrometer, Tubes                      ”    ”    91

     XI. Aurora-spectra, Candle-spectrum                      ”    ”   102

    XII. Aurora-spectrum, Solar spectrum, and
           Candle-spectrum                                    ”    ”   104

   XIII. Vogel’s Aurora-lines, Aurora-lines near G, and in
           the red and green                                  ”    ”   108

    XIV. Aurora, Hydrocarbons, Oxygen                         ”    ”   110

     XV. Aurora and Air-tubes, &c.                            ”    ”   115

    XVI. Aurora, Phosphoretted Hydrogen, Iron, &c.            ”    ”   117

   XVII. Effect of Magnet on Tubes and Spark                  ”    ”   134

  XVIII. Same, and Oxygen-spectrum                            ”    ”   154




PART I.

THE AURORA AND ITS CHARACTERS.




CHAPTER I.

THE AURORA AS KNOWN TO THE ANCIENTS.


[Sidenote: Seneca’s ‘Quæstiones Naturales,’ Lib. I. c. xiv. Description
of Auroræ.]

In Seneca’s ‘Quæstiones Naturales,’ Lib. I. c. xiv., we find the
following:—“Tempus est, alios quoque ignes percurrere, quorum diversæ
figuræ sunt. Aliquando emicat stella, aliquando ardores sunt, aliquando
fixi et hærentes, nonnunquam volubiles. Horum plura genera conspiciantur.
Sunt _Bothynoë_[1], quum velut corona cingente introrsus igneus cœli
recessus est similis effossæ in orbem speluncæ. Sunt _Pithitæ_[2], quum
magnitudo vasti rotundique ignis dolio similis, vel fertur vel in uno
loco flagrat. Sunt _Chasmata_[3], quum aliquod cœli spatium desedit,
et flammam dehiscens, velut in abdito, ostentat. Colores quoque omnium
horum plurimi sunt. Quidam ruboris acerrimi, quidam evanidæ ac levis
flammæ, quidam candidæ lucis, quidam micantes, quidam æqualiter et sine
eruptionibus aut radiis fulvi.

…

[Sidenote: Seneca, c. xv.]

C. xv. “Inter hæc ponas licet et quod frequenter in historiis legimus,
cœlum ardere visum: cujus nonnunquam tam sublimis ardor est ut inter ipsa
sidera videatur, nonnunquam tam humilis ut speciem longinqui incendii
præbeat.

“Sub Tiberio Cæsare cohortes in auxilium Ostiensis coloniæ cucurrerunt,
tanquam conflagrantis, quum cœli ardor fuisset per magnam partem noctis,
parum lucidus crassi fumidique ignis.”

[Sidenote: Translation.]

We may translate this:—“It is time other fires also to describe, of which
there are diverse forms.

“Sometimes a star shines forth; at times there are fire-glows, sometimes
fixed and persistent, sometimes flitting. Of these many sorts may be
distinguished. There are Bothynoë, when, as within a surrounding corona,
the fiery recess of the sky is like to a cave dug out of space. There are
Pithitæ, when the expanse of a vast and rounded fire similar to a tub
(dolium) is either carried about or glows in one spot.

“There are Chasmata, when a certain portion of the sky opens, and gaping
displays the flame as in a porch. The colours also of all these are
many. Certain are of the brightest red, some of a flitting and light
flame-colour, some of a white light, others shining, some steadily and
yellow without eruptions or rays.

…

“Amongst these we may notice, what we frequently read of in history, the
sky is seen to burn, the glow of which is occasionally so high that it
may be seen amongst the stars themselves, sometimes so near the Earth
(humilis) that it assumes the form of a distant fire. Under Tiberius
Cæsar the cohorts ran together in aid of the colony of Ostia as if it
were in flames, when the glowing of the sky lasted through a great part
of the night, shining dimly like a vast and smoking fire.”

[Sidenote: Auroræ frequently read of in history.]

From the above passages many striking particulars of the Aurora may be
gathered; and by the division of the forms of Aurora into classes it is
evident they were, at that period, the subject of frequent observation.
The expression “et quod frequenter in historiis legimus” shows, too, that
the phenomena of Auroral displays were a matter of record and discussion
with the writers of the day.

Various forms of Aurora may be recognized in the passages from Chap.
xiv.; while in those from Chap. xv. a careful distinction is drawn
between the Auroræ seen in the zenith or the upper regions of the sky,
and those seen on the horizon or apparently (and no doubt in some cases
actually) near the Earth’s surface.

[Sidenote: A spurious Aurora.]

The description of the cohorts running to the fire only to find it an
Aurora, calls to mind the many similar events happening in our own days.
Not, however, but that a mistake may sometimes occur in an opposite
direction. In the memoirs of Baron Stockmar an amusing anecdote is
related of one Herr von Radowitz, who was given to making the most of
easily picked up information. A friend of the Baron’s went to an evening
party near Frankfort, where he expected to meet Herr von Radowitz.
On his way he saw a barn burning, stopped his carriage, assisted the
people, and waited till the flames were nearly extinguished. When he
arrived at his friend’s house he found Herr von Radowitz, who had
previously taken the party to the top of the building to see an Aurora,
dilating on terrestrial magnetism, electricity, and so forth. Radowitz
asked Stockmar’s friend, “Have you seen the beautiful Aurora Borealis?”
He replied, “Certainly; I was there myself; it will soon be over.” An
explanation followed as to the barn on fire: Radowitz was silent some ten
minutes, then took up his hat and quietly disappeared.

[Sidenote: Auroræ as portents.]

It is probable that many of the phantom combats which are recorded to
have appeared in forms of fire in the air on the evenings preceding great
battles might be traced to Auroræ, invested with distinct characteristics
by the imagination of the beholders. Auroræ are said to have appeared
in the shape of armies of horse and foot engaged in battle in the sky
before the death of Julius Cæsar, which they were supposed to foretell.
For more than a year before the siege and destruction of Jerusalem by
Titus Vespasian, the Aurora was said to have been frequently visible in
Palestine.

Josephus, in his ‘Wars of the Jews’ (Whiston’s Translation, Book VI.
chap. v. sect. 3), in referring to the signs and wonders preceding the
destruction of Jerusalem, speaks of a star or comet, and that a great
light shone round about the altar and the holy house, which light lasted
for half an hour, and that a few days after the feast of unleavened bread
a certain prodigious and incredible phenomenon appeared—“for before
sunsetting chariots and troops of soldiers in their armour were seen
running about among the clouds, and surrounding of cities.” (This, if an
Aurora, must have been an instance of a daylight one.)

We find in Book II. of Maccabees, chap. v. verses 1, 2, 3, 4 (B.C. about
176 years):—

“1. About this same time Antiochus prepared his second voyage into Egypt:

“2. And then it happened that through all the city, for the space almost
of forty days, there were seen horsemen running in the air, in cloth of
gold, and armed with lances like a band of soldiers.

“3. And troops of horsemen in array, encountering and running one against
another, with shaking of shields and multitude of pikes, and drawing
of swords and casting of darts, and glittering of golden ornaments and
harness of all sorts.

“4. Wherefore every man prayed that that apparition might turn to good.”

[Sidenote: Early descriptions of Auroræ.]

In Aristotle’s ‘De Meteoris,’ Lib. I. c. iv. and v., the Aurora is
described as an appearance resembling flame mingled with smoke, and of a
purple red or blue colour. Pliny (Lib. II. c. xxvii.) speaks of a bloody
appearance of the heavens which seemed like a fire descending on the
earth, seen in the third year of the 107th Olympiad, and of a light seen
in the nighttime equal to the brightness of the day, in the Consulship
of Cæcilius and Papirius (Lib. II. c. xxxiii.), both of which may be
referred to Auroræ.

In the ‘Annals of Philosophy,’ vol. ix. p. 250, it is stated that
the Aurora among English writers is first described by Matthew of
Westminster, who relates that in A.D. 555 lances were seen in the air
(“quasi species lancearum in aëre visæ sunt a septentrionali usque ad
occidentem”).

In the article in the ‘Edinb. Encyc.’ vol. iii. (1830), the Aurora (known
to the vulgar as “streamers” or “merry dancers”) is distinguished in
two kinds—the “tranquil” and the “varying.” Musschenbroek enumerates as
forms:—_trabs_, “the beam,” an oblong tract parallel to the horizon;
_sagitta_, “the arrow;” _faces_, “the torch;” _capra saltans_, “the
dancing goat;” _bothynoë_, “the cave,” a luminous cloud having the
appearance of a recess or hollow in the heavens, surrounded by a corona;
_pithiæ_, “the tun,” an Aurora resembling a large luminous _cask_. The
two sorts of Auroræ distinguished as the “bothynoë” and “pithiæ” are
evidently taken from the passage in Seneca’s ‘Quæstiones’ before quoted.
In ‘Liberti Fromondi Meteorologicorum’ (London, 1656), Lib. II. cap. v.
“De Meteoris supremæ regionis aëris,” art. 1. De Capra, Trabe, Pyramide,
&c., these and other fantastic forms attributable to Auroræ are more
fully described.

In the article “Aurora Polaris,” Encyc. Brit. edit. ix., we find noted
that from a curious passage in Sirr’s ‘Ceylon and the Cingalese,’
vol. ii. p. 117, it would seem that the Aurora, or something like it,
is visible occasionally in Ceylon, where the natives call it “Buddha
Lights,” and that in many parts of Ireland a scarlet Aurora is supposed
to be a shower of blood. The earliest mentioned Aurora (in Ireland)
was in 688, in the ‘Annals of Cloon-mac-noise,’ after a battle between
Leinster and Munster, in which Foylcher O’Moyloyer was slain.

In the article in the Edinb. Encyc. before referred to it is stated that
it was not much more than a century ago that the phenomenon had been
noticed to occur with frequency in our latitudes.

Dr. Halley had begun to despair of seeing one till the fine display of
1716.

[Sidenote: Early notices of Auroræ not frequent in our latitudes.]

The first account on record in an English work is said to be in a book
entitled ‘A Description of Meteors by W. F. D. D.’ (reprinted, London,
1654), which speaks of “burning spears” being seen January 30, 1560. The
next is recorded by Stow as occurring on October 7, 1564; and, according
to Stow and Camden, an Aurora was seen on two nights, 14th and 15th
November, 1574.

Twice, again, an Aurora was seen in Brabant, 13th February and 28th
September, 1575. Cornelius Gemma compared these to spears, fortified
cities, and armies fighting in the air. Auroræ were seen in 1580 and 1581
in Wirtemberg, Germany.

Then we have no record till 1621, when an Aurora, described by Gassendi
in his ‘Physics,’ was seen all over France, September 2nd of that year.

In November 1623 another, described by Kepler, was seen all over Germany.

From 1666 to 1716 no appearance is recorded in the ‘Transactions of the
French Academy of Sciences;’ but in 1707 one was seen in Ireland and at
Copenhagen; while in 1707 and 1708 the Aurora was seen five times.

The Aurora of 1716, occurring after an interval of eighty years, which
Dr. Halley describes, was very brilliant and extended over much country,
being seen from the west of Ireland to the confines of Russia and the
east of Poland, extending nearly 30° of longitude, and from about the
50th degree of latitude, over almost all the north of Europe, and in all
places exhibiting at the same time appearances similar to those observed
in London. An Aurora observed in Bologna in 1723 was stated to be the
first that had ever been seen there; and one recorded in the ‘Berlin
Miscellany’ for 1797 is called a very unusual phenomenon. Nor did Auroræ
appear more frequent in the Polar Regions at that time, for Cælius
states that the oldest inhabitants of Upsala considered the phenomenon
as quite rare before 1716. Anderson, of Hamburg, writing about the same
time, says that in Iceland the inhabitants themselves were astonished
at the frequent Auroræ then beginning to take place; while Torfæus, the
Icelander, who wrote in 1706, was old enough to remember the time when
the Aurora was an object of terror in his native country.

According to M. Mairan, 1441 Auroræ were observed between A.D. 583 and
1751, of which 972 were observed in the winter half-years and 469 during
the summer half-years. In our next Chapter we propose to give some
general descriptions of Auroræ from comparatively early sources.




CHAPTER II.

SOME GENERAL DESCRIPTIONS OF AURORÆ.


[Sidenote: Sir John Franklin’s description.]

Sir John Franklin (‘Narrative of a Journey to the Shores of the Polar Sea
in the years 1819, 1820, 1821, 1822’) describes an Aurora in these terms:—

[Sidenote: Parts of the Aurora: beams, flashes, and arches.]

“For the sake of perspicuity I shall describe the several parts of the
Aurora, which I term beams, flashes, and arches.

“The beams are little conical pencils of light, ranged in parallel lines,
with their pointed extremities towards the earth, generally in the
direction of the dipping-needle.

[Sidenote: Formation of the Aurora.]

“The flashes seem to be scattered beams approaching nearer to the
earth, because they are similarly shaped and infinitely larger. I have
called them flashes, because their appearance is sudden and seldom
continues long. When the Aurora first becomes visible it is formed like
a rainbow, the light of which is faint, and the motion of the beams
undistinguishable. It is then in the horizon. As it approaches the zenith
it resolves itself into beams which, by a quick undulating motion,
project themselves into wreaths, afterwards fading away, and again and
again brightening without any visible expansion or contraction of matter.
Numerous flashes attend in different parts of the sky.”

[Sidenote: Arches of the Aurora.]

Sir John Franklin then points out that this mass would appear like an
arch to a person situated at the horizon by the rules of perspective,
assuming its parts to be equidistant from the earth; and mentions a
case when an Aurora, which filled the sky at Cumberland House from
the northern horizon to the zenith with wreaths and flashes, assumed
the shape of arches at some distance to the southward. He then
continues:—“But the Aurora does not always make its first appearance as
an arch. It sometimes rises from a confused mass of light in the east
or west, and crosses the sky towards the opposite point, exhibiting
wreaths of beams or coronæ boreales on its way. An arch also, which is
pale and uniform at the horizon, passes the zenith without displaying any
irregularity or additional brilliancy.” Sir John Franklin then mentions
seeing three arches together, very near the northern horizon, one of
which exhibited beams and even colours, but the other two were faint and
uniform. (See example of a doubled arc Aurora observed at Kyle Akin,
Skye, Plate VII.)

He also mentions an arch visible to the southward exactly similar to one
in the north. It appeared in fifteen minutes, and he suggests it probably
had passed the zenith before sunset. The motion of the whole body of
the Aurora from the northward to the southward was at angles not more
than 20° from the magnetic meridian. The centres of the arches were as
often in the magnetic as in the true meridian. A delicate electrometer,
suspended 50 feet from the ground, was never perceptibly affected by the
Aurora.

[Sidenote: Aurora does not often appear until some hours after sunset.]

Sir John Franklin further remarks that the Aurora did not often appear
immediately after sunset, and that the absence of that luminary for some
hours was in general required for the production of a state of atmosphere
favourable to the generation of the Aurora.

[Sidenote: Aurora seen in daylight.]

On one occasion, however (March 8th, 1821), he observed it distinctly
previous to the disappearance of daylight; and he subsequently states
that on four occasions the coruscations of the Aurora were seen very
distinctly before daylight had disappeared.

[In the article “Aurora Polaris,” Encyc. Brit. edit. ix., the
Transactions of the Royal Irish Academy, 1788, are referred to, where Dr.
Usher notices that the Aurora makes the stars flutter in the telescope;
and that, having remarked this effect strongly one day at 11 A.M., he
examined the sky, and saw an Auroral corona with rays to the horizon.

Instances are by no means rare of the principal Aurora-line having
been seen in waning sunlight, and in anticipation of an Aurora which
afterwards appeared.]

[Sidenote: The Rev. James Farquharson’s observations. Auroral arch.
Passage across the zenith.]

The Rev. James Farquharson, from the observation of a number of
Auroræ in Aberdeenshire in 1823 (‘Philosophical Transactions,’ 1829),
concluded:—that the Aurora follows an invariable order in its appearance
and progress; that the streamers appear first in the north, forming an
arch from east to west, having its vertex at the line of the magnetic
meridian (when this arch is of low elevation it is of considerable
breadth from north to south, having the streamers placed crosswise in
relation to its own line, and all directed towards a point a little
south of the zenith); that the arch moves forward towards the south,
contracting laterally as it approaches the zenith, and increasing its
intensity of light by the shortening of the streamers and the gradual
shifting of the angles which the streamers near the east and west
extremities of the arch make with its own line, till at length these
streamers become parallel to that line, and then the arch is seen in
a narrow belt 3° or 4° only in breadth, stretching across the zenith
at right angles to the magnetic meridian; that it still makes progress
southwards, and after it has reached several degrees south of the zenith
again enlarges its breadth by exhibiting an order of appearances the
reverse of that which attended its progress towards the zenith from the
north; that the only conditions that can explain and reconcile these
appearances are that the streamers of the Aurora are vertical, or nearly
so, and form a deep fringe which stretches a great way from east to
west at right angles to the magnetic meridian, but which is of no great
thickness from north to south, and that the fringe moves southward,
preserving its direction at right angles to the magnetic meridian.

[Sidenote: M. Lottin’s observations.]

Dr. Lardner, in his ‘Museum of Science and Art,’ vol. x. p. 189 _et
seq._, alludes to a description of “this meteor” (_sic_) supplied by M.
Lottin, an officer of the French Navy, and a Member of the Scientific
Commission to the North Seas. Between September 1838 and April 1839,
being the interval when the sun was constantly below the horizon, this
savant observed nearly 150 Auroræ. During this period sixty-four were
visible, besides many concealed by a clouded sky, but the presence of
which was indicated by the disturbances they produced upon the magnetic
needle.

The succession of appearances and changes presented by these “meteors” is
thus graphically described by M. Lottin:—

[Sidenote: Formation of the auroral bow.]

“Between four and eight o’clock P.M. a light fog, rising to the altitude
of six degrees, became coloured on its upper edge, being fringed with the
light of the meteor rising behind it. This border, becoming gradually
more regular, took the form of an arch, of a pale yellow colour, the
edges of which were diffuse, the extremities resting on the horizon.
This bow swelled slowly upwards, its vertex being constantly on the
magnetic meridian. Blackish streaks divided regularly the luminous arc,
and resolved it into a system of rays. These rays were alternately
extended and contracted, sometimes slowly, sometimes instantaneously,
sometimes they would dart out, increasing and diminishing suddenly in
splendour. The inferior parts, or the feet of the rays, presented always
the most vivid light, and formed an arc more or less regular. The length
of these rays was very various, but they all converged to that point
of the heavens indicated by the direction of the southern pole of the
dipping-needle. Sometimes they were prolonged to the point where their
directions intersected, and formed the summit of an enormous dome of
light.

[Sidenote: It ascends to the zenith. Reaches the zenith.]

“The bow then would continue to ascend toward the zenith. It would
suffer an undulatory motion in its light—that is to say, that from
one extremity to the other the brightness of the rays would increase
successively in intensity. This luminous current would appear several
times in quick succession, and it would pass much more frequently from
west to east than in the opposite direction. Sometimes, but rarely, a
retrograde motion would take place immediately afterward; and as soon
as this wave of light had run successively over all the rays of the
Aurora from west to east, it would return in the contrary direction
to the point of its departure, producing such an effect that it was
impossible to say whether the rays themselves were actually affected by
a motion of translation in a direction nearly horizontal, or if this
more vivid light was transferred from ray to ray, the system of rays
themselves suffering no change of position. The bow, thus presenting
the appearance of an alternate motion in a direction nearly horizontal,
had usually the appearance of the undulations or folds of a ribbon or
flag agitated by the wind. Sometimes one, and sometimes both of its
extremities would desert the horizon, and then its folds would become
more numerous and marked, the bow would change its character and assume
the form of a long sheet of rays returning into itself, and consisting
of several parts forming graceful curves. The brightness of the rays
would vary suddenly, sometimes surpassing in splendour stars of the
first magnitude; these rays would rapidly dart out, and curves would be
formed and developed like the folds of a serpent; then the rays would
affect various colours, the base would be red, the middle green, and the
remainder would preserve its clear yellow hue. Such was the arrangement
which the colours always preserved. They were of admirable transparency,
the base exhibiting blood-red, and the green of the middle being
that of the pale emerald; the brightness would diminish, the colours
disappear and all be extinguished, sometimes suddenly and sometimes by
slow degrees. After this disappearance fragments of the bow would be
reproduced, would continue their upward movement and approach the zenith;
the rays, by the effect of perspective, would be gradually shortened;
the thickness of the arc, which presented then the appearance of a large
zone of parallel rays, would be extended; then the vertex of the bow
would reach the magnetic zenith, or the point to which the south pole of
the dipping-needle is directed. At that moment the rays would be seen in
the direction of their feet. If they were coloured they would appear as
a large red band, through which the green tints of their superior parts
could be distinguished, and if the wave of light above mentioned passed
along them their feet would form a long sinuous undulating zone; while
throughout all these changes the rays would never suffer any oscillation
in the direction of their axis, and would constantly preserve their
mutual parallelisms.

[Sidenote: Multiple bows. Corona formed.]

“While these appearances are manifested new bows are formed, either
commencing in the same diffuse manner or with vivid and ready formed
rays; they succeed each other, passing through nearly the same phases,
and arrange themselves at certain distances from each other. As many as
nine have been counted having their ends supported on the earth, and in
their arrangement resembling the short curtains suspended one behind the
other over the scene of a theatre, and intended to represent the sky.
Sometimes the intervals between these bows diminish, and two or more of
them close upon each other, forming one large zone traversing the heavens
and disappearing towards the south, becoming rapidly feeble after passing
the zenith. But sometimes also, when this zone extends over the summit of
the firmament from east to west, the mass of rays appear suddenly to come
from the south, and to form, with those from the north, the real boreal
corona, all the rays of which converge to the zenith. This appearance of
a crown, therefore, is doubtless the mere effect of perspective; and an
observer placed at the same instant at a certain distance to the north or
to the south would perceive only an arc.

“The total zone, measuring less in the direction north and south than in
the direction east and west, since it often leans upon the corona, would
be expected to have an elliptical form; but that does not always happen:
it has been seen circular, the unequal rays not extending to a greater
distance than from eight to twelve degrees from the zenith, while at
other times they reach the horizon.

“Let it then be imagined that all these vivid rays of light issue forth
with splendour, subject to continual and sudden variations in their
length and brightness; that these beautiful red and green tints colour
them at intervals; that waves of light undulate over them; that currents
of light succeed each other; and in fine, that the vast firmament
presents one immense and magnificent dome of light, reposing on the
snow-covered base supplied by the ground, which itself serves as a
dazzling frame for a sea calm and black as a pitchy lake. And some idea,
though an imperfect one, may be obtained of the splendid spectacle which
presents itself to him who witnesses the Aurora from the Bay of Alten.

[Sidenote: Duration of corona.]

“The corona when it is formed only lasts for some minutes; it sometimes
forms suddenly, without any previous bow. There are rarely more than two
on the same night, and many of the Auroras are attended with no crown at
all.

[Sidenote: Disappearance of Aurora.]

“The corona becomes gradually faint, the whole phenomenon being to
the south of the zenith, forming bows gradually paler and generally
disappearing before they reach the southern horizon. All this most
commonly takes place in the first half of the night, after which the
Aurora appears to have lost its intensity; the pencils of rays, the
bands, and the fragments of bows appear and disappear at intervals. Then
the rays become more and more diffused, and ultimately merge into the
vague and feeble light which is spread over the heavens, grouped like
little clouds, and designated by the name of auroral plates (plaques
aurorales). Their milky light frequently undergoes striking changes in
the brightness, like motions of dilatation and contraction, which are
propagated reciprocally between the centre and the circumference, like
those which are observed in marine animals called Medusæ. The phenomena
become gradually more faint, and generally disappear altogether on the
appearance of twilight. Sometimes, however, the Aurora continues after
the commencement of daybreak, when the light is so strong that a printed
book may be read. It then disappears, sometimes suddenly; but it often
happens that, as the daylight augments, the Aurora becomes gradually
vague and undefined, takes a whitish colour, and is ultimately so mingled
with the cirro-stratus clouds that it is impossible to distinguish it
from them.”

Lieutenant Weyprecht has grandly described forms of Aurora in Payer’s
‘New Lands within the Arctic Circle’ (vol. i. p. 328 _et seq._) as
follows:—

[Sidenote: Lieut. Weyprecht’s description. Formation of arches.]

“There in the south, low on the horizon, stands a faint arch of light.
It looks as it were the upper limit of a dark segment of a circle; but
the stars, which shine through it in undiminished brilliancy, convince
us that the darkness of the segment is a delusion produced by contrast.
Gradually the arch of light grows in intensity and rises to the zenith.
It is perfectly regular; its two ends almost touch the horizon, and
advance to the east and west in proportion as the arch rises. No beams
are to be discovered in it, but the whole consists of an almost uniform
light of a delicious tender colour. It is transparent white with a
shade of light green, not unlike the pale green of a young plant which
germinates in the dark. The light of the moon appears yellow contrasted
with this tender colour, so pleasing to the eye and so indescribable in
words, a colour which nature appears to have given only to the Polar
Regions by way of compensation. The arch is broad, thrice the breadth,
perhaps, of the rainbow, and its distinctly marked edges are strongly
defined on the profound darkness of the Arctic heavens. The stars shine
through it with undiminished brilliancy. The arch mounts higher and
higher. An air of repose seems spread over the whole phenomenon; here and
there only a wave of light rolls slowly from one side to the other. It
begins to grow clear over the ice; some of its groups are discernible.
The arch is still distant from the zenith, a second detaches itself from
the dark segment, and this is gradually succeeded by others. All now
rise towards the zenith; the first passes beyond it, then sinks slowly
towards the northern horizon, and as it sinks loses its intensity. Arches
of light are now stretched over the whole heavens; seven are apparent at
the same time on the sky, though of inferior intensity. The lower they
sink towards the north the paler they grow, till at last they utterly
fade away. Often they all return over the zenith, and become extinct just
as they came.

[Sidenote: Band of light appears. Second band and rays.]

“It is seldom, however, that an Aurora runs a course so calm and so
regular. The typical dark segment, which we see in treatises on the
subject, in most cases does not exist. A thin bank of clouds lies on
the horizon. The upper edge is illuminated; out of it is developed a
band of light, which expands, increases in intensity of colour, and
rises to the zenith. The colour is the same as in the arch, but the
intensity of the colour is stronger. The colours of the band change in
a never-ceasing play, but place and form remain unaltered. The band is
broad, and its intense pale green stands out with wonderful beauty on
the dark background. Now the band is twisted into many convolutions, but
the innermost folds are still to be seen distinctly through the others.
Waves of light continually undulate rapidly through its whole extent,
sometimes from right to left, sometimes from left to right. Then, again,
it rolls itself up in graceful folds. It seems almost as if breezes high
in the air played and sported with the broad flaming streamers, the ends
of which are lost far off on the horizon. The light grows in intensity,
the waves of light follow each other more rapidly, prismatic colours
appear on the upper and lower edge of the band, the brilliant white of
the centre is enclosed between narrow stripes of red and green. Out of
one band have now grown two. The upper continually approaches the zenith,
rays begin to shoot forth from it towards a point near the zenith to
which the south pole of the magnetic needle, freely suspended, points.

[Sidenote: Corona formed.]

“The band has nearly reached it, and now begins a brilliant play of rays
lasting for a short time, the central point of which is the magnetic
pole—a sign of the intimate connexion of the whole phenomenon with the
magnetic forces of the earth. Round the magnetic pole short rays flash
and flare on all sides, prismatic colours are discernible on all their
edges, longer and shorter rays alternate with each other, waves of light
roll round it as a centre. What we see is the auroral corona, and it
is almost always seen when a band passes over the magnetic pole. This
peculiar phenomenon lasts but a short time. The band now lies on the
northern side of the firmament, gradually it sinks, and pales as it
sinks; it returns again to the south to change and play as before. So
it goes on for hours, the Aurora incessantly changes place, form, and
intensity. It often entirely disappears for a short time, only to appear
again suddenly, without the observers clearly perceiving how it came and
where it went; simply, it is there.

[Sidenote: Single-rayed band.]

“But the band is often seen in a perfectly different form. Frequently
it consists of single rays, which, standing close together, point in an
almost parallel direction towards the magnetic pole. These become more
intensely bright with each successive wave of light; hence each ray
appears to flash and dart continually, and their green and red edges
dance up and down as the waves of light run through them. Often, again,
the rays extend through the whole length of the band, and reach almost
up to the magnetic pole. These are sharply marked, but lighter in colour
than the band itself, and in this particular form they are at some
distance from each other. Their colour is yellow, and it seems as if
thousands of slender threads of gold were stretched across the firmament.
A glorious veil of transparent light is spread over the starry heavens;
the threads of light with which this veil is woven are distinctly marked
on the dark background; its lower border is a broad intensely white band,
edged with green and red, which twists and turns in constant motion. A
violet-coloured auroral vapour is often seen simultaneously on different
parts of the sky.

[Sidenote: Aurora in stormy weather. Fragments.]

“Or, again, there has been tempestuous weather, and it is now, let us
suppose, passing away. Below, on the ice, the wind has fallen; but the
clouds are still driving rapidly across the sky, so that in the upper
regions its force is not yet laid. Over the ice it becomes somewhat
clear; behind the clouds appears an Aurora amid the darkness of the
night. Stars twinkle here and there; through the opening of the clouds we
see the dark firmament, and the rays of the Aurora chasing one another
towards the zenith. The heavy clouds disperse, mist-like masses drive on
before the wind. Fragments of the northern lights are strewn on every
side: it seems as if the storm had torn the Aurora bands to tatters,
and was driving them hither and thither across the sky. These threads
change form and place with incredible rapidity. Here is one! lo, it is
gone! Scarcely has it vanished before it appears again in another place.
Through these fragments drive the waves of light: one moment they are
scarcely visible, in the next they shine with intense brilliancy. But
their light is no longer that glorious pale green; it is a dull yellow.
It is often difficult to distinguish what is Aurora and what is vapour;
the illuminated mists as they fly past are scarcely distinguishable from
the auroral vapour which comes and goes on every side.

[Sidenote: Bands. Rays reach the pole. No noise.]

“But, again, another form. Bands of every possible form and intensity
have been driving over the heavens. It is now eight o’clock at night,
the hour of the greatest intensity of the northern lights. For a moment
some bundles of rays only are to be seen in the sky. In the south a
faint, scarcely visible band lies close to the horizon. All at once it
rises rapidly, and spreads east and west. The waves of light begin to
dart and shoot, some rays mount towards the zenith. For a short time it
remains stationary, then suddenly springs to life. The waves of light
drive violently from east to west, the edges assume a deep red and green
colour, and dance up and down. The rays shoot up more rapidly, they
become shorter; all rise together and approach nearer and nearer to the
magnetic pole. It looks as if there were a race among the rays, and that
each aspired to reach the pole first. And now the point is reached, and
they shoot out on every side, to the north and the south, to the east and
the west. Do the rays shoot from above downwards, or from below upwards?
Who can distinguish? From the centre issues a sea of flames: is that sea
red, white, or green? Who can say? It is all three colours at the same
moment! The rays reach almost to the horizon: the whole sky is in flames.
Nature displays before us such an exhibition of fireworks as transcends
the powers of imagination to conceive. Involuntarily we listen; such
a spectacle must, we think, be accompanied with sound. But unbroken
stillness prevails; not the least sound strikes on the ear. Once more
it becomes clear over the ice, and the whole phenomenon has disappeared
with the same inconceivable rapidity with which it came, and gloomy night
has again stretched her dark veil over everything. This was the Aurora
of the coming storm—the Aurora in its fullest splendour. No pencil can
draw it, no colours can paint it, and no words can describe it in all its
magnificence.”

A reproduction of the woodcut in Payer’s ‘Austrian Arctic Voyages,’
illustrating some of the features of the above description, will be found
on Plate I.

In the ‘Edinburgh Encyclopædia,’ article “Aurora,” we find:—

[Sidenote: Descriptions of Auroræ in high Northern latitudes.]

“In high Northern latitudes the Auroræ Boreales are singularly
resplendent, and even terrific.

“They frequently occupy the whole heavens, and, according to the
testimony of some, eclipse the splendour of stars, planets, and moon, and
even of the sun itself.

[Sidenote: In Siberia.]

“In the south-eastern districts of Siberia, according to the description
of Gmelin, cited and translated by Dr. Blagden (Phil. Trans. vol. lxxiv.
p. 228), the Aurora is described to begin with single bright pillars,
rising in the north, and almost at the same time in the north-east,
which, gradually increasing, comprehend a large space of the heavens,
rush about from place to place with incredible velocity, and finally
almost cover the whole sky up to the zenith, and produce an appearance
as if a vast tent were expanded in the heavens, glittering with gold,
rubies, and sapphires. A more beautiful spectacle cannot be painted; but
whoever should see such a northern light for the first time could not
behold it without terror.”

[Sidenote: Maupertius’s description at Oswer-Zornea.]

Maupertius describes a remarkable Aurora he saw at Oswer-Zornea on the
18th December, 1876. An extensive region of the heavens towards the south
appeared tinged of so lively a red that the whole constellation of Orion
seemed as if dyed in blood. The light was for some time fixed, but soon
became movable, and, after having successively assumed all the tints of
violet and blue, it formed a dome of which the summit nearly approached
the zenith in the south-west.

[Sidenote: Red Auroræ rare in Lapland.]

Maupertius adds that he observed only two of the red northern lights in
Lapland, and that they are of very rare occurrence in that country.

The observations of Carl Bock, the Norwegian naturalist, kindly
communicated by him to me, and detailed in Chapter III., quite confirm
this observation of Maupertius as to the rare occurrence of red Auroræ in
Lapland, he having only seen one.

[Illustration: Plate I.]




CHAPTER III.

SOME SPECIFIC DESCRIPTIONS OF AURORÆ, INCLUDING RESULTS OF THE ENGLISH
ARCTIC EXPEDITION, 1875-76.


_Captain Sabine’s Auroræ._

[Sidenote: Captain Sabine’s Auroræ.]

Captain Sabine describes Auroræ seen at Melville Island (Parry’s first
voyage, January 15). Towards the southern horizon an ordinary Aurora
appeared. The luminous arch broke into masses streaming in different
directions, always to the east of the zenith.

[Sidenote: Curvature of arches towards each other.]

The various masses seemed to arrange themselves in two arches, one
passing near the zenith and a second midway between the zenith and the
horizon, both north and south, but curving towards each other. At one
time a part of the arch near the zenith was bent into convolutions like a
snake in motion and undulating rapidly.


_Aurora seen at Sunderland, February 8th, 1817._

(‘Annals of Philosophy,’ vol. ix. p. 250.)

[Sidenote: Aurora seen at Sunderland, Feb. 8, 1817. Formation of corona.]

It began about 7 P.M. during a strong gale from the N.W., with single
bright streamers in the N. and N.W., which covered a large space and
rushed about from place to place with amazing velocity, and had a fine
tremulous motion, illuminating the hemisphere as much as the moon does
eight or nine days from change. About 11 o’clock part of the streamers
appeared as if projected south of the zenith and looked like the pillars
of an immense amphitheatre, presenting a most brilliant spectacle and
seeming to be in a lower region of the atmosphere, and to descend and
ascend in the air for several minutes. (This appears to have been the
formation of a corona.) One streamer passed over Orion, but neither
increased nor diminished its splendour.


_Description of Aurora by Dr. Hayes, 6th January, 1861._

[Sidenote: Dr. Hayes’s Aurora, 6th January, 1861.]

‘Recent Polar Voyages’ contains a narrative of the voyage of Dr. Hayes,
who sailed from Boston on the 6th of July, 1860, and wintered at Port
Foulbe. He witnessed a remarkable display of the Aurora Borealis on the
morning of the 6th January, 1861.

[Sidenote: Development of Aurora.]

The darkness was so profound as to be oppressive. Suddenly, from the rear
of the black cloud which obscured the horizon, flashed a bright ray.
Presently an arch of many colours fixed itself across the sky, and the
Aurora gradually developed.

[Sidenote: Rays changed to glow.]

The space within the arch was filled by the black cloud; but its borders
brightened steadily, though the rays discharged from it were exceeding
capricious, now glaring like a vast conflagration, now beaming like the
glow of a summer morn. More and more intense grew the light, until, from
irregular bursts, it matured into an almost uniform sheet of radiance.
Towards the end of the display its character changed. Lurid fires flung
their awful portents across the sky, before which the stars seemed to
recede and grow pale.

[Sidenote: Mixed colours. Colours change. Tongues of white flame formed.]

The colour of the light was chiefly red; but every tint had its turn,
and sometimes two or three were mingled; blue and yellow streamers shot
across the terrible glare, or, starting side by side from the wide
expanse of the radiant arch, melted into each other, and flung a strange
shade of emerald over the illuminated landscape. Again this green subdues
and overcomes the red; then azure and orange blend in rapid flight,
subtle rays of violet pierce through a broad flash of yellow, and the
combined streams issue in innumerable tongues of white flame, which mount
towards the zenith.

The illustration which accompanies this description in the work is
reproduced on Plate II., and forcibly reminds one of the “curtains” of
the Aurora described in the preceding Chapter by Mons. Lottin.

[Illustration: Plate II.]


_Prof. Lemström’s Aurora of 1st September, 1868._

[Sidenote: Prof. Lemström’s Aurora, 1st September, 1868.]

In the first Swedish Expedition, 1868, some remarkable observations were
made on the appearance of luminous beams around the tops of mountains,
which M. Lemström showed by the spectroscope to be of the same nature as
Auroræ.

[Sidenote: Aurora from earth’s surface. Yellow-green line seen.]

On the 1st September, 1868, on the Isle of Amsterdam in the Bay of
Sweerenberg, there was a light fall of snow, and the snowflakes were
observed falling obliquely. All at once there appeared a luminous
phenomenon which, starting from the earth’s surface, shot up vertically,
cutting the direction of the falling snowflakes, and this appearance
lasted for some seconds. On examination with a spectroscope the
yellow-green line was found by Lemström (but of feeble intensity) when
the slit of the instrument was directed towards a roof or other object
covered with snow, and even in the snow all round the observer.

[Sidenote: Lemström’s conclusions.]

M. Lemström concluded that an electric discharge of an auroral nature,
which could only be detected by means of the spectroscope, was taking
place on the surface of the ground all around him, and that, from a
distance, it would appear as a faint display of Aurora.

[It should, however, here be noted that the reflection of an Aurora from
a white or bright surface would give, in a fainter degree, the spectrum
of the Aurora itself; and, apart from the phenomena seen by the eye, the
case fails to be conclusive that an Aurora on the surface of the ground
was examined.]


_Mr. J. R. Capron’s Aurora of October 24th, 1870._

[Sidenote: Mr. J. R. Capron’s Aurora, Oct. 24, 1870. Silver glow in
north. Phosphorescent cloud-streamers. Crimson masses on horizon.
Coloured streamers. Corona formed. Aurora fades away.]

The description, from my notes made at the time of this fine display, is
as follows:—“Last evening (October 24) the Aurora Borealis was again most
beautifully seen here (Guildford). At 6 P.M. indications of the coming
display were visible in the shape of a bright silver glow in the north,
which contrasted strongly with the opposite dark horizon. For two hours
this continued, with the addition from time to time of a crimson glow in
the north-east, and of streamers of opaque-white phosphorescent cloud,
shaped like horse-tails (very different from the more common transparent
auroral diverging streams of light), which floated upwards and across
the sky from east and west to the zenith. At about 8 o’clock the display
culminated; and few observers, I should think, ever saw a more lovely
sky-picture. Two patches of intense crimson light about this time massed
themselves on the north-east and north-west horizon, the sky between
having a bright silver glow. The crimson masses became more attenuated
as they mounted upwards; and from them there suddenly ran up bars or
streamers of crimson and gold light, which, as they rose, curved towards
each other in the north, and, ultimately meeting, formed a glorious arch
of coloured light, having at its apex an oval white luminous corona
or cloud of similar character to the phosphorescent clouds previously
described, but brighter. At this time the spectator appeared to be
looking at the one side of a cage composed of glowing red and gold bars,
which extended from the distant parts of the horizon to a point over
his head. Shortly after this the Auroral display gradually faded away,
and at 9 o’clock the sky was of its usual appearance, except that the
ordinary tint seemed to have more of indigo, probably by contrast with
the marvellous colours which had so lately shone upon it.”

[Sidenote: T. F.’s description of same at Torquay. Mr. Gibbs’s report in
London.]

T. F., describing the same Aurora from Torquay, says it showed itself at
sundown, attained its maximum at 8, and lasted until 11. At 8 o’clock
more than half the visible heavens was one sea of colour; the general
ground greenish yellow and pale rose, with extensive shoals of deep rose
in the east and west; while from the north, streaming upwards to and
beyond the zenith, were tongues and brushes of rosy red, so deep that the
sky between looked black. Mr. Gibbs reported that in London, at about 8
o’clock, brilliant crimson rays shot up to the zenith, and the sky seemed
one mass of fire.

A facsimile of my water-colour sketch of this fine discharge is given on
Plate III.

[Illustration: Plate III.]


_Mr. Barker’s (superposed) red and white Auroræ, 9th November (1870?)._

[Sidenote: Mr. Barker’s Auroræ, 9th November (1870?). Red Aurora. White
Aurora.]

On the 9th November (1870?) Mr. Barker saw at New Haven (U. S.) a most
magnificent crimson Aurora. At about a quarter to 6 P.M. it consisted of
a brilliant streamer shooting up from the north-western horizon. This
was continued in a brilliant red, but rather nebulous, mass of light
passing upwards and to the north. Its highest points were from 30° to 40°
in altitude. A white Aurora, consisting of bright streamers, appeared
simultaneously and extended round to the north-east. Prof. Newton
informed Mr. Barker that he had observed an equally brilliant red patch
of auroral light in the north-east five or ten minutes earlier.

[Sidenote: Red seen through white.]

Since the lower end of the red streamers was much lower than that of the
white, it would seem as if the red were seen through the white, the red
being most remote.

[Sidenote: Crimson line not seen in white Aurora.]

Spectroscopic observations of this Aurora were made. The crimson Aurora
lasted less than half an hour, and then disappeared. In the white Aurora,
which remained, the crimson line could not be seen.

[Sidenote: Carl Bock’s vibrating rays.]

It may be here noted that during the Aurora seen by Carl Bock in
Lapland, and painted by him by its own light (described, p. 25), he had
the impression of sets of vibrating rays behind each other, and in the
drawing it looks as if streamers were seen behind an arc.

[Illustration: Plate IV.]


_Mr. J. R. Capron’s Aurora of February 4th, 1872._

[Sidenote: Mr. J. R. Capron’s Aurora, Feb. 4, 1872. Masses of
phosphorescent vapour. Rose tints appeared. Aurora from behind clouds.
Formation of corona. Duration of corona. Streamers from corona. Rain
during Aurora. Wind during night. Phosphorescent clouds preceded the
Aurora in daylight.]

My description of this Aurora as seen at Guildford, and as given at the
time, is as follows:—“Last evening, returning from church a little before
8 P.M., the sky presented a weird and unusual aspect, which at once
struck the eye. A lurid tinge upon the clouds which hung around suggested
the reflection of a distant fire; while scattered among these, torn and
broken masses of vapour, having a white and phosphorescent appearance,
and quickly changing their forms, reminded me of a similar appearance
preceding the great Aurora of 24th October, 1870. Shortly some of these
shining white clouds or vapours partly arranged themselves in columns
from east to west, and at the same time appeared the characteristic
patches of rose-coloured light which are often seen in an auroral
display. About 8 o’clock the clouds had to a certain extent broken away,
and the Aurora shone out from behind heavy banks of vapour, which still
rested on the eastern horizon, the north-west horizon being free from
cloud and glowing brightly with red light. And now, at about 8.15, was
presented a most beautiful phenomenon. While looking upwards, I saw a
corona or stellar-shaped mass of white light form in the clear blue sky
immediately above my head[4], not by small clouds or rays collecting,
but more in the way that a cloud suddenly forms by condensation in the
clear sky on a mountain top, or a crystal shoots in a transparent liquid,
having too, as I thought, an almost traceable nucleus or centre, from
which spear-like rays projected. From this corona in a few seconds shot
forth diverging streamers of golden light, which descended to and mingled
with the rosy patches of the Aurora hanging about the horizon. The spaces
of sky between the streamers were of a deep purple (probably an effect of
contrast). The display of the corona, though lasting a few minutes only,
was equal to, if not excelling in beauty, the grand display of October
1870, before described, in which case, however, a ring or disk of white
light of considerable size took the place of the stellar-shaped corona.
What struck me particularly was the corona developing itself as from a
centre in the clear sky, and the diverging streamers apparently shooting
downwards, whereas in general the streamers are seen to shoot up from the
horizon and converge overhead. The effect may have been an illusion; but,
if so, it was a remarkable one. The general Aurora lasted for some time,
till it was lost in a clouded sky; and, in fact, rain was descending at
one time while the Aurora was quite bright. Strong wind prevailed during
the night[5]. The Aurora was probably very extensive, as the evening,
notwithstanding the clouds, was nearly as bright as moonlight. The
peculiar clouds referred to must have preceded the Aurora in daylight,
as I recollect seeing them at 6.30 as we went to church.”

[Sidenote: Aurora predicted.]

They had even then a peculiarly wild, ragged, and phosphorescent
appearance, and so much resembled some I had seen to accompany the Aurora
of October 1870, that I predicted (as came to pass) a display later in
the evening. A _facsimile_ of my water-colour sketch of this Aurora is
given on Plate IV. fig. 1, while the corona and rays are represented
(with rather too hard an outline) on Plate V. fig. 2.

[Illustration: Plate V.]


_Description of an Aurora seen at Cardiff._

[Sidenote: Aurora seen at Cardiff. Formation of corona.]

An Aurora was seen at Cardiff. A dusky red aspect of the sky towards
the north, and extending itself across the zenith westward, made its
appearance about half-past 5 P.M. The lights reached their greatest
intensity at 6 o’clock, when the sky was suffused with a rich crimson
glow, a broad band of colour reaching from N.E. to W. A corona of deep
hue, having rugged sharply defined edges, stood out prominently in the
zenith, apparently on a parallel plane to the earth, and having its
centre almost immediately over the head of the spectator.

[Sidenote: Radii thrown out from corona.]

From this corona, elliptic in form, and in its broadest diameter about
four times the size of the moon, there were thrown out brilliant silvery
blue radii, extending to the N.E. and W. horizon, and presenting the
appearance of a vast cupola of fire.

[Sidenote: Rain fell when Aurora died out.]

At half-past 6 the lights died completely out, leaving masses of cloud
drifting up from the south, and a shower of rain fell. The corona was
remarked upon as unusual. At Edinburgh the sky was brilliant for several
hours. (The date of this Aurora is uncertain, as the account is from an
undated newspaper cutting. It is supposed to be in February 1872, but
could hardly have been on the 4th, as the Aurora of that date did not
reach its maximum development at Edinburgh till 8 P.M.)


_Mr. J. R. Capron’s Aurora, seen at Guildown, Guildford, February 4th,
1874._

[Sidenote: Silvery brightness in N.E. Light-cloud, which moved from E. to
W. Formation of arc in N. Streamers. Horizontal clouds of misty light.]

About 7 P.M. my attention was drawn to a silvery brightness in the
north-east. Above, and still more to the east, was a bright cloud of
light, which looked dense and misty, and gave one the impression of an
illuminated fog-cloud. The edges were so bright that the adjacent sky,
but for the stars shining in it, might, by contrast, have been taken for
a dark storm-cloud. The light-cloud expanded upwards until its apex
became conical, and then moved rapidly from east, along the northern
horizon, until it reached the due west, where it rested, and formed for
some time a luminous spot in the sky. About the same time a long low arch
of light formed along the northern horizon, having a brighter patch at
each extremity; and these being higher in the sky, the arch and turned-up
ends were in shape like a Tartar bow. This bow was permanent; and later
on a cloud of rose-coloured light formed in the east, looking like the
reflection of a distant fire. From the bow also shot up curved streamers
of silver light towards the zenith, which at one time threatened to form
a corona. This, however, did not happen, and the Aurora gradually faded
away, until, when the moon rose about 8, a silver tinge in the east alone
remained. I should also mention that fleecy horizontal clouds of misty
light floated in the north above the bow across the streamers.

Mr. H. Taylor informed me he saw a similar Aurora some three weeks
before, in which the bright horizontal light and short white streamers
were the main characters. I am not sure that the horizontal light-clouds
were not actual mist-clouds illuminated by reflection of the Aurora;
not so, however, I think, the first-mentioned cloud, which had more the
appearance of the _aura_ in the large end of an illuminated Geissler tube.

[Sidenote: Spectrum of the Aurora described.]

I examined the Aurora with a Browning direct-vision spectroscope, and
found Ångström’s line quite bright, and by the side of it three faint
and misty bands towards the blue end of the spectrum upon a faintly
illuminated ground. I could also see at times a bright line beyond
the bands towards the violet. There was not light enough to take any
measurements of position of the lines.

I made a pencil sketch of this Aurora, at the time when the light-cloud
had moved W. and the arc formed, and of the spectrum. These drawings are
reproduced on Plate VI. figs. 1 and 1_a_.

[Illustration: Plate VI.]


_Mr. Herbert Ingall’s Aurora, July 18th, 1874._

[Sidenote: Mr. Herbert Ingall’s Aurora, July 18, 1874. Haze canopy
formed. Bright bluish flames appeared. Beams and streamers appeared.
Oscillatory motion of rays.]

An Aurora of July 18th, 1874, seen by Mr. Herbert Ingall at Champion
Hill, S.E., was described by him as an extraordinary one. About 11
the sky was clear; at midnight the sky was covered by a sort of haze
canopy, sometimes quite obscuring the stars, and then suddenly fading
away. Mr. Ingall was shortly after remarking the sky in the S.E. and S.
horizon as being more luminous than usual, when his attention was drawn
to a growing brightness in the S.W., and a moment afterwards bright
bluish flames “swept over the S.W. and W. horizon, as if before a high
wind. They were not streamers, but bright blue flames.” They lasted
about a minute and faded; but about two minutes afterwards a glowing
luminosity appeared in the W.S.W., and broke into brilliant beams and
streamers. The extreme rays made an angle of 90° with each other, the
central ray reaching an altitude of 50°. The extreme divergence of the
streamers (indicating their height above the earth’s surface), and
their direction (from W.S.W. to E.N.E.) at right angles to the magnetic
meridian, suggested to Mr. Ingall a disturbance of an abnormal character.
The rays had an oscillatory motion for about fifteen seconds, and then
disappeared, “as if a shutter had suddenly obscured the source of light.”

[Sidenote: Mr. Ingall’s remarks corroborated.]

Mr. Ingall’s remarks were corroborated by an observer in lat. 54° 46´
6″·2 N., long. 6h 12m 19s·75 W. The display, however, was more brilliant,
and the intensity of light at midnight illuminated the whole district
as with an electric light. The rays, too, bore tints differing from one
another; the largest seemed to partake of the nature of the blue sky,
while the smaller ones, running parallel with the horizon, were ever
changing from blue to orange-red.

[Sidenote: Rev. C. Gape saw flashes or streaks of a pale blue colour.]

On June 25 (same year?), between 9 and 10 o’clock, the Rev. Chas. Gape
saw at Rushall Vicarage, Scole, Norfolk, in the E.S.E., very frequent
flashes or streaks of a pale blue colour darting from the earth towards
the heavens like an Aurora. The day had been dull and close, with distant
thunder. In the E.S.E. it was dark, but overhead and everywhere else it
was clear and starry.


_Mr. J. R. Capron’s White Aurora of September 11th, 1874._

[Sidenote: Mr. J. R. Capron’s white Aurora of Sept. 11, 1874.]

On September 11, 1874, we were at Kyle Akin, in the Isle of Skye. The
day had been wet and stormy, but towards evening the wind fell and the
sky became clear. About 10 P.M. my attention was called to a beautiful
Auroral display.

[Sidenote: Double arc of pure white light in the N.]

No crimson or rose tint was to be seen, but a long low-lying arc of the
purest white light was formed in the north, and continued to shine with
more or less brilliancy for some time. The arc appeared to be a double
one, by the presence of a dark band running longitudinally through it.

[Sidenote: White streamers. Auroral bow believed to be near the earth.]

Occasional streamers of equally pure white light ran upwards from
either end of the bow. The moon was only a day old, but the landscape
was lighted up as if by the full moon; and the effect of Kyle Akin
lighthouse, the numerous surrounding islands, and the still sea between
was a true thing of beauty. The display itself formed a great contrast
to the more brilliant but restless forms of Auroræ generally seen. I
particularly noticed a somewhat misty and foggy look about the brilliant
arc, giving it almost a solid appearance. The space of sky between the
horizon and the lower edge of the arc was of a deep indigo colour,
probably the effect of contrast. I had a strong impression that the bow
was near to the earth, and was almost convinced that the eastern end and
some fleecy clouds in which it was involved were between myself and the
peaks of some distant mountains.

I have not seen any other account of this Aurora, of which I was able at
the time to obtain a sketch. This is reproduced on Plate VII. It was a
lovely sight, and wonderfully unlike the cloud-accompanied and crimson
Auroræ which I had seen in the South.

It is noticed in Parry’s ‘Third Voyage’ that the lower edge of the
auroral arch is generally well defined and unbroken, and the sky beneath
it so exactly like a dark cloud (to him often of a brownish colour), that
nothing could convince to the contrary, if the stars, shining through
with undiminished lustre, did not discover the deception.

[Sidenote: No trace of brown colour in segment of sky below the arc.]

I saw no trace of brown colour. The segment below the arch resting on the
horizon was of a deep indigo colour.

[Illustration: Plate VII.]


_Dr. Allnatt’s Aurora, June 9th, 1876._

[Sidenote: Dr. Allnatt’s Aurora, June 9, 1876. Band of auroral light
appeared. Streaks of cirro-stratus divided the Aurora. Want of electric
manifestations attributed to absence of sun-spots.]

Dr. Allnatt, writing to the ‘Times’ from Abergele, North Wales, near
the coast of the Irish Channel, reported an Aurora on the night of the
9th June, 1876. After a cool and gusty day, with a strong N.E. wind
and a disturbed sea, there appeared at 11 P.M. in the N. horizon a
broad band of vivid auroral light, homogeneous, motionless, and without
streamers. About midnight a long attenuated streak of black cirro-stratus
stretched parallel with the horizon, and divided the Aurora into nearly
symmetrical sections. On the preceding day the sky was covered with dark
masses of electric cloud of weird and fantastic forms. The season had
been singularly unproductive of high electric manifestations, which Dr.
Allnatt thought might be attributable to the comparative absence of spots
on the solar disk. [It may here be noted how conspicuous the years 1877
and 1878 have been for absence of Sun-spots and of Auroræ.]

[Illustration: Plate VIII.]


_Herr Carl Bock’s Lapland Aurora, 3rd October, 1877._

[Sidenote: Herr Carl Bock’s Lapland Aurora, 3rd Oct. 1877. Lapland Auroræ
generally of the yellow type.]

In January 1878 I had the pleasure to meet, at the Westminster Aquarium,
Herr Carl Bock, the Norwegian naturalist, who accompanied four
Laplanders, two men and two women, with sledges, tents, &c., on their
visit to this country. The Laplanders (as mentioned elsewhere) did not
confirm the accounts of noises said to have been observed by Greenlanders
and others during the Aurora. Carl Bock mentioned to me that the displays
he saw in Lapland were most brilliant, but generally of the yellow type
(the Laplanders called the Aurora “yellow lights”). He saw only one red
Aurora. He kindly lent me a picture (probably in its way unique), an
oil-painting of an Aurora Borealis, entirely sketched by the light of the
Aurora itself.

[Sidenote: A picture painted by light of the Aurora. Movement of the
rays. Inner edge of arc fringed with rays.]

The painting is remarkable for the tender green of the sky, an effect
probably due to a mixture of the ordinary sky colour with the yellow
light of the Aurora. This picture was taken at Porsanger Fjord, in lat.
71° 50´, on 3rd October, 1877. It lasted from 9 P.M. till about 11
P.M. The rays kept continually moving, and certain of them seemed in
perspective and behind the others. It will be noticed that the _inner_
edge of the arc is fringed with rays, contrary to the sharp and definite
margin which is usually presented. Probably two Auroræ or auroral forms
were seen—a quiescent arc in front, and a set of moving streamers beyond.
Two larger and brighter patches of light are seen at each extremity of
the arc, as in the case of the Aurora seen by me at Guildown, February
4th, 1874, which, indeed, the display much resembles. A reduced facsimile
of Herr Bock’s excellent picture is given on Plate VIII.

[Sidenote: Aurora of longitudinal rays.]

Herr Bock also acquainted me that on the following day he saw an
Aurora in which the lines of light, instead of being vertical, were
longitudinal, and were continually swept along in several currents. They
were not so strong as in the former case.


_Rev. T. W. Webb’s Aurora._

[Sidenote: Rev. T. W. Webb’s Aurora. Arc resolved into sets of streamers
moving in opposite directions.]

The Rev. T. W. Webb has described to me in a letter an Aurora very like
that seen by Carl Bock in Lapland, and apparently the prevailing type
in those regions. An arc similar to that figured by Carl Bock appeared
in the N.W., and seemed to resolve itself into two sets of streamers
moving in opposite directions (or the one set might be fixed and the
other moving), like the edges of two great revolving toothed wheels. This
lasted but for a few seconds; but during that interval the tints were
varied and brilliant, including blue and green.


_The English Arctic Expedition 1875-76, under Capt. Sir George Nares._

[Sidenote: English Arctic Expedition, 1875-76. Instructions for use of
officers. Appendix B. Capt. Sir G. Nares’s report. True Auroræ seldom
observed, and displays faint. Citron-line observed on only two displays.
Appendix C.]

In anticipation of the starting of this Expedition, some instructions
for the use of the officers in connexion with the hoped for display of
brilliant Auroræ were prepared:—as to general features of the Auroræ, by
Professor Stokes; as to Polarization, by Dr. William Spottiswoode; and as
to Spectrum work, by Mr. Norman Lockyer and myself. As these instructions
were somewhat elaborate, and will apply to all Auroral displays, I have
supplied a copy of them in Appendix B. They were unfortunately not
brought into requisition, for want of the Auroræ themselves. Capt. Sir
George Nares has reported to the Admiralty, under date 5th December,
1877, as follows:—“Although the auroral glow was observed on several
occasions between 25 October, 1875, and 26 February, 1876, true Auroræ
were seldom observed; and the displays were so faint, and lasted so short
a time, and the spectrum observations led to such poor results, that no
special report has been considered necessary. Although the citron-line
was observed occasionally, on only two displays of the Aurora was it well
defined, and then for so short a time that no measure could be obtained.”
(For Sir George Nares’s further Report see Appendix C, containing
extracts from blue-book, ‘Results derived from the Arctic Expedition,
1875-6.’)


_Aurora Australis._

[Sidenote: Aurora Australis. Mr. Forster’s description. Long columns of
white light spreading over the whole sky.]

In an article on Auroræ in high Southern latitudes (Phil. Trans. No.
461, and vol. liv. No. 53), we find that Mr. Forster, who as naturalist
accompanied Capt. Cook on his second voyage round the world, says:—“On
February 17th, 1773, in south latitude 58°, a beautiful phenomenon was
observed during the preceding night, which appeared again this, and
several following nights. It consisted of long columns of a clear white
light shooting up from the horizon to the eastward almost to the zenith,
and gradually spreading over the whole southern part of the sky. These
columns were sometimes bent sideways at their upper extremities; and
though in most respects similar to the northern lights of our hemisphere,
yet differed from them in being always of a whitish colour, whereas ours
assume various tints, especially those of a fiery and purple hue. The sky
was generally clear when they appeared, and the air sharp and cold, the
thermometer standing at the freezing point.” This account agrees very
closely in particulars with Capt. Maclear’s notice of Aurora Australis
[after referred to], and especially in the marked absence of red Auroræ.

The height of the barometer does not appear to be mentioned, the
temperature being apparently much the same as in the more recent cases.

[Sidenote: Capt. Maclear’s Aurora Australis, 3rd March, 1874. Light of
pale yellow tint only.]

In a letter dated from H.M.S. ‘Challenger,’ North Atlantic, April 10th,
1876, Capt. Maclear was good enough to communicate to me some particulars
of an Aurora Australis seen 3rd March 1874, in lat. 54° S., long. 108° E.
The letter is mainly descriptive of the spectrum (which will be described
in connexion with the general question of the spectrum of the Aurora).
It states that the red line was looked for in vain, and that the light
appeared of a _pale yellow_, and had none of the rosy tint seen in the
northern displays.

[Sidenote: Capt. Maclear’s Auroræ described in ‘Nature.’]

Capt. Maclear has since contributed to ‘Nature,’ of 1st November 1877, a
description of four Auroræ seen from the ‘Challenger’ in high southern
latitudes (including the one communicated to me). He speaks of the
opportunity of observing as not frequent, either from the rarity of
the phenomena, or because the dense masses of cloud prevalent in those
regions prevented their being seen except when exceptionally bright.
There were four appearances described:—

[Sidenote: Feb. 9, 1874.]

(1.) At 1.30 on the morning of February 9th, 1874, preceded by a watery
sunset, lat. 57° S. and long. 75° E., bar. 29·0 in., ther. 35°; brilliant
streaks to the westward. Day broke afterwards with high cirrus clouds and
clear horizon.

[Sidenote: Feb. 21, 1874.]

(2.) At 9.30 P.M., February 21, 1874, lat. 64° S., long. 89° E., bar.
28·8 in., ther. 31°; one bright curved streamer. The Aurora preceded a
fine morning with cumulo-stratus clouds, extending from Jupiter (which
appeared to be near the focus) through Orion and almost as far beyond.
Under this a black cloud, with stars visible through it. Real cumuli
hid great part of the remainder of the sky, but there were two vertical
flashing rays which moved slowly to the right (west). Generally the
Aurora was still bright.

[Sidenote: March 3, 1874. Auroral line found in light to southward.]

(3.) At midnight, March 3rd, 1874, lat. 53° 30´ S., long. 109° E., bar.
29·1, ther. 36°, after some days’ stormy weather, a brilliant sunset,
followed by a fine morning. Soon after 8 P.M. the sky began to clear
and the moon shone out. Noticing the light to the southward to be
particularly bright, Capt. Maclear applied the spectroscope, and found
the distinguishing auroral line.

[Sidenote: Brilliant white clouds seen.]

About midnight the sky was almost clear, but south were two or three
brilliant light clouds, colour very white-yellow, shape cumulo-stratus.
From about west to near south extended a long feathery light of the same
colour, parallel with the horizon, and between south and west there
appeared occasionally brilliant small clouds. The upper edges seemed
hairy, and gave one the idea of a bright light behind a cloud. The forms
changed, but no particular order was noticed.

(Here follows a description of the spectrum, and the mode in which a
delineation by the lines was obtained.)

[Sidenote: March 6, 1874. Capt. Maclear suggests whether a low barometer
has to do with the absence of red.]

(4.) At 8 P.M., March 6th, 1874. This was a slight Aurora, seen to the
southward; after this the clouds changed to high cirrus. Capt. Maclear
suggests whether a low barometer has any thing to do with the absence
of red in the spectrum, the normal state of the barometer being an inch
lower in those regions than in more temperate latitudes.

[Sidenote: Barometer falls after the Aurora, and strong gale from the S.
or S.W. follows.]

Edin. Encyc. vol. iii. article “Aurora.” Dr. Kirwan observed that the
barometer commonly falls after the Aurora. Mr. Winn, in the seventy-third
volume of the Phil. Trans., makes the same remark, and says that in
twenty-three instances, without fail, a strong gale from the south or
south-west followed the appearance of an Aurora. If the Aurora were
bright, the gale came on within twenty-four hours, but was of no long
continuance; if the light was faint and dull, the gale was less violent,
longer in coming, and longer in duration.

[Sidenote: Pale yellow glow rare in the Aurora Borealis.]

The pale yellow-coloured glow referred to by Capt. Maclear is, in my
experience, rare in the Aurora Borealis. It is probably the “æqualiter et
sine eruptionibus aut radiis _fulvi_,” described by Seneca (_antè_, p.
1), and may probably belong to more southern climes.

[Sidenote: Spectrum of Auroræ Australes extends more into the violet.]

We shall see too, by-and-by, that these Auroræ Australes as to spectrum
extend more into the violet than the Aurora Borealis. The yellow, as
complementary to violet, is likely thus to make (in the absence of the
red) its appearance.

It is, however, somewhat singular that Carl Bock found almost exclusively
yellow Auroræ in Lapland.

In Proctor’s ‘Borderland of Science,’ article “The Antarctic Regions,”
we find quoted a passage from a letter by Capt. Howes, of the ‘Southern
Cross,’ in which a graphic description is given of a Southern Aurora:—

[Sidenote: Capt. Howes’s description of a Southern Aurora.]

“At about half-past one on the 2nd of last September the rare phenomenon
of the Aurora Australis manifested itself in a most magnificent manner.
Our ship was off Cape Horn, in a violent gale, plunging furiously into
a heavy sea, flooding her decks, and sometimes burying her whole bows
beneath the waves. The heavens were as black as death, not a star was to
be seen, when the brilliant spectacle first appeared.

[Sidenote: Balls of electric fire resting on mast-heads &c.]

“I cannot describe the awful grandeur of the scene; the heavens gradually
changed from murky blackness till they became like vivid fire, reflecting
a lurid glowing brilliancy over every thing. The ocean appeared like
a sea of vermilion lashed into fury by the storm, the waves dashing
furiously over our side, ever and anon rushed to leeward in crimson
torrents. Our whole ship—sails, spars, and all—seemed to partake of
the same ruddy hues. They were as if lighted up by some terrible
conflagration. Taking all together—the howling, shrieking storm, the
noble ship plunging fearlessly beneath the crimson-crested ways, the
furious squalls of hail, snow, and sleet, drifting over the vessel, and
falling to leeward in ruddy showers, the mysterious balls of electric
fire resting on our mast-heads, yard-arms, &c., and, above all, the awful
sublimity of the heavens, through which coruscations of auroral light
would shoot in spiral streaks, and with meteoric brilliancy,—there was
presented a scene of grandeur surpassing the wildest dreams of fancy.”

The foregoing picture presents a singular contrast to the yellow-white
Auroræ described as seen in high southern latitudes by Capt. Maclear,
and is interesting as a southern Aurora of a red or ruddy tint. Looking,
however, at the extreme rarity of red Auroræ in those latitudes, and
the description of “mysterious balls of electric fire resting on our
mast-heads, yard-arms, &c.” (a phenomenon not often noticed in connexion
with the Aurora), it suggests itself that the case in question may have
been an instance not of a true Aurora, but of an electric display, with
conditions approaching those experienced by travellers who have found
themselves in mountainous districts surrounded by storm-clouds charged
with electricity[6].


_Prof. Piazzi Smyth’s Typical Auroræ._

[Sidenote: Prof. Piazzi Smyth’s typical Auroræ.]

Prof. Piazzi Smyth was kind enough lately to send me the fourteenth
volume of the ‘Astronomical Observations made at the Royal Observatory,
Edinburgh, during the years 1870-1877.’ This volume, amongst its other
interesting matter, affords some valuable information on the subject
of the Aurora Borealis. The Aurora plates are five in number, three
comprising some well-executed chromo-lithographs of typical Auroræ, from
sketches made by Prof. Smyth, the other two plates being of the Aurora
spectrum. The Auroræ delineated are thus described:—

[Sidenote: Aug. 6, 1871, quiescent arc. August 21, 1871, active arc.]

Plate 5. (August 6, 1871.) An example of a mild quiescent kind of auroral
arc, with dark cavernous substratum. (August 21, 1871.) An example of a
bright large active arc darting out rays.

[Sidenote: Sept. 7, 1871, arc streamers and clouds. May 8, 1871, double
arc (longitudinal).]

Plate 6. (September 7, 1871.) An auroral arc, with streamers and dark
clouds, and maintaining a bright appearance though in proximity to
the moon. (May 8, 1871.) A double-arched auroral arc (the arches are
longitudinally arranged).

[Sidenote: April 28, 1871, multiple arc. Oct. 25, 1870, coloured Aurora.]

Plate 7. (April 28, 1871.) A multiple-arched arc of Aurora with
moonlight. (October 25, 1870.) A case of grandest coloured Auroræ, or
Aurora superb and almost universal.

All the foregoing drawings are very vivid and striking, and form a most
interesting set of typical forms of Auroræ.

According to my own experience, the Aurora with arches arranged
longitudinally, thus, [Illustration], is the rarest of all the forms. I
have not met with it myself, nor do I recollect an illustration of one
other than Prof. Smyth’s.




CHAPTER IV.

PHENOMENA SIMULATING AURORÆ.

_Auroric Lights (Kinahan)._


[Sidenote: Mr. Kinahan’s Auroric Lights. White and red. White light
appears in pencils radiating from a point.]

Mr. G. Henry Kinahan writes to ‘Nature,’ from Ovoca, under date January
27th, 1877, and speaks of two distinct kinds of light so classed—one
brilliant and transparent, of a white yellowish-blue or yellowish-red
colour, while the other is semi-opaque and of a bloody red colour, the
latter being considered in Ireland a forerunner of bad weather. The first
kind generally appears as intermittent pencils of light that suddenly
appear and disappear.

[Sidenote: Frequently not stationary, but jumping about.]

Usually they proceed or radiate from some point near the north of the
horizon; but Mr. Kinahan has frequently seen them break from a point in
the heavens, not stationary, but jumping about within certain limits.
Sometimes these lights occur as suddenly flashing clouds of light of a
white colour, but at other times of blue and reddish yellow.

[Sidenote: In daylight like sun-rays. Red light appears in clouds
floating upwards or diffused.]

If this class of lights is watched into daylight, they appear somewhat
like faint rays of a rising sun. One morning, while travelling in West
Galway in the twilight, they were very brilliant, and quite frightened
Mr. Kinahan’s car-driver, who thought the sun was going to rise in the
north instead of the east. The second, or bloody red light, usually
occurs in clouds floating in one direction up into the heavens, but often
diffused over a portion of the sky. Mr. Kinahan has never seen them
coming from the east, and on only a few occasions from the south, but
generally from the west, north-west, or north.

[Sidenote: Red light appears as dirty misty clouds in daylight, or as a
mist or misty rays.]

If both kinds of light appear at the same time, the second while passing
over the first dims it. If the second class is watched into daylight,
they appear as dirty misty clouds that suddenly form and disappear
without the spectator being able to say where they come from or where
they go to, or as a hazy mist over a portion of the sky, that suddenly
appears and disappears, or as misty rays proceeding from a point in the
horizon. Generally, when these clouds occur, there is a bank of black
clouds to the westward.

[Sidenote: Season since October 1876 prolific in auroric light.]

Mr. Kinahan then speaks of the season as having been prolific in auroric
light, as there had been few nights since the 1st October then last
(1876) in which they did not appear. On many occasions they were late
in the night, being very common and brilliant during the dark days of
December, a few hours before dawn (about 5 o’clock). Each time there was
a fine day they appeared also, and the weather broke again.

[Sidenote: Mr. J. Allan Broun questions nature of these lights, as Aurora
is seldom seen at 5 A.M. in this country. On 77 occasions seen only twice
so early. Season was of marked infrequency elsewhere.]

Mr. Jno. Allan Broun refers to this graphic account of Mr. Kinahan’s,
and concludes there must have been some mistake as to the nature of
these “auroric lights,” as the Aurora Borealis is very rarely seen at
5 A.M. in this country. In the years 1844 and 1845, during which the
Aurora was sought for at Makerstown every hour of the night, it was
observed in 77 nights on an average of nearly three hours each night;
but it was seen only twice so early, and that with a bright or brilliant
Aurora, which remained during five hours on the first occasion, and
from 6 P.M. to 6 A.M. on the second. Parts of the phenomenon seen by
Mr. Kinahan, Mr. Broun also could not say he had ever seen; and if Mr.
Kinahan’s observations could have been confirmed it would have been most
important, especially as made so frequently at the epoch of minimum.
The description is in many respects a sufficiently recognizable one of
auroral discharges; but the frequent appearance in early morning is
certainly unusual, and few if any Auroræ seem to have been recorded as
appearing elsewhere in Great Britain during the time which Mr. Kinahan
refers to as so prolific (see, however, Dr. Allnatt’s, _antè_, p. 24). In
fact, the season in question was one of marked infrequency (see English
Arctic Expedition Report, _antè_, p. 26). Mr. Buchan furnished Mr. Broun
with a note of Auroræ seen in the stations of the Scottish Meteorological
Society during the year 1876, and they were 42 in number, 26 in the first
half, and 16 in the second half of the year. The greater part were seen
in the most northerly stations, including the Orkney, Shetland, and Faroe
Islands, and only 9 south of the Forth.


_Luminous Arch._

[Sidenote: Luminous arch, Sept. 11, 1814. Height above horizon 6 to 9
miles.]

In the ‘Annals of Philosophy,’ vol. iv. p. 362, there is a minute
description of a luminous arch which appeared in the sky on the night
of Sunday, September 11th, 1814, and was seen in the west of England
opposite the Irish Sea, the west part of the south of Scotland, and part
of the west of Ireland. It was described as a part of either a body of
dense greyish-white light, or a mass of luminous matter in the shape of
an arch. Its height above the horizontal line was estimated at not more
than 9 nor less than 6 miles.

[Sidenote: It moved southward, and was assumed to differ from the Aurora.]

Its direction when first seen was N. 80° E., and S. 80° W. It moved to
the southward. It was assumed to differ from the Aurora Borealis in
wanting coruscations, and in its having a much paler light.




CHAPTER V.

SOME QUALITIES OF THE AURORA.


_Noises attending Auroræ._

[Sidenote: Noises attending Auroræ. Gmelin affirms them. Other testimony
to them. Musschenbroek. Cavallo. Nairne. Belknap.]

In the Edinb. Encyc., Gmelin is stated, in continuation of his
description of an Arctic Aurora, to add:—“For however fine the
illumination may be, it is attended, as I have heard from the relation of
many persons, with such a hissing, cracking, and rushing noise through
the air, as if the largest fireworks were playing off.” To describe what
they then heard, the natives are said to use the expression, “Spolochi
chodjat”—that is, The raging host is passing. The hunter’s dogs, too,
are also described as so much frightened when the Auroræ overtake the
hunters, that they will not move, but lie obstinately on the ground till
the noise has passed. This account of noises seems to be confirmed by
other testimony. They are stated to have been heard at Hudson’s Bay and
in Sweden; and Musschenbroek mentions that the Greenland whale-fishers
assured him they had frequently heard the noise of the Aurora Borealis,
but adds that “no person in Holland had ever experienced this
phenomenon.” Mr. Cavallo declares he “has repeatedly heard a crackling
sound proceeding from the Aurora Borealis” (Elements of Nat. or Exper.
Phil. vol. iii. p. 449). Mr. Nairne mentions that in Northampton, when
the northern lights were very bright, he is confident he perceived a
hissing or whizzing sound. Mr. Belknap of Dover, New Hampshire, North
America, testifies to a similar fact (American Trans. vol. ii. p. 196).

[Sidenote: Sir John Franklin negatives them.]

Sir John Franklin mentions, in his ‘Journey to the Shores of the Polar
Sea’:—“Nor could we distinguish its (the Aurora’s) rustling noise, of
which, however, such strong testimony has been given to us that no doubt
can remain of the fact.”

[Sidenote: March 11th. Hissing noise heard during Aurora’s passage.
Explained to arise from the snow.]

In detail, he mentions he never heard any sound that could be
unequivocally considered as originating in the Aurora, although he had
had an opportunity of observing that phenomenon for upwards of 200 nights
(the Aurora was registered at Bear Lake 343 times without any sound being
heard to attend its motions); but the uniform testimony of the natives
and all the older residents in the country induced him to believe that
its motions were sometimes audible. On the 11th March, at 10 P.M., a
body of Aurora rose N.N.W.; and after a mass had passed E. by S., the
remainder broke away in portions, which crossed about 40° of the sky
with great rapidity. A hissing noise, like that of a bullet passing
through the air, was heard, which seemed to proceed from the Aurora;
but Mr. Wentzel assured the party the noise was occasioned by severe
cold succeeding mild weather, and acting upon the surface of the snow
previously melted in the sun’s rays. A similar noise was heard the next
morning.

[Sidenote: Capt. Sabine also negatives noise.]

In Parry’s first voyage, Captain Sabine describes an Aurora seen at
Melville Island, and adds that the Aurora had the appearance of being
_very near_ the party, but _no sound could be heard_.

[Sidenote: Article “Aurora Polaris,” Encyc. Brit., suggests noises as not
improbable.]

In the article “Aurora Polaris,” Encyc. Brit, edition ix., the writer
admits the evidence of scientific Arctic voyagers having listened in vain
for such noises; but, referring to the statements of Greenlanders and
others on the subject, concludes there is no _à priori_ improbability
of such sounds being occasionally heard, since a somewhat similar sound
accompanies the brush-discharge of the electric machine.

[Sidenote: Payer negatives and discredits noises.]

Payer, of the Austrian Polar Expedition (1872-1874), states that the
Aurora was never accompanied by noise, and discredits the alleged
accounts of noises in the Shetlands and Siberia.

[Sidenote: As also Weyprecht.]

Lieut. Weyprecht, of the same expedition, says (_antè_, p.
14):—“Involuntarily we listen; such a spectacle must, we think, be
accompanied with sound, but unbroken silence prevails, not the least
sound strikes on the ear.”

[Sidenote: Herr Carl Bock negatives noises in the case of Lapland Auroræ.]

Herr Carl Bock, who accompanied the Laplanders visiting this country (at
the Westminster Aquarium) in 1877-78, and who witnessed many brilliant
auroral displays in Lapland, assured me he could trace no noise, except
on one occasion, when he heard a sort of rustling, which he attributed to
the wind. The Laplanders themselves did not associate any special noise
with the Aurora.

[Sidenote: Auroral noises in telephone. Ringing sound in vacuum-tube
under influence of magnet.]

It has been recently stated, in an article on the Telephone in ‘Nature,’
that Professor Peirce “has observed the most curious sounds produced
from a telephone in connexion with a telegraph wire during the Aurora
Borealis;” but no further details are given. In experimenting with a
silicic fluoride vacuum-tube between the poles of an electro-magnet, I
found, on the magnet being excited, that the capillary stream of blue
light was decreased in volume and brightness, and at the same time from
within the tube a peculiar whistling or slightly metallic ringing sound
was heard.

[Sidenote: Adverse conclusion as to noises accompanying Aurora.]

I certainly have never met with an instance of noise accompanying an
Aurora and traced to it. On the whole the balance of evidence seems quite
adverse to any proof of noises proper ordinarily accompanying an Aurora.


_Colours of the Aurora._

[Sidenote: Colours of the Aurora. Sir John Franklin’s views. Other
observers have described all colours of spectrum. Violet rare. Crimson
indicates coming Aurora.]

Sir John Franklin considered the colours in the Polar Aurora did not
depend on the presence of any luminary, but were generated by the motion
of the beams, and then only when that motion was rapid and the light
brilliant. The lower extremities, he says, quivered with a fiery red
colour, and the upper with orange. He also saw violet in the former.
Other observers have, in their various descriptions of Auroræ, mentioned
the colours of the rays or beams as red, crimson, green, yellow, &c.; in
fact, comprising the range of the spectrum. Violet seems less frequently
mentioned. The red or crimson colour is frequently the first indication
of the coming Aurora, and is usually seen on or near the horizon. The
colours have frequently been observed to shift or change.

[Sidenote: Prof. Piazzi Smyth describes colours of Aurora of Feb. 4,
1872, as seen at Edinburgh.]

Prof. Piazzi Smyth, in a letter to ‘Nature,’ describing the Aurora of
February 4th, 1872, as seen at Edinburgh, says that when the maximum
development was reached all the heavens were more or less covered with
pink ascending streamers, except towards the N., which was dark and
grey—first by means of a long low arch of blackness, transparent to large
stars, and then by the streamers which shot up from this arch, which were
green and grey only for several degrees of their height, and only became
pink as they neared the zenith. The red streamers varied from orange to
rose-pink, red rose, and damask rose.

The Professor pointed out that the spectroscope knew no variety of reds
giving one red line only, and attributed this to the mixing up of rays
and streamers of blackness out of the long low arch. When the Aurora
faded away a true starlight-night sky appeared; so that evidently the
dark arch and streamers were as much part of the Aurora as the green and
red lights.

[Sidenote: Dr. Allnatt at Frant describes vivid colours of same Aurora.]

Dr. Allnatt, at Frant, found in the case of the same Aurora the
south-western part of the heavens tinged by a bright crimson band.
A dark elliptical cloud extending from S. to S.E. was illuminated
at its upper edge with a pale yellow light, and sent up volumes of
carmine radii interspersed with green and the black alternating matter
characteristic of elemental electricity. Almost due E., and of about 25
degrees elevation, was a bright insulated spot of vivid emerald-green,
which appeared almost sufficiently intense to cast a faint shadow from
intercepting objects. At 7 o’clock the Aurora had passed the zenith, and
the sky presented a weird and wonderful appearance. A dark rugged cloud,
some 8 degrees E. of the zenith, was surrounded by electric light of all
hues—carmine, green, yellow, blood-red, white, and black; and the bright
spot still existed in the south.

[Sidenote: Descriptions at Blackburn and Cambridge. Lapland Auroræ
yellow.]

At Blackburn, in Lancashire, the rays were described as glowing in the
N.E. from silvery white to deepest crimson; and at Cambridge the same
Aurora was described as of a brilliant carmine tint. The Auroræ seen
in Lapland by Herr Carl Bock, were, he informed me, almost invariably
yellow; he saw only one red one.

[Sidenote: Hydrogen vacuum-tube suggestive of Aurora colours.]

The behaviour of a hydrogen Geissler vacuum-tube will be subsequently
referred to in the Chapter on the comparison of some tubes with the
Aurora spectrum, and is suggestive as to Aurora colours.

[Sidenote: Variation of tints in.]

The capillary part of this tube, when lighted by a small coil, was found
to vary in tint—silver-white, bright green, and crimson being each in
succession the dominant colour, according to the working of the break of
the coil. When a spectroscope was used, the red, blue, and violet lines
of the gas were seen to change in intensity in accordance with the light
colour seen in the tube.

[Sidenote: Variation of colour in nitrogen tube under influence of
magnet.]

A Geissler nitrogen vacuum-tube was also so arranged that the capillary
part of it should be vertically between the conical extremities of the
armatures of a large electro-magnet, the armatures just being clear of
the outside of the tube. The tube was then lighted up by a small coil,
and the magnet excited by four large double-plate bichromate cells.

[Sidenote: Change from rosy to violet hue.]

The stream of light was steady and brilliant, and, except at the violet
pole, of the rosy tint peculiar to a nitrogen vacuum-tube. On excitation
of the electro-magnet, the discharge was seen to diminish in volume, with
an apparent increase in impetuosity; and not only the capillary part, but
in a less degree the bulbs also of the tube, changed from a rosy to a
well-marked violet hue.

[Sidenote: Photographic plates taken. Difference in.]

We several times connected and disconnected the magnet with its
batteries, but always with the same result. Of the spectrum of the
capillary part of this tube we took photographic plates with quartz
prisms and lenses, taking care that all things should be as equal as
possible, the apparatus undisturbed, and the time of exposure exactly
the same. One plate was taken with the tube in its normal condition, the
other while it was under the influence of the magnet. The spectra were
identical, except that the plate of the tube influenced by the magnet
was decidedly the brightest, and was found to penetrate more into the
violet region (the Author’s ‘Photographed Spectra,’ p. 60, plate xxv.).
These plates effectually corroborated the change of colour, as the violet
ray would have more photographic effect than the rosy. The identity of
the spectra of the capillary part proved that the change in colour could
not have proceeded from an extension of the violet glow. (A similar
experiment will be found also detailed in Part III. Chapter XII.)


_Height of the Aurora._

[Sidenote: Height of Aurora. Sir John Franklin considers it within the
region of the clouds. At no great elevation.]

Sir John Franklin (Narrative of a Journey on the Shores of the Polar Sea
in the years 1819, ’20, ’21, ’22) says:—“My notes upon the appearance of
the Aurora coincide with those of Dr. Richardson in proving that that
phenomenon is frequently seated within the region of the clouds, and that
it is dependent in some degree upon the cloudy state of the atmosphere.”
And further:—“The observations of Dr. Richardson point particularly to
the Aurora being formed at no great elevation, and that it is dependent
upon certain other atmospheric phenomena, such as the formation of one or
other of the various modifications of cirro-stratus.”

Sir John Franklin also refers to notes from the Journal of Lieut. Robert
Hood, R.N., on an Aurora:—

[Sidenote: Observations of Lieut. Robert Hood and Dr. Richardson. A beam
not more than 7 miles from the earth. An arch 7 miles from the earth.]

The observations were made at Basquian House, and at the same time by Dr.
Richardson at Cumberland House, quadrants and chronometers having been
prepared for the purpose. On the 2nd April the altitude of a brilliant
beam was 10° 0´ 0″ at 10h 1m 0s at Cumberland House. Fifty-five miles
S.S.W. it was not visible. It was estimated that the beam was not more
than 7 miles from the earth, and 27 from Cumberland House. On the 6th
April the Aurora was for some hours in the zenith at that place, forming
a confused mass of flashes and beams; and in lat. 53° 22´ 48″ N., long.
103° 7´ 17″, it appeared in the form of an arch, stationary, about 9°
high, and bearing N. by E. It was therefore 7 miles from the earth.

[Sidenote: An arch between 6 and 7 miles from the earth.]

On the 7th April the Aurora was again in the zenith before 10 P.M. at
Cumberland House, and in lat. 53° 36´ 40″ N., long. 102° 31´ 41″. The
altitude of the highest of two concentric arches at 9h P.M. was 9°, at 9h
30m it was 11° 30´, and at 10h 0m 0s P.M. 15° 0´ 0″, its centre always
bearing N. by E. During this time it was between 6 and 7 miles from the
earth. [The bearings are true, not magnetic.]

[Sidenote: Sir John Franklin’s remarks.]

Sir J. Franklin says this was opposed to the general opinion of
meteorologists of that period: he also noticed he had sometimes seen an
attenuated Aurora flashing across the sky in a single second, with a
quickness of motion inconsistent with the height of 60 or 70 miles, the
least that had hitherto been ascribed to it.

[Sidenote: Dr. Richardson’s conclusions.]

The needle was most disturbed, February 13, 1821, P.M., at a time when
the Aurora was distinctly seen passing between a stratum of cloud and
the earth; and it was inferred from this and other appearances that the
distance of the Aurora from the earth varied on different nights. Dr.
Richardson concludes that his notes prove, independent of all theory,
that the Aurora is occasionally seated in a region of the air below a
species of cloud which is known to possess no altitude; and is inclined
to infer that the Aurora Borealis is constantly accompanied by, or
immediately precedes, the formation of one or other of the forms of
cirro-stratus.

[Sidenote: Captain Parry observed Auroræ near the Earth’s surface. Sir W.
R. Grove’s observation at Chester. Mr. Ladd’s observation at Margate. The
author’s observation at Kyle Akin, Skye.]

Captain Parry observed Auroræ near the earth’s surface; and records that
he and two companions saw a bright ray of the Aurora shoot down from the
general mass of light between him and the land, which was distant some
3000 yards. Sir W. R. Grove (‘Correlation of Physical Forces’) saw an
Aurora at Chester, when the flashes appeared close, so that gleams of
light continuous with the streamers were to be seen between him and the
houses—“he seemed to be in the Aurora.” Mr. Ladd, of Beak Street, Regent
Street, has related to me an appearance he was struck with, and examined
carefully. Standing in the evening in Margate Harbour, he saw a white ray
of the Aurora, which, apparently shooting downwards, was clearly placed
between his eye and the opposite head of the pier, which projected into
the sea. Mr. Ladd also informed me that Prof. Balfour assured him that
such an appearance was not unusual. In the double-arc Aurora seen by me
in the Isle of Skye, September 11, 1874 (described _antè_, p. 23), I had
a strong impression that the bow was near the earth, and thought that
the eastern end, and some fleecy clouds in which it was involved, were
between myself and the peaks of the distant mountains.

[Sidenote: Dalton’s calculation of 100 miles. Backhouse’s 50 to 100
miles. Prof. Newton, mean 130 miles. Elevation of Auroræ cannot exceed a
few miles.]

In the article “Aurora Polaris,” Encyc. Brit., edition ix., Dalton is
instanced as having calculated the height of an Aurora in the north of
England at 100 miles; and Backhouse as having made many calculations,
with the result of an average height of 50 to 100 miles. Prof. Newton,
too, is quoted for the height of 28 Auroræ (calculated by one observation
of altitude and amplitude of an arch) as ranging from 33 to 281 miles,
with a mean of 130 miles. It is, however, pointed out that a height of
62 miles above the earth’s surface would imply a vacuum attainable with
difficulty, even with the Sprengel pump. This difficulty is then met by a
reference to the observed altitude of some meteors, and to a suggestion
of Prof. Herschel’s that electric repulsion may carry air or other matter
up to a great height. Dr. Lardner (‘Museum of Science and Art,’ vol. x.
p. 192) speaks of the height of Auroræ as not certainly ascertained; but
considers them atmospheric phenomena scarcely above the region of the
clouds, and does not think it probable that their elevation in any case
can exceed a few miles.

[Sidenote: M’Clintock’s observations. Capt. Ross saw Auroræ on an
ice-cliff, which he attributed to electric action.]

M’Clintock, after noticing that the beams of the Aurora were most
frequently seen in the direction of open water, says that in some cases
patches of light could be plainly seen a few feet above a small mass of
vapour over an opening in the ice. Captain Ross, in his Antarctic voyage,
saw the bright line of the Aurora forming a range of vertical beams along
the top of an ice-cliff; and suggested this was produced by electrical
action taking place between the vaporous mist thrown upwards by the waves
against the berg, and the colder atmosphere with which the latter was
surrounded.

[Sidenote: Bergman estimates height as 468 miles.]

Bergman, from a mean of 30 computations, makes the height of the
phenomenon to be 72 Swedish (about 468 English) miles.

[Sidenote: Boscovich 825 miles.]

Father Boscovich calculated the height of an Aurora Borealis observed on
the 16th December, 1727, to have been 825 miles.

[Sidenote: Mairan 600 miles. Euler several thousand miles. Dr. Blagden
about 100 miles.]

Mairan supposed the far greater number of Auroræ to be at least 600
miles above the surface of the earth. Euler assigned them an elevation
of several thousands of miles. Dr. Blagden, however, limited their
height to about 100 miles, which he supposed to be the region of
fireballs—remarking that instances were upon record in which northern
lights had been seen to join and form luminous balls, darting about with
great velocity, and even leaving a train behind them like common meteors
(Phil. Trans. vol. lxxiv. p. 227).

[Sidenote: Dalton 150 miles.]

Mr. Dalton, from an observation of the luminous arches on a base of 22
miles, found the altitude of the Aurora to be about 150 miles (Dalton’s
‘Meteorological Observations and Essays,’ 1793, pp. 54, 153).

[Sidenote: Dr. Thompson assumes considerable height. His table. Average
of 31 observations, 500 miles.]

Dr. Thompson, ‘Annals of Philosophy,’ vol. iv. p. 429 (1814), assumes
that the height of the beams above the surface of the earth was much
greater than that of most other meteorological appearances, and gives
(p. 430) a table of Auroræ, mainly taken from Bergman, Opusc. v. p. 291,
of 31 Auroræ observed in the years 1621 to 1793, with heights in English
miles. The lowest is, 23rd February, 1784, London (Cavendish), 62 miles;
the highest, 23rd October, 1751, Fournerius, 1006 miles! The average
of the 31 estimated observations gives a height of about 500 miles. It
is not stated how these observations were obtained, though methods are
mentioned how they might be.

[Sidenote: Prof. Heis’s instrument for determining height of Auroræ.]

Prof. Heis, of Münster, exhibited at the recent Scientific Loan
Collection at South Kensington (‘Official Catalogue,’ 3rd edit. p. 296,
No. 1231) an instrument for the determination of the position of the
point of convergence of the rays of the Aurora, and for determining the
height of the Aurora. A ball resting in a pan was to be brought into
position, so that several diverging pencils of Aurora, when properly
viewed, were covered by the rod which passed through the centre of the
ball. The point of the rod (which could be moved up and down in the
ball), when the instrument was set to the astronomical meridian, showed
the azimuth and altitude of the converging point of the pencils of light.
This point of convergence does not coincide with the point to which the
inclination-needle directs. From the deviation of the two points, the
height of the Aurora could be calculated.

[Sidenote: Professor Newton’s method of calculating height.]

Professor H. A. Newton (Sil. Journ. of Science, 2nd ser. vol. xxix. p.
286) has proposed a method of calculating the height of Auroræ by one
observation of altitude and amplitude of an arch. It assumes that the
auroral arches are arcs of circles, of which the centre is the magnetic
axis of the earth, or at least that they are nearly parallel to the
earth’s surface, and probably also to the narrow belt or ring surrounding
the magnetic and astronomical poles. Professor Newton finds that, _d_
being the distance from the observer to the centre of curvature of the
nearest part of this belt (for England, situated about 75° N. lat., 50°
W. long.), _h_ the apparent altitude of the arch, 2_a_ its amplitude on
the horizon, _x_ its height, R the earth’s radius, and _c_ the distance
of the observer from the ends of the arch:—

  sin φ   = sin _d_ cos _a_ cosec(_d_ + _h_)   (1)
  tan _c_ = _z_ sin _h_ sin φ sec ²φ           (2)
  _x_     = R - (sec _c_ - 1)                  (3)

[Sidenote: Gave a height from 33 to 281 miles, and a mean of 130 miles.]

This method with 28 Auroræ gave a height from 33 to 281 miles and a mean
of 130 miles.

Galle has suggested (Pogg. Ann. cxlvi. p. 133) that the height of Auroræ
might be calculated from the amount of divergence between the apparent
altitude of the auroral corona and that indicated by the dipping-needle,
a principle which has been adopted in Prof. Heis’s apparatus before
described. The results do not differ materially from Professor Newton’s.

The conclusions to be arrived at from the foregoing instances and
opinions are certainly very puzzling. The terrestrial character of some
Auroræ seems well established. The height to which these phenomena _may_
ascend is left almost a matter of conjecture, and further observations
are very desirable.


_Phosphorescence._

[Sidenote: Phosphorescence. Phosphorescent bands. Storm-clouds which
threw out cirri. Shone with a sort of phosphorescence. Storm-cloud
surrounded by glories of a phosphorescent whiteness.]

In the voyage of the ‘Hansa’ (‘Recent Polar Voyages,’ p. 420), on the
9th September, 1869, at 10 P.M., Aurora gleams appeared in the west,
shooting towards the south. “Radiant sheaves and phosphorescent bands
mounted towards the zenith,” but the phantasmagoria quickly vanished.
M. Silbermann (‘Comptes Rendus,’ lxviii. p. 1120) mentions storm-clouds
which threw out tufts of cirri from their tops, which extended over
the sky, and resolved into, first, fine, and afterwards more abundant
rain. (I saw a fine day example of this on the Lago di Guarda, ending
in a copious discharge of rain attended with loud thunder and vivid
lightning.) Usually the fibres were sinuous; but in much rarer cases they
became perfectly rectilinear and surrounded the cloud like a glory, and
occasionally shone _with a sort of phosphorescence_. On the night of 6th
September, 1865, at 11 P.M., a stormy cloud was observed in the N.N.W.,
and lightning was seen in the dark cumulous mass. Around this mass
extended _glories of a phosphorescent whiteness_, which melted away into
the darkness of the starry sky. Round the cloud was a corona, and outside
this two fainter coronæ. After the cloud had sunk below the horizon the
glories were still visible.

[Sidenote: Sabine’s luminous cloud at Loch Scavaig, Skye. Other
observations of luminous clouds.]

Sabine mentions a cloud frequently enveloping Loch Scavaig, in Skye, as
being at night perfectly self-luminous, and that he saw rays, similar to
those of the Aurora, but produced in the cloud itself. Sabine also refers
to luminous clouds mentioned in Gilbert’s Annals, and to observations by
Beccaria, Deluc, the Abbé Rozier, Nicholson, and Colla; and to luminous
mists as observed by Dr. Verdeil at Lausanne in 1753, and by Dr. Robinson
in Ireland.

[Sidenote: Aurora at Melville Island.]

He also describes (Parry’s First Voyage) an Aurora seen at Melville
Island, and says the light was estimated as equal to that of the moon
when a week old. Besides the pale light, _which resembled the combustion
of phosphorus_, a slight tinge of red was noticed when the Aurora was
most vivid; but no other colours. This Aurora was repeatedly seen _on the
following day_.

[Sidenote: Procter suspects Aurora is formed in a mist. M’Clintock:
Aurora is never visible in a perfectly clear atmosphere.]

Mr Procter, in a letter to me, suspects that the Aurora is generally
formed in a sort of “mist or imperfect vapour;” and this mist or
imperfect vapour seems in many instances to form part of the Aurora, and
to partake of its self-luminous character. M’Clintock does not imagine
that the Aurora is ever visible in a perfectly clear atmosphere. He has
often observed it just silvering or rendering luminous the upper edge of
low fog or cloud-banks, and with a few vertical rays feebly vibrating.

[Sidenote: Aurora of Feb. 4, 1874. Illuminated fog-cloud. Capt.
Oliver’s meteor-cloud. Auroral display, 24th Oct., 1870. Streamers of
phosphorescent cloud.]

An instance of apparent phosphorescence is supplied by the Aurora of the
4th February, 1874 (_antè_), when a bright cloud of light was seen which
gave the impression of an “_illuminated fog-cloud_.” Captain S. P. Oliver
saw at Buncrana, Co. Donegal, on February 4, 1874, what he describes as
a meteor-cloud, viz. “a broad band of silvery white and luminous cloud.”
This appearance, as described by another correspondent, was evidently
an imperfectly formed (perhaps actually forming) Auroral arc. The great
Auroral display of the 24th of October, 1870, as seen by me, included,
according to my notes made at the time, “streamers of opaque white
phosphorescent cloud, very different from the more common transparent
Auroral diverging streams of light.”

[Sidenote: Aurora of Feb. 4, 1872, at Frant. Radii of phosphorescent
light.]

Describing the Aurora of February 4, 1872, at Frant, Dr. Allnatt
says:—“At a later hour of the night the canopy of cirro-stratus had
separated, and was transformed into luminous masses of radiant cumulus.
At 10.40 the Aurora reappeared in the N., and sent luminous radii of
white _phosphorescent_ light from the periphery of a segment of a
perfectly circular arch”[7].

[Sidenote: The author’s description of same Aurora. Masses of
phosphorescent vapour.]

Again, February 4th, 1872, as described by me, the first signs of the
Aurora were (in dull daylight) a lurid tinge upon the clouds, which
suggested the reflection of a distant fire; while scattered among
these, “torn and broken masses of white vapour having a phosphorescent
appearance” reminded me of a similar observation in October 1870.

[Sidenote: Day Auroræ must have a phosphorescent glow. Ångström considers
yellow-green line due to fluorescence or phosphorescence. Oxygen and some
of its compounds phosphorescent.]

The day Auroræ, which are elsewhere described, and are not very uncommon,
could, we may presume, hardly be seen without the presence of some
phosphorescent glow. Professor Ångström, in his Aurora Memoir (discussed
elsewhere), in discussing the yellow-green line, considers the only
probable explanation to be that it owes its origin to fluorescence
or phosphorescence. He says that some fluorescence is produced by
the ultra-violet rays; and adds, “an electric discharge may easily
be imagined, which, though in itself of feeble light, may be rich in
ultra-violet light, and therefore in a condition to cause a sufficiently
strong fluorescent light.” And he refers to the fact that oxygen and some
of its compounds are phosphorescent.

[Sidenote: A phosphoretted hydrogen spectrum-band is close to
yellow-green auroral line. Phosphorescent or fluorescent after-glow of
electric discharge.]

In the examination of certain spectra connected with the Aurora,
detailed in Part II., I have shown that the bright edge of one of the
phosphoretted hydrogen bands is in close proximity to the yellow-green
Auroral line. I have also referred to the peculiar brightening by
reduction of temperature of one of the bands in the red end of the
spectrum of phosphoretted hydrogen, so that from almost invisible
it became bright, and to the peculiar brightening of a line in the
yellow-green in certain “Aurora” and phosphorescent tubes. It has
also been observed that the electric discharge has a phosphorescent
or fluorescent after-glow (isolated, I believe, by Faraday). It seems
difficult to avoid in some way connecting all these circumstances with
the yellow-green line of the Aurora, if not also with the line in the red.

[Sidenote: Sorby’s experiments on fluorescence and absorption.
Bonelleine, spectrum of. Coloured layer of fungi. Spectrum of
_Oscillatoriæ_.]

Mr. Sorby, in his experiments on the connexion between fluorescence and
absorption (‘Monthly Microscopical Journal’), found in the spectrum of a
solution in alcohol of a strongly fluorescent substance called bonelleine
(the green colouring-matter found in the _Aurelia Bonellia-viridis_) two
bright bands, the one red and the other green, with centres respectively
at 6430 and 5880, and their limits towards the blue end at 6320 and 5820.
On adding an acid the red band changed its place to 6140. The superficial
membranous coloured layer of the fungi _Russula nitida_ and _vesca_ in
alcohol gave an absorption band with centre at 5540, while the spectrum
of fluorescence extended to 4400. A solution of _Oscillatoriæ_ in water
gave a spectrum of absorption with bands at 6200 and 5690; while the
spectrum of fluorescence showed two bright bands having their centres at
6470 and 5800, and their limits towards the blue end at 6320 and 5710.

[Sidenote: Sea phosphorescence, a continuous spectrum.]

These instances of course cannot be connected with the Aurora except
as showing the spectrum region and lines of fluorescence. The sea
phosphorescence, according to Professor Piazzi Smyth, has a continuous
spectrum extending from somewhat below E to near F (Plate V. fig. 3).

[Sidenote: Ångström finds the sky almost phosphorescent.]

Ångström, on the occasion of the starry night when he found traces of
the green line in all parts of the heavens, speaks of the sky as being
“almost phosphorescent.”

[Sidenote: Author of article in Encyc. Brit. suggests that the
phosphorescent or fluorescent light may be due to chemical action.
Herschel’s observation of phosphorescence in Geissler and “garland”
tubes.]

The author of the Aurora article in the Encyc. Brit. suggests that the
phosphorescent or fluorescent light attributed to the Aurora may be
due to chemical action. He also questions Ångström’s assumption that
water-vapour is absent in the higher atmosphere, and thinks that it and
other bodies may, by electric repulsion, be carried above the level they
would attain by gravity. He then continues that if discharges take place
between the small sensible particles of water or ice in the form of cirri
(as Silbermann has shown to be likely) surface decomposition would ensue,
and it is highly probable the nascent gases would combine with emission
of light. He adds “that it has been almost proved that in the case of
hydrogen phosphide the very characteristic spectrum (light?) produced
by its combustion is due neither to the elements nor to the products of
combustion, but to some peculiar action at the instant of combination;
and it is quite possible that under such circumstances as above described
water might also give an entirely new spectrum.” Professor Herschel has
referred to the phosphorescent light which remains glowing in Geissler
tubes after the spark has passed, and to the fact that one of the globes
of a “garland” tube which was heated did not shine after the spark had
passed, apparently because of the action of heat on the ozone to which
the phosphorescence might be due. (See experiments on Mr. Browning’s
bulbed tube, Part III. Chap. XV.)


_Aurora and Ozone._

[Sidenote: Aurora and Ozone. Smells of sulphur during Auroræ attributed
to ozone.]

Accounts are given by travellers in Norway of their being enveloped
in the Aurora, and perceiving a strong smell of sulphur, which was
attributed to the presence of ozone. M. Paul Rollier, the aëronaut,
descended on a mountain in Norway 1300 metres high, and saw brilliant
rays of the Aurora across a thin mist which glowed with a remarkable
light. To his astonishment, an incomprehensible muttering caught his ear;
when this ceased he perceived a very strong smell of sulphur, almost
suffocating him (‘Arctic Manual,’ p. 726).

[Sidenote: Question whether the oxygen of the air may be changed into
ozone.]

In the case of the Aurora, the question naturally arises whether the
oxygen of the air may be changed into ozone, perhaps also whether the
nitrogen may not be modified in some similar manner.

[Sidenote: Ozone destroyed by heat.]

The absorption spectra of oxygen, and of the same gas in its form of
ozone, may possibly differ; but this can hardly happen in the case of
incandescent oxygen, for ozone is at once destroyed by heat at 300°, and
slowly at 100°, and must be partially at least destroyed by the heat of
the discharge. If any lines were due to ozone in such a spectrum, we
should expect they would be weakened by heat and brightened by cold.

[Sidenote: Ozone in a large bell-receiver not manifested in spectrum.]

In the case of a continued discharge in a large exhausted bell-receiver,
the presence of ozone in considerable quantities was manifested to us by
its odour when the receiver was removed from the pump; but the spectrum
of the stream of light did not appear to differ from that in Geissler
tubes.

[Sidenote: Professor Dewar demonstrates that ozone is condensed oxygen.]

In a course of lectures at the Royal Institution in March 1878, on the
Chemistry of the Organic World, Prof. Dewar appears to have demonstrated,
by Prof. Andrews’ apparatus, that ozone is really condensed oxygen, and,
further, that during this condensation heat is absorbed, which is evolved
during the decomposition or re-expansion.

[Sidenote: Refers to the silent discharge between the atmosphere and the
earth.]

He also exhibited the oxidizing power of ozone in its action on mercury,
and commented on its similar action upon organic matter in forming
nitrates, and on its remarkable bleaching properties, but added there
was as yet no proof of its combining with free nitrogen. That peroxide
of hydrogen accompanies the formation of ozone by the slow combustion of
phosphorus, and that this peroxide acts with ozone in decomposing organic
bodies, though in an inexplicable manner, the Professor considered to be
proved. He also referred to the silent discharge probably perpetually
going on between the upper and lower strata of the atmosphere, and also
between these and the earth, accounting, as the Professor considered, for
some of the chemical actions whereby nitrogenous compounds are formed in
the soil.

[Sidenote: No spectrum of ozone obtained.]

As far as I am aware, no information as to a possible spectrum of ozone,
or a modification of the oxygen or other spectra by its presence, has, up
to the present time, been obtained[8].

[Sidenote: Suggestion to subject electric discharge to influence of cold.]

It has been suggested by Mr. Procter and myself that the electric
discharge in an exhausted moist tube, if subjected to a considerable
degree of cold, might produce a modification of the air-spectrum, perhaps
even a spectrum analogous to that of the Aurora.

For some further notes on this subject see Appendix D (Aurora and Ozone).


_Polarization of the Aurora Light._

[Sidenote: Polarization of the Aurora light. Mr. Ranyard found none.]

In ‘Nature,’ vol. vii. p. 201, is contained an account of observations
of the polarization of the zodiacal light and of the Aurora, by Mr. A.
Cowper Ranyard, who, using both a double-image prism and a Savart on the
great Aurora of February 4th, 1872, detected no trace of polarization. He
also examined a smaller one of 10th November, 1871, with a like result.

[Sidenote: Prof. Alexander found strong polarization in latitude 60°.]

Mr. Fleming (who refers to these observations) remarks that the only
other account he had met with was contained in Prof. Stephen Alexander’s
Report on his Expedition to Labrador, given in Appendix 21 of the U.S.
Coast Survey Report for 1860, p. 30. Professor Alexander found strong
polarization with a Savart’s polariscope, and thought that the dark parts
of the Aurora gave the strongest polarization. This was in latitude
about 60°, at the beginning of July, and near midnight. It is not stated
whether there was twilight or air-polarization at the time, nor is the
plane of polarization given.

[Sidenote: Mr. Shroeder found no polarization.]

The question naturally arises, especially as the darkest parts of the
Aurora are usually situated low down near the horizon, whether the
polarization in the latter case did not proceed from the atmosphere and
not from the Aurora itself. Mr. Shroeder found no traces of polarization
in the Aurora of February 4th, 1872. Further examinations of the Aurora
with some delicate form of polariscope would seem very desirable.

[Sidenote: Polarization not found in the zodiacal light; except faint
traces by Mr. Burton.]

The evidence of polarization in the case of the zodiacal light seems also
almost entirely negative—Mr. Ranyard pointing out observations of his
own, of Captain Tupman, and of Mr. Lockyer with this result. Mr. Burton,
using a Savart set so as to give a black centre when the bands were
parallel to the plane of polarization, believed he detected faint traces
of polarization in the brightest parts of the zodiacal light (as seen in
Sicily), the bands being black-centred when their direction coincided
with the axis of the cone of light. Mr. Burton saw no trace of bands when
examining the slight remaining twilight apart from the zodiacal light.
Mr. Ranyard was not able to confirm Mr. Burton’s observations on the same
evening and with the same instrument.


_Number of Auroræ._

[Sidenote: Number of Auroræ. Sir John Franklin’s observations.]

Sir John Franklin saw in the Arctic Regions, in the years 1819, 1820,
1821, 1822:—In the month of September two Auroræ, in October three, in
November three, in December two, in January five, in February seven, in
March sixteen, in April fifteen, and in May eleven.

[Sidenote: Periodicity as to days not established.]

Periodicity as to days seems to have no certain law; and though certain
days in February and March are marked as those of fine returning
displays, they must be looked on as accidental.

[Sidenote: Maxima and minima.]

Two well-marked annual maxima seem to occur in March and October (the
latter the greater), and two minima in June and January, the greater in
June (Encyc. Brit.). The 4th of February, 1872, and same day 1874, are,
however, curious instances of a recurring remarkable display.

[Sidenote: Kæmtz’s table.]

A table by Kæmtz, showing the number of Auroras in each month of the
year, with the maxima and minima as above stated, will be found on Plate
V. fig. 5.

[Sidenote: Dr. Hayes’s observations in winter of 1860-61.]

Dr. Hayes has observed that in the winter of 1860-61 (when the ten or
eleven years’ inequality was at its maximum) only three Auroræ were seen
and recorded, and they were feeble and short in duration.

[Sidenote: Captain Maguire’s observations at Point Barrow as to number
and time of appearances.]

Captain Maguire, at Point Barrow (1852-54), reports that the Aurora was
seen six days out of seven, and on 1079 occasions, being nearly one third
of the hourly observations. It was seldom seen between 9 A.M. and 5 P.M.,
not at all between 10 A.M. and 4 P.M. It increased regularly and rapidly
from 5 P.M. until 1 A.M., and then diminished in the same way until 9 A.M.

The winters of 1877 and 1878 and the springs of 1878 and 1879 have been
singularly deficient in Auroræ. I have seen none at Guildown.


_Duration of Aurora._

[Sidenote: Duration of Aurora. Sometimes a few minutes; at other times
the whole night or even days.]

In the article in the ‘Edinb. Encyc.’ before referred to some remarks
are made on the duration of the Aurora. Sometimes it is formed and
disappears in the course of a few minutes. At other times it lasts for
hours or during the whole night, or even for two or three days together.
Musschenbroek observed one in 1734 which he considered to have lasted ten
days and nights successively, and another in 1735 which lasted from the
22nd to the 31st March.

[Sidenote: Auroræ may run on into the day without being noticed.]

With respect to Captain Maguire’s observations (_antè_) it may be
remarked that Auroræ may doubtless frequently run on into and through the
day without their being noticed (instances, however, are known of Auroræ
seen in daylight); and hence it is difficult to judge of the limit of
duration of a particular Aurora unless indications are sought for during
the day (by the shapes of clouds, action of the magnet, &c.) as well as
during the night. Probably Auroræ seen during successive nights may be
parts of a continuous discharge.


_The Travelling of Auroræ._

[Sidenote: Travelling of Auroræ. Donati’s investigations.]

Donati undertook to study the Aurora with reference to the mode of its
extension; and he arrived at the result that the Aurora of February 4,
1872, was not observed in different regions of the earth in the same
physical moment; _but everywhere at the same local hour_, as in the case
of celestial phenomena, which do not share in the earth’s rotation.

[Sidenote: Questions sent to Italian Consuls.]

The Minister of Foreign Affairs sent a circular to all Italian Consuls,
asking them the necessary questions; and in reply received reports from
forty-two places in our hemisphere and from four in the southern, the
places embracing in one latitude the considerable extent of 240 degrees
of longitude.

An epitome of the tables (in which the results are divided into three
zones) is as follows:—

[Sidenote: Table of results.]

  +---------+--------------------+----------+------------+------------+
  |  Zone.  |   Mean longitude   |  No. of  |  Mean hour |  Mean hour |
  |         |     of zone.       | stations.| of maximum.|   of end.  |
  +---------+--------------------+----------+------------+------------+
  | Eastern | 2 hrs.  5 mins. E. |     9    |  9½ hrs.   |  12¼ hrs.  |
  | Middle  | 0 hr.  20 mins. E. |    17    |  8½ hrs.   |  11½ hrs.  |
  | Western | 5 hrs. 38 mins. W. |    13    |  8¾ hrs.   |   9¾ hrs.  |
  +---------+--------------------+----------+------------+------------+

[Sidenote: Extensions of the Aurora. The Aurora passed through four
periods. First period of origin, light weak. Second period, increase of
intensity. Third period, continuous brightness. Fourth period, decrease.]

Donati summed up the facts:—That the light phenomena of this Aurora began
to show themselves in the extreme east of the southern hemisphere in
Eden and Melbourne; shortly after, they were observed in the east of our
hemisphere in China (but not in Japan); from China the Aurora passed over
the whole of Asia and Europe, and crossed the Atlantic and the American
Continent as far as California. It was invisible in Central and South
America. During these immense extensions it passed through four periods.
In the first (called by Donati the period of origin) the light of the
Aurora was pretty weak, and spread from Shanghai to Bombay; in the second
period, during which it passed on from Bombay to Taganrog, it acquired a
sudden increase of intensity; in the third period (called by Donati the
normal) the Aurora passed over Europe from east to west with regularity
and a continuous brightness; the fourth period, that of decrease,
was observed in America. The Aurora had a tendency to end earlier in
reference to the local hour in the western stations than in the eastern.
The acceleration on an average of the end of the phenomenon was twenty
minutes for every hour of longitude.

[Sidenote: Donati’s conclusions. Explanation of mode of propagation of
same Aurora.]

Donati concluded that these facts were not reconcilable with the theory
of the Aurora depending on meteorological and electro-magnetic phenomena
of the globe. Since, too, we have not a yearly, but a ten-yearly period
of the Aurora, which coincides with that of sun-spots and terrestrial
magnetism, Donati supposed that the cosmic causes of the polar lights
were electro-magnetic currents between the sun and the earth. This would
explain the mode of propagation of the Aurora of 4th February. Conceive
an electric current going from the earth to the sun, or _vice versâ_;
certain phenomena of the Aurora could only be observed in those parts
of the atmosphere which have a determinate position or direction with
reference to this current; and consequently these phenomena would be
successively visible on the different meridians, as these meridians,
by reason of the earth’s rotation, assume the same position to the
current. For the Aurora to be visible certain meteorological and telluric
circumstances must, however, doubtless work together with the cosmical
cause.


_Geographical Distribution of Auroræ (Fritz and Loomis)._

[Sidenote: Geographical Distribution of Auroræ. Prof. Fritz’s and Prof.
Loomis’s line of frequency.]

Professors Fritz and Loomis have investigated this subject; and
Petermann’s ‘Mittheilungen,’ vol. xx. (1874), contains a paper by the
former, from which it appears that the northern limit of Auroræ chosen
by Professor Loomis nearly coincided, except in England, with a line of
frequency in Professor Fritz’s paper. This line nearly passes through
Toronto, Manchester, and St. Petersburg. Professor Loomis places it
as far north as Edinburgh. On a line across Behring’s Straits, and
coming down below 60° N. in America and the Atlantic, and just north of
the Hebrides, to Dröntheim, and including the most northern points of
Siberia, the frequency is represented by 100.

[Sidenote: Within this another zone of greatest frequency and intensity.]

Within this is another zone of greatest frequency and intensity, which
passes just south of Point Barrow, in lat. 72° N., on the northern coast
of America, and by the Great Bear Lake to Hudson’s Bay, where it reaches
a latitude of 60°, then on to Nain, on the coast of Labrador, and to the
south of Cape Farewell; then bending sharper to the northward, it passes
between Iceland and the Faroe Islands, near to the North Cape, on by
the northern ice-sea to Nova-Zembla and Cape Tschejuskin, and on just
to the north of the Siberian coast to the south of Kellett Land, thence
returning to Point Barrow.

[Sidenote: Lines on which annually nearly the same number of Auroræ are
seen.]

More or less parallel with this line are the lines on which annually
nearly the same number of Auroræ are seen. The line for one Aurora
annually went from Bordeaux, through Switzerland, past Krakau, south of
Moscow and Tobolsk, to the northern end of Lake Baikal, on to the Sea of
Ochotsk and to the Southern Aleutes, thence through Northern California
to the mouth of the Mississippi and to Bordeaux. The line for five Auroræ
annually went from Brest through Belgium, Stettin, Wologda, between
Tobolsk and Beresow, parallel to the previous line to Ochotsk, and on to
Brest, &c. Almost exactly with the line of greatest frequency coincides
the line forming the boundary of the direction of visibility of the
Northern Light towards the Pole or towards the Equator; while northwards
of this line the Polar Light is seen in the direction towards the
Equator; and from all stations the Northern Lights are seen in directions
which are pretty much normal to that curve and the entire system of
isochasms.

[Sidenote: Assumed connexion between Aurora and ice-formation.]

Professor Fritz has remarked that the curves of greater frequency tend
towards the region of atmospheric pressure, and also that they bear some
relation to the limit of perpetual ice—tending most southward where, as
in North America, the ice limit comes further south. He also endeavours
to show a connexion between the periods of maximum of Auroræ and those
of ice-formation, and considers ice to be an important local cause
influencing their distribution. These being most frequently seen over
open water in the Arctic regions, has been referred to as noticed by
Franklin and others.


_Extent and principal Zone of the Aurora._

[Sidenote: Extent and principal zone of Aurora. M. Moberg’s Finland
observations (1846-55) compared by Prof. Fritz with those in other
regions.]

The Finland observations, published by M. Moberg in his ‘Polarlichter
Katalogue’ of Northern Lights in the years 1846-55, numbering 1100, have
been compared by Prof. Fritz, in his paper in the ‘Wochenschrift für
Astronomie,’ with the auroral phenomena of the same period in all other
regions. The Table shows that of 2035 days of the months August to April
on which Northern Lights were seen, 1107 days were those of Northern
Lights for Finland. On 794 they were visible simultaneously in America,
and mostly also in Europe; on 101 days in Europe only, and on 212 days
in Finland only. On 958 days Northern Lights were visible in Europe and
America which were not visible in Finland. All these numbers refer only
to the months August to April, as in the remaining months the brightness
of the night in Finland makes such observations impossible.

The conclusion is arrived at that a large portion of Auroræ have no very
great extension, or that the causes producing the phenomena must often be
of a very local character; while in another portion of the phenomena the
extent, or the regions of simultaneous appearance are very considerable.

[Sidenote: Number limited to Finland only small.]

The number limited to Finland, for which hitherto corresponding
observations from other lands are wanting, is very small—212, or 19 per
cent. of the whole number seen in Finland. With the increase of frequency
of the phenomena at the time of maximum, the number observed in Finland
and America on the same day increases; while those observed in Finland
and Europe only, or in Finland only, decreased, in accordance with the
known law that with the frequency the intensity and extent also increase.

[Sidenote: One third of Auroræ seen in America and Europe simultaneously.]

Between 1826 and 1855, of 2878 days on which, in America, the Northern
Lights were seen, there were 1065 on which they were also visible in
Europe; so that at least every third day of Auroræ was common to both
these portions of the globe. In the years 1846 to 1855, and 1868 to 1872,
there were in the first period 657 Northern-Light days common to America
and Europe out of 1691, and in the second 397 out of 715.

[Sidenote: Local occurrence of the Aurora not in favour of its assumed
cosmical nature.]

The comparison by Prof. Fritz of M. Moberg’s Finland observations has
been lately reviewed in ‘Nature’ (March 8, 1878) and the result arrived
at that, “After ten years, in spite of the vastly accumulated material of
careful observations, there appears no necessity to change Herr Fritz’s
system of curves in any essential detail; indeed certain parts of the
same, which were at first only based on probability and supposition
(the part of the principal zone between the north of Norway and Nishen
Kolynisk as an instance), we now know with perfect certainty to be
correct.” It has been remarked that the local occurrence of Auroræ is
not in accordance with the hypothesis of the phenomenon being one of a
cosmical nature.

The winter of 1870 was remarkable for brilliant displays; and the
displays of October 24th and 25th, 1870, were remarkably brilliant in
England and in America also, and the Aurora Australis was seen on the
same days at Madras. These displays were seen in England and America
in the daytime as patches or coronæ of white light, with streamers
stretching upward from them.




CHAPTER VI.

THE AURORA IN CONNEXION WITH OTHER PHENOMENA.


_Auroræ and Clouds._

[Sidenote: Auroræ and clouds. Dr. Richardson’s observations. Aurora
constantly accompanied by or immediately precedes the formation of
cirro-stratus.]

Dr. Richardson (‘Sir John Franklin’s Narrative’), so long ago as the
years 1819-1822, made many recorded observations on the connexion of
clouds with the Aurora Borealis in the Polar regions. Some of these are
alluded to in Chapter V., section “Height of the Aurora,” for the purpose
of showing the moderate distance he found it to be above the earth; and
his inference is there mentioned, “that the Aurora Borealis is constantly
accompanied by or immediately precedes the formation of one or other
of the various kinds of cirro-stratus.” On the 13th November and 18th
December, 1820, the connexion of an Aurora with a cloud intermediate
between cirrus and cirro-stratus is mentioned. It is, however, also
mentioned that the most vivid coruscations of the Aurora were observed
when there were only a few attenuated shoots of cirro-stratus floating in
the air, or when that cloud was so rare that its existence was only known
by the production of a halo round the moon. (An instance of attenuated
streaks of cirro-stratus in connexion with an auroral arc will be found
in the Aurora seen at Guildown on the 4th February 1874, a sketch of
which is reproduced on Plate VI. fig. 1.)

[Sidenote: Polarity discerned in cirro-stratus clouds.]

Dr. Richardson goes on to express his opinion that he, on some occasions,
discerned a polarity in the masses of clouds belonging to a certain kind
of cirro-stratus (approaching cirrus), by which their long diameters,
having all the same direction, were made to cross the magnetic meridian
nearly at right angles.

[Sidenote: Apparent polarity of Aurora might perhaps be ascribed to the
clouds themselves.]

Dr. Richardson further suggests that if it should be thereafter proved
that the Aurora depends upon the existence of certain clouds, its
apparent polarity might perhaps be ascribed to the clouds themselves
which emit the light; or, in other words, the clouds might assume their
peculiar arrangement through the operation of one cause (magnetism, for
instance), while the emission of light might be produced by another—a
change in their internal constitution perhaps connected with a motion of
the electric fluid.

Dr. Richardson further remarks that, generally speaking, the Aurora
appeared in small detached masses for some time before it assumed that
convergency towards the opposite parts of the horizon which produced the
arched form.

[Sidenote: Sir John Franklin’s observations.]

Sir John Franklin says in his Polar expeditions he often perceived the
clouds in the daytime disposed in streams and arches such as the Aurora
assumes.

[Sidenote: Dr. Low’s.]

Dr. Low (‘Nature,’ iv. p. 121) considers he witnessed a complete display
of auroral motions in cirrus cloud, and considers all clouds subject
to magnetic or diamagnetic polarization; he states that when the lines
converge towards the magnetic pole fine weather follows, and when at
right angles to it wet and stormy.

[Sidenote: M. Silbermann’s observations, 15th April, 1869. Cirrus clouds
took the place of the Aurora.]

In the Encyc. Brit. edition 9, article “Aurora Polaris,” after referring
to the evidence of Franklin, Richardson, and Low, M. Silbermann (‘Comptes
Rendus,’ lxviii. p. 1051) is quoted in detail for observed connexion
between the Aurora and cirrus cloud. 15th April, 1869, at 11h 16m, an
Aurora appeared and disappeared; but it seemed as if the columns were
still visible, and it soon became obvious that fan-like cirrus clouds,
with their point of divergence in the north, had taken the place of the
Aurora. Between 1 and 2 A.M. the clouds had passed the zenith, and let
fall a little fine frozen rain. At 4 A.M. the cirrus of the false Aurora
was still visible, but deformed towards the top, and presenting a flaky
aspect. The cirrus never appeared to replace the Aurora either from right
or left, but to substitute itself for it like the changes of a dioramic
view.

[Sidenote: Payer thinks the transition of Aurora into clouds not proved.]

Payer, in his ‘Austrian Arctic Voyages,’ thinks that the occurrence of
the Aurora during the day (i. e. _light clouds with its characteristic
movement_) had been rather imagined than actually observed, and that the
transition of white clouds into auroral forms at night has never been
satisfactorily proved. He, however, mentions the mist-like appearance of
the Aurora.

[Sidenote: Dr. Allnatt’s observations, 4th February, 1872, at Frant.
Aurora passed into cirro-stratus.]

Dr. Allnatt observed the splendid Aurora of 4th February, 1872, at
Frant, and noticed the weird and wonderful appearance of the phenomena.
At 6 P.M. the Aurora commenced by the S.W. portion of the heavens being
tinged with a bright carmine hue, and in a short time the whole visible
hemisphere was lighted up. A dark elliptical cloud extending from S. to
S.E. and S.W. sent up volumes of coloured radii. At 7 the Aurora had
passed the zenith, and a dark, broken, rugged cloud some 8° E. of zenith
was surrounded by electric light of all hues. At 7.40 the Aurora began to
wane, and passed into a homogeneous cirro-stratus of sufficient density
to obscure the stars, disappearing at 7.45.

[Sidenote: Later, cirro-stratus was transformed into luminous cumulus.]

At a later hour of the night the canopy of cirro-stratus had separated
and was transformed into luminous masses of radiant cumulus; so that, as
Dr. Allnatt observes, there were called in requisition almost all the
most prominent cloud-modifications during the progress of the phenomena.
The succession of formation, transformation, and reformation from Aurora
to cloud and from cloud to Aurora was, Dr. Allnatt concluded, conclusive
of the theory before advanced of the electric origin of the recurrent
rayed cloud-modifications in the place of the magnetic meridian, over
which so much mystery had been cast.


_Aurora and Thunder-storms._

[Sidenote: Aurora and thunder-storms. Silbermann’s theory.]

Silbermann asserts that Auroræ are produced by the same general phenomena
as thunder-storms, and concludes that the Auroræ of 1859 and 1869 assumed
the character of thunder-storms which, instead of bursting in thunder,
had been drawn into the upper parts of the atmosphere, and their vapour
being crystallized in tiny prisms by the intense cold, the electricity
became luminous in flowing over these icy particles.

[Sidenote: Prof. Piazzi Smyth on monthly frequency of Auroræ and storms.]

Professor Piazzi Smyth has observed that the monthly frequency of Auroræ
varies inversely with that of thunder-storms. His Table of comparisons is
as follows:—

[Sidenote: His table of observations.]

   Month.            Lightning.   Auroræ.

  January              24·0        29·7
  February             14·4        42·5
  March                 7·0        35·0
  April                15·4        27·5
  May                  37·4         4·8
  June                 48·0         0·0
  July                 55·2         0·5
  August               38·4        12·6
  September            22·4        36·6
  October              20·8        49·4
  November             15·0        32·4
  December             15·0        28·8
                       ----        ----
  Mean of whole year   24·0        20·1

[Sidenote: Silbermann’s observations 15th April, 1869. 30th April, 1865.]

Silbermann, on 15th April, 1869, observed a fall of rain (tiny crystals
of ice) on the disappearance of an Aurora and its change into cloud
forms (see section, “Auroræ and Clouds,” p. 53). He also observed a rain
of little sparkling ice-prisms on 30th April, 1865, at Paris, the city
being then enveloped in a cirrus of vertical fibres similar to that which
frequently accompanies the Aurora.

On the occasion of the Aurora seen by me at Guildown, 4th February, 1872,
rain fell immediately succeeding the formation of the corona.

The falling of rain as an immediate sequence of an Aurora seems, however,
to be rather the exception than the rule; but possibly this may vary with
the character of the Aurora itself—whether it be of the crimson class,
passing into cloud and accompanied with much electric disturbance, or of
the more quiescent white.

[Sidenote: A falling barometer observed to follow Auroræ.]

A falling barometer following a display of Auroræ has been noticed by Sir
John Franklin and others; and in some cases (notably one in Sicily before
referred to) storms and floods have accompanied this.

[Sidenote: Professor Christison’s observations.]

In a paper read before the Royal Society of Edinburgh in 1868, Prof.
Christison mentioned, as a fact of importance to agriculturists, that the
first great Aurora after autumn is well advanced, and following a period
of fine weather, is a sign of a great storm of rain and wind in the
forenoon of the second day afterwards.

Mr. C. L. Prince, in his ‘Climate of Uckfield,’ p. 218, remarks that
displays of Auroræ are almost invariably followed by very stormy weather,
after an interval of from 10 to 14 days.


_Aurora and the Magnetic Needle._

[Sidenote: Aurora and the magnetic needle. Sir John Franklin’s
observations. Motion communicated to the needle was neither sudden
nor vibratory. Return of needle to its former position very gradual.
Different positions of the Aurora had considerable influence on the
direction of the needle. Needle disturbed when Aurora not visible.
Quiescent yellow Aurora produced no perceptible effect on needle. Return
of needle more speedy after formation of a second arch. Slow when
disturbance was considerable.]

Sir John Franklin, in his ‘Narrative’ (before referred to), gives
Lieutenant Robert Hood, R.N., the credit of being “the first who
satisfactorily proved, by his observations at Cumberland House (before
mentioned), the important fact of the action of the Aurora upon the
compass-needle,” and also “to have proved the Aurora to be an electrical
phenomenon, or at least that it induces a certain unusual state of
electricity in the atmosphere.” Sir John Franklin then mentions that
the motion communicated to the needle was neither sudden nor vibratory.
Sometimes it was simultaneous with the formation of arches, prolongation
of beams, or certain other changes of form or of activity of the Aurora.
But generally the effect of these phenomena upon the needle was not
visible immediately; but in about half an hour or an hour the needle had
obtained its maximum of deviation. From this its return to its former
position was very gradual, seldom regaining it before the following
morning, and frequently not until the afternoon, unless it was expedited
by another arch of the Aurora operating in a direction different from
the former one. The magnetic needle in the open air was disturbed by the
Aurora whenever it approached the zenith. Its motion was not vibratory
(as observed by Mr. Dalton), perhaps owing to the weight of the card.
It moved slowly to the E. or W. of the magnetic meridian, and seldom
recovered its original direction in less than eight or nine hours. The
greatest extent of its aberration was 45´. The arches of the Aurora were
remarked commonly to traverse the sky nearly at right angles to the
magnetic meridian; but deviation was not rare, and it was considered
that the different positions of the Aurora had considerable influence on
the direction of the needle. When an arch was nearly at right angles to
the magnetic meridian, the motion of the needle was towards the W. This
motion was greater when the extremity of the arch approached from the
west towards the magnetic north. A westerly motion also took place when
the extremity of an arch was in the true north, or about 36° to the west
of the magnetic north. The motion of the needle was towards the east
when the same end of an arch originated to the southward of the magnetic
west, and when of course its opposite extremity approached nearer to
the magnetic north. In one case only a complete arch was formed in the
magnetic meridian. In another the beam shot up from the magnetic north to
the zenith. In both these cases the needle moved towards the west. The
needle was most disturbed on February 13, 1821, at a time when an Aurora
was distinctly seen passing between a stratum of clouds and the earth.
Sometimes the needle deviated though no Aurora was visible; but it was
uncertain whether there might not have been a concealed Aurora at the
time. Clouds were sometimes observed during the day to assume the form of
the Aurora, and deviations of the needle were occasionally remarked at
such times. An Aurora sometimes approached the zenith without producing
any change of position of the needle; while at other times a considerable
alteration took place, though the beams or arches did not come near the
zenith. The Aurora was frequently seen without producing a perceptible
effect on the needle. At such times it was generally an arch or a
horizontal stream of dense yellowish light with little or no internal
motion. The disturbance of the needle was not always proportionate to the
agitation of the Aurora, but was always greater when the quick motion
and vivid light were observed to take place in a hazy atmosphere. In a
few instances the needle commenced at the instant a beam started from
the horizon upwards; and its return was according to circumstances. If
an arch formed immediately afterwards, having its extremities placed on
opposite sides of the magnetic north and south to the former one, the
return of the needle was more speedy, and it generally went beyond the
point from which it first started. When the disturbance was considerable,
it seldom regained its usual position before 3 or 4 P.M. on the following
day. On one occasion only the needle had a quick vibratory motion
(between 343° 50´ and 344° 40´). The disturbance produced by the Aurora
was so great that no accurate deductions as to diurnal variation could be
made.

[Sidenote: Magnetic observations on board the ‘Tegetchoff.’]

Payer, in his ‘New Lands within the Arctic Circle’ (vol. i. pp. 327,
328), gives the result of the magnetic observations on board the Austrian
ship ‘Tegetchoff’ in the years 1872-74, made by means of a magnetic
theodolite, a dipping-needle, and three variation instruments. The
extraordinary disturbances of the needle rendered the determination of
exact mean values for the magnetic constants impossible. The following
were the principal results of these observations:—

[Sidenote: Disturbances great.]

(1) The magnetic disturbances were of extraordinary magnitude and
frequency.

[Sidenote: Greater as the rays were rapid. Quiescent arches exercised no
influence.]

(2) They were closely connected with the Aurora, and they were greater
as the motion of the rays was more rapid and fitful and the prismatic
colours more intense. Quiescent and regular arches, without changing rays
or streamers, exercised mostly no influence on the needle.

[Sidenote: Declination-needle, effects on.]

(3) In all the disturbances the declination-needle moved towards the
east, and the horizontal intensity decreased while the inclination
increased.

Sir John Franklin sums up his information as to the needle to much
the same effect, viz. that brilliant and active coruscations cause a
deflection almost invariably if they appear through a hazy atmosphere
and if the prismatic colours are exhibited in the beams or arches. On
the contrary, when the air is clear and the Aurora presents a steady
dense light of a yellow colour and without motion, the needle is often
unaffected by its appearance.

[Sidenote: Parry’s experience.]

Parry (Third Voyage) found his variation-needle (extremely light and
delicately suspended) in no instance affected by the Auroræ; but he seems
to have principally met with the quiescent form of that phenomenon.

M. Lottin, the French savant (whose description of an Auroral display has
been given in Chapter II.), observed in the North Sea, between September
1838 and April 1839, while the sun was below the horizon, 150 Auroræ.
During this period 64 were visible, “besides many which a cloudy sky
concealed, but the presence of which was indicated by the disturbances
they produced upon the magnetic needle” (Lardner’s ‘Museum of Science and
Art,’ vol. x. p. 189).

[Sidenote: Grand displays accompanied by motion of needle to the west.]

It has been remarked by some observers that grand displays of the Aurora
are frequently preceded or accompanied by an extraordinary motion of the
needle to the westward.

Captain Maguire found at Point Barrow (1852-54) that the appearance of
the Aurora in the south was connected with the motion of the magnet to
the east of the magnetic north, and if in the north to the west of the
same.

[Sidenote: Solar disturbances and Aurora.]

On an occasion in 1859 great solar disturbances were observed, the
Greenwich magnets were much disturbed, and a fine Aurora was visible.

[Sidenote: Cipoletti’s observation.]

Cipoletti, of Florence, remarks on the strong magnetic disturbances
at Vienna and Munich during the Auroræ of 4th February, 1872, and 4th
February, 1874.

[Sidenote: Dr. Thompson concludes that cylinders of Aurora cannot be
doubted to be magnets.]

Dr. Thompson, in his ‘Annals of Philosophy,’ vol. iv. p. 431 (1814),
mentions as an authenticated fact that during the prevalence of the
Aurora the magnetic needle was frequently observed to become unsteady,
and (p. 432) concludes that cylinders of Aurora cannot be doubted to be
magnets. The only three bodies capable of assuming magnetic properties
are iron, nickel, and cobalt. When meteors are considered, it is not
altogether extravagant to conjecture that bodies similar in their nature
to some of the solid bodies which constitute our globe may exist in some
unknown state in the atmosphere.

During the Aurora of 13th May, 1869, the declination at Greenwich varied
1° 25´, while the vertical force experienced four successive maxima, and
the greatest oscillation amounted to 0·04 of the total mean value. The
horizontal force varied only 0·014 of its mean value.

During the Aurora of 15th April, 1869, the declination at Stonyhurst
varied 2° 23´ 14″ in nine minutes.

[Illustration: Plate IX.]


_Auroræ, Magnetic Disturbances, and Sun-spots._

[Sidenote: Auroræ, magnetic disturbances, and sun-spots in Italy.]

Auroræ were frequent in Italy in April 1871. On the 10th a remarkable one
was seen, with declinometer deflected towards the east, and 63 sun-spots
were counted. On the morning of the 10th the deflection continued, and at
midday 97 sun-spots were counted.

On the 18th a brilliant Aurora lasted to 10 o’clock at night. From this
time till the 23rd the Aurora appeared constantly, giving a reddish
tinge in the north and north-west. A brilliant display took place on
the evening of the 23rd. On the evenings when the Aurora appeared the
magnetometers were disturbed throughout Italy, and ended by a violent
agitation during the whole of the 24th. Sun-spots were observed at Rome,
Palermo, and Moncalieri, but the greater number on the days of the
Auroræ. A brilliant display at Moncalieri on June 18 was accompanied by
very violent magnetic disturbance.

[Sidenote: Proctor’s sun-spots and Aurora.]

September 25, 1870, Mr. Proctor counted 102 spots on the solar disk; and
on the night of the 24th and morning of the 25th an Aurora of unwonted
magnificence was visible at various stations in England, France, and
Germany.

[Sidenote: Sun-spots and the magnet. 11 years’ period. Schwabe’s sun-spot
period.]

With respect to sun-spots and the magnet, the frequency of magnetic
storms, causing oscillation of the needle, gradually increased from a
minimum in 1843 to a maximum in 1848, giving a variation of something
near 11 years altogether. Schwabe observed the sun-spots for 24 years,
and found they had a regular maximum and minimum every five years, and
that the years 1843 and 1848 were minimum and maximum years coinciding
with the magnetic variation at those periods.

[Sidenote: Prof. Loomis considers connexion established between magnetic
declination, auroral displays, and sun-spots.]

Professor Loomis (‘American Journal of Science,’ vol. v. April 1873)
considers that a comparison between the mean daily range of the magnetic
declination and the number of Auroras observed in each year, and also
with the extent of the black spots on the surface of the sun, establishes
a connexion between these phenomena, and indicates that auroral displays
(at least in the middle latitudes of Europe and America) are subject
to a law of periodicity, that their grandest displays are repeated at
intervals of about 60 years, and that there are also other fluctuations,
less distinctly marked, which succeed each other at an average interval
of about 10 or 11 years, the times of maxima corresponding quite
remarkably with the maxima of solar spots.

[Sidenote: Illustrative table of coincidences.]

An illustration of the result of these observations is given on Plate
IX. fig. 2. The curves are in close correspondence, and the coincidence
at the times of maximum and minimum is remarkable. The auroral maximum
generally occurs a little later than the magnetic maximum; and the
connexion between the auroral and magnetic curves appears somewhat more
intimate than between the auroral and sun-spot curves.

[Sidenote: Prof. Loomis considers a sun-spot a solar disturbance
affecting the earth’s magnetism.]

Professor Loomis contends “that the black spot is a result of a
disturbance of the sun’s surface, which is accompanied by an emanation
of some influence from the sun, which is almost instantly felt upon the
earth in an unusual disturbance of the earth’s magnetism, and a flow of
electricity, developing the auroral light in the upper regions of the
earth’s atmosphere.”

[Sidenote: Carrington and Hodgson’s observations of bright spots on
the sun, accompanied by magnetic disturbance at Kew, and followed by
wide-spread Auroræ.]

This connexion between the sun’s spots and the earth’s magnetism has been
considered as proved; and one instance at least of an intense disturbance
and outbreak of the sun’s surface having been observed simultaneously
with the occurrence of a terrestrial magnetic storm is a matter of
record. This will be found detailed in the ‘Monthly Notices of the Royal
Astronomical Society,’ vol. xx. pp. 13 and 15, and is so interesting
in its character that it may be briefly referred to here. Mr. R. C.
Carrington, September 1, 1859, 11h 18m, while observing and drawing a
group of solar spots, saw suddenly two patches of intense bright light
break out in the middle of the group. The brilliancy was fully equal to
that of direct sunlight. Seeing the outbreak was on the increase, Mr.
Carrington left the telescope, to call some one to witness it. On his
return within sixty seconds it was nearly concluded. The spots travelled
from their first position, and vanished as two rapidly fading dots of
white light. In five minutes the two spots traversed a space of about
35,000 miles. Mr. Carrington found no change in the group itself. His
impression was that the phenomena took place at an elevation considerably
above the general surface of the sun, and above and over the great group
of spots on which it was seen projected. It broke out at 11h 18m, and
vanished at 11h 23m. Mr. R. Hodgson independently on the same day, and
at close upon the same time, saw a very brilliant star of light, much
brighter than the sun’s surface, most dazzling to the protected eye,
illuminating the upper edges of the adjacent spots and streaks. The rays
extended in all directions, and the centre might be compared to α Lyræ
when seen in a large telescope. It lasted for some five minutes.

At the very moment of this solar disturbance the instruments at Kew
indicated a _magnetic storm_; and Proctor, in his volume on the Sun,
page 206, details how this magnetic storm was accompanied by very
widely-spread indications of electrical disturbance in many parts of
the globe. Vivid Auroræ were seen not only in both hemispheres, but in
latitudes and places where they are seldom witnessed. Rome, Cuba, and
the West Indies, the tropics within 18° of the equator, and even South
America and Australia, are thus referred to for displays. At Melbourne,
on the night of September 2nd, the greatest Aurora ever seen there made
its appearance.

It was observed, too, that magnetic communication was at the same time
disturbed all over the earth. Strong currents, continually changing
their direction, swept along the telegraphic wires. At Washington and
Philadelphia the signal-clerks received severe shocks, and the wires had
to give up work. At a station in Norway the transmitting apparatus was
set fire to; and at Boston, in North America, a flame of fire followed
the pen of Baine’s electric telegraph.

[Sidenote: Mr. John Allan Broun’s magnetic oscillation-curves; showing
that the sun’s magnetic action has lately become more constant. In
diagram, curves gradually flatten.]

In an interesting communication to ‘Nature’ (January 3rd, 1878), entitled
“The Sun’s Magnetic Action at the Present Time,” Mr. John Allan Broun has
contributed some magnetic oscillation-curves, deduced from observations
made in the Trevandrum Observatory (nearly on the magnetic equator),
by which, if confirmed by other observations, it would appear that the
sun’s magnetic action has lately become gradually more constant. The
curves are three in number,—no. 1 for the years 1855-58, no. 2 for the
years 1865-68, no. 3 for the years 1874-77. In no. 1 curve the minimum
is very clearly marked by two points corresponding to April 1 and May 1,
1856, and there is little difference in the rapidity with which the curve
descends to and ascends from the minimum. In no. 2 curve the epoch of
minimum is by no means so well marked; it occurs between the points for
April 1 and September 1, 1866. There is also a considerable difference
in the rapidity of variation in the descending and ascending branches
of the curve. The descent is nearly as rapid as in curve no. 1; but the
ascent is very much slower. In curve no. 3 the lowest point is that for
December 1, 1875; but it is even now, with points a year and a half
later, difficult to say whether this is the minimum or not, the point
for January 1, 1877, being only 0·02 (two hundredths of a minute of arc)
higher. In this curve the change of range in diurnal oscillation is quite
insignificant from November 1, 1874, to April 1, 1877, an interval of
three years and five months. In the diagram given by Mr. Broun the curves
show themselves gradually flattening, no. 3 being almost a straight line.

[Sidenote: Mr. Broun never found an Aurora without a corresponding
irregularity in the declination-needle.]

Mr. Broun remarks upon the report of Sir George Nares as to the
insignificant nature of the Auroræ seen in the Arctic Expedition in the
winter of 1875-76, and the accompanying statement that, as far could be
discovered, they were totally unconnected with any magnetic or electric
disturbance; and states, as the result of his own experience in the south
of Scotland, that several of the Auroræ observed by him were of the very
faintest kind, “were traces” which he could never have remarked had he
not been warned by very slight magnetic irregularities to examine the sky
with the greatest attention. Again, in no case had he seen the faintest
trace of an Aurora without finding at the same time a corresponding
irregularity in the movement of the force or declination-magnet.

[Sidenote: Prof. Piazzi Smyth comments on variance in the cycles.]

Prof. Piazzi Smyth, commenting on this article, makes the inquiry how
the sun-spot cycle and the terrestrial magnetic oscillation cycle can
be considered as agreeing, the sun-spot cycle, according to Prof. Wolf,
being 11·111 years, and the magnetic cycle 10·5 years according to Mr.
Broun.

[Sidenote: M. Faye’s remarks to a similar effect.]

Another correspondent writes and quotes M. Faye, in ‘La Météorologie
Cosmique,’ for the remark, “La période des taches portée à 11 ans ·1 par
M. Wolf n’étant pas égale à celle des variations magnétiques (10 ans
·45), ces deux phénomènes n’ont aucun rapport entre eux.”

[Sidenote: Mr. Broun’s rejoinder and explanation.]

Mr. Broun, in a further letter, rejoins that if we could accept Dr.
Wolf’s view we should find that the mean duration of a cycle for _both_
phenomena since 1787 would be 11·94 years, while the sun-spot results for
eight cycles determined by Dr. Wolf during eighty years before 1787 give
10·23 or, if we take nine cycles, 10·43 years for the mean duration. It
is by mixing these two very different means that the Zurich philosopher
finds 11·1 years, a mean which Mr. Broun considers can evidently have
no weight given to it. On the other hand, if Dr. Wolf is in error (as
Mr. Broun believes he is) as to the existence of a maximum in 1797,
the mean durations for the eighty years after and for the eighty years
before 1787 agree as nearly as the accuracy of the determinations for the
beginning of the eighteenth century will permit. Mr. Broun then repeats
his conviction that the sun-spot maxima and minima are really synchronous
with those of the magnetic diurnal observations.

[Sidenote: Mr. Jenkins’s explanation of Prof. Loomis’s chart.]

Mr. B. G. Jenkins, in a letter to ‘Nature,’ refers Prof. Smyth to Prof.
Loomis’s chart of magnetic oscillations given in Prof. Balfour Stewart’s
paper in ‘Nature’ (vol. xvi. p. 10), for the purpose of showing that
there are exactly seven minimum periods from 1787 to 1871, the mean
of which is twelve years, the mean of the seven corresponding maximum
periods being 11·8 years. The true magnetic declination-period is, then,
the mean of these, viz. 11·9 years. In exactly the same manner he finds
that the mean period of sun-spots is 11·9 years.

[Sidenote: Jupiter’s suspected connexion with sun-spots.]

The auroral displays also have the same period. Mr. Jenkins also refers
to Wolf, De La Rue, Stewart, and Loewy, as having stated their belief
that Jupiter is the chief cause in the production of sun-spots, and draws
attention to the period of 11·9 years as being Jupiter’s anomalistic
year, or the time which elapses between two perihelion passages.

[Sidenote: Infrequency of Auroræ and absence of sun-spots in 1876-78.]

The infrequency of Auroræ during the years 1876-78, and a corresponding
comparative absence of sun-spots, may be added to the evidence on the
subject. I have seen no account of important Auroræ during the years
mentioned, and day after day has recently (1878) passed with a perfectly
clean sun-disk.


_Aurora and Electricity._

[Sidenote: Aurora and electricity. Sir John Franklin’s experience with
electrometer.]

Sir John Franklin failed to get indications of electricity connected
with the Aurora with a pith-ball electrometer; but with another form of
electrometer specially constructed for the purpose he seems to have got
some, though not very strong or regular, indications of repulsion between
the needle of the instrument and the conductor when Auroræ were seen. He
does not decide whether the electricity was received from or summoned
into action by the Aurora.

[Sidenote: Parry’s experience.]

Parry, at Fort Bowen, with a gold leaf electroscope connected with a
chain attached by glass rods to the skysail mast-head, 115 feet above
sea-level, found no effect.

[Sidenote: Dr. Allnatt’s experience, February 4, 1872.]

Dr. Allnatt, at Frant, during the display of 4th February, 1872,
found the earth’s electricity so powerful that the gold leaves of the
electrometer remained divergent for a considerable time.

[Sidenote: M’Clintock found electroscope affected in Baffin’s Bay and
Port Kennedy.]

M’Clintock observes that on six occasions of Aurora in Baffin’s Bay, the
electroscope was strongly affected, and on three occasions of Aurora at
Port Kennedy. The electricity was always positive.

Dec. 18.—Dr. Walker called him to see the electroscope. The charge was at
first weak, but afterwards strong enough to keep the leaves diverged. Dr.
Walker found two periods of minimum electrical disturbance about 9 P.M.
and noon.

[Sidenote: Electric currents in telegraphic wires during Auroræ.]

Electric currents have been reported as produced in telegraph wires
during Auroræ. Though transient they are said to be often very powerful,
and to interrupt the ordinary signals. Loomis (Sillim. Journ. vol.
xxxii.) mentions cases where wires have been ignited, brilliant flashes
produced, and combustible materials kindled by their discharge.

Here, too, we may note the account of electric phenomena in the case of
the Aurora Australis described (_antè_, p. 28) by Mr. Proctor.

[Sidenote: Mr. George Draper’s report as to disturbed condition of the
Indian Submarine and other cables during Aurora of February 4, 1872.]

Mr. George Draper, of the British-Indian Submarine Telegraph Company,
speaking of the Aurora of February 4, 1872 (and writing to the ‘Times’
under date February 5th), states that the Aurora visible in London
was also visible at Bombay, Suez, and Malta, and that the Company’s
electrician at Suez reported that the earth-currents there were equal
to 170 cells (Daniell’s battery), and that sparks came from the cable.
The electrical disturbances lasted until midnight, and interrupted the
working of both sections of the British Indian Cable between Suez and
Aden, and Aden and Bombay. For some days previously the signals on the
British Indian cables had been much interfered with by electrical and
atmospheric disturbances.

At Malta there was a severe storm on the morning of the 4th, so that it
was necessary to join the cable to earth for some hours, and the Aurora
was very large and brilliant there.

The electrical disturbances on the cables in the Mediterranean and on
those between Lisbon and Gibraltar, and Gibraltar and the Guadiana, were
also very great. The signals on the land line between London and the
Land’s End were interrupted for several hours on the night of the 4th by
atmospheric currents. Similar effects accompanied the displays of Oct. 24
and 25, 1870.


_Aurora and Meteoric Dust._

[Sidenote: Aurora and meteoric dust. Theories of Dr. Zeyfuss and M.
Gröneman.]

A theory has been propounded independently by Dr. Zeyfuss and by M.
Gröneman, of Gröningen, according to which the light of the Aurora is
caused by clouds of ferruginous meteoric dust ignited by friction with
the atmosphere. Gröneman shows that these might be arranged along the
magnetic curves by the action of the earth’s magnetic force during their
descent, and that their influence might produce the observed magnetic
disturbances.

[Sidenote: Ferruginous dust in the Polar Regions.]

The arches might be accounted for by the effects of perspective; and the
iron spectrum shows correspondence with some of the lines of the Aurora.
Ferruginous particles have been found in the dust of the Polar regions
according to Professor Nordenskiöld, but whether derived from stellar
space or from volcanic eruption is uncertain. A difficulty has been
suggested that while meteors are more frequent in the morning, or on the
face of the earth which is directed forward on its orbit, the reverse
prevails in the case of Auroræ. Gröneman meets this by supposing that in
the first case the velocity may be too great to allow of arrangement by
the earth’s magnetic force. He accounts for the infrequency of the Aurora
in equatorial regions by the weakness of the earth’s magnetic force, and
the fact that when it does occur the columns must be parallel to the
earth’s surface.

[Sidenote: Baumhauer’s proposition.]

Baumhauer (Compt. Rend. vol. lxxiv. p. 678) advances, as regards Polar
Auroræ, the proposition, that not only solid masses large and small, but
clouds of “uncondensed” (meteoric) matter probably enter our atmosphere.

If from our knowledge of the meteoric stones which fall to the earth’s
surface we may draw any conclusion respecting the chemical constitution
of these clouds of matter, it would appear that they may contain a
considerable portion of the magnetic minerals iron and nickel. Let
such a cloud approach our earth, regarded as a great magnet, it would
be attracted towards the Pole, and, penetrating our atmosphere, the
particles which have not been oxidized, and are in a state of extremely
fine division, would by their oxidation generate light and heat,
producing the polar Auroræ. Baumhauer suggests it would be interesting
in support of this theory to detect in the soil of polar areas the
presence of nickel. The presence of iron and nickel in meteoric masses
in considerable quantities is frequent; and cases are also on record by
Eversmann of hailstones containing crystals of a compound of iron and
sulphur, by Pictet of hailstones containing nuclei which proved to be
iron, and by Cozari of hailstones containing nuclei of an ashy-grey
colour, the larger ones of which were attracted by the magnet, and found
to contain iron and nickel. Nickel was found by Reichenbach in parts of
Austria on hills consisting of beds of sandstone and limestone, and quite
free from metallic veins.

[Sidenote: Mr. Lefroy’s description of a phenomenon ascribed by him to
streams of cosmic dust.]

Mr. J. W. N. Lefroy, in ‘Nature,’ describes a phenomenon seen by him
at Fremantle, West Australia, in the month of May, which he designates
“A Lunar Rainbow, or an Intra-lunar convergence of Streams of slightly
illuminated Cosmic Dust?”

It lasted about three quarters of an hour, and consisted of one grand
central feather, of very bright white cloud, springing out of the horizon
at W.N.W., and crossing the meridian at about 20° north of the zenith,
with a width of 7° to 8°.

On either side of this was a system of seven or eight minor beams of
light, extending from the W. to the E. horizon, subtending a chord common
to themselves and to the main stream, and converging towards the axis
of the central stream so as to intersect it at a point about 30° or
40° below the western horizon, at which the whole system subtended an
azimuth of about 20°. Near the zenith, where its transverse section was a
maximum, that section subtended an angle of about 40°.

The idea strongly suggested itself to Mr. Lefroy of converging streams of
infinitely minute particles of matter passing through space at a distance
from the earth at which its aerial envelope may have still a density
sufficient by its resistance to give cosmic dust passing through it that
illumination which it possessed. In about twenty minutes the streams of
light had attained their maximum brightness. Their apparent figure was
that of a nearly circular (slightly flattened) arc of an amplitude of 15°
or 20°, as viewed from the middle point of its chord.

The brightness and the convergence of the streams were both more marked
towards the western horizon than the eastern. This same phenomenon was
described in the ‘South-Australian Register’ as a beautiful lunar rainbow
visible in the western heavens.

Mr. Lefroy and other observers concurred in the impression that the minor
lateral streams on the N. side of the main one intervened between the
earth and the moon, and that one or more of them in their slow vibrations
swept the surface of the moon and sensibly obscured its light. There can
be hardly any question that the phenomenon observed was in fact an Aurora.

[Sidenote: Suggestion as to collecting iron and nickel particles from the
atmosphere.]

It may be a question whether iron and nickel particles of meteoric origin
do not ordinarily exist in the atmosphere in a greater degree than we
suspect, and might be detected if special means, such as magnets, plates
of glass covered with glycerine, &c., were adopted for the purpose of
collecting and examining the cosmic dust. Larger gatherings than usual
of iron and nickel particles during the presence of Auroræ would be in
support of Mr. Lefroy’s theory.


_The Aurora and the Planets Venus and Jupiter._

[Sidenote: The Aurora and planets Venus and Jupiter. The planet Venus’s
halo during Aurora.]

During a brilliant Aurora seen at Sunderland, February 8, 1817 (‘Annals
of Philosophy,’ p. 250), about 8 o’clock, Venus was about 8° above the
horizon, and displayed a very peculiar appearance. Her rays passed
through a thin mist or cloud, probably electric, of a deep yellow tint.
Her apparent magnitude seemed increased, and a halo was formed round her
as sometimes appears round the moon in moist weather; but the stars that
were in that part of the heavens shone with their accustomed brilliancy.

[Sidenote: Dr. Miles’s observation of Venus during an Aurora.]

The Rev. T. W. Webb, in his ‘Celestial Objects’ (1859), p. 43, quoting
from the Philosophical Transactions, mentions that, “January 23rd,
1749-50, there was a splendid Aurora Borealis about 6 P.M. The Rev. Dr.
Miles, at Tooting, had been showing Jupiter and Venus to some friends
with one of Short’s reflectors, greatest power 200, when a small red
cloud of the Aurora appeared, rising up from the S.W. (as one of a deeper
red had done before), which proceeded in a line with the planets and soon
surrounded both. Venus appearing still in full lustre, he viewed her
again with the telescope without altering the focus, and saw her much
more distinctly than ever he had done upon any occasion. His friends
were of the same opinion. They all saw her spots plain (resembling those
in the moon), which he had never seen before, and this while the cloud
seemed to surround it as much as ever.”

I think this effect might perhaps have arisen from the Aurora acting as
a screen, and removing the glare with which so bright an object as Venus
is always accompanied; but the case is a singular one, and one would be
glad of further experience. I suggested observations on this head during
Sir Geo. Nares’s Arctic Expedition; but the suggestion, for some reason
of which I am not aware, was not included in the official instructions
issued.

[Sidenote: Brightness of stars during Auroræ.]

Remarks are frequent of the brightness of stars as seen through Auroræ.
Payer, of the Austrian Expedition, remarks that falling stars passed
through the Aurora without producing any perceptible effect or undergoing
any change.

[Sidenote: Aurora of Oct. 24, 1870, and Jupiter.]

A grand display of the Aurora took place 24th October, 1870. About
this time the belts of Jupiter were observed to be highly coloured. As
observed by me on the night of November 2, 1870, at 9 P.M., with an
8¼-in. Browning reflector, achromatic eyepieces 144, 305, and 450, the
equatorial zone was of a distinctly dark ochre colour, deepening to
red-brown as it approached the lower (N.) edge. Two thin belts above were
slate-purple, and a darker belt below was of a deep purple colour.

[Sidenote: According to Lassell and others, Jupiter’s belts exhibit the
brightest colours at period of Auroræ.]

Lassell, Proctor, and others have reported Jupiter’s belts to exhibit the
brightest colours at the period of Auroræ. Mr. Browning gives a drawing
of Jupiter as seen on January 31, 1870 (a year noted for Auroræ), with
the belts brightly coloured. The finest view of Jupiter I ever had was
on the 8th February, 1872 (a fine Aurora was on the 4th), when, with the
8¼-inch Browning reflector, I saw the whole surface of the planet (by
glimpses) cloud-mottled. The equatorial belt was, however, then slightly
tinted only. In Dr. Miles’s observation (p. 66) he does not seem to have
noticed the colouring of Jupiter’s belts.

[Sidenote: Infrequency of Auroræ and lightness in tint of Jupiter’s
belts.]

The three past years, 1876, 1877, and 1878, have been distinguished by
the infrequency of Auroræ; and Jupiter’s equatorial zone and belts have
been mainly reported of light tints.

The subject apparently deserves more attention than it has hitherto
received.


_The Aurora and the Zodiacal Light._

[Sidenote: The Aurora and the Zodiacal Light. Ångström’s observation on
spectrum.]

Ångström in 1867 found the spectrum of the Zodiacal Light to be
monochromatic, consisting of a single line in the green, to which he
assigned approximately the position 1259 on Kirchhoff’s scale, the same
that he had determined for the green line of the Aurora Borealis; and
Respighi, on the Red Sea, on the evening of the 11th and the morning of
the 12th January 1872, perceived in the Zodiacal Light not only this
green line, but near it, towards the blue, a band or zone of apparently
continuous spectrum.

[Sidenote: Respighi’s at Campidoglio.]

At the Observatory of the Royal University of Campidoglio, February 5th,
1872, Respighi, at 7 P.M., was able to discern the same spectrum; and on
directing the spectroscope to other points he found that this spectrum
showed itself in all parts of the heavens from the horizon to the zenith,
more or less defined in different parts, but everywhere as bright as in
the Zodiacal Light. The Observatory Assistant, Dr. di Legge, likewise
observed this spectrum distinctly in various parts of the heavens.
Respighi’s observations corroborating Ångström’s in 1867, appeared to him
to demonstrate the identity of the Zodiacal Light with the Aurora, and to
establish the identity of their origin.

[Sidenote: Pringle thinks the Aurora may be considered as allied to the
Zodiacal Light.]

Pringle, in a letter to ‘Nature’ from South Canara, October 3, 1871,
alludes to the Aurora as being considered by many allied to the Zodiacal
Light, and does not think the evidence then hitherto adduced against the
theory at all conclusive. He says:—“Assume the auroral light to consist
of solid particles of matter, planet dust, shining by reflected light,
and it is not difficult to imagine the Aurora playing amongst these tiny
worlds, each of which would have its own small magnetic system swayed
like our own by the monster magnet the sun.”

[Sidenote: Phosphorescence of sky when Zodiacal Light has been seen
bright.]

He notices he has never found it to have a decided outline, nor traced
it east or west to 180° from the sun. He also refers to others having
noticed that when the Zodiacal Light has been seen unusually bright, a
“phosphorescence” of the sky was everywhere visible.

[Sidenote: Pringle failed to find bright lines or bands in the Zodiacal
Light.]

He does not seem at that time to have examined the matter
spectroscopically; and on June 23, 1872, he writes again, pointing out
the peculiarity in Respighi’s observation that the green line was seen
everywhere as bright as in the Zodiacal Light, and suggesting that it
was due to a concealed Aurora present at the time of Ångström’s and
Respighi’s observations. He further states he had examined the Zodiacal
Light with a Browning 5-prism spectroscope (I presume a compound
direct-vision form is meant) since the last December, and, brilliant
as the phenomenon had frequently been, failed to detect the slightest
appearance of bright lines or bands. A faint diffuse spectrum about as
intense as that of a bright portion of the Milky Way was all he had
obtained.

[Sidenote: Prof. Piazzi Smyth confirms this.]

Professor Piazzi Smyth, in the clear sky of Italy, and with an instrument
specially designed for showing faint spectra, found no lines or bands,
but only a faint continuous spectrum extending from about midway between
D and E in the solar spectrum to nearly F (see Plate V. fig. 3, in which
the continuous spectrum is graphically shown, white on a black ground).

[Sidenote: Colour of the Zodiacal Light.]

It may here be mentioned that the Zodiacal Light is usually described as,
in these latitudes, of a golden yellow or pale lemon tinge.

[Sidenote: Rev. Mr. Webb’s observation, February 2, 1862. He found no
green line of the Aurora.]

On one occasion, however, it has been described as not having this
tinge, but rather resembling the light of the Milky Way, but brighter.
On another occasion I saw the whole cone of a crimson hue without any
mixture of yellow. The Rev. Mr. Webb thought that a display seen at
Hardwick Vicarage, February 2nd, 1862, showed a ruddy tinge not unlike
the commencement of a crimson Aurora—“it was certainly redder or yellower
than the galaxy.” He examined it with a pocket spectroscope which would
show distinctly the green line of the Aurora (probably Browning’s
miniature), but nothing of the kind was visible, nor could any thing be
traced beyond a slight increase of general light, which, on closing the
slit, was extinguished long before the auroral band would have become
imperceptible.

[Sidenote: A. W. Wright’s observations and conclusions.]

A. W. Wright examined the Zodiacal Light with a Duboscq single-prism
spectroscope, the telescope and collimator having a clear aperture of
2·4 centimetres, magnifying-power of telescope 9 diameters. Special
precautions were taken about the observations, and the conclusions
arrived at were:—

(1) The spectrum of the Zodiacal Light is continuous, and is sensibly the
same as that of faint sunlight or twilight.

(2) No bright line or band can be recognized as belonging to this
spectrum.

(3) There is no evidence of any connexion between the Zodiacal Light and
the Polar Aurora.

[Sidenote: Polarization of Zodiacal Light. Burton’s observation confirmed
by Wright and Tacchini.]

The Polarization of the Zodiacal Light has been already referred to under
the head of “Polarization of the Aurora:” but it may be here noted that
Mr. Burton’s observation of polarization of the light there mentioned
has been confirmed by Wright and Tacchini, and the presence of reflected
sunlight established. In this respect it differs from the Aurora, in
which no trace of polarization has hitherto been detected; and looking at
this, and at the weight of evidence in the spectroscopic observations,
the theory of a connexion between the Aurora and the Zodiacal Light must,
as the matter stands, be given up.




CHAPTER VII.

AURORA-LIKE PATCHES ON THE PARTIALLY-ECLIPSED MOON.


[Sidenote: Aurora-like patches on the partially-eclipsed moon, Feb. 27,
1877.]

In anticipation of the total eclipse of the Moon on the 27th February,
1877, several articles appeared in the leading journals of the day
describing, for the public benefit, the appearances which might be
expected during the occurrence of the phenomenon.

[Sidenote: Formerly it was thought the moon was illuminated by auroral
light.]

Among these was one by Mr. R. A. Proctor, in which the following passage
occurs:—“That dull, or occasionally glowing red colour, shown by the moon
when she is fully and even deeply immersed in the shadow of the earth,
is a phenomenon whose explanation is not without interest. Formerly it
was thought that the moon possessed an inherent light, or _perhaps was
illuminated by auroral light_, which only became discernible at the time
of total eclipse. Indeed even Sir W. Herschel fell into the mistake of
supposing this the only available explanation, having miscalculated the
efficiency of the true cause.”

[Sidenote: Author’s notes of the eclipse. Colour-tints described. A
crimson-scarlet tint reminded author of an auroral glow.]

This passage was only pointed out to me by a friend after the eclipse
had actually taken place, and I had sent him some notes of what I then
saw. My notes on the occasion comprised, amongst others, the following
remarks:—“The tints of colour also during partial eclipse, owing, no
doubt, to the moon’s considerable altitude, were singularly bright and
well contrasted. Silver-grey, dusky copper-red, and the same tint clearer
and brighter were ranged side by side with a lovely jewel effect. _We
noticed also at times a crimson-scarlet tint, deeper and less mixed with
yellow than the copper colour._ This last tint reminded me much of a
_crimson glow common to the Aurora_, and which I also once distinctly
remarked (of course in a weaker degree) in the zodiacal light” (_antè_,
p. 68).

[Sidenote: Eclipse, Aug. 23-24, 1877. Sky clear, but eclipsed moon misty
and indistinct until total obscuration. Succession of colours.]

On the occasion of the eclipse of August 23-24, 1877, we were favoured
at Guildown, in common with many other places, by a singularly clear sky
during the progress of the moon’s obscuration and subsequent clearing.
In the early part of the evening, however, the moon, from some cause
(possibly atmospheric vapour), seemed to have, as the earth’s shadow
advanced on its disk, an unexpectedly misty and indistinct appearance,
which lasted up to and including total obscuration. Golden yellow, yellow
copper, dull copper, ruddy copper, and dull red were successively the
principal colours observed at different times and at various portions of
the moon’s surface.

[Sidenote: As shadow passed off, indistinctness gave way to a sharpness
of the moon’s features as seen through shadow. Two patches of crimson
light described.]

After referring to some spectroscopic appearances, my notes then ran on
thus:—“As the shadow began to pass off, and the bright sharp crescent
of the illuminated portion of the moon to appear, the general aspect of
the moon’s disk seemed to me to greatly change. The certain amount of
indistinctness noticeable during approach and continuance of totality,
gave way to a considerable sharpness of the moon’s features as seen
through the shadow. The shadowed part glowed with a richer copper tint,
on which were seen dark, almost black, spots and patches.” Then follows a
description of these; and the notes continue:—“Two features here struck
me—the one a continuation of the upper limb of the illuminated crescent,
so that it seemed to form a bead of light just on the centre of the upper
edge of the moon; the other _two patches of crimson light_, similar to
those I described as having been seen in the last total eclipse. One
of these, quite a small one, was just under the elongated bead before
described; the other, a much larger and more diffused one, was seen
towards the south-west limb of the moon, about midway between it and the
centre. The spots or patches were of a decidedly crimson-red, in contrast
to the ordinary copper-red of the disk, and were noticed by my friend as
well as by myself.”

[Sidenote: Patches well seen in field-glass; lost in small refractor.
They gradually deepened in tint.]

These were eye observations. The patches were quite well seen (but not
so brightly as with the eye) with a double achromatic field-glass.
With a 3¼-inch Cooke refractor and low power, they seemed lost in the
general moon tint; but they were then diminishing in brightness. From
a comparison of my two sketches, the patches seem to have gradually
deepened in tint, and we considered them to have disappeared in a like
gradual manner.

[Sidenote: Two sketches taken.]

My first sketch was taken shortly after end of total phase; the second
about ten minutes later. I have reproduced the original sketches in
preference to any drawing prepared from them (Plate IV. figs. 2 and 3).

The patches did not last long, but were lost as the shadow swept off
the moon. I saw nothing of the sort during the approach of or pending
totality, nor until a small crescent of the moon began to appear behind
the shadow.

I have looked for other accounts of these patches, but cannot find any.
Most observers have described the deeper colour of the shadowed moon by
the word “copper.” Some extend this colour to red; but there is probably
much in the state of the atmosphere affecting this.

[Sidenote: Dec. 3, 1703, moon’s colour described.]

At Avignon, December 3, 1703, the moon appeared, pending eclipse,
“extraordinarily illuminated and of a very bright red,” while other and
different features were seen at Montpelier.

On March 19, 1848, observers in England, Ireland, and Belgium described
the moon’s disk as “intensely bright coppery red.” On the occasion of
August 23-24, 1877, before mentioned, an article in one of the public
papers described the moon’s disk, during totality, as of a “dull copper
colour.”

[Sidenote: Mr. Keye’s observation.]

Mr. Henry Keye, in the Engadine, at a height of 4500 feet above
sea-level, and with the purest air, saw the partially covered moon
(before totality) as a “dull copper colour.”

[Sidenote: Prof. Pritchard’s. M. Faye’s. Dr. Allnatt’s at Frant.]

Prof. Pritchard, writing from the Oxford University Observatory, says
that at 12h 10m (about the time my sketches were taken) there was a
good deal of light on the moon’s following limb, and the colour was
“more red than copper,” and apparently redder than it had been at a
similar distance of time before totality. Mons. Faye reported to the
French Academy of Sciences that “a striking phenomenon not previously
noticed was that the reddish tinge, resembling that of a fine sunset,
was deepest at the margin of the disk, a circumstance which he could not
explain.” Dr. Allnatt, writing from Frant, says:—“At totality the moon’s
disk presented a most extraordinary appearance: the western limb was
comparatively transparent, but the main body appeared as though enveloped
in a semi-opaque clot of coagulated blood, through which the lunar
features were dimly visible.”

[Sidenote: Observations as to the patches.]

The observations of Prof. Pritchard and Mons. Faye point more immediately
to redness; and this is the nearest approach I can find to the patches I
noticed. These patches do not seem to me easy of explanation. They could
not well be colours or details due to the actual surface of the moon
itself. The moon, we are aware, has only a certain portion of the visible
disk slightly tinted. The Mare Serenitatis is certainly of a slight green
tinge; and to the Palus Somni and certain other districts is attributed a
pale red or pink; but these tints could hardly have sufficed to produce
the effect seen, as the patches were conspicuous for a bright and decided
colour. The positions, moreover, did not correspond; while the ease
with which other details of the surface were seen at the time would, if
the tints had arisen from the surface itself, probably have enabled the
circumstance to be detected.

[Sidenote: Refraction of sun’s rays not a satisfactory explanation.]

The refraction of the sun’s rays by passage through the earth’s
atmosphere is, too, not a satisfactory explanation. This, as judged by
the appearance of the covered moon immediately before and at totality,
gives a disk of shadow deeper in tone in the centre and lightening
towards the edges, but in other respects fairly uniform, so that the
whole disk seems to partake of the same tint and its graduations; and
this is what might have been expected under the circumstances. The
patches, on the other hand, were quite local.

[Sidenote: Question of lunar atmosphere.]

The theory of the moon’s possessing no atmosphere whatever is now very
generally, but perhaps too readily, received (mainly upon the evidence
of the spectroscopic observations of occulted stars[9]), as there still
seems a reasonable doubt whether our satellite may not possess an
atmosphere, possibly rarefied, but yet sufficiently dense to permit of
the formation of cloud or vapour.

[Sidenote: Instance of patch of vapour or cloud on moon’s surface.]

A curious case, in which a patch of vapour or cloud was supposed to be
detected on the moon’s surface, is reported by the Rev. J. B. Emmett in
a communication to the ‘Annals of Philosophy’ (New Series, vol. xii. p.
81). It is dated “Great Ouseburn, near Boroughbridge, July 5, 1826,”
the observation being made with “the greatest care with a very fine
telescope.”

On the 12th April 8h, while observing the part of the moon called Palus
Mœotis by Nevelius, with an excellent Newtonian reflector of 6 inches
aperture, at a particular part of the Palus, which he minutely describes,
he saw, with powers 70 and 130, “a very conspicuous spot wholly enveloped
in black nebulous matter, which, as if carried forward by a current of
air, extended itself in an easterly direction, inclining a little towards
the south, rather beyond the margin of Mœotis.” April 13th 8h to 9h, the
cloudy appearance was reduced both in extent and intensity, and the spot
from which it seemed to issue had become more distinctly visible. On
April 17th scarcely a trace of the nebulous matter remained; but so long
after as June 10th 8h “a little blackness” remained about the spot. Mr.
Emmett suggested “smoke of a volcano or cloudy matter.” A copy of the
drawing annexed to the paper is given on Plate X. fig. 10 (black patch on
moon). If this observation was (as it certainly appears to be) critical
and exact, there must have been a disturbance of the moon’s surface,
indicating some sort of cloud- or vapour-supporting atmosphere; and
probably, for the purposes of Auroræ, an atmosphere of a very rarefied
condition would suffice[10].

[Sidenote: Prof. Alexander’s evidence in favour of a lunar atmosphere.]

According to the ‘New York Tribune,’ at a recent semi-annual meeting of
the American Academy of Sciences, Professor Alexander “brought forward a
variety of evidence tending to indicate some envelope like an atmosphere
for the moon. The evidence was principally drawn from observations
during eclipses. The explanations usually offered for the bright band
seen around the moon at such times was fully considered, and shown to
be inadequate, though good as far as they would apply. The ruddy band
of light is much too broad to be the sun’s chromosphere. It was most
apparent in those instances where the moon was nearest the earth. It
would best be accounted for by supposing an atmosphere to the moon, a
thin remnant of ancient nebulosity, comparable to that which accompanies
the earth and gives rise to the appearance of the Aurora Borealis.” Is
it not, however, possible that the appearance might have arisen from
Auroræ in action within the region of the earth’s own atmosphere during
the passage of the sun’s rays through it at the time of the eclipse?
The whole subject is difficult of explanation, and should be one of the
points for attention on the occasion of the next total Lunar eclipse. It
seemed to me appropriate for introduction into the present history of the
Aurora, whatever its solution may ultimately be.

[Sidenote: Mars and Jupiter.]

In the case of Mars and Jupiter, whose atmospheres are sufficiently
recognized, red- and scarlet-tinted patches are frequently noticed. In
Mars this is generally attributed to the geological character of the
surface of the planet itself; but I have observed on Mars’s surface
during the recent opposition a local rosy tint of a more diffused and
indefinite character; and in the case of Jupiter the appearances seem
almost always connected with the clouds’ belts, as distinguished from the
regions lying nearer to the planet’s surface.

[Sidenote: Prof. Dorna’s “Lunar Aurora.”]

Professor Dorna, of Turin, ascribed a flickering light seen on the
reddened disk of the moon during the Lunar eclipse of February 1877
to the action of a _Lunar Aurora_, holding that the refraction of the
sun’s rays within the cone of the earth’s shadow was not an adequate
explanation (‘L’Opinione Nazionale,’ March 3, 1877).

[Sidenote: Spectroscopic observations bearing on the subject. Mr.
Christie’s observations at Greenwich.]

The spectroscope might have afforded some information on the question;
but my own telescopes (8¼ and 3¼ in.) were not of sufficient aperture
to give a sensible spectrum of a portion of the moon’s eclipsed
surface, and my observations were chiefly made on the entire disk
with hand-spectroscopes without a slit. Mr. Christie, at the Royal
Observatory, Greenwich, made a set of observations during totality, and
also during subsequent partial phase, with a single-prism spectroscope.
During totality a strong absorption band was seen in the yellow, and
the red and blue ends of the spectrum were completely cut off, while
the orange was greatly reduced in intensity. The yellow and green
were comparatively bright, and seemed to constitute the whole visible
spectrum. The absorption band became narrowed as the end of the total
phase approached, and during partial phase was reduced to a mere line.
The red end of the spectrum was cut off by a dark band commencing about
halfway from D to C, in which a black line was suspected. The bands
observed were characteristic of the spectrum of light which has passed
through a thick stratum of air. In the description of the spectrum of the
Aurora in Part II., it will be seen that the conspicuous red and green
lines of the Aurora are either coincident with, or very close to, some of
these atmospheric lines. It does not appear that Mr. Christie examined
the crimson patches specifically, nor that he saw bright lines on any
part of the moon’s eclipsed disk.

[Sidenote: Mr. Pratt’s notes of Lunar Eclipse, August 23, 1877.]

Mr. Henry Pratt has also kindly handed me for use his notes of the Lunar
eclipse of August 23, 1877, as seen at Brighton on a splendid night. They
were made as the phenomenon progressed, are 58 in number, and in many
instances only a few minutes, or even seconds, apart. A selection of
them is here given:—9h 13m 50s, first contact of shadow. 9h 30m, shadow
very dark; no details of disk easily seen. 9h 40m, first appearance of
red. 9h 50m, _red_ all over disk, except margin bluish and S. part green
tint. 10h 2m, _a sudden brightening of whole disk_, in strong contrast to
two minutes previously. 10h 15m, _E. limb much darker_. 10h 35m, _south
pole decidedly brightest_. 10h 44m, _S.E. limb much brighter_. 10h 48m,
_whole disk much darker_. 10h 51m, _S.E. limb brightening again_. 11h 1m,
_N.E. limb brightening_. 11h 3m, _N.E. limb has darkened and brightened
three times during last two minutes_. 11h 20m, N. pole has _darkened_.
11h 21m, N. pole has _brightened_. 11h 24m 30s, N. pole darker _red_. 11h
35m, N. pole _bright_. 11h 35m 30s, same _dark_ and _red_. 11h 42m, N.E.
limb especially bright for a few seconds, and then _reddened_ and shaded
again. 11h 49m, _S. pole reddened_. 12h 1m, _S.W. limb reddest part; S.
pole red; N. pole paler red_. 12h 3m 50s, first appearance of E. limb
(my first sketch was made shortly after this, and my second about ten
minutes later). 12h 21m, a bright patch on N.N.W. separated from N. pole.
12h 24m, _S.W. region is reddest part of eclipse_. 12h 40m, _redness_ of
shadow fading out.

With a small Browning star-spectroscope Mr. Pratt saw the red and blue
ends of the spectrum cut off, but nothing else. Mr. Pratt adds that the
_red_ colour was not an effect of contrast or an optical delusion in any
way, as was proved by using at times a limited field containing only the
red portion under examination. In reference to the curious brightening
and darkening of the disk, and the change from time to time of local
colour, he says that with much experience he has seen nothing of the
same marked character on other occasions, and that “the whole matter was
at the time astonishing to me, but none the less real.” The local red
patches seen by me seem also to have been observed by Mr. Pratt.

[Sidenote: Mr. Pratt’s observation on the floor of Plato.]

As an addition to the instances of Tycho, Picard, &c., mentioned in the
note on p. 73, Mr. Pratt has also sent me his notes of some observations
by him, of “local obscuration of the floor of Plato.” As somewhat
condensed, they are as follows:—1872, July 16. While in other parts of
the floor spots and streaks were well visible, “the N.W. portion was
in such a hazy condition that nothing could be defined upon it.” 1873,
Nov. 1. 27 light streaks seen (7 new): the brightness of the streaks was
in excess of their usual character, as compared with the craterlets;
“an _obliteration_ or _invisibility_ of _all_ the light streaks in the
neighbourhood of craterlet no. 1 was very noticeable;” and also “a
similar obliteration of the N. end of the streak called the Sector, near
craterlet 3.” 1874, January 1. 18 light streaks seen, including 3 new,
“some of which outshone other longer known ones. This was curious; for
had they been as bright within the last two years as on this occasion
I must have noticed them.” Mr. Pratt points out, as worthy of remark,
that some months previous to November 1st, 1873, neither craterlets
nor streaks on the floor of Plato “had maintained their previous
characteristic brightness,”—a fact which he thinks ought to be considered
together with the outbreak of brilliancy of both orders on that day, as
well as the apparently sudden existence of new ones.

[Sidenote: Observation by Mr. Hirst of a dark shade on the moon.]

The ‘Observatory,’ March 1, 1879, p. 375, contains an account, by Mr. H.
C. Russell, of some Astronomical Experiments made on the Blue Mountains,
near Sydney, N. S. W. Among these it is noticed that on 21st October,
1878, at 9 A.M., when looking at the moon, Mr. Hirst found that a large
part of it was covered with a dark shade, quite as dark as the shadow
of the earth during an eclipse of the moon. Its outline was generally
circular, and fainter near the edges. Conspicuous bright lunar objects
could be seen through it; but it quite obliterated the view of about half
the moon’s terminator, while those parts of the terminator not in the
shadow were distinctly seen.

No change in the position of the shade could be detected after three
hours’ watching. The observation is made, “One could hardly resist the
conviction that it was a shadow; yet it could not be the shadow of any
known body. If produced by a comet, it must be one of more than ordinary
density, although dark bodies have been seen crossing the sun which were
doubtless comets.” The diameter of the shadow from the part of it seen on
the moon was estimated at about three quarters that of the moon[11].




CHAPTER VIII.

AURORA AND THE SOLAR CORONA.


[Sidenote: Aurora and the solar corona. Mr. Norman Lockyer’s ‘Solar
Physics.’]

Mr. Norman Lockyer, in his ‘Solar Physics,’ a work of 666 pages, gives
but little space to the Aurora. The index comprises:—“Aurora Borealis,
connexion with sun-spots, pp. 82-102.” “Affirmed coincidence of spectrum
with that of the corona, pp. 244, 256.”

[Sidenote: Extracts from as to Aurora’s connexion with sun-spots and with
solar corona.]

Page 82. After referring to Gen. Sabine as having shown that there are
occasional disturbances in the magnetic state of the earth, and that
these disturbances have a periodical variation, coinciding in period
and epoch with the variation in frequency and magnitude of the solar
spots as observed by Schwabe, the author proceeds to state, “and the
same philosopher has given us reason to conclude that there is a similar
coincidence between the outburst of solar spots and of the Aurora
Borealis.”

Page 102. “We have also shown that sun-spots or solar disturbances appear
to be accompanied by disturbances of the earth’s magnetism, and these
again by auroral displays.”

[Sidenote: Evidence of American observers on nature of the corona
considered.]

Page 243. “What, then, is the evidence furnished by the American
observers on the nature of the corona (solar)? It is bizarre and puzzling
to the last degree. The most definite statement on the subject is
that it is nothing more nor less than a _permanent Solar Aurora_! the
announcement being founded on the fact that three bright lines remained
visible after the image of a prominence had been moved away from the
slit, and that one (if not all) of these lines is coincident with a line
(or lines) noticed in the spectrum of the Aurora Borealis by Professor
Winlock.” Mr. Lockyer then adds, that amongst the lines he had observed
up to that time, some forty in number, this line was among those which he
had most frequently recorded, and was, in fact, the first iron line which
made its appearance in the part of the spectrum he generally studied,
when the iron vapour is thrown into the chromosphere.

[Sidenote: Mr. Lockyer’s conclusion adverse to the question being
settled.]

Hence he thought he should always see it if the Aurora were a permanent
solar corona, and gave out this as its brightest line, and on this ground
alone should hesitate to regard the question as settled.

[Sidenote: Prof. Young’s communication to ‘Nature.’]

Page 256 is an extract from a communication by Prof. Young to ‘Nature,’
March 24, 1870, in which the Professor refers to the bright line 1474 as
being always visible with proper management. He also thinks it probable
that this line coincides with the Aurora line reported by Prof. Winlock
at 1550 of Dr. Huggins’s scale, though he is by no means sure of it. He
had only himself seen it thrice, and then not long enough to complete a
measurement. He was only sure that its position lay between 1460 and 1490
of Kirchhoff.

[Sidenote: He does not abandon his hypothesis, it having other elements
of probability.]

For this reason he did not abandon the hypothesis, which appeared to have
other elements of probability, in the general appearance of the corona,
the necessity of immense electrical disturbances in the solar atmosphere
as the result of the powerful vertical currents known to exist there,
as well as the curious responsiveness of our terrestrial magnets to
solar storms; yet he did not feel in a position to urge it strongly, but
rather awaited developments. Father Secchi was disposed to think the line
hydrogen, while Mr. Lockyer still believed it to be iron.

[Sidenote: Dr. Schellen reviews the subject in eclipse of 1869.]

Dr. Schellen, in his ‘Spectrum Analysis,’ treats the matter more in
detail. Referring to the eclipse of 1869 as confirming the previous
observations that the coronal spectrum was free from dark lines, he
points out that Pickering, Harkness, Young, and others were agreed that
with the extinction of the last rays of the sun all the Fraunhofer lines
disappeared at once from the spectrum. He further says:—

[Sidenote: Young observed three bright lines in the spectrum of the
corona. Coincidence of these lines with three bright lines observed by
Winlock in the Aurora. Corona self-luminous, and probably of a gaseous
nature. Corona supposed to be a permanent polar light existing in the
sun. Polar light in the sun attributed to electricity. Dr. Schellen
thinks nature of the corona still a problem.]

“The small instruments employed by Pickering and Harkness, with a large
field of view, exhibited a spectrum obtained at once from the corona,
the prominences, and the sky in the neighbourhood of the sun. These
instruments showed during totality a faint continuous spectrum free
from dark lines, but crossed by two or three bright lines. Young, with
a spectroscope of five prisms, observed the three bright lines in the
spectrum of the corona, and deduced the following positions according to
Kirchhoff’s scale:—1250 ± 20, 1350 ± 20, and 1474. It had been already
explained why the last and brightest of these lines was thought to belong
to the corona and not to that of the prominences, and it seemed probable
that the other two lines belonged also to the light of the corona, from
the fact that they were both wanting in the spectrum of the prominences
when observed without an eclipse. But what invested these three lines
with a peculiar interest was the circumstance that they appeared to
coincide exactly with the first three of the five bright lines observed
by Professor Winlock in the spectrum of the Aurora Borealis. These lines
of the Aurora were determined by Winlock according to Huggins’s scale;
and if these be reduced to Kirchhoff’s scale, the positions of the lines
would be 1247, 1351, and 1473, while the lines observed by Young were
1250, 1350, and 1474.” Dr. Schellen then points out that if it be borne
in mind that Young found the positions of the two fainter lines more by
estimation than by measurement, the coincidence between the bright lines
of the corona and those of the Aurora would be found very remarkable.
The brightest of the lines, 1474, was the reversal of a strongly marked
Fraunhofer line, ascribed by Kirchhoff and Ångström to the vapour of
iron. Dr. Schellen then details Professor Pickering’s observations with
the polariscope, showing that the corona must be self-luminous, and that
from the bright lines seen in its spectrum it is probably of a gaseous
nature, and forms a widely diffused atmosphere round the sun; and then
adds, “It has been supposed, from the coincidence of the three bright
lines of the corona with those of the Aurora Borealis, that the corona
is a permanent polar light existing in the sun analogous to that of our
earth.” Dr. Schellen here adds:—“Lockyer, however, justly urges against
this theory the fact that although the brightest of these three lines,
which is due to the vapour of iron, is very often present among the
great number of bright lines occasionally seen in the spectrum of the
prominences, it is by no means constantly visible, which ought to be the
case if the corona were a permanent polar light in the sun.” (Professor
Young’s answer to this, on the ground of line 1474 being always visible,
has been already given.) “A yet bolder theory is the ascription of such
a polar light in the sun to the influence of electricity, which has been
proved, it is well known, by the relation of the magnetic needle, and
the disturbance of the electric current in the telegraph wires, to play
an important part in the phenomenon of the Aurora Borealis;” and Dr.
Schellen then concludes with an opinion that the nature of the corona was
still a problem[12].

[Sidenote: Various places of wave-length assigned to these lines.]

On reference to the ‘American Journal of Science,’ vol. xlviii. pp. 123
and 404, it seems that the auroral observations before referred to were
made on 15th April, 1869, by C. S. Pierce, with “an ordinary chemical
spectroscope, with the collimator pointed directly to the heavens,”
and were reported by Winlock. The lines were 1280, 1400, and 1550 of
Huggins’s scale, and were reduced to Kirchhoff’s scale by Young. These
lines have had all sorts of places of wave-length assigned to them
by different writers. Proctor gives 5570, 5400, 5200; Pickering and
Alvan Clarke, 5320 (assumed to be 5316, coronal line); Barker, 5170,
5200, 5020; Backhouse, 5320, 4640, and 4310. In my ‘Aurora Spectrum,’
Plate XII., I have assigned two, with a?, to 5320 (Alvan Clarke) and
5020 (Barker). The third might perhaps be placed at 4640 (Backhouse and
Winlock).

[Sidenote: Doubts raised as to closeness of the observations for the
purpose of comparison.]

The coincidences relied on in the foregoing observations depend, of
course, upon (1) the accuracy of the observations themselves, and
(2) the subsequent reduction of the lines for comparison. Assuming
the correctness of the latter, what have we as to the former? Two of
Professor Young’s positions of coronal lines, as stated, seem to have far
too much of the ± element to make them sufficiently accurate. Pierce’s
auroral observation does not state how the lines were positioned. As
they _all_ end with a cypher, the suspicion naturally arises that the
measurements did not extend beyond the first three places of the figures,
and, if so, could not be used for accurate comparison. The auroral lines,
too, are generally rather wide and nebulous, and not easy of comparison
with sharper ones.




CHAPTER IX.

SUPPOSED CAUSES OF THE AURORA.


[Sidenote: Supposed causes of the Aurora. Sulphurous vapours. Magnetic
effluvia.]

At first the Aurora was described to be sulphurous vapours issuing from
the earth; and Musschenbroek pointed out that certain chemical mixtures
sent forth a phosphorescent vapour, in some respects resembling the
Aurora. Dr. Halley originally proposed a similar theory, but ultimately
concluded that the Aurora might be occasioned by the circulation of the
magnetic effluvia of the earth from one pole to another.

[Sidenote: Zodiacal light.]

M. de Mairan, in 1721, in a treatise, ascribed the Aurora to the impulse
of the zodiacal light upon the atmosphere of the earth.

[Sidenote: Luminous particles of our atmosphere.]

Euler combated this theory, and ascribed the Aurora to the luminous
particles of our atmosphere driven beyond its limits by the light of the
sun, and sometimes ascending to the height of several thousand miles.

[Sidenote: Electric fluid _in vacuo_ resembles Aurora.]

Mr. Hawksbee very early showed that the electric fluid assumes, _in
vacuo_ or in highly rarefied atmosphere, an appearance resembling the
Aurora. Mr. Canton contrived an imitation of the Aurora by means of
electricity transmitted through the Torricellian vacuum in a long glass
tube, and showed that such a tube would continue to display strong
flashes of light for 24 hours and longer without fresh excitation.

[Sidenote: Experiment with electrical machine and exhausted receiver.]

In the ‘Edinburgh Encyclopædia,’ date 1830, is mentioned an experiment
in which an electrical machine and air-pump are so disposed that strong
sparks pass from the machine to the receiver of the air-pump.

[Sidenote: Dr. Franklin’s theory.]

As the exhaustion proceeds the electricity forces itself through the
receiver in a visible stream, at first of a deep purple colour; “but as
the exhaustion advances it changes to blue, and at length to an intense
white, _with which the whole receiver becomes completely filled_.” [It
will be noticed that this experiment bears a close resemblance to Prof.
Ångström’s exhausted flask referred to later in treating of the spectrum
of the Aurora.]

Dr. Franklin gave a different form to the electric theory of the Aurora,
supposing that the electricity which is concerned in the phenomenon
passes into the Polar regions from the immense quantities of vapour
raised into the atmosphere between the tropics (Exper. and Observ. 1769,
p. 43).

[Sidenote: Mr. Kirwan’s theory.]

Mr. Kirwan (Irish Trans. 1788) supposed that the light of the Aurora
Borealis and Australis was occasioned by the combustion of inflammable
air kindled by electricity.

[Sidenote: Mons. Monge’s.]

Mons. Monge proposed the theory that the Auroræ were merely clouds
illuminated by the sun’s light falling upon them after numerous
reflections from other clouds placed at different distances in the
heavens (Leçons de Physique par Prejoulz, 1805, p. 237).

[Sidenote: Mons. Libes’.]

Mons. Libes propounded a theory that the electric fluid, passing through
a mixture of azote and oxygen, produced nitric acid, nitrous acid or
nitrous gas, and that these substances, acted upon by the solar rays,
would exhibit those red and volatile vapours which form the Aurora
Borealis (Traité de Physique, ou Dictionnaire de Physique, par Libes;
Rozier’s Journal, June 1790, February 1791, and vol. xxxviii. p. 191).

[Sidenote: Mr. Dalton’s.]

Mr. Dalton considered the Aurora a magnetic phenomenon whose beams were
governed by the magnetism of the earth. He observed that the luminous
arches were always perpendicular to the magnetic meridian (Dalton’s
Meteorological Observations and Essays, 1793, pp. 54, 153).

[Sidenote: Abbé Bertholon’s.]

The Abbé Bertholon ascribed the Aurora Borealis to a phosphorico-electric
light (Encyc. Méthod. art. Auroræ).

[Sidenote: Dr. Thompson concluded the arches to be an optical deception.]

Dr. Thompson (Annals of Philosophy, vol. iv. p. 429), from the
observations of Mr. Cavendish and Mr. Dalton, concluded there was no
doubt that the arched appearance of the Aurora was merely an optical
deception, and that in reality it consisted of a great number of straight
cylinders parallel to each other and to the dipping-needle at the place
where they were seen.

[Sidenote: Artificial Auroræ produced in exhausted tubes.]

With many of us (at least it was so in my own case) our first viewed
Auroræ have been artificial ones, devised by electricians and having
their locus at the Royal Polytechnic in Regent Street or in some
scientific lecture-room. The effects in these cases are produced in tubes
nearly exhausted by means of an air-pump, and then illuminated by some
form of electric or galvanic current.

[Sidenote: Tubes described.]

In one instance the tube is usually of the form shown on Plate X. fig.
9, supported on a base with a brass ball electrode at the lower end, and
a pointed wire at the upper. In another case the tube is of the form
shown on same Plate, fig. 8. After exhaustion it is permanently closed,
the current passing through it by means of the platinum-wire electrodes
introduced into each end of the tube. The first form of tube is usually
excited by a frictional plate machine; the second by a galvanic current
from a Grove or bichromate battery, which, by the aid of a Ruhmkorff
coil, has had its character changed from quantity to intensity. In each
instance, upon connexion with the source supply of the electric current,
a very similar effect is produced.

[Sidenote: Effects described.]

Brilliant streams of rose-coloured light pass between the electrodes,
sometimes as a single luminous misty band, sometimes in divided vibrating
sprays or streams, and sometimes in a flaky column of striæ.

All this, before the spectroscope took its part in the investigation, we
were content to accept as a very fair and probable explanation of the
Aurora accompanied by a mimic representation of the phenomenon.

These appearances may, of course, be produced at will in tubes having
electrodes; but it is, moreover, possible to produce them, though with
less effect, in certain other forms of tube having no such direct
communication with the external electric machine.

One electrode only may be connected with the coil or electrical machine.
The appearance is then a faint representation of what happens when the
current entirely passes (but see experiments with a single wire detailed
in Part III.).

[Sidenote: Tube without electrodes.]

In the case of an exhausted tube having no electrodes, the wires from
the coil may be made into a little helix and placed at each end of the
tube, and the induced currents within will show themselves in flashes and
streams of light, varying in colour and tint according to the gaseous or
other contents of the tube.

[Sidenote: Tube excited by friction.]

In some cases the ordinary forms of galvanic or electrical machine for
supplying the current of electricity may be dispensed with. A long
straight tube exhausted and closed at each end, and without electrodes,
Plate X. fig. 6, being slightly warmed and then excited by friction with
the dry hand or a piece of flannel, silk handkerchief, or the like, is
soon filled with the most brilliant flashes of light playing in the
interior, and when once thoroughly charged needs but little further
excitation to keep up the effect.

[Sidenote: Geissler’s mercury tube.]

Geissler has introduced a form of tube in which electricity in its form
of flashes and glow of light is produced by the friction of mercury. The
outer tube is strong, and contains within it a smaller tube of uranium
glass with balls blown upon it (Plate X. fig. 7). The tubes are exhausted
and a small quantity of mercury is introduced which has access to both
surfaces of the inner tube, as well as to the inner surface of the outer
tube. Upon the tube being reversed end for end or shaken, the mercury
runs up or down the tube and causes a very considerable display of
whitish light.

The before-described tubes are also referred to, and their spectra
described, in the section “On the comparison of some tube and other
Spectra with the Aurora” (Part II.).

The aura or brush from the electrical machine has been considered as
resembling the Aurora, while the hissing and crackling accompanying it
has been supposed to corroborate the reports of similar noises having
been heard during an auroral display.

[Sidenote: Prof. Lemström’s instrument to demonstrate the nature of
Auroræ.]

Prof. Lemström, of the University of Helsingfors, has devised an
instrument for the purpose of demonstrating that Auroræ are produced by
electrical currents passing through the atmosphere. An illustration of
this instrument (for which I am indebted to the Editor of ‘Nature’) is
introduced (fig. 1).

The instrument was exhibited at the recent Scientific Loan Collection at
South Kensington, and a full description of it, together with an essay by
Prof. Lemström, “On the Theory of the Polar Light,” will be found in the
third edition of the Official Catalogue, p. 386. no. 1751. The apparatus
is intended to show that an electric current passing from an insulated
body does not produce light in air of normal pressure; but as it rises to
the rarefied air in the Geissler tubes a phenomenon very like the real
Polar Light is produced.

[Illustration: Fig. 1.]

A is an electrical machine, the negative pole being connected with a
copper sphere and the positive with the earth.

_s s´_ is of ebonite as well as R R _d_, so that B is quite insulated as
the earth is in space. B is surrounded by the atmosphere. _a´ a´ a´ a´
a´ a´_ are a series of Geissler tubes with copper ends above and below.
All the upper ends are connected with a wire which goes to the earth;
consequently a current runs in the direction of the arrows through the
air, and the Geissler tubes become luminous when the electrical machine
is set into operation.

The Geissler tubes represent the upper part of the atmosphere which
becomes luminous when the Aurora Borealis is observed in the northern
hemisphere. The phenomena produced by the Lemström apparatus are
considered consistent with the theory advocated by Swedish observers that
electrical currents emanating from the earth and penetrating into the
upper regions produce Auroræ in both hemispheres. The experiment differs
from the apparatus of M. de la Rive, who placed his current _in vacuo_,
and did not show the property of ordinary atmospheric air, in allowing to
pass unobserved, at the pressure of 760 millims., a stream of electricity
which illuminates a rarefied atmosphere.

[Sidenote: M. de la Rive’s apparatus described.]

De la Rive’s apparatus was also exhibited at the same time, and will be
found described at p. 385 of the Catalogue, No. 1749. A large sphere of
wood represented the earth, and iron cylinders the two extremities of the
terrestrial magnetic axis. These penetrated into two globes filled with
rarefied air, simulating the higher regions of the Polar atmosphere. The
electric discharge turned around a point situate in the prolongation of
the axis, in a different direction at either pole, when the two cylinders
were charged by means of a horseshoe electro-magnet, in accordance with
observations on the rotation of the rays of the Aurora.

[Sidenote: De la Rive’s magnet in an electric egg.]

De la Rive placed an electro-magnet in an electric egg. As soon as the
magnet was set in action the discharge which had before filled the egg
was concentrated into a defined band of light, which rotated steadily
round the magnet.

[Sidenote: Gassiot’s experiment with 400 Grove cells and exhausted
receiver between poles of magnet.]

Gassiot describes an experiment with his great Grove’s battery of 400
cells, in which an exhausted receiver was placed between the poles of the
large electro-magnet of the Royal Institution.

“On now exciting the magnet with a battery of 10 cells, effulgent strata
were drawn out from the positive pole, and passed along the under or
upper surface of the receiver according to the direction of the current.

“On making the circuit of the magnet and breaking it immediately, the
luminous strata rushed from the positive pole and then retreated, cloud
following cloud with a deliberate motion, and appearing as if swallowed
up by the positive electrode.” Mr. Marsh considered this bore a very
considerable resemblance to the conduct of the auroral arches, which
almost invariably drift slowly southward.

[Sidenote: Mr. Marsh considered the Aurora an electric discharge between
the magnetic poles of the earth.]

He considered it probable that the Aurora was essentially an electric
discharge between the magnetic poles of the earth, leaving the immediate
vicinity of the north magnetic pole in the form of clouds of electrified
matter, which floated southward, bright streams of electricity suddenly
shooting forth in magnetic curves corresponding to the points from
which they originated, and then bending southward and downward until
they reached corresponding points in the southern magnetic hemisphere,
and forming pathways by which the electric currents passed to their
destination; and, further, that the magnetism of the earth caused these
currents and electrified matter composing the arch to revolve round the
magnetic pole of the earth, giving them their observed motion from east
to west or from west to east.

[Sidenote: Varley’s observation on a glow-discharge _in vacuo_. Spark
surrounded by an aura which could be separated.]

Varley showed that when a glow-discharge in a vacuum tube is brought
within the field of a powerful magnet, the magnetic curves are
illuminated beyond the electrodes between which the discharge is taking
place, as well as in the path of the current, and also thought that this
illumination was caused by moving particles of matter, as it deflected a
balanced plate of talc on which it was caused to infringe. It has also
been shown that in electrical discharges in air at ordinary pressure,
while the spark itself was unaffected by the magnet, it was surrounded by
a luminous cloud or aura which was driven into the magnetic curve, and
which might also be separated from the spark by blowing upon it.

Most of the foregoing interesting results and experiments will be found
repeated and verified in Part III.


_Prof. Lemström’s Theory._

[Sidenote: Prof. Lemström’s theory.]

Prof. Lemström thinks that terrestrial magnetism plays only a
comparatively secondary part in the phenomena of the Polar Light, this
part consisting essentially in a direct action upon the rays.

That the experiments of M. de la Rive do not all furnish the proof that
the rays of the light are really united under this influence.

[Sidenote: Character of the Polar Light.]

That the Polar Light considered as an electrical discharge gives the
following results:—

(1) An electric current arising from the discharge itself, which takes
place slowly.

(2) Rays of light consisting of an infinite number of sparks, each spark
giving rise to two induction currents going in opposite directions.

(3) A galvanic current going in an opposite direction to that of the
discharge, and having its origin in the electromotive force discovered
by M. Edlund in the electric spark. That these currents require a closed
circuit; but this is not necessary in the case of the Aurora, as the
earth and rarefied air of the upper regions are immense reservoirs of
electricity producing the same effect as if the circuit were closed. That
permanent moisture in the air, a good conductor of electricity, is the
cause of a slow and continuous discharge assuming the form of an Aurora,
instead of suddenly producing lightning as in equatorial regions and mean
latitudes.

[Sidenote: Polar Light due to electric discharges only.]

He sums up, that the electric discharges which take place in the Polar
regions between the positive electricity of the atmosphere and the
negative electricity of the earth are the essential and unique cause
of the formation of the Polar Light, light the existence of which is
independent of terrestrial magnetism, which contributes only to give to
the Polar Light a certain direction, and in some cases to give it motion.

This Prof. Lemström maintains contrary to those who believe they see
in terrestrial magnetism, or rather in the induction currents, what is
capable of developing the origin of the Polar Light.


_Theories of MM. Becquerel and De la Rive._

[Sidenote: Theories of MM. Becquerel and De la Rive.]

M. Becquerel’s theory is that solar spots are cavities by which hydrogen
and other substances escape from the sun’s protosphere. That the hydrogen
takes with it positive electricity which spreads into planetary space,
even to the earth’s atmosphere and the earth itself, always diminishing
in intensity because of the bad conducting-power of the successive layers
of air and of the earth’s crust. That would then only be negative, as
being less positive than the air. The diffusion of electricity through
planetary space would be limited by the diffusion of matter, since it
cannot spread in a vacuum. That gaseous matter extends further than the
limits usually assigned to the earth’s atmosphere, is proved by the
observation of Auroræ at heights of 100 and 200 kilometres, where some
gaseous matter must exist. M. de la Rive agrees with M. Becquerel as to
the electric origin of the Aurora, but considers the earth is charged
with negative electricity and is the source of the positive atmospheric
electricity, the atmosphere becoming charged by the aqueous vapour rising
in tropical seas. The action of the sun he considers is an indirect one,
varying with the state of the sun’s surface, as shown by coincidences in
the periods of Aurora and sun-spots.


_M. Planté’s Electric Experiments._

[Sidenote: M. Planté’s experiments. Effects produced resembling Auroræ.]

M. Planté has performed some experiments with a very considerable series
of secondary batteries. By inserting the positive electrode after the
negative in a vessel of salt water, luminous and other effects were
observed which were considered to have a strong resemblance to those of
Auroræ.

M. Planté advocates the theory that the imperfect vacuum of the upper
regions, acting like a large conductor, plays the part of the negative
electrode in his experiments, while the positive electricity flows
towards the planetary spaces, and not towards the ground, through the
mists and ice-clouds which float above the Poles.

[Sidenote: M. Planté’s experiments producing a corona, an arc, or a
sinuous line.]

In an article in ‘Nature,’ March 14, 1878, a further account is given
of M. Planté’s experiments, under the head of “Polar Auroræ;” and it is
stated that, in these experiments, the electric current, in presence
of aqueous vapour, yielded a series of results altogether analogous to
the various phases of Polar Auroræ. If the positive electrode of the
secondary battery was brought into contact with the sides of a vessel
of salt water, there was observed, according to the distance of the
film (electrode?) from the liquid, either a corona formed of luminous
particles arranged in a circle round the electrode (fig. 2, p. 90), an
arc bordered with a fringe of brilliant rays (fig. 3), or a sinuous line
which rapidly folded and refolded on itself (fig. 4). This undulatory
movement, in particular, formed a complete analogy with what had been
compared in Auroræ to the undulations of a serpent, or to those of
drapery agitated by the wind. The rustling noise accompanying the
experiment was analogous to that sometimes said to accompany Auroræ, and
was caused by the luminous electric discharge penetrating the moisture.
As in Auroræ, magnetic perturbations were produced by bringing a needle
near the circuit, the deviation increasing with the development of the
arch.

The Auroræ were produced by positive electricity, the negative electrode
producing nothing similar.

Illustrations of these miniature Auroræ are given in ‘Nature,’ and
reproduced on p. 90. No mention of any spectroscopic observations is made.

[Sidenote: Mr. Holden’s views.]

In a communication to the Metropolitan Scientific Association
(‘Observatory,’ March 1, 1879, p. 389), Mr. A. P. Holden, after
supporting the theory of a connexion between the waxing and waning of the
solar corona and sun-spots, adopts Mr. F. Pratt’s hypothesis “that the
Aurora is simply light filmy cirrus cloud, first deposited at the base
of a vast upper body of highly rarefied vapour, and illuminated by the
free electricity escaping in the condensation through the very rarefied
medium above, towards the north or south. The Aurora would, according to
this theory, have its origin in a vast electrical storm, resulting from
a violent condensation of vapour which causes a flow of electricity from
the pole to restore equilibrium.” The Aurora would thus, in Mr. Holden’s
opinion, “depend on storm phenomena of an intense character; and the
frequency of Auroræ at the sun-spot maxima would indicate the connexion
of the latter with the weather.”

[Illustration: Fig. 2. The corona.

Fig. 3. The arc and rays.

Fig. 4. The sinuous line.]




PART II.

THE SPECTRUM OF THE AURORA.




CHAPTER X.

SPECTROSCOPE ADAPTED FOR THE AURORA.


[Sidenote: Must be of moderate dispersion, with ready mode of measuring
line-positions.]

Any form of spectroscope of moderate dispersion will suffice for
observations of the spectrum of the Aurora; but, for sake of convenience,
a hand or direct-vision spectroscope is to be preferred, and it is
desirable also to have some quick and ready mode of measuring the
position of the lines while the Aurora lasts.

[Sidenote: Mr. Browning’s instrument described.]

Mr. John Browning arranged for me a form of instrument which I have
found very convenient for observations by hand of the Aurora-lines, and
also, when fixed on a stand, for tube and chemical investigations. A
representation of this instrument is given on Plate X. fig. 1. A brass
tube carries a large compound (5) direct-vision prism (shown dark in the
drawing). An arrangement is made so that a second prism can at will be
slipped into the tube (shown in outline in the drawing). With one prism
and a fine slit the D lines are widely separated, and the field of view
extends at one glance from near C to near G. When the second prism is
inserted and used in combination, the nickel line can be seen between the
two D lines, and the instrument may be used for solar work. A photograph
of the sun’s spectrum, taken with one prism only, shows a great number
of the dark solar lines and many of the bright ones, ascribed by Prof.
Draper to oxygen and nitrogen.

[Illustration: Plate X.]

[Sidenote: Diaphragm micrometer described. Mode of use of the micrometer.]

The collimator and observing telescope are respectively 6 inches in
length, and carry achromatic lenses of one inch aperture. The telescope
traverses the field so that the extremities of the spectrum may be
observed. The dispersion of the instrument was ascertained by a set
of observations of the principal solar and some metallic lines, made
with an excellent filar micrometer. For the Auroral observations, Dr.
Vogel has described an instrument (see Appendix E) in which the usual
spider’s-web wires are replaced by a needle-point, as being easily seen
upon a faint spectrum. Illuminated wires may also be used; but I was led
ultimately to employ, in preference, a diaphragm micrometer which the
spectrum itself illuminates, as being adapted for speedy, yet fairly
accurate, observations. It was made in this manner:—A card was first of
all prepared (Plate X. fig. 2), and within a circle described on this,
a scale was drawn of moderately wide white spaces, with black divisions
between, short and long, so as to read off easily by eye. The upper half
of the circle was then entirely filled in with black; and from the card
as thus prepared a reduced negative photograph was made. In this the
spaces and lower half of the circle were opaque, and the upper half of
the circle and the lines between the spaces were transparent (Plate X.
fig. 4). This photograph was about the size of a shilling (fig. 3, same
Plate). It was mounted carefully in Canada balsam, with a thin glass
cover, and then placed in the focus of the eyepiece. In use, the spectrum
is brought upon the scale so that the upper half shows above the scale
without any interruption at all; while the lower half illuminates the
scale and renders the divisions visible, showing the spectrum-lines
falling either upon them or the spaces between. The photographed scale
was next enlarged to a considerable size and printed upon faintly ruled
paper; and the enlargement was so arranged as to comprise five of the
faint ruled lines between each division of the scale. Each of these faint
lines in turn represented a certain portion of the spectrum as read off
with the filar micrometer; so that the scales as constructed with the
filar micrometer and with the photographed micrometer corresponded for
all parts of the spectrum included in the field of the eyepiece.

[Sidenote: Advantage of the method.]

One of the photographed enlargements being laid on the table under the
spectroscope, the observed lines were marked off with ease and accuracy
upon it; and as the photograph was an exact copy of the scale, any want
of exactitude in the divisions was of no moment.

One great advantage of this method was, that all the lines seen could
be recorded at one time and with all in view, and without the risk of
slight shift in the instrument, which frequently happens when lines are
read off seriatim.

I found this plan most effective for the rapid and correct recording of
a faint and evanescent spectrum, and it gave close results when compared
with traversing-micrometer measured spectra. The records, too, admitted
of subsequent examination at leisure.

[Sidenote: Double-slit plate arrangement.]

Mr. Browning subsequently constructed for me a double-slit plate (lately
in the Scientific Loan Collection at South Kensington) for the same
instrument (Plate X. fig. 5). The lower half of the plate is fixed. The
upper half traverses the lower by the aid of a micrometer-screw. The slit
is widened or closed at pleasure by loosening the small screws by which
the jaw-plates are attached. A scale is engraved on the fixed lower half
of the plate for an approximate measurement, while the division of the
micrometer-screw-head completes it.

In use, one half of the spectrum slides along the other, and a bright
line in the upper spectrum is selected as an index. The distances between
the lines of the lower half of the spectrum are read off by means of the
bright line above. This form of micrometer was suggested by Mr. Procter
(in ‘Nature’) many years ago as a substitute for a more complicated
apparatus by Zöllner. Other instruments on a similar principle have been
lately introduced, but for Aurora purposes I prefer a fixed scale.

[Sidenote: Photographed spectrum suggested.]

In ‘Photographed Spectra’ I have pointed out that we shall probably
obtain no spectrum of the Aurora to be absolutely depended upon for
comparison with other spectra until we succeed in a photographed one.
From experiments made with a special prism of the Rutherfurd form,
constructed for me by Mr. Browning (with which many gas-spectra have
been already photographed), I see no reason, should an unusually bright
Aurora favour us with a visit, why its spectrum may not be recorded in a
permanent form, and with lines sufficiently well marked to be compared
with other spectra. Rapid dry plates would be especially useful for such
a purpose, and some Auroræ, if wanting in brilliancy, would doubtless
compensate by their period of endurance.

[Sidenote: Mr. Hilger’s half-prism spectroscope.]

Mr. Adam Hilger has also made for me one of his “half-prism”
spectroscopes, in which considerable dispersion is obtained with but very
little loss of light. This instrument has a simple and rapid micrometer
arrangement, with a bright line as an index. I have (for want of Auroræ)
had no opportunity of trying it, but I doubt not it is well adapted for
such a purpose.


_Spectrum of the Aurora described._

[Sidenote: Lines or bands and continuous spectrum.]

The spectrum of the Aurora consists of a set of lines or bands upon a
dark ground at each extremity of the spectrum, but with more or less of
faint continuous spectrum towards the centre. The extreme range of the
spectrum, as observed up to the present time, is from “_a_” (between C
and D) in the red to “_h_” (hydrogen) in the violet.

[Sidenote: Lines nine in number.]

The lines have been classified and arranged by Lemström and others as
nine in number, but I believe not more than seven have ever been seen
simultaneously.

The author of the article “Aurora Polaris,” in the ‘Encyclopædia
Britannica,’ classes the lines as nine, and gives a table with the
following results (to these I have added Herr Vogel’s lines, for the
purpose of identification and comparison):—

[Sidenote: Table from Encyc. Brit.]

  +------+-------------+------+--------+-------+
  |No. of|  Number of  | Mean |Probable|Vogel’s|
  |line. |observations.| W.L. | error. |lines. |
  +------+-------------+------+--------+-------+
  |  1.  |      5      | 6303 | ± 8·1  | 6297  |
  |  2.  |     10      | 5569 | ± 2·9  | 5569  |
  |  3.  |      4      | 5342 | ±16    | 5390  |
  |  4.  |      6      | 5214 | ± 5·4  | 5233  |
  |  5.  |      4      | 5161 | ± 9·7  | 5189  |
  |  6.  |      6      | 4984 | ±11    | 5004  |
  |  7.  |      4      | 4823 | ± 9·3  |       |
  |  8.  |      8      | 4667 | ± 9·8  | 4663  |
  |  9.  |      8      | 4299 | ± 9·3  |       |
  +------+-------------+------+--------+-------+

The probable errors are large, and it is a question whether any thing is
gained by thus endeavouring to average the lines.

[Sidenote: Ångström’s line. Zöllner’s line in the red. Other Lines of the
spectrum.]

The principal and brightest line, in the yellow-green, is generally
called “Ångström’s,” and his (probably the first) measurement of its
position at 5567 adopted. This was in the winter of 1867-68, and he saw
in addition, by widening the slit, traces of three very feeble bands
situated near to F. Zöllner is credited with the first observation of
the line in the red. These two lines are generally described as with
similar characteristics, and in about the same respective positions, by
all observers, and have never been remarked to spread into bands. The
other lines in the spectrum are difficult to position, owing to the many
discordant observations of them. They seem also variable in intensity
as well as in number (sometimes even in the same Aurora), and are not
unfrequently observed to have their places supplied by bands.

[Sidenote: Second German expedition observations. Austro-Hungarian.]

The spectroscope was used in the second German expedition, but only the
one brightest line seen—Dr. Börgen stating he had never seen a trace of
the weak lines in the blue and red, which were observed so distinctly
with the same spectroscope on 25th October, 1870, after the return of the
expedition. Lieutenant Weyprecht used a small spectroscope during the
Austro-Hungarian Expedition, and saw only the well-known yellow-green
line.

[Sidenote: Swedish expedition, 1868. Lemström’s observations.]

In the Swedish Expedition, 1868, Lemström mentions that in the
Aurora spectrum there are nine lines (he does not say he saw them
simultaneously), which he considers to agree with lines belonging to the
air-gases. He also thinks the Aurora could be referred to three distinct
types, depending on the character of the discharge.

[Sidenote: Spectrum or Aurora seen at Tronsa.]

At Tronsa an Aurora was seen October 21st, 1868, which commenced in the
north and became very brilliant. The spectroscope showed:—

1. A yellow line at 74·9.

2. A very clear line in the blue at 65·90.

3. Two lines of hair’s breadth, with very pronounced (horizontal?) striæ
on the side of the yellow, one at 125 and the other about 105.

[I presume the striæ were really vertical, and that the explanation
intended to convey that these lines shaded off towards the yellow. From
a comparison of the figures they must have been in the red, and are the
only instance recorded of two auroral lines in that region. They are
subsequently spoken of as “shaded rays.”—J. R. C.]

[Sidenote: MM. Wijkander and Parent’s observations.]

M. Auguste Wijkander and Lieut. Parent, of the Swedish Expedition in
1872-73, under Professor Nordenskiöld, used a direct-vision spectroscope,
with a micrometer-screw movement of the prisms, the reading being
afterwards reduced to wave-lengths upon Ångström’s line-values.

The following Table gives the results, with Dr. Vogel’s lines added for
the sake of comparison:—

  +------+------------------------+-----------------------+-------+------+
  |      |Observations, Wijkander.| Observations, Parent. |       |      |
  |      |-------+-----+----------+-------+-----+---------|Mean of|      |
  |Lines.|       |     | Probable |       |     |Probable | both. |Vogel.|
  |      |Number.|W.L. |  error.  |Number.|W.L. | error.  |       |      |
  +------+-------+-----+----------+-------+-----+---------+-------+------+
  |  ..  |  ..   |  .. |    ..    |  ..   |  .. |   ..    |   ..  | 6297 |
  |  ..  |  ..   |  .. |    ..    |  ..   |  .. |   ..    |   ..  | 5569 |
  |  (1) |   5   | 5359|    ±3    |  ..   |  .. |   ..    |  5359 | 5390 |
  |  (2) |   6   | 5289|    ±5    |   3   | 5280|   ± 1   |  5286 |  ..  |
  |  (3) |   6   | 5239|    ±4    |   2   | 5207|   ±11   |  5231 | 5233 |
  |  ..  |  ..   |  .. |    ..    |  ..   |  .. |   ..    |   ..  | 5189 |
  |  (4) |   5   | 4996|    ±9    |  ..   |  .. |   ..    |  4996 | 5004 |
  |  (5) |   1   | 4871|    ..    |   1   | 4873|   ..    |  4872 |  ..  |
  |  (6) |   8   | 4692|    ±2    |  10   | 4708|   ± 5   |  4701 | 4663 |
  |  (7) |   1   | 4366|    ..    |  ..   |  .. |   ..    |  4366 |  ..  |
  |  (8) |   4   | 4280|    ±3    |   3   | 4286|   ±16   |  4284 |  ..  |
  +------+-------+-----+----------+-------+-----+---------+-------+------+

The brightest line in all Auroræ, 5567, was intentionally not included in
the Tables. The red line was not seen. Nos. 5 and 7 were only seen once,
and not in the same Aurora.

[Sidenote: Spectrum of Aurora of October 24th, 1870.]

The Aurora of October 24th, 1870, came at a time when spectroscopes of a
direct-vision form were being introduced, and a number of observations
were communicated at the time to ‘Nature.’

[Sidenote: T. F.’s observations. W. B. Gibbs’s observation. Elger’s
observation.]

A correspondent, T. F., writing from Torquay, saw, with a direct-vision
spectroscope, one strong red line near C, one strong pale yellow line
near D, one paler near F, and a still paler one beyond, with a faint
continuous spectrum from about D to beyond F. The C line was very
conspicuous and the brightest of the whole. It was intermediate in
position and colour to the red lines of the lithium and calcium spectra.
Plainly there were two spectra superposed, for while the red portions of
the Aurora showed the four lines with a faint continuous spectrum, the
greenish portions showed only one line near D on a faint ground. W. B.
Gibbs saw, in London, only two bright lines, one a greenish grey, situate
about the middle of the spectrum, and the other a red line very much like
C (hydrogen). Thomas G. Elger, at Bedford, on the 24th and 25th, saw:—(1)
a broad and well-defined red band near C; (2) a bright white band near D
(same as Ångström’s W.L. 5567), on 25th visible in every part of the sky;
(3) a faint and rather nebulous line, roughly estimated to be near F; (4)
a very faint line about halfway between 2 and 3. The red band was absent
from the spectrum of the white rays of the Aurora, but the other lines
were seen.

[Sidenote: J. R. Capron’s observation.]

With a small Browning direct-vision spectroscope on the 24th, I found
no continuous spectrum, but two bright lines, one in the green (like
that from the nebulæ, but more intense, and considerably flickering),
the other in the red (like the lithium line, but rather duskier: Plate
V. fig. 6). The latter was only well seen when the display was at its
height; it could, however, be faintly traced wherever the rose tint of
the Aurora extended. The line in the green was well seen in all parts of
the sky, but was specially bright in the Auroral patches of white light.

[Sidenote: Mr. Browning’s observation. Alvan Clarke’s, jun.,
observations.]

Mr. Browning also saw the red line, but found comparison difficult. On
the evening of the 24th October, Mr. Alvan Clarke, jun., at Boston,
used a chemical spectroscope of the ordinary form, with one prism and a
photographed scale illuminated with a lamp. Four Auroral lines were seen
at points of his scale numbered 61, 68, 80, and 98. These were reduced to
wave-lengths by Professor Pickering, with the following results:—

  +-----+---------+--------+-------+-------------------+--------+
  |Line.| Reading | Wave-  |Assumed|     Comments.     |Probable|
  |     |on scale.|lengths.| line. |                   | error. |
  |-----+---------+--------+-------+-------------------+--------|
  |  1. |    61   |  5690  |  5570 |Common Aurora-line.| -20    |
  |  2. |    68   |  5320  |  5316 |   Corona line.    | + 1    |
  |  3. |    80   |  4850  |  4860 |   F, hydrogen.    | - 3    |
  |  4. |    98   |  4350  |  4340 |   G, hydrogen.    | + 6    |
  +-----+---------+--------+-------+-------------------+--------+

[61 is evidently wrong, and was probably a mistake for 63.]

[Sidenote: G. F. Barker’s observations.]

George F. Barker, observing at New Haven (U.S.A.), saw, on November 9th,
a crimson and white Aurora, which he examined with a single glass-prism
spectroscope, by Duboscq, of Paris. The line positions were obtained by
an illuminated millimetre scale. In the white Aurora were four lines (the
red one being absent); in the red Aurora five. The wave-lengths of the
Aurora-lines were run out as follows:—

  (1.) Between C and D, 6230 (Zöllner’s 6270).
  (2.)    ”    D and E, 5620 (Ångström’s 5570).
  (3.)    ”    E and _b_, 5170 (Winlock’s 5200).
  (4.)    ”    _b_ and F, 5020.
  (5.)    ”    F and G, 4820 (Alvan Clarke’s, jun., 4850).

[Sidenote: Spectrum of Aurora of Feb. 4, 1872. Prof. Piazzi Smyth’s
observations.]

Mr. Procter’s Aurora-lines will be found noticed in connexion with the
spectrum of oxygen; and Lord Lindsay’s lines, with a comparison scale
drawing, are separately described further on in this Chapter. The Aurora
of February 4th, 1872, had many observers; some of whom communicated
at the time spectroscopic notes. Professor Piazzi Smyth minutely
describes the display as seen in Edinburgh, and saw “Ångström’s green
Aurora-line perpetually over citron acetylene[13] at W.L. 5579, and the
red Aurora-line between lithium _a_ and sodium _a_, but nearer to the
latter, say at W.L. 6370.” Extremely faint greenish and bluish lines also
appeared at W.L. 5300, 5100, and 4900 nearly.

[Sidenote: Rev. T. W. Webb’s observations.]

The Rev. T. W. Webb, with a very fine slit, saw the green Auroral line
even in the light reflected from white paper. With a wider slit he saw a
crimson band in the brighter patches of that hue, and beyond an extent of
greenish or bluish light, which he suspected to be composed of contiguous
bands.

[Sidenote: R. J. Friswell’s observations.]

R. J. Friswell, coming up the Channel at 9.40, with a Hoffman’s
direct-vision spectroscope (the observing telescope removed), saw the
green line, a crimson line near C, and faint traces of structure in the
blue and violet.

[Sidenote: The Rev. S. J. Perry’s observations.]

The Rev. S. J. Perry observed at Stonyhurst four lines, and, on examining
one of the curved streamers, found the red line even more strongly marked
than the green. A magnetic storm was observed to be at its height from 4
to 9 P.M. of the same day.

[Sidenote: J. R. Capron’s observations.]

With a Browning 7-prism direct-vision spectroscope I saw the green line
in all parts of the Aurora, attended with a peculiar flickering movement.
I did not see the other lines.

[Sidenote: His catalogue of lines up to Nov. 9, 1872.]

In a letter to ‘Nature,’ dated November 9th, 1872, I catalogued the lines
observed up to that date as follows:—

1. A line in the red between C and D. W.L., Ångström, 6279.

2. A line (the principal one of the Aurora) in the yellow-green, between
D and E. W.L., Ångström, 5567.

3. A line in the green, near E (corona line?). W.L., Alvan Clarke, jun.,
and Backhouse, 5320.

4. A faint line in the green, at or near _b_. W.L., Barker, 5170.

5. A faint line or band in the green, between _b_ and F. W.L., Barker,
5020 (chromospheric?).

6. A line in the green-blue, at or near F. W.L., Alvan Clarke, jun., 4850.

7. A line in the indigo, at or near G. W.L., Alvan Clarke, jun., 4350.

8. The continuous spectrum from about D to beyond F.

[Sidenote: Dr. H.C. Vogel’s observations of Auroral lines. Spectrum
described.]

Dr. H. C. Vogel, formerly of the Bothkamp Observatory, near Kiel,
and since of the Astrophysical Observatory, Potsdam, made several
observations of the Auroral lines, October 25th, 1870. Besides the bright
line between D and E, he found several other fainter lines stretching
towards the blue end of the spectrum on a dimly-lighted ground. February
11th, 1871, he observed the same set of lines, and an average of six
readings gave 5572 as the W.L. of the Ångström line. February 12th gave
5576 as Dr. Vogel’s reading, and 5569 as Dr. Lohse’s. April 9th gave
5569, and April 14th 5569. The Aurora of April 9th, 1871, was exceedingly
brilliant, so that micrometer measurements of the lines were taken. The
spectrum consisted of one line in the red, five in the green, and a
somewhat indistinct broad line or band in the blue. The lines are thus
described:—

[Sidenote: Table of lines.]

Table of Dr. Vogel’s lines. Aurora, April 9th, 1871.

  +-----+--------+------------------------------------------------+
  |W.L. |Probable|                   Remarks.                     |
  |     | error. |                                                |
  +-----+--------+------------------------------------------------+
  |6297 |   14   |Very bright stripe.             }               |
  |     |        |                                }               |
  |5569 |    2   |Brightest line of the spectrum, }               |
  |     |        | became noticeably fainter at   } On a faintly  |
  |     |        | appearance of the red line.    } lighted       |
  |     |        |                                } ground.       |
  |5390 |   ..   |Extremely faint line;           }               |
  |     |        | unreliable observation.        }               |
  |     |        |                                }               |
  |5233 |    4   |Moderately bright.              }               |
  |     |        |                                                |
  |5189 |    9   |This line was very bright when the red line     |
  |     |        | appeared at the same time; otherwise equal     |
  |     |        | in brilliancy with the preceding one.          |
  |     |        |                                                |
  |5004 |    3   |Very bright line.                               |
  |     |        |                                                |
  |4694}|        |{Broad band of light, somewhat less brilliant   |
  |4663}|    3   |{ in the middle; very faint in those parts of   |
  |4629}|        |{ the Aurora in which the red line appeared.    |
  +-----+--------+------------------------------------------------+

A translation of Dr. Vogel’s interesting paper will be found printed _in
extenso_ in Appendix E, and his lithographed drawings of the spectrum in
the green and red portions of the Aurora respectively on Plate VI. figs.
2 and 3. The observations of April 9th by Dr. Vogel are probably, up to
the present time, the most exact of any one Aurora, and I have therefore
in most cases used them for comparison.

[Sidenote: Mr. Backhouse’s catalogue of lines.]

Mr. Backhouse, in a letter to ‘Nature,’ commenting upon my catalogue
of lines, gave the following as the latest determinations from his own
observations:—

  No. 1. Wave-length 6060
      2.      ”      5660
      3.      ”      5165
      4.      ”      5015
      6.      ”      4625
      7.      ”      4305

(6060 must be a mistake for 6260, and 5660 for 5560.—J. R. C.) Mr.
Backhouse never saw a line at 5320 again. He found the continuous
spectrum to reach from No. 2 to No. 7, being brightest from a little
beyond No. 2 to No. 6. This part of the spectrum did not give him so much
the idea of a true “continuous spectrum” as of a series of bright bands
too close to be distinguished.

[Sidenote: Subsequent full catalogue of Auroral lines.]

I have subsequently, in another section of this Chapter, added a full
catalogue of the Auroral lines, prepared by myself from the foregoing and
other sources and observations; and I also append to it a Plate [Plate
XII.], in which these lines are positioned and the wave-lengths and names
of observers are given. The numbers of the lines on the Plate correspond
with those in the catalogue. The solar spectrum and the spectrum of the
blue base of a candle-flame are added for purposes of comparison. [The
telluric bands in the solar spectrum are shown more distinctly than they
actually appear, and do not profess to give details.]


_Flickering of the Green Line._

[Sidenote: Flickering of the green line. Herschel’s observation. J. R.
Capron’s observation.]

A. S. Herschel noticed this, April 9th (1871?). He says:—“A remarkable
circumstance connected with the appearance of the single line observed on
this occasion was the flickering and frequent changes with which it rose
and fell in brightness; apparently even more rapidly than the swiftly
travelling waves, or pulsations of light, that repeatedly passed over the
streamers, near the northern horizon, towards which the spectroscope was
directed.” In the spectrum of the Aurora of 20th October, 1870, I saw
and noted the green line as “considerably flickering;” and in the Aurora
of 4th February, 1872, I again saw and noted “the peculiar flickering”
I had remarked in 1870. I have not seen the peculiarity noted by other
observers.


_Mr. Backhouse’s graphical Spectra of four Auroræ._

[Sidenote: Mr. Backhouse’s graphical spectra of Auroræ.]

Mr. Backhouse has been good enough to supply me with some details of
four several Auroræ seen by him at Sunderland, accompanied by drawings,
showing in a graphical way the spectrum of each display as seen with a
spectroscope with rather a wide slit and as drawn by eye. I have reduced
the four drawings to the same scale, and in this way they are extremely
interesting for comparison (Plate V. fig. 4). The line on the left in
each spectrum is Ångström’s bright Auroral line, and is supposed to be
considerably prolonged. The height of the lines denotes intensity.

[Sidenote: April 18, 1873.]

April 18th, 1873, was a bright Aurora. No. 3 is a faint band, which Mr.
Backhouse had not perceived before. No. 5 had not been visible lately,
and Mr. Backhouse thought it must belong to Auroræ of a different type
from those which had appeared latterly.

[Sidenote: Feb. 4, 1874.]

February 4th, 1874. In the spectrum of this Aurora Mr. Backhouse saw
seven lines, all that he had ever seen. (The red line, not shown in the
diagram, makes the seventh.)

The spectrum is represented as seen between 6.50 and 7.5 P.M. Mr.
Backhouse had only once before seen No. 4, and it became quite invisible
between 7.45 and 7.55, though the other lines were as bright as before
and the red line had appeared.

[Sidenote: Oct. 3, 1874.]

October 3rd, 1874. This spectrum was examined, and diagram made between
10 and 10.25 P.M. Five lines only are indicated.

It is mainly distinguished from the two preceding spectra by the
brightness of the continuous spectrum on which the lines 2, 3, and 4 lie,
and by the weakness of No. 5.

[Sidenote: Oct. 4, 1874.]

October 4th, 1874. Taken between 11.10 and 11.20 P.M.; distinguished,
like the last, by a considerable amount of continuous spectrum and by a
faint line (No. 3), not seen in the last spectrum, while No. 3 in the
last is missing in this spectrum.

[Sidenote: Mr. Backhouse’s remarks as to comparative frequency of some of
the Auroral lines.]

Mr. Backhouse, as to both these last spectra, remarks that the lines were
very variable in intensity, and sometimes some were visible and sometimes
others. They varied also in relative brightness in different parts of
the sky at the same time. Mr. Backhouse, in a communication to ‘Nature,’
referring to a statement of Mr. Procter’s, that the bands of the Auroral
spectrum are seldom visible, except the bright line at 5570, says that
he always found two bands, “doubtless Winlock’s 4640 and 4310,” to be
invariably visible when the Aurora was bright enough to show them. Of
thirty-four Auroræ examined by Mr. Backhouse, fourteen showed the lines
4640 and 4310, and three others at least one of these, while eight showed
the red line. (Ångström only once saw this line.) In five Auroræ, all
more or less red, he saw a faint band, the wave-length of which he placed
at 5000 or 5100. He never saw the line 5320 (also Winlock’s coronal
line), unless it were once, probably from want of instrumental power.
With regard to these observations, I may say that with a Browning’s
miniature spectroscope I saw only two lines (the red and the green) in
the grand display of the 24th October, 1870; and with an instrument of
larger aperture the green line only on the 4th February, 1872; while I
saw the green line and three others towards the violet with the same
instrument during the Aurora of 4th February, 1874. (See description of
this Aurora, _antè_ p. 21, and drawing of spectrum, Plate VI. fig. 1 _a_.)


_Lord Lindsay’s Aurora-Spectrum, 21st October, 1870._

[Sidenote: Lord Lindsay’s Aurora of 21st Oct., 1870.]

Lord Lindsay observed a fine Aurora at the Observatory at Dun Echt on the
night of the 21st October, 1870. It commenced about 9.30, reached its
maximum about 11, and faded away suddenly about 11.30 P.M.

[Sidenote: Spectrum described.]

A spectrum obtained in the north-west gave five bright lines with a
Browning’s direct-vision spectroscope—two strong, one medium, two very
faint. A tallow candle was used to obtain a comparison spectrum of sodium
and carburetted hydrogen.

A drawing of the spectrum obtained is given on Plate XI. fig. 2. No. 1 is
a sharp well-formed line visible with a narrow slit.

No. 2, a line very slightly more refrangible than F. The side towards D
is sharp and well defined, while on the other side it is nebulous.

No. 3, slightly less refrangible than G, is a broad ill-defined band,
seen only with a wide slit.

No. 4, a line near E, woolly at the edges, but rather sharp in the
centre. This, says Lord Lindsay, should be at or near the position of the
line 1474 of the solar corona.

No. 5, a faint band, coincident with _b_, extending equally on both sides
of it.

The lines are numbered in order of intensity. It is questionable,
from observations with instruments carrying a scale, whether the
line-positions are exact; but the description of their characters is
valuable.

[Sidenote: Candle-spectrum.]

As a candle blue-base spectrum is at times a ready and handy mode of
reference in Auroral observations (as was found in this instance), I
have, on Plate XI. fig. 5, given a representation of it as seen with my
Auroral spectroscope. Dr. Watts’s corresponding carbon-spectrum is added
on the lower margin. The numbers on the upper margin refer to my scale.

[Illustration: Plate XI.]


_Spectrum of the Aurora Australis._

[Sidenote: Captain Maclear’s spectra of Aurora Australis.]

Captain Maclear, on examining the streamer seen by him Feb. 9th, 1874
(_antè_, p. 27), with the spectroscope, found three prominent lines in
the yellow-green, green, and blue or purple, but not the red line. In
the Aurora of March 3rd, 1874 (p. 27), he could trace four lines, three
bright and one rather faint. They must have been exceedingly bright to
show so plainly in full-moon light.

[Sidenote: Instrument used, and mode of registering lines.]

The spectroscope used was a Grubb single-prism with long collimator. A
needle-point in the eyepiece marked the position of the lines; and a
corresponding needle-point, carried on a frame by a screw movement in
concord with the point of the eyepiece, scratched the lines on a plate of
blackened glass. Two plates were taken. On the first were scratched the
auroral lines and the solar lines as seen in the moonlight; on the second
plate were scratched the auroral lines, the Solar lines from the moon,
and the carbon lines in a spirit-lamp.

[Sidenote: Copies of the two spectra obtained. Discrepancy in the
spectra. Remarks on the spectra.]

The next morning the solar lines were verified in sunlight. I subjoin
(Plate XI. fig. 3) copies of the two spectra as printed in ‘Nature,’ the
auroral lines being marked A, the solar lines by the usual designating
letters, and the carbon by _Car_. To these spectra I have added for
comparison Dr. Vogel’s spectrum of the Aurora Borealis. Captain Maclear
could not account for the different positions of the auroral lines in the
two plates; for the prism, as far as he was aware, was not moved during
the observations. As the solar lines are indicated in the same place in
both spectra, the case would seem one of actual change of position of the
auroral lines during observation. A comparison of the two spectra gives
the impression that the lower one is the same as the upper, except that
the dispersion is greater, the lines remaining relatively in position.
One does not, however, see how the dispersion could have so varied in a
single-prism instrument, and the position of the solar lines is adverse
to such an explanation.

There is a suspicion that Auroræ are not always identical in position
of some of the lines; but the line in the green (considerably out of
place in the lower Australis spectrum) has always, within small limits,
the same position. It will be noticed how much further the Australis
spectrum runs into the violet than Vogel’s Borealis, the latter having no
lines much beyond F.

The faint line (No. 2) mentioned by Captain Maclear possibly corresponds
with Dr. Vogel’s band. The absence of the four lines of the Aurora
Borealis in the green part of the spectrum of the Australis is peculiar;
and in this respect, too, the two Australis spectra agree.

[Sidenote: Comparison of the lines.]

The nearest approaches to Captain Maclear’s lines (of the upper spectrum)
which I can find are:—

  Line      Corresponding line.

    1.      5567, Ångström.
    2.      (The faint line.) Vogel’s band, 4694-4629.
    3.      I find no approximately corresponding line.
    4.      4350, Alvan Clarke.

But the comparisons are not by any means close. Further observations of
the Australis spectrum are very desirable.


_Prof. Piazzi Smyth’s Aurora-Spectra._

[Sidenote: Prof. Piazzi Smyth’s chemical and auroral spectra.]

Prof. Piazzi Smyth, in volume xiv. of the ‘Edinburgh Astronomical
Observations,’ 1870-77, has compared simultaneously the Aurora-spectrum
with the sets of bright lines seen in the blue base of flame—the lines of
potassium, lithium, sodium, thallium, and indium being also introduced
for comparison. The spectra are drawn as seen under small dispersion,
and will prove most useful in cases where an Aurora is not bright enough
to admit of the lines being measured by micrometer, and the eye and
comparison spectrum are obliged to be resorted to.

[Illustration: Plate XII.]


_Author’s Catalogue of the Auroral Lines._

(See Plate XII.)

1. W.L. 6297, Vogel. Very bright stripe; first noticed by Zöllner. Seen
only in red Auroræ; stands out on a dark ground, without other lines
near it. Character of line sharp and well defined; varies in colour from
dusky red to bright crimson. Intensity, Herschel, 0 to 4 or 8. According
to same, position coincident with atmospheric absorption-group “_a_” in
solar spectrum (between C and D). I confirm this position according to
my scale of solar lines, and a drawing of the coincidence (in which,
and in Plate XII., the absorption-lines are drawn too dark) is given on
Plate XIII. fig. 2. Herschel says this line coincides with a red band in
the negative glow-discharge, but its identity is doubtful. Its isolation
and want of adjacent lines seem to separate it from the air-spectrum and
gas-spectra in general. At the appearance of this line, 5569 (No. 2)
becomes noticeably fainter. When this line is bright, 5189 (No. 5) is
bright also (Vogel).

If we propose to assign to this line, as well as to 5569, a
phosphorescent origin, it would be strongly confirmatory of such a theory
(in connexion with the phosphoretted-hydrogen spectrum) to find it
brighten at low temperatures.

_Note._—Sir John Franklin says, in his ‘Polar Expeditions,’ that a low
state of temperature is favourable for the production of brilliant
coruscations. It was seldom witnessed that the Auroræ were much agitated,
or that the prismatic tints were very apparent, when the temperature was
above zero.

2. Line in the yellow-green. Brightest of all lines in the
Aurora-spectrum. W.L. 5567, Ångström; 5569, Vogel. Intensity 25,
Herschel. To me more pale green than yellow, sometimes flickering
and changing in brightness (Herschel and Capron). Seen in all Auroræ
usually sharp and bright, but Procter has once recorded it nebulous. Its
character as to width, sharpness, and intensity, if carefully observed,
might indicate height and structure of Aurora. Becomes noticeably
fainter at appearance of red line (Vogel). Found by me to correspond in
position with a faint atmospheric absorption-band (see Plate XIII. fig.
2). According to Ångström and Herschel, arising from a phosphorescent
and fluorescent light, emitted when air is subjected to the action of
electrical discharge.

3. Line in green near last. W.L. 5390. An extremely faint and unreliable
observation (Vogel). Seen only by him, unless Alvan Clarke’s 5320
(coronal?) be the same.

4. Line in green-blue. W.L. 5233, moderately bright (Vogel); 5200,
Winlock. Intensity, 2 or 0? to 6, Herschel. Coincides with line in the
negative glow according to same. Frequently observed.

5. Line in green-blue. W.L. 5189. This line is very bright when the red
line appears at the same time; otherwise equal in brilliancy with No.
3 (Vogel); Winlock, 5200. Not so frequently observed as No. 3. Barker
gives a band extending from 5330 to 5200. Intensity of 5189, 0 to 8,
Herschel, who considers it coincident with a constant strong line in the
spark-discharge.

6. Line in blue. W.L. 5004. Very bright line, Vogel; 5020, Barker
(coronal?). Intensity 2 or 0? to 8, Herschel. Coincides with line of
nitrogen in the nebulæ according to same. Barker gives a band extending
from 5050 to 4990.

7. Line in the blue not found by Vogel in Aurora, April 9th, 1871. W.L.
4850, Alvan Clarke; 4820, Backhouse and Barker. Intensity, Herschel, of
4820-4870, 0 to 4? Herschel suspects this and No. 4 to be seen only in
Auroral streamers of low elevation. Barker gives a band extending from
4930 to 4850.

8. 4694, 4663, 4629. Broad band of light, somewhat less bright in the
middle; very faint in those parts of the Aurora in which the red line
appears (Vogel). Intensity 3-6 (Herschel). A double band, consisting of
two lines, the first rather more frequently noted than the second in
Auroral spectra, agrees well in position with the principal band in the
negative glow-spectrum (same). Barker gives a band extending from 4740 to
4670; Backhouse and Winlock give a line at 4640, situate within the same.

9. There seems a good deal of confusion about a fairly bright line
(intensity 0-6, Herschel) seen in most Auroræ (not, however, by Vogel,
April 9th, 1871), and situate somewhere near G in the solar spectrum.
Alvan Clarke places it at 4350, on the less refrangible side of G;
Backhouse and Barker at or very near to G; while Lemström and others
position it on the more refrangible side of G. Accurate observations, for
which a quartz spectroscope might be useful, are much wanted. Herschel
makes this line, at 4285, correspond with a strong band in the violet in
the negative glow-spectrum.

Herschel also refers to an apparently additional line near the
hydrogen-line, or between G and H₁, in the solar spectrum, as mentioned
once by Lemström at Helsingfors. I am not aware of any other observation
of this line, which must be considerably beyond that at or near G, and
would probably be difficult to detect, except in instruments specially
adapted for examination of the violet end of the spectrum.


_Theories in relation to the Aurora and its Spectrum._

[Sidenote: Lemström’s.]

Lemström (1):—That the Polar light is caused by an electric current
passing from the upper rarefied layers of the air to the earth, producing
light-phenomena that do not arise in the denser layers of the air. (2)
That there are nine rays (lines or bands) in the Aurora-spectrum, which
in all probability agree with lines which belong to the gases of the air.
(3) That the Aurora-spectrum can be referred to three distinct types,
which depend on the character of the discharge.

[Sidenote: Vogel’s.]

Dr. Vogel:—(1) That the Auroræ are electric discharges in rarefied-air
strata of very considerable thickness. (2) That the Aurora-spectrum is a
modification of the air-spectrum, involving the question of alteration of
the spectrum by conditions of temperature and pressure.

[Sidenote: Ångström’s.]

Ångström:—(1) seems to adopt the hypothesis that the Aurora has its final
cause in electrical discharges in the upper strata of the atmosphere, and
that these, whether disruptional or continuous, take place sometimes on
the outer boundary of the atmosphere, and sometimes near the surface of
the earth.

(2) That the Aurora has two different spectra.

(3) That the green line is due to fluorescence or phosphorescence, and
that there is no need to resort to Dr. Vogel’s variability of gas-spectra
according to circumstances of pressure and temperature.

(4) That an agreement exists between the lines of the Aurora (except the
red and green before mentioned) and the lines or bands of the violet
light which proceed from the negative pole in dry air.

[Sidenote: Zöllner’s remark as to temperature of Aurora and character of
spectrum.]

Zöllner has pointed out that the temperature of the incandescent gas of
the Aurora must be exceedingly low, comparatively, and concludes that the
spectrum does not correspond with any known spectrum of the atmospheric
gases—only because, though a spectrum of our atmosphere, it is one of
another order, and one which we cannot produce artificially.




CHAPTER XI.

THE COMPARISON OF SOME TUBE AND OTHER SPECTRA WITH THE SPECTRUM OF THE
AURORA.

[In part from an Article in the ‘Philosophical Magazine’ for April 1875.]


[Sidenote: Testing Ångström’s Aurora theory. Battery and spectroscope
described. Vogel’s spectrum selected for comparison.]

In order to test Professor Ångström’s theory of the Aurora, referred to
in the last Chapter, in an experimental way, I examined, in the winter of
1874, some tube and other spectra, not only for line-positions, but also
for general resemblance to an Aurora-spectrum. It did not seem desirable
to use powerful currents. A ½-inch-spark coil, worked by a quart
bichromate-cell, was found sufficient to illuminate the tubes steadily.
The spectroscope used was one made for me by Mr. Browning specifically
for Auroral purposes, and of the direct-vision form, being the same
instrument as is described _antè_, p. 91, and figured in Plate X. fig. 1.
The micrometer was the diaphragm one, also before described and figured
on same Plate, figs. 2, 3, and 4. I selected Dr. Vogel’s spectrum for
comparison, it being, so far as I am aware, the most accurately mapped,
with regard to wave-length, at one observation, of any Auroral spectrum.
It seemed an unsafe plan to attempt to obtain an average Aurora by
comparison of different observations made at various times by different
observers with all sorts of instruments—the difficulty, too, being
increased by the suspicion that the spectrum itself at times varies in
number and position as well as intensity of its lines.

[Sidenote: Central part only of spectrum mapped.]

In most cases the central part of the spectrum only (corresponding to
the central lines of the Aurora) was mapped, the red line in the Aurora
not being found to correspond with any prominent line in the gas-spectra
examined, and the Auroral line near solar G being so indefinitely fixed
as to render comparison almost valueless. (See Plate XIII. fig. 1.)

Dr. Vogel’s spectrum does not comprise the line near G; but I have added
this (in an approximate place only) in order to complete the set of
lines. For drawing of Dr. Vogel’s spectrum, with its scales attached, see
Plate XIII.

[Illustration: Plate XIII.]


_Hydrogen-tube._

[Sidenote: Hydrogen-tube. Colour of glow varied with intensity of
current.]

This tube was one of Geissler’s and of rather small calibre. On
illumination the wide ends were easily lighted with a silver-grey
glow, having a considerable amount of stratification. The capillary
part glowed brilliantly with silver-white, bright green, and crimson
light, according to the intensity of the current. With the commutator
slowly working, white running into green and bright green were the main
features of the thread of light; on the current passing more rapidly,
the capillary thread became of an intense crimson, at the same time
apparently increasing in diameter (an effect probably due to irradiation).

[Sidenote: Spectrum described.]

The spectrum was very brilliant, consisting of the three bright lines
usually distinguished as Hα, Hβ, and Hγ, and a number of shaded bands
and fainter lines between these, with a bright continuous spectrum as a
background to the whole.

[Sidenote: Lines α, β, and γ varied in intensity with colour as seen by
eye. Fainter lines or bands described.]

The lines Hα, Hβ, and Hγ were found to vary in intensity with the
current, and in accordance with the colour of the light as seen by the
eye—a fact, as I think, not without bearing on the question of the
Aurora, the varying tints of which are so well known. The fainter lines
or bands were mostly stripes of pretty equal intensity throughout, and
all about the width of the Hβ line. I did not trace any marked degrading
on either side of the lines, though the edges were not uniformly so sharp
as Hα and Hβ. Some of the lines were found coincident in position with
lines of the air-spectrum.

[Sidenote: Purity of subsidiary lines questioned.]

It is a question whether these subsidiary lines are hydrogen, or are due
to some tube impurity. A photograph I have taken of this tube-spectrum
shows 17 lines in the part of the spectrum between F and H₂, some of
which are repeated in the hydrocarbon-tube spectra.

[Sidenote: Coincidence of lines with Aurora-spectrum.]

No principal line, and one subsidiary line only, actually coincide with
the Aurora-spectrum, this last being that to which Dr. Vogel assigns an
identical wave-length, viz. 5189. Other of the subsidiary lines, however,
fall somewhat near the Aurora-lines 5569, 5390, 5233, and 5004, two faint
lines also falling within the band 4694 to 4629.

[Sidenote: Comparison of the lines.]

The lines (adopting Dr. Vogel’s wave-lengths for the H lines) were, when
compared, as under:—

                                           {4694}
  Aurora     5569  5390  5233  5189  5004  { to } band.
                                           {4629}
  Hydrogen   5555  5422   ...  5189  5008   4632

I remarked that a line (5596) described by Dr. Vogel as “very bright” in
his H spectrum does not appear in my tube, though in most other respects
our H spectra agree.

[Sidenote: Effect of distance on the spectrum.]

I thought this tube afforded a good opportunity for testing the effect
of distance upon the spectrum. The slit was made rather fine. At 6
inches distance from it the line α in the blue-green (F solar) was very
bright. The lines marked β, γ, δ, ε, and ζ also survived, but were faint.
At 12 inches from the slit α and γ were alone seen, and at 24 inches
α stood by itself upon a dark ground. I also noticed that the red and
yellow parts of the spectrum first lost their light on the tube being
withdrawn from the slit; and this appeared to account for β disappearing
while γ survived. For drawing of the hydrogen-tube spectrum see Plate
XIV. spectrum 1.

The question of effect of distance upon the spectroscopic appearance of
a glowing light, as tested for this and other tubes, seems an important
one. It may possibly account for the generally faint aspect of the lines
in the more refrangible part of the Auroral spectrum.

[Illustration: Plate XIV.]


_Carbon- and Oxygen-tubes._

[Sidenote: Carbon- and oxygen-tubes. Tubes described. Carbon-tubes lighted
up. Spectra of the carbon-tubes described.]

The following tube-observations were taken together, because my friend
Mr. Henry R. Procter (to whom I am indebted for many profitable hints and
suggestions in Auroral work) was disposed to regard the spectra found
in the carbon-tubes and in those marked “O” as identical, suggesting
that pure O, with the ordinary non-intensified discharge, gives only a
continuous spectrum; and that the O tubes are in fact generally lighted
up by a carbon-spectrum, as the result of impurity from accidental
causes. The tubes examined for the purpose of comparison were as
follows:—A coal-gas tube, a tube marked “C.A.,” three O tubes (two of,
I believe, London make, and the third from Geissler), and an OH₂ tube,
also from Geissler. The carbon-tubes were both brilliantly and steadily
lighted by the current. The C.A. tube glowed with a peculiar silvery-grey
green light in the capillary part, and with a grey glow, considerably
stratified, in the bulbs. The coal-gas tube illumination was whiter and
still more brilliant than the C.A., and with even finer stratification in
the bulbs. The spectra of both tubes were conspicuous for the same three
well-known principal bright lines or bands in the yellow, green, and blue
(with one fainter in the violet), all shading off towards the violet, and
in both cases with fainter intervening bands or lines. These last bands
or lines only partially coincided when the two tubes were compared.

The spectra in both cases were rich and glowing, with a certain amount of
continuous spectrum between the lines; and the three principal bands or
lines showed well and distinctly their respective place-colours.

[Sidenote: Tubes tested for distance.]

_Tubes tested for distance._—In the case of the C.A. tube, at 18 inches
from the slit the continuous spectrum and fainter lines disappeared,
while the four principal lines still shone out, that in the green being
the strongest. At 24 inches the same lines were still visible, though
somewhat faintly.

In the case of the coal-gas tube, at 24 inches the whole spectrum was
quite brilliant, the four principal lines being very bright and even
preserving their distinctive colours. The H line, near the line or band
in the blue, was also plainly seen.

[Sidenote: O tubes lighted up.]

The O tubes, when treated by the same current as the carbon-tubes, were
found to be all three identical in general features. The discharge
lighted up each of the tubes feebly and somewhat intermittently. Grey
in the bulbs and a faint but decidedly pinkish white in the capillary
part were the distinguishing light colours; while nothing could be more
marked than the difference in brilliancy between these and the preceding
carbon-tubes.

[Sidenote: OH₂ tubes lighted up. O tubes spectra described.]

The OH₂ tube presented very much the same character; but the discharge
occasionally varied from a pinkish white to a yellow colour, somewhat
like that which artists call brown-pink, and reminding one of the “golden
rays” in certain Auroræ. These O spectra presented, in common with the
carbon-tubes, three principal bright lines or bands in the yellow, green,
and blue, with a fainter one in the violet, all shading off towards
the violet. The bands, however, showed but very little trace of local
colour, and the whole spectrum had a faint and washed-out look, very
different from the carbon-spectra. (I certainly, by a little management,
subsequently succeeded in getting the same look to the C.A. spectrum; but
it was only by removing the tube to some distance from the slit, and thus
depriving the spectrum of very much of its brightness.) The hydrogen line
(solar F) was bright, more so than any of the O lines.

The intensity of the three principal lines seemed to me to run in the
following order:—

               Yellow.  Green.  Blue.

  Coal-gas        β       α       γ
  Oxygen          γ       α       β

Between the lines γ and α in the Geissler O tube I found a rather bright
line, which I shall have occasion to refer to hereafter.

At 12 inches distance from the slit the O spectrum lost nearly all
its light; the H line, and the three lines γ, α, and β, alone faintly
remaining, α being decidedly the brightest. At 24 inches no spectrum at
all was to be seen.

[Sidenote: Comparison of spectra of coal-gas and O tubes.]

I carefully compared together the three principal lines of the two
spectra of coal-gas and O by means of:—

1st, the photographed micrometer before described;

2nd, a comparison-prism on the slit plate;

3rd, a piece of very fine brass foil cut as a pointer and fixed in the
focus of a positive eyepiece.

The lines or bands in both tubes were found to be slightly nebulous
towards the less-refrangible end (where they were measured), and the O
tube was not bright under a moderately high power (positive eyepiece).
Subject to these remarks, the three principal lines in both tubes were
found to correspond in position within the limits of my instrument. The
spectra did not, however, I am bound to say, _look_ alike.

[Sidenote: Dr. Vogel’s O spectrum reduced and compared.]

Puzzled by these observations, it then occurred to me to reduce Dr.
Vogel’s spectrum of O, given in his memoir, to the same scale with my
own. This I did independently, and I then compared the result with my own
spectrum as mapped out. From the comparison, I judge that if my O tubes,
one and all, showed a carbon-spectrum, the learned Doctor’s tube must
have been subject to a similar infirmity, as the tubes all agreed in main
features.

There is, however, one point to which I desire to draw attention, which
is this, that common to both the Doctor’s and my own Geissler spectrum I
found the before-mentioned rather bright line between γ and α. This line
I found no equivalent for in either of the carbon-tubes. For spectra of
coal-gas and oxygen-tubes, see Plate XIV. spectra 2, 3, & 4.

[Sidenote: Tube- and flame-spectra of carbon do not correspond.]

In comparing the spectra, it should be remembered that the tube- and
flame-spectra of carbon do not correspond. Compare, for instance, the
spectrum of coal-gas or CO₂ in tube, and the well-known lines or bands
in the blue base of a candle-flame. The sharper edge of the yellow
line or band of the carbon-tubes will be found about midway between
the two bright yellow candle-lines or bands. The first of the very
beautiful group of lines or bands in the green in the candle-flame
falls considerably behind the sharper edge of the green line or band
in the tube, while the third bright band in the tube, alone of the
three, corresponds with a very faint band in the candle-flame. A line
or band in the violet in the tube-spectrum finds no equivalent in the
candle-spectrum. For comparison of the carbon-tube and flame spectra (the
principal lines of the tube being alone shown), see Plate XVI. spectra 6
& 7.

[Sidenote: Prof. Piazzi Smyth’s measurements of the components of the
citron-band in a coal-gas flame.]

_Note._—Prof. Piazzi Smyth has been good enough, at my instance, to
measure the components of the citron band of the carbo-hydrogen spectrum
(near Ångström’s Aurora-line), as seen in a coal-gas blowpipe-flame urged
with common air.

The spectroscope used had prisms giving 22° of dispersion between A and
H, and the observing telescope magnified 10 times. The following is a
table of the results communicated to me by the Professor:—

                                            Reading of
                                Intensity.  Micrometer.

  Reference line, lithium β          4         16·55
        ”         sodium, α1        10         18·45
        ”            ”    α2        10         18·51

          Citron band. Carbo-hydrogen.

  Line 1, exquisitely clear          6         21·28
       2,         ”                  5         21·88
       3,         ”                  3         22·44
       4, faint but clear            2         22·95
       5, faint                      1         23·38
       6, faint and hazy             1         23·70
       7, doubtful                   ?         23·92
  Reference line, thallium α        10         25·08

[Sidenote: Comparison of Dr. Vogel’s O lines and Dr. Watts’s
carbon-lines.]

From Dr. Watts’s ‘Index of Spectra’ I have extracted the three principal
carbon-tube bands or lines; and they compare with Dr. Vogel’s oxygen-tube
as under:—

                                         Yellow.  Green.  Blue.
  Dr. Vogel’s oxygen-lines                 5603    5189    4829
  Dr. Watts’s carbon-tube bands or lines   5602    5195    4834

Now these wave-length differences are so small that they raise a
presumption of the possibility of the spectra being identical. On the
other hand, assuming the spectra are not identical, the comparison tells
the other way, viz. that the differences are so minute as to escape
detection in instruments of moderate dispersion. With my own instrument
I found the O spectrum too faint to increase the dispersive power with
advantage. Considering the extremely different character of the two
discharges, the identity of all the O tubes, and the presence of the line
found between γ and α in the O spectrum, I think the two spectra are
independent, though I admit there is room for doubt.

[Sidenote: O and CO₂ spectra photographed.]

_Note._—Since this examination I have photographed both spectra side
by side (see ‘Photographed Spectra,’ Plate XXXI., text, pp. 69, 70).
The pictures include, of course, only the blue and violet parts of
the spectrum; but they are widely different in aspect, and show that,
photographically at least, in this part of the spectrum there is a
complete want of identity. Subsequent investigations, however, by
Schuster and others (detailed later in this Chapter), go to establish
that the principal lines shown in mine and Dr. Vogel’s tubes were due to
(probably hydrocarbon) impurity. The exception is the single line common
to mine and Dr. Vogel’s tubes, but absent from the coal-gas spectrum.
This line proves to be oxygen. Compare oxygen-tube spectra (Plate XIV.
spectra 3 and 4) with Schuster’s oxygen-tube spectrum (Plate XVIII. fig.
15). The line in question is found identical in the three tubes.

The tube OH₂ was found to give the principal lines of the O and H spectra
combined on a faint continuous spectrum.


_Geissler Mercury-tube_ (Plate X. fig. 7) _and Barometer Mercurial
vacuum_.

[Sidenote: Mercury- and barometer-tubes examined. Mercury-tube described.
Barometer-tube.]

I next examined two vacuum-tubes of an entirely different character. The
one was a tube from Geissler of stout glass, some fifteen inches long,
without electrodes, and an inch across. Within this tube was a second
of uranium glass, with bulbs blown in it. In contact with both tubes a
quantity of fluid mercury ran loose (Plate X. fig. 7). Upon shaking this
tube with the hand brilliant flashes of blue-white light, like summer
lightning, flashed out. These were discernible (though faintly) even in
daylight. The fine terminal wires of the coil being wrapped round each
end of this tube, when the current passed, a bright and white induced
discharge, with a considerable amount of stratification, was seen in the
tube. The other tube was that of a mercurial siphon-barometer. This being
placed in a stand, one terminal wire was placed in the mercury in the
short leg of the siphon, while the other terminal was made into a little
coil and placed on the upper closed extremity of the barometer-tube. On
passing the current, the entire short space above the mercury was filled
with a grey-white light, not stratified, but showing a conspicuous bright
ring just above the level of the mercury.

[Sidenote: Spectrum of both these tubes described.]

Both these tubes, when examined with the spectroscope, showed four bright
rather uniform bands (the central one being the brightest), which I
assigned to the carbon-spectra (see Plate XIV. spectra 5 and 6).

The Geissler tube was probably filled designedly with coal-gas. In the
case of the barometer-tube the spectrum must be assumed to be the result
of some carbon impurity.

No lines of mercury could be detected in either case.

An effort was made to examine the light of the Geissler mercury-tube as
excited by motion only, but the spectrum could not be kept in the field;
the four lines were, however, seen to flash out as the light passed
before the slit.

[Illustration: Plate XV.]


_Air-tubes._

[Sidenote: Air-tube illuminated.]

The first tube I examined was an ordinary Geissler tube charged with
rarefied air. The bulbs, on passing the discharge, were filled with the
well-known rose-tinged light like to the Aurora-streams. This in the
capillary part was condensed into a brighter and whiter thread, while the
platinum wire of the negative pole was surrounded by its characteristic
mauve or violet glow.

[Sidenote: Spectrum described.]

The spectrum, even with a weak current, was quite bright, and consisted
mainly of the nitrogen-lines and bands, with the lines Hα, Hβ, and Hγ,
and some of the intermediate lines of the H tube.

The double line α was undoubtedly the brightest in the spectrum when
taken in the capillary part of the tube. After this followed β, and then
γ(H), δ, and ε. I was, however, uncertain as to the relative brightness
of the last three, and marked their intensities with hesitation. I tested
them several times independently with differing results, and suspected
them of variability with the current.

The rest of the lines were very much of the same intensity. (For drawing
of spectrum of air-tube in capillary part see Plate XV. spectrum 1.)


_Violet [negative] Pole, same tube._

[Sidenote: Violet (negative) pole: spectrum described.]

I next turned my attention to the violet or negative-pole glow; and here
a remarkable change took place in the spectrum, not only in the position
of the principal bands or lines, but in their relative intensity (see
Plate XV. spectrum 2).

The double line α in the capillary part was replaced in the violet glow
by a shaded band of second intensity β, the sharp edge of which was
extended towards the red, and formed (except for some faint indications)
the limit of the spectrum in that direction. The somewhat faint line
next α in the capillary tube had its faint representative in the violet
pole; but the next two lines (capillary) were represented by the bright
band γ in the violet pole lying in a position between them. Next γ in
the violet pole came three faint lines, representing β, γ, and δ in the
capillary spectrum; and then the bright band α, which was the brightest
of the violet-pole group, and represented a medium-intensity band in the
capillary spectrum. After this was a faint band near α, representing
two rather bright ones in the capillary spectrum, this last being
succeeded by other bands in the violet. α, β, and γ in the violet pole
were examined carefully for relative brightness, and were, I believe,
correctly marked.


_Red [positive] Pole._

[Sidenote: Red (positive) pole: spectrum described.]

The red [positive] pole was next examined, but presented no peculiar
features. It appeared as a fainter representation of the capillary
air-spectrum, with some few lines or bands absent, and (as will be seen
after) was also a fair representation of a diffused air-spectrum (see
Plate XV. spectrum 3).

Examined for comparative intensity, at 24 inches from the slit, the whole
capillary air-spectrum showed faintly. The marked lines in the centre of
the spectrum generally retained their prominence; but after α I judged
ε next in brightness. On examining the violet pole at 12 inches from
the slit, the whole spectrum was faint and the bands α and β were alone
distinctly seen.


_Aurora (air)-tube._ (Plate XV. spectrum 4.)

[Sidenote: Aurora-tube: discharge described. Spectrum described.]

Next to the Geissler air-tube I examined an “aurora”-tube, about 15
inches long and 1¼ inch across, with platinum terminals, and of the same
diameter throughout (Plate X. fig. 8). The discharge was of a rosy-red
colour, and the long flickering stream from pole to pole certainly
much reminded one optically of an auroral streamer. Spectroscopically
examined, the discharge presented a faint banded air-spectrum similar to
that of the positive pole (see Plate XV. spectrum 4); but the relative
intensity of the lines was somewhat altered, while a very bright line
in the green (seen also in the tube next described) was characteristic
of the spectrum, and in this respect distinguished it from the ordinary
air-spectrum.


_Phosphorescent tube._

[Sidenote: Phosphorescent tube described. Discharge described. Spectrum
described.]

Following this last tube I examined one purchased as “phosphorescent.”
It was rather short (6½ inches), of equal calibre, and about the size of
the bulb of a Geissler tube. It was filled with a white powder (probably
one of the Becquerel compounds). On passing the current between the
electrodes, a bright rose-coloured stream appeared; and wherever this
was in contact with the powder, the tube glowed with a brilliant green
light. On stopping the current, the tube still continued to shine, but
with a fainter green glow, which gave only a continuous spectrum. When
examined in full glow, the tube-spectrum was also in the main continuous
and of a green tinge; but upon it were bright lines in the blue and
violet portions of the spectrum, while in the red, yellow, and green a
faint but distinct air-spectrum was seen; and with this was also found
the same bright line in the green which distinguished the “aurora”-tube.
[Five out of six of the lines in the blue and violet will be also found
in Schuster’s oxygen-tube, violet pole (Plate XVIII. fig. 15). The
air-spectrum probably arose from impurity.]

[Illustration: Plate XVI.]


_Spark in Air._

[Sidenote: Spark in air: spectrum described.]

I next took a ½-inch spark in air between platinum terminals (see Plate
XV. spectrum 6). The principal lines in this spectrum were the line α
(by far the brightest), corresponding to γ in the violet pole; next was
β, a line in the yellow, not appearing in the tube-spectrum, and then
other lines of less intensity. In the “aurora” and “phosphorescent” tubes
was found, as before mentioned, a line in the green prominent for its
brightness, and, indeed, in the “aurora”-tube the only one which survived
when it was moved away from the slit. This line also appeared in the
spark-spectrum, but there only of an average brightness. I examined it
carefully for position in the respective tubes; and on comparing them by
means of a pointer in the eyepiece, found it coincident with the ridge or
centre of the wedge-like bright-green broad band which is so conspicuous
in the air-tube spectrum.

I think this edge-like centre has actually a line coincident with the
line I refer to; but if so, its intensity little exceeds that of the band
itself.


_Spark over Water._

[Sidenote: Spark over water: spectrum described.]

To complete the set of air-experiments, I examined the same spark taken
from the surface of a small meniscus of water, placed in a glass cup upon
the lower platinum wire. In this case the air-spectrum was plainly, but
not brightly, seen at the violet end of the spectrum—the red, yellow,
green, and blue being filled with a continuous spectrum, through which
some of the air-lines faintly showed (see Plate XV. spectrum 7).


_Phosphoretted-Hydrogen Flame._

[Sidenote: Phosphoretted-hydrogen flame.]

This was obtained from a hydrogen-bottle fitted with glass tubing, two or
three minute pieces of phosphorus being placed with the zinc. The flame
was of a bright yellow colour, with a cone of vivid green light in its
centre.

[Sidenote: Spectrum described.]

The spectrum was found to consist mainly of three bright bands in the
yellow, green, and green-blue respectively (see Plate XVI. spectrum 3).

[Sidenote: Mons. Lecoq de Boisbaudran’s remarks on the spectrum
increasing in brilliancy when the flame is cooled.]

The central band was very striking in its emerald-green colour, while
all the bands were remarkable as being very broad in proportion to the
slit (which, however, was not fine). The yellow band had a rich glow of
colour. My spectrum was mapped out at ordinary temperature, and I found
the bands sufficiently bright; but Mons. Lecoq de Boisbaudran, in his
‘Spectres Lumineux’ (texte, p. 188), has described how the brilliancy
of these bands is increased when the flame is artificially cooled
(_refroidie_).

The idea of cooling the flame was due to M. Salet, who effected it either
by a jet of water or by an air-blast.

The less refrangible bands seem the most susceptible to increase of
brilliancy.

Mons. Boisbaudran also makes the important remark that the relative
intensities of the bands are in such case altered, adding:—“La
plus importante de ces modifications consiste en un renforcement
très-considérable de la bande rouge δ 97·03 (W.L. 5994) qui devient
vive de presque invisible qu’elle était en l’absence du refroidissement
artificiel de la flamme.”

Full details of the changes are given by M. de Boisbaudran.

The bearing of these observations as connected with the variable
character of the red line in the Aurora-spectrum seems to me in the
highest degree noteworthy.


_Iron-Spectrum._

[Sidenote: Iron-spectrum.]

A comparison of this spectrum suggested itself, partly from the suspected
relations between the Aurora and solar corona, and partly from a
consideration of the views expressed by M. Gronemann and others in favour
of the Aurora having its origin in the fall of an incandescent meteoric
powder.

[Sidenote: How obtained. Spectrum described. Mons. Lecoq de Boisbaudran’s
spectra also given.]

The spectrum was obtained from a spark taken over a solution of
perchloride of iron in a small glass cup, and was remarkable for its
brightness in and about the green region. The lines varied considerably
in intensity, and with a fine slit the principal ones were sharp,
distinct, and clear. A group of three lines (α) stood out boldly in the
green as the most marked, and next to these a group of three others more
towards the violet end of the spectrum (see Plate XVI. spectrum 4). By
the side of my phosphoretted-hydrogen and iron spectra I have placed the
principal lines of Mons. Lecoq de Boisbaudran’s same spectra (reduced
to my scale), and with figures of wave-lengths for comparison with the
Aurora-spectrum (see Plate XVI. spectra 1 and 2).

[Sidenote: Comparison of iron- and Aurora-spectrum.]

A difficulty in comparing the iron-spectrum with that of the Aurora
arises from the large number of fine lines found in the former spectrum.
In a photograph (taken with the same prism as before described) of a
small piece of meteoric iron fused in an electric arc by the aid of 40
Grove cells, about 154 lines are easily counted in the blue and violet
parts of the spectrum. Double this number at least would be seen with a
spectroscope of moderate dispersion in the region comprising the entire
set of auroral lines.


_Spectrum of Mercury._

[Sidenote: Mercury-spectrum. How obtained.]

This spectrum is given as useful for comparison with the bright and
principal Aurora-line. It is easy to obtain with a small coil, the metal
being used as one electrode. The yellow lines are distinct and steady;
but the green, which is very bright, is apt to flicker as the spark moves
on the surface of the metal (see Plate XVI. spectrum 5).


_The following Table was compiled for the purpose of comparing the
foregoing results with the Aurora-spectrum._

[Sidenote: Table of coincidences.]

TABLE showing comparative position of Aurora-lines with the principal
lines in the examined spectra. C. means coincident within the limits of
my instrument and scale, N. near, and VN. very near.

  -------------+--------+------+------+------+------+------+--------+------
              {|        |      |      |      |      |      | 4694   |
  Aurora-lines{|  6297  | 5569 | 5390 | 5233 | 5180 | 5004 |to 4629 | 4350?
              {|   β.   |  α.  |  ζ.  |  δ.  |  δ.  |  γ.  |  ε.    |  ε.
  -------------+--------+------+------+------+------+------+--------+------
               |        |      |      |      |  C., |      | Band   |
  Hydrogen-tube|        |  N.  |      |  N.  | same |      |includes|
               |        |      |      |      | W.L. |      |2 lines.|
  -------------+        +------+------+------+------+------+--------+
               |        |      |      |      |      |      | Band   |
  Coal-gas     |        |      |      |  N.  |  VN. |      |includes|
   tube        |        |      |      |      |      |      |1 line. |
  -------------+        +------+------+------+------+------+--------+
  Oxygen-tube  |        |      |      |      |  VN. |      |        |
  -------------+        +------+------+------+------+------+--------+
               |        |    Band     |      |      |      | Band   |
  Air,         |        |  includes   |      |  N.  |      |includes|
   capillary   |        |             |      |      |      |2 lines.|
  -------------+        +------+------+------+------+------+--------+
               |        |      |      |      |Band  |      | Band   |
  Air, violet- |   *    |      |      |      | in-  |  C.  |includes|  **
   pole        |        |      |      |      |cludes|      |1 line. |
  -------------+        +------+------+------+------+------+--------+
  Air, red-pole|        | See Air, capillary.                       |
  -------------+        +-------------------------------------------+
  Aurora-tube} |        | See Air, capillary; and note bright line. |
   and phos- } |        |                                           |
   phorescent} |        |                                           |
   tube      } |        |                                           |
  -------------+        +------+------+------+------+------+--------+
               |        |      |      |      |      |      | Band   |
  Air, spark   |        |  N.  |      |      |  N.  |  C.  |includes|
               |        |      |      |      |      |      |2 lines.|
  -------------+        +------+------+------+------+------+--------+
  Air, spark   |        | Continuous spectrum and faint    | Band   |
   over water  |        | air-lines.                       |includes|
               |        |                                  |2 lines.|
  -------------+        +------+------+------+------+------+--------+
  Phosphoretted|        |  N.  | Faint| Band |      |      |        |
   hydrogen    |        |      | band.| in-  |      |      |        |
               |        |      |      |cludes|      |      |        |
  -------------+        +------+------+------+------+------+--------+
  Iron         |        | VN.  |   N. |  VN. |  VN. |      |        |
  -------------+--------+------+------+------+------+------+--------+------

  *  No results in the examined spectra; but see Plate XIII. fig. 2.

  ** Too uncertain in position for comparison (see Plate XIII. fig. 1).

Tested by coincidence, or close proximity of lines to those of the
Aurora, we arrange the spectra in the following order:—(1) iron, (2)
air-spark, (3) hydrogen, (4) air-tube, (5) phosphoretted hydrogen, (6)
carbon and oxygen.

The air-tube spectrum might perhaps stand higher in the scale but for its
broad bands, which make comparison doubtful. Lines of oxygen possibly
escape detection in the Aurora from the faint character of its spectrum.

The phosphorus and iron spectra are especially interesting in connexion
with Professor Nordenskiöld’s “metallic and magnetic cosmic dust in the
Polar regions” (see Phil. Mag. ser. 4, vol. xlviii. p. 546).

[Sidenote: Additional Table of compared spectra.]

As an addendum to the foregoing, on Plate IX. fig. 1 will be found a
Table I have prepared, in which a type Aurora and also Vogel’s and
Barker’s Auroræ are compared with eight other spectra, viz.:—

  S.   Solar spectrum.
  N.   Nitrogen (air): Watts.
  O.   Oxygen (air): Watts.
  C.H. Carburetted-hydrogen vacuum-tube: Watts.
  C.I. Carburetted-hydrogen flame: Watts.
  C.C. Blue base of candle-flame: Capron.
  O.P. Oxygen vacuum-tube: Procter.
  I.   Iron: Watts.

The divisions and vertical lines will guide the eye in making comparison
of the spectra.




CHAPTER XII.

SOME NOTES ON PROFESSOR ÅNGSTRÖM’S THEORY OF THE AURORA-SPECTRUM.

    [The substance of these appeared in the ‘Philosophical
    Magazine’ for April 1875, in conjunction with the “Comparison
    of the Tube and other Spectra” (Chapter XI.), but they are now,
    for the sake of convenience, made a separate article.]


[Sidenote: Professor Ångström’s propositions.]

In a contribution by the late Professor Ångström to a solution of the
problem of the Aurora-spectrum (an abstract of which appeared in ‘Nature’
of July 16, 1874), the Professor is stated, amongst other things, to have
laid down certain propositions in substance as follows:—

[Sidenote: That the Aurora has two spectra.]

1st. That the Aurora has two different spectra—the one comprising the one
bright line in the yellow-green only, and the other the remaining fainter
lines.

[Sidenote: That bright line does not coincide with HC₂.]

2ndly. That the bright line falls within a group of hydrocarbon lines,
but does not actually coincide with any prominent line of such group, and
that Dr. Vogel’s finding this line to coincide with a not well-marked
band in the air-spectrum must be regarded as a case of accidental
coincidence.

[Sidenote: That moisture is _nil_ in Aurora region.]

3rdly. That moisture in the region of the Aurora must be regarded as
_nil_, and that oxygen and hydrogen must alone there act as conductors of
electricity.

[Sidenote: Ångström’s flask-experiment described.]

Professor Ångström then details the examination of an exhausted dry
air-flask filled with a discharge analogous to the glow of the negative
pole of a vacuum air-tube.

[Sidenote: Flask-spectrum compared with Aurora-spectrum.]

The experiment is described as follows:—“Into a flask, the bottom of
which is covered with a layer of phosphoric anhydride, the platinum wires
are introduced, and the air is pumped out to a tension of only a few
millimetres. If the inductive current of a Ruhmkorff coil be sent through
the flask, the whole flask will be filled, as it were, with a violet
light, which otherwise only proceeds from the negative pole, and from
both electrodes a spectrum is obtained composed chiefly of shaded violet
bands.” The comparison of the spectrum of this violet glow with that of
the Aurora gives, according to Ångström, the following results:—

  Aurora-lines, wave-lengths      4286  4703  5226
  Violet light, wave-lengths      4272  4707  5227

Two weak light bands, found by Dr. Vogel at 4663 and 4629, are also
compared with other lines in the violet light 4654 and 4601; and the
Professor then concludes that it may be in general assumed that the
feeble bands of the Aurora-spectrum belong to the spectrum of the
negative pole, possibly changed more or less by additions from the banded
or the line air-spectrum.

[Sidenote: Bright line is due to fluorescence or phosphorescence.]

4thly. That the only probable explanation of the bright line is, that it
owes its origin to fluorescence or phosphorescence. The Professor remarks
on this point that “an electric discharge may easily be imagined which,
though in itself of feeble light, may be rich in ultra-violet light, and
therefore in a condition to cause a sufficiently strong fluorescence.” He
notes also that oxygen and some of its compounds are fluorescent.

[Sidenote: No need of Dr. Vogel’s theory of variability.]

5thly. That there is no need, in order to account for the spectrum of the
Aurora, to have recourse to the “very great variability of gas-spectra
according to the varying circumstances of pressure and temperature” (Dr.
Vogel’s theory). Professor Ångström does not admit such variability, and
does not admit that the way a gas may be brought to glow or burn can
alter the nature of the spectrum.

[Sidenote: Professor Ångström’s conclusions tested.]

In order to test some of the Professor’s conclusions in an experimental
way, I examined some tube and other spectra not only for line-positions,
but also for general resemblance to an Aurora-spectrum.

These experiments are detailed in the last Chapter, and the results are
comprised in Plates XIV., XV., and XVI., in which the spectra obtained
are represented in black for white.

[Sidenote: Result of examination of the Professor’s propositions.]

The result of the examination of Professor Ångström’s principal
propositions seems to be this:—

1st. Two Auroral spectra. I agree in this, but question whether the
fainter lines may not possibly comprise more than one spectrum.

2nd. I agree also that the bright yellow-green line falls, as Professor
Ångström describes, just behind the second line in the hydrocarbon yellow
group (see Plate V. fig. 7). And I find, in common with the Professor, no
well-marked or prominent line in the air-spectrum with which it accords.

3rd. This may be conveniently divided into two parts, viz.:—

A. The proposition that “moisture in the region of the Aurora must be
regarded as _nil_.”

[Sidenote: Moisture probably not _nil_ in the Aurora region. Reasons for
this given. Aurora in vapour or mist. Frequently near to earth’s surface.]

Here I see reason to differ, since (to quote a letter of Mr. Procter’s)
“the vapour-density of OH₂ is only 9 against 14 for N and 16 for O;” and
again, “electrical or heat-repulsion may carry water-dust up to enormous
heights.” There are, too, I think, circumstances connected with the
Aurora itself which make the assumption of moisture being _nil_ in the
Auroral regions untenable. The first of these is the fact that the white
arc, streamers, and floating patches of light, found in some Auroræ,
have frequently the peculiarly dense and solid look of vapour-clouds—a
circumstance with which I have been frequently struck. Mr. Procter and
others have also remarked that the Aurora is generally formed in a sort
of “mist or imperfect vapour.” The second, that Auroræ, or portions of
them, are frequently near to the earth’s surface. Instances of this are
given in the section on the Height of the Aurora, notably the experiences
of Sir W. Grove and Mr. W. Ladd.

[Sidenote: Coincidence of Auroral lines with telluric solar lines.]

On this point, too, note the peculiarities of the red line, which (and,
as I find, the green line also) are coincident with, or very close
to, telluric bands or groups of lines in the solar spectrum usually
attributed to moisture. (See Plate XIII. fig. 2.)

[Sidenote: Continuous spectrum.]

I think we may also claim the continuous spectrum in the Aurora in
further proof of water-vapour (see Plate XV. spectrum 7). The continuous
spectrum of the Aurora is also, to my observation, more local and dense
in the spectroscope than the glow generally seen between the lines or
bands in gas-spectra.

[Sidenote: Violet-pole spectrum discussed. Most spectra have a general as
well as special character.]

B. The question of the violet-pole spectrum. Here I make the remark that
in comparing other spectra with that of the Aurora, it is, I think, too
much the practice to trust to the coincidence (more or less perfect) of
one or perhaps two lines out of many; whereas we know by experience that
most spectra have so well-marked a general as well as special character
that, when once seen, they are recognized afterwards with the greatest
ease and without measurements. An experience and proof of this is found
in a set of “Photographed Spectra” which the Autotype Company have
reproduced for me.

[Sidenote: Coincidence of one or two lines not sufficient to establish
identity.]

Of course no two given spectra can be considered identical unless their
principal lines coincide; but, on the other hand, the coincidence
of one or two lines out of many, without other features, cannot be
satisfactorily or conclusively held to establish identity.

[Sidenote: Ångström’s compared spectra.]

In Professor Herschel’s letter (Phil. Mag. ser. 4, vol. xlix. p. 71),
Professor Ångström’s representation of the “spectrum of the glow
discharge round the negative pole of air-vacuum tubes” is given, in
comparison with the Aurora-lines and those of olefiant gas. This
illustration is here introduced.

Ångström’s representation of the Spectrum of the glow discharge round the
negative pole of Air-vacuum tubes, and its comparison with the Spectrum
of the Aurora.

[Illustration: Wave-lengths, in hundred-thousandths of a millimetre.]

It is unfortunate that in this illustration and in Professor Herschel’s
paper the wave-lengths of the Aurora-lines are not given in figures, but
must be roughly calculated from the scale. Professor Herschel speaks of
Ångström’s drawing as representing a _normal_ spectrum, and as derived
from authentic sources, such as Vogel, Barker, and others; but beyond
this we are not certain as to its origin.

In illustration of the difficulty of constructing any thing like a
general typical Aurora-spectrum I append a Table of eight Auroral spectra
taken at hazard:—

[Sidenote: Table of compared Aurora.]

Auroral lines and bands.

  ---------------------+----+---------+------+-----+-----------+-----------
      Observers.       |Red.| Yellow. |Green.|Blue.|  Indigo.  |  Violet.
  ---------------------+----+----+----+------+-----+-----+-----+------+----
                       |    |    |    |      |     |     |     |{4694}|
  Vogel, April 9, 1871 |6297|5569|5390| 5233 | 5189| 5004|  —  |{ to }|  —
                       |    |    |    |      |     |     |     |{4629}|
                       |    |    |    |      |     |     |     |      |
  Barker, Nov. 9, 1871 |6230|5620|  — |   —  | 5170| 5020| 4820|   —  |  —
                       |    |    |    |      |     |     |     |      |
                       |    |    |    |{5330}|     |{5050| 4930| 4740}|
  Barker, Oct. 14, 1873|6300|5550|  — |{ to }|   — |{ to |  to |  to }|4310
                       |    |    |    |{5200}|     |{4990| 4850| 4670}|
  A. Clarke, junr.,    |    |    |    |      |     |     |     |      |
    Oct. 24, 1870      |    |5690|  — | 5320 |   — |   — | 4850|   —  |4350
                       |    |    |    |      |     |     |     |      |
  Backhouse, 1873      |6060|5660|  — |   —  | 5165| 5015|   — | 4625 |4305
                       |    |    |    |      |     |     |     |      |
  Backhouse, Feb. 4,   |    |    |    |      |     |     |     |      |
    1874               |  * |5570|  — |   —  | 5180| 4980| 4830| 4640 |4320
                       |    |    |    |      |     |     |     |      |
  H. R. Procter, 1870  |  * |  * |  — |   *  |   — |   — |   — |   *  |  *
                       |    |    |    |      |     |     |     |      |
  Lord Lindsay, 1870   |  — |  * |  * |   *  |   * |   — |   — |   —  |  *
  ---------------------+----+----+----+------+-----+-----+-----+------+----

  * Mr. Procter’s and Lord Lindsay’s lines had no wave-lengths.

[Sidenote: Ångström’s drawing discussed.]

On examining Ångström’s diagram it certainly seems to me that, upon the
showing of the drawing itself, the coincidences are not very exact.
All three of the violet-pole bands appear to be less refrangible than
the Aurora-lines with which they are compared-the middle one (at 47)
considerably so, the one near E (at about 52·30) appreciably so, and the
third (at 43) slightly so.

[Sidenote: Diagram of Vogel’s Aurora and violet-pole spectrum.]

As it seemed desirable to adopt a specific Aurora-spectrum for
comparison, and to show such comparison on a somewhat larger scale than
Ångström’s drawing, I prepared the diagram shown on Plate XI. fig. 1. The
upper spectrum is Vogel’s, already described and figured on Plate XIII.
The lower spectrum is that of “Air, violet pole,” Plate XV. spectrum 2,
graphically shown. I can only find one absolute coincidence in the two
compared spectra in this diagram.

It should, too, I think, be borne in mind that there is a great
difference in the character of the compared spectra, whether as shown in
Ångström’s drawing or mine—the bands of the violet-pole spectrum mostly
degrading towards the violet, while the lines or bands of the Aurora in
no way possess that character[14].

[Sidenote: Dr. Vogel’s violet-pole and Aurora-lines.]

To assist in the foregoing violet-pole comparison I add the following
Table derived from Dr. Vogel’s memoir:—

       Violet-pole lines.                Aurora-lines.
   W.L.                             W.L.
  6100,} broad, moderately
  5945,} bright stripe             6297, very bright stripe.

  5459,} broad, moderately        {5569, brightest line of spectrum.
  5289,} bright stripe            {5390, extremely faint line.

  5224,  very bright line          5233, moderately bright.
  5147,  faint line                5189, moderately bright.
  5004,  bright line               5004, very bright line.
  4912,  fainter than last.

  4808,  very faint line.         {6694,}
  4704,  very intense line.       {4663,} band less brilliant in
  4646,  very faint line.         {4629,}   the middle.

  4569,  moderately bright.
  4486,  moderately bright.
  4417,  quite faint line.
  4346,  moderately bright line.
  4275,  very bright line.

On examination of these figures it will be seen that 5224 and 5233 are
fairly close, and that 5004 is coincident. Beyond these there is little
to identify the spectra.

[Sidenote: Conclusions arrived at adverse to the violet-pole theory.]

As the general result of my observations and a comparison of the
foregoing spectra and tables, I see no reason for giving to the
violet-pole glow any special or distinguished place in a comparison with
the Aurora, and certainly not for assigning to it the nearly absolute
monopoly of the spectrum. It is true that the line γ in the violet-pole
glow (Plate XV. spectrum 2), which, by the way, degrades towards the
red, is in close coincidence with one of the Aurora-lines; but how
are the brighter bands α and β accounted for? These, as I have before
pointed out, alone survive when the tube is placed at a distance from
the slit. It is true they are thus reduced to shaded-off lines in lieu
of bands; but the difficulty still remains, that they are conspicuous
for their absence in the Aurora-spectrum. On the whole, I cannot but
conclude that Professor Ångström’s theory fails. At all events, if the
violet-pole glow-spectrum is to represent the Aurora-spectrum, it must
be under conditions different from those by which it obtains in dry-air
vacuum-tubes or flasks at ordinary temperature.

[Sidenote: Phosphorescence or fluorescence of the yellow-green line.]

4th. I feel more in accord with Professor Ångström’s memoir upon the
subject of the phosphorescence or fluorescence of the bright yellow-green
Aurora-line.

[Sidenote: External features of Auroræ confirmatory of this.]

I do not notice that the Professor touches upon the external features of
the Aurora in respect of this question.

October 20, 1870.—I noted the grand Auroral display of that evening,
including “streamers of opaque-white _phosphorescent_ cloud very
different from the more common transparent Auroral diverging streams of
light.”

February 4, 1872.—A fine display. The first signs were (in dull daylight)
“a lurid tinge upon the clouds, which suggested the reflection of a
distant fire, while, scattered among these, torn and broken masses of
white vapour, _having a phosphorescent appearance_, reminded me of a
similar appearance in October 1870.” (Other instances of this effect
will be found in the section Aurora and Phosphorescence.) Day Auroræ,
too, we might suppose could hardly be seen without the presence of some
phosphorescent glow.

[Sidenote: Other confirmatory circumstances. Conclusion in favour of the
theory.]

Having regard to the near proximity of the phosphoretted-hydrogen band to
the bright Aurora-line, to the circumstance of this band brightening by
reduction of temperature (a phenomenon probably connected with ozone),
to the peculiar brightening of one line in the green in the “Aurora” and
“phosphorescent” tubes (the phosphorescent tubes probably containing
O), and to the observed circumstance that the electric discharge has
a phosphorescent or fluorescent after-glow (isolated, I believe, by
Faraday), I feel there is strong evidence in favour of such an origin to
the principal Aurora-line, if not to the red line as well.

[Sidenote: Invariability of gas-spectra questioned.]

5th. Professor Ångström opens a wide door to discussion in his
proposition of the invariability of gas-spectra, and I do not now attempt
to follow in detail this interesting part of the present subject. Suffice
it to say, that if the Professor lays down this proposition in its
strictest sense (I can hardly suppose he so meant it), there is, so far
as I am aware, no one spectrum that can at all claim comparison with the
Aurora-spectrum. Giving greater latitude to the Professor’s words, I
reply, upon competent authority, that lines vary in number and brilliancy
with temperature, and in breadth with pressure. Kirchhoff, too, in
speaking of vapour-films as increasing the intensity of lines, states
“it may happen that the spectrum appears to be totally changed when the
mass of vapour is altered.” We may, too, now add magnetism as capable of
effecting a change in certain spectra, not only as to brilliancy, but
even as to position of lines. (Chautard’s Researches, ‘Philosophical
Magazine,’ 4th series, vol. 1. p. 77, and experiments detailed in Chap.
III. of this work.)




CHAPTER XIII.

THE OXYGEN-SPECTRUM IN RELATION TO THE AURORA (PROCTER AND SCHUSTER).


[Sidenote: Procter’s oxygen-spectrum.]

In a communication to ‘Nature,’ Mr. H. R. Procter has pointed out an
apparent coincidence in position of several of the Auroral lines with
those of a spectrum occasionally obtained from air at low pressure with
a feeble discharge. It is, he says, sometimes exhibited in lumière
(phosphorescent?) tubes, and he believed it, in part at least, to be the
spectrum described by Wüllner (Philosophical Magazine, June 1869) as a
new spectrum of oxygen.

[Sidenote: How obtained.]

He had obtained it very vividly in pure electrolyzed oxygen with a feeble
discharge, but some perplexing observations made him doubtful of its
origin.

Plate XI. fig. 4 gives a representation of this spectrum as shown by Mr.
Procter, except that my drawing is in black for white.

[Sidenote: Compared spectra described.]

The upper spectrum is that above mentioned, the centre one that of the
Aurora, the lower one the lines of Na and H for comparison. The Auroral
yellow-green line, in January 1870, was found by Mr. Procter coincident
with a bright line or band in the tube (with a spectroscope of a 60°
bisulphide prism, and magnifying-power about six). The third and fifth
lines in the Aurora seemed also to correspond with tube-lines. As to
these Mr. Procter says they were not bright enough to be compared with
the same accuracy as the yellow-green line, but that the positions could
not be far wrong.

[Sidenote: Mr. Procter’s subsequent views. Yellow-green line traced to
some form of hydrocarbon.]

Mr. Procter subsequently (‘Edinburgh Encyclopædia,’ art. “Aurora”)
considered he traced the yellow-green tube-line to some form of
hydrocarbon. On examination with instruments of greater dispersion, it
was found that, though more refrangible than the first band of citron
acetylene (candle-flame), it was less so than the Aurora-line. The
tube-band, too, was shaded towards the violet, which was not the case
with the Aurora-line.

The question as between hydrocarbon and oxygen I did not then consider as
disposed of. With the lumière tubes the question might be open, but I did
not see how it could be in the case of the electrolyzed oxygen-spectrum.

From a comparison of the tube-spectra, I have shown that although
the spectra of the carbon and oxygen tubes are proved to be,
photographically, as a whole, distinct, they have, as to position of
some of the principal lines in the central part of the spectrum, a very
close resemblance.

[Sidenote: Probability that O may play a part in the Aurora-spectrum.]

That oxygen may in some form play a part in the Aurora seems highly
probable; how far it is spectroscopically detected seems a different
question.

[Sidenote: Difference between air-spark and tube-spectra.]

Ångström and Herschel suggest its presence in the Aurora in connexion
with phosphorescence or fluorescence. With a spark-discharge in air at
ordinary pressure, a mixed spectrum of bright lines of N and O is found;
while in the case of Geissler vacuum-tubes (representing a glow-discharge
in a much more rarefied atmosphere) the N lines appear mainly to usurp
the spectrum.

[Sidenote: H₂O tube referred to.]

It must, however, be borne in mind that a Geissler tube, as to
temperature at least, in no way represents the conditions of the Aurora;
and when we remember the association of oxygen and ozone, and the way in
which the latter is affected by heat, it may well be that temperature
plays an important part in the matter. In proof of this conduct of
oxygen, it may be cited that, in the case of a H₂O tube, the H lines
come out sharp and brilliant in the spectrum, while what is seen of the
O lines is comparatively weak, misty, and ill-defined. Vogel, it will be
remembered, makes 5189 of the Aurora coincident with an O line.

[Sidenote: Residual phosphorescence in Geissler tubes. Garland tube.]

Professor Herschel has pointed out, and I have confirmed, that the
residual phosphorescence in Geissler tubes, after the spark has passed,
is probably associated with oxygen. He also alludes to the fact that when
one of the globes of a “Garland” tube was heated, it did not shine after
the spark had passed, apparently because of the destruction of the ozone
by heat.

[Some experiments with a tube of this description will be found detailed
in Part III. Oxygen was not, I think, the gas it was filled with.]

[Sidenote: Dr. Schuster’s tubes described.]

Subsequently to my examination and comparison of the O and CO₂ spectra
before detailed, Dr. Arthur Schuster was good enough to send me three
vacuum-tubes of his own preparation, showing an oxygen-spectrum.

One, with large disk-shaped brass electrodes, was unfortunately broken in
transit. Dr. Schuster informed me it showed the carbonic-oxide spectrum
as well as that of oxygen. The other two tubes had aluminium electrodes.
They were similar in shape to ordinary Geissler tubes, but had attached
to each a supplemental bulb containing dry oxide of manganese.
Illuminated by the larger coil, one of these tubes (which had a slight
crack in the manganese bulb) lighted up faintly; the other was fairly
bright, and the glow had a somewhat reddish tint.

Plate XVIII. fig. 15 represents as the upper spectrum Vogel’s Aurora,
with W.L. numbers, as the middle spectrum the capillary part of Dr.
Schuster’s O tube, and as the lower spectrum the negative (violet) pole
of the same tube.

[Sidenote: Spectra described.]

The tube-spectra were mapped out with the aid of the diaphragm micrometer
before described.

[Sidenote: Capillary.]

The capillary spectrum was mainly distinguished by four bright sharp
lines—one in the red, between the red Aurora-line and D, two in
the green, but considerably more refrangible than the yellow-green
Aurora-line, while the fourth was found to be hydrogen F. The other lines
in the spectrum were considerably fainter, and misty and band-like. The
red line, though not brilliant, was fairly bright and sharp.

The place of the less refrangible of the two bright bands in the
violet-pole spectrum was occupied in the capillary spectrum by a faint
glow only.

[Sidenote: Violet-pole.]

The violet-pole spectrum was recognized by two very bright broad
bands of light in the green, each including within its limits one of
the Aurora-lines. The bright red line in the capillary had a faint
representative in the violet-pole spectrum, as also had the two bright
lines in the green. Other fainter lines appeared in the blue, and three
fairly bright ones towards the violet.

[Sidenote: Dr. Schuster’s remarks on the spectra.]

Dr. Schuster remarks that one of these O bright bands is closely
coincident with a band in the CO spectrum, but that the CO band is bright
towards one edge and fades off gradually thence, while the O band is of
pretty uniform strength throughout. Dr. Schuster finds the wave-lengths
of the violet-pole O bands to be as follows:—

  5205·0}
  5292·5} Brightest part 5255.

  5552·8}
  5629·6} Brightest part 5586.

[Sidenote: His tubes free from impurity.]

He also gives as weak bands 5840-5900 and 5969-6010. Dr. Schuster comes
to the conclusion that the green line of the Aurora is not due to
oxygen, as, under considerable dispersion and with good definition, the
oxygen-bands can be broken up into a series of lines, when the brightest
part is found to lie at 5586, which is too much towards the red to
compare with the Aurora-line. He notices that the more refrangible of the
O bands corresponds with a line sometimes seen in the Aurora (Vogel’s
5233). The same remark will, however, apply to this last as to the other
coincidence, viz., that a broad band can hardly represent a line—at
least, the line can only be said to coincide in a loose and indefinite
way. It is evident that Dr. Schuster’s tubes were free from what must
now be considered an impurity in those examined by me and by Dr. Vogel,
and that Mr. Procter’s suspicions of carbon impurities in these, and the
ordinary oxygen-tubes, are thereby quite confirmed.

[Sidenote: Experiments with an open Geissler tube.]

In some experiments which we made (after receiving Dr. Schuster’s tubes)
with an open Geissler tube, so arranged as to connect with an air-pump
and gas-receiver, and thus from time to time to wash out the tube and
vary its contents, we found the same impure spectrum as in the case
of the sealed O tubes; and it seems to require a very large amount of
precaution to avoid these impurities.

[Sidenote: Spectra of Dr. Schuster’s O tube examined.]

Dr. Schuster was kind enough to examine the spectra I mapped out, and
which are shown in Plate XVIII. fig. 15, with the following results:—The
lines Oα, Oβ, Oγ are those he has referred to under that designation in
his communications to ‘Nature,’ and undoubtedly belong to oxygen. The
bands A, B, and C are the bands characteristic of the negative pole.
He finds A divided into two parts by a dark space. The spectrum of the
negative pole, under good exhaustion, stretches into the capillary part;
hence B appears in the capillary as a faint band. A similar thing happens
with nitrogen. I., II., III., and possibly 8 and 9, he thinks, are due
to the spark-spectrum of oxygen, obtained when the jar and a break are
interposed, the brighter lines of the line-spectrum being always present
at the negative pole. These last-mentioned lines I have already referred
to, as having been found by me in a tube showing phosphorescence after
the spark has passed. (Compare Plate XVIII. fig. 15, O violet pole, with
Plate XV. spectrum 5.) Nos. 1 and 2, he thinks, are due to some foreign
matter, as they are not in all his tubes.

Dr. Schuster often finds that a spectrum due to the aluminium electrodes
is seen in tubes under great exhaustion; and this he considers is the
spectrum of aluminium oxide. A drawing of this spectrum is found in
Watts’s ‘Index of Spectra,’ plate iii., “Aluminium first Spectrum.”
To this, he thinks, are also due the bands, or sets of lines in my
aluminium-arc spectrum (‘Photographed Spectra,’ plate ii.), and he
believes lines 3, 4, 5, 6, and 7 in the mapped-out spectra are due to it.
It would thus appear that the lines due to O are few in number, and do
not well compare with the Aurora-spectrum.




PART III.

MAGNETO-ELECTRIC EXPERIMENTS IN CONNEXION WITH THE AURORA.




INTRODUCTION.


[Sidenote: Object of experiments. Description of apparatus employed.
Electro-magnet. Battery.]

The set of experiments detailed in Chapters XIV. to XIX. was mainly
conducted for the purpose of testing, in connexion with the Aurora,
the action of a magnet upon the electric glow _in vacuo_ and on the
spark at ordinary pressure. It also includes some observations on the
glow from the violet pole with and without the magnet, and on the
glow obtained from one wire only. The apparatus employed was a Ladd’s
electro-magnet, with poles 10¼ inches high by 2 inches across, each
pole being surrounded by a movable helix, composed of two sets of stout
copper wire wound together, so that they could be used either in one
length or as independent coils excited at the same time. The latter form
of arrangement was employed by us. In most of the experiments conical
armatures were employed for the purpose of bringing the action of the
poles to bear upon the subjects examined. A contact-maker was added to
the magnet, so that it could be put rapidly in or out of action without
disturbing the wires. The battery used to excite the magnet was of the
form known as that of Dr. Huggins, and consisted of four vulcanite
cells in a frame, each holding seven pints of bichromate solution, and
containing two carbon and one zinc plate, each 13½ by 6 inches.

[Sidenote: Small coils. Larger coil. Magnetic curves obtained.]

A winch and pulley enabled the whole set of plates to be lowered into
the liquid and withdrawn at pleasure, and the large quantity of solution
gave the battery a considerable amount of constancy. We found it could be
used for two evenings’ work, of four hours each, without any material
dropping in power. For obtaining the glow in the Geissler and other small
tubes, a Ruhmkorff coil, giving a ½-inch spark, excited by one plate of a
½-gallon bichromate (bottle form), was used. For the glow in the larger
tubes and the spark in air a larger coil, giving a two-to three-inch
spark, and worked by two ½-gallon double-plate bichromates, was employed.
Notes were taken of the experiments, and drawings of the effects at the
time; and these are reproduced almost literally in the text and Plates
comprised in this Part. To ascertain the direction and extent of the
magnetic curves, we covered large sheets of cardboard, placed over the
poles, with iron filings; excited the magnet so as to obtain the curves,
and then obtained permanent prints from the filings by spraying the
cardboard with tannin solution. The magnetic effects were thus found to
extend to a radius of at least ten inches (see diagram, Plate XVII. fig.
1, showing magnet-poles and curves on a ¼ scale).

[Sidenote: Chautard’s investigations kept in view. Evidence obtained of
change in colour, form, &c. of Aurora. Ångström’s flask-experiment tried.]

In the vacuum-tube experiments we held Mons. J. Chautard’s investigations
(on the action of magnets on rarefied gases in capillary tubes rendered
luminous by the induced current, Phil. Mag. 4th series, vol. 1. p. 77)
in view. We obtained in our experiments plenty of evidence of a change
of colour and form in the discharge under the magnetic influence;
and both simple and compound spectra were found to be much varied by
the exaltation or suppression of some parts of the spectrum, so that
apparently new lines sprang up; but we failed to trace actual change of
position or wave-length in any given line, though we carefully looked for
it. A portion of our researches was directed to the subject of Ångström’s
experiment of filling a dry flask with a violet glow, analogous to
that from the negative pole. We entirely failed in obtaining the same
result while two wires and an uninterrupted circuit were employed. When,
however, we attached a negative wire only (the other wire being left
free) to an exhausted globular receiver, we obtained an effect very
similar to that referred to in Prof. Ångström’s memoir.

[Sidenote: General results of experiments. Bulb effects noticed as a mode
of analysis of gases.]

The general result of the experiments was to prove, assuming the Aurora
to be an electric discharge, the great influence the magnetic forces may
exercise on the colours, form, motions, and probably the spectrum also
of that phenomenon. It is easy to conceive that the variation in number,
and intensity of the lines which has been remarked in Auroral spectra
may have its origin in such a cause. The influence of the magnet on the
capillary stream was mainly in colour and intensity; but in the bulbs the
effects were still more marked and striking, and, in a greater or less
degree, different in the case of each gas which we examined. A careful
and extended study of these effects, conjointly with the changes in the
spectrum, might possibly form a new and valuable mode of analysis of
compound gases. This is well illustrated in the case of the iodine and
sulphur tubes which we examined.

[Illustration: Plate XVII.]




CHAPTER XIV.

EXAMINATION OF GEISSLER TUBES UNDER ACTION OF THE MAGNET.


_Nitrogen-tubes._

[Sidenote: Nitrogen-tube No. 1. Discharge described. Spectrum described.
Capillary stream. Positive bulb. Violet-pole glow.]

(1) A small Geissler tube (No. 1) was lighted up by the small coil.
The capillary part showed a very bright, slightly rosy-tinted stream.
Negative bulb was filled with rosy-purple light, the violet-pole glow
being confined to the extent of the electrode. Positive bulb of the
same rosy-purple colour, but stream slightly contracted in volume. Glow
throughout quiescent, and no stratification in the tube. A compound-prism
spectroscope, taking in the whole of the spectrum, showed in the
capillary stream, from yellow to red, a fairly bright wedge, having a
dark band in the centre, and six bright columns, with dark lines at
intervals, shading off on either side. On the more refrangible side of
the yellow, the spectrum was composed of a set of bright bands and lines
in the green, blue, and purple, one line only (in the green) standing
out very bright. In the yellow and red no bright line stood out alone.
The positive bulb gave a fainter spectrum of the same character, mainly
confined to the centre, the violet, yellow, and red not being well seen.
When the violet-pole glow was examined, the general character of the
spectrum was quite changed: a brilliant broad band in the violet, a
bright narrower one in the blue, and two bright lines in the green, with
intermediate fainter lines throughout, were the main features. The yellow
and red part of the spectrum was also changed. The yellow was fairly
and evenly distinct up to the dark band; then came a somewhat brighter
orange band, and after that the red, but rather obscure and cut off. No
absolutely bright line could be traced in the red.

[Sidenote: Nitrogen-tube No. 2. Glow described. Difference of spectra of
capillary stream and violet-pole glow. Junction of the violet-pole glow
and capillary stream.]

(2) To compare the capillary stream and the violet-glow, a second
nitrogen-tube (No. 2) was used. This tube was larger in bulk and bore
than No. 1. The glow in the bulbs was considerably fainter and more
salmon-coloured; and there was much stratification in both, extending
to the capillary bore. (This stratification was considered due to H,
as the three principal lines of that gas came out very brightly in the
spectrum.) The difference of the spectra of the capillary stream and of
the violet-pole glow was extremely well marked—the former consisting of
a set of bright lines and bands of fairly uniform intensity, while the
latter was split up into a few bright bands with fainter lines between.
The yellow and red of the violet-glow were very weak as compared with
the same region of the capillary spectrum. No bright line appeared in
the red. The tube being properly adjusted for the purpose, the junction
of the violet-pole glow and the capillary red-glow was easily observed.
The bright bands of the violet-pole were seen to run into the capillary
line-spectrum, and then, gradually getting finer and more pointed, to
fade out.

[Sidenote: Tube No. 1 between the poles of the magnet. Change of colour
in capillary stream. “Tailing-over” of capillary stream.]

(3) The capillary part of tube No. 1 was arranged between the poles of
Ladd’s electro-magnet, the conical ends of the armatures almost touching
the tube (Plate XVII. fig. 1). With the magnet not excited, the capillary
stream was bright and of a slightly rosy-yellow tinge. It varied a little
in apparent diameter with the current. As soon as the magnet was excited
the capillary stream, as also (in a less degree) that in the bulbs,
were seen to contract, and to change from a _rosy_ tint to a distinctly
_blue-violet_. The polished armatures, acting as reflectors, showed this
change of tint in a most marked manner each time the magnet was excited.
At the same time the capillary stream was seen to run into the negative
bulb, as if overflowing, and with an effect resembling the “tailing-over”
of a gas-flame. This effect took place each time the magnet was excited,
and was not found at the positive-bulb end.

[Sidenote: Spectrum examined.]

Occasionally, when the magnet was excited, flashes of light were
discharged in the negative bulb from the capillary towards the
violet-pole. The spectrum was then carefully examined. No change was
seen in the actual position of any of the lines or bands when the tube
was influenced by the magnet, but those towards the violet end of the
spectrum were conspicuously brightened.

[Sidenote: Negative bulb between poles of the magnet. Positive bulb
within action of the magnet.]

(4) The extremity of the negative bulb was now placed between the poles
of the magnet. A bright violet-coloured arc, following the magnetic
curve, was at once formed, as in the case of the large Plücker tubes; and
at the same time a straight stream of not very bright light ran along the
bulb. The positive bulb was next placed within the action of the magnet;
and immediately a brilliant spiral of flickering light appeared in the
bulb, lighting it up, and reminding one in shape of the spiral which
water forms on being poured from a lipped jug (see Plate XVII. fig. 9).

[Sidenote: Spiral formed.]

This was repeated each time the magnet was excited. The spiral, though
flickering in character, was permanent in form, and inclined to the side
of the tube which was in contact with the N pole of the magnet.


_Oxygen-tubes._

[Sidenote: O tube No. 1; spectrum described.]

A tube (No. 1) was lighted up and examined with the spectroscope, and
found to give the spectrum shown on Plate XIV. spectrum 3, but with a
strong set of H lines in addition.

[Sidenote: O tube No. 2; spectrum described.]

A second tube (No. 2) was then lighted up. The spectrum was a bright one,
similar to the foregoing, the principal H lines being present, but not
strong.

[Sidenote: Tube-glow described. Effect upon glow when magnet excited.
Bulbs between poles of the magnet. Effect of magnet on spectrum.]

The red region was indistinct, and showed no prominently bright line.
The bulbs were mainly of a slightly blue-grey tint, with a steady glow.
Capillary stream quite pale white, with a very slight tinge of red.
Violet-glow small and confined to the electrode. Upon the magnet being
excited, the capillary stream became intensely brighter, and the glow
in both bulbs contracted into a single bright stream, which curved
towards the sides of the bulbs at right angles to the magnetic poles, and
changed from side to side with the current. This effect was very marked,
and was more apparent in the positive than the negative pole. A faint
stratification was seen in both bulbs. Upon either bulb being placed
between the armatures, the glow left the electrode point and condensed
into one bright stream, running along the side of the tube and curving at
each end (Plate XVII. fig. 10). No trace whatever of tendency to form a
spiral was seen. The spectrum with the magnet on was very conspicuously
brightened up throughout. A set of fluted bands with a bright line
among them appeared in the red, and several lines or bands appeared in
the violet which could not be seen before. The bright red line, upon
measurement, proved to be the hydrogen-line C. It thus seemed brighter
in proportion than the F line, although, with the magnet off, the latter
was well seen, while the C was not. No actual change in position of the
spectrum-lines could be detected.

[It is to be noticed that the O tubes employed were those used by me
in former experiments, and had the bright lines now attributed to
hydrocarbon impurity. Their bulb-effects differed, however, entirely from
those of the CO₂ tube. (Compare figs. 10 and 11, Plate XVII.)]


_Hydrogen-tubes._

[Sidenote: H tube, No. 1; glow described.]

A small H Geissler tube (No. 1) was selected, and lighted up by the small
coil. The capillary was a bright white-pink stream, with a tendency to
redden at times. The bulbs were both of a faint blue-grey tint, with
coarse lenticular stratification. The violet-pole glow was pale and
white as compared with that of N.

[Sidenote: When the magnet was excited, whole character of tube changed.
Unexcited spectrum described. Effect when magnet was excited.]

When the magnet was excited, the whole character of the tube changed.
The capillary stream diminished in brightness and in apparent volume,
and changed to a deep amber-yellow. The bulbs lost some of their light,
and their coarse stratification; being, in lieu, filled with a vertical
condensed stream of moderate light, in which a fine stratification only
was seen. The stream in the positive bulb had a tendency to the spiral
form. The capillary, each time the magnet was excited, “tailed over”
into the negative bulb, as in the case of N, looking as if it were
squeezed out of the capillary bore. The unexcited spectrum was found to
consist of the usual principal lines of H on a continuous glow, with the
intermediate bands and finer lines, which are usually suspected to be due
to impurity. The sodium-line was also seen. When the magnet was excited,
the spectrum grew much fainter—the continuous glow in the red and blue,
and the red and blue lines, nearly disappearing, and the line in the
green alone shining out conspicuously. No change of place in the lines
could be noticed.

[Sidenote: No. 2 H tube; effects described.]

A longer H tube (No. 2) was then tried, with similar effects, except that
the diminution in brightness was not so conspicuous. When the negative
bulb of the tube No. 1 was placed between the poles of the magnet, a
stream of light was formed, and the stratification became finer. The same
effect took place with the positive bulb, with a tendency to the spiral
form.


_Water-Gas (H₂O) tube._

[Sidenote: Water-gas (H₂O) tube; effects produced described.]

A faint purple glow was seen in each bulb, the tube not lighting-up
brightly. The capillary showed a slightly rosy-tinted, grey stream of
brighter light. With the magnet on, the glow in the bulbs was condensed
into a single bright stream. The capillary brightened up, and assumed a
yellow tint—this effect being principally confined to that portion which
was between the conical ends of the armatures, and gradually diminishing
as the distance increased from these. Without the magnet, the principal
H lines showed brightly in the spectrum, the O lines being misty and
indistinct. With the magnet on, the O lines and spectrum generally
brightened up.


_Ammonia-tube._

[Sidenote: Ammonia-tube; lighting-up described. Spectrum described.]

This tube was difficult to light up. Hardly any light was seen in
the bulbs, except a very faint purple glow at the electrodes. In the
capillary part a fairly bright stream of purple-white light appeared. The
spectrum was a faintly shown one of N and H. The effect of the magnet
was to reduce the brightness of the glow in the capillary, but with
little marked action on the bulbs, except to condense the faint glow into
a slightly bright stream running along the side of the tube.

On a subsequent examination the tube and spectrum both brightened up
under the influence of the magnet. The N lines, which were faint without
the magnet, shone out under its influence distinctly—the red and yellow
parts of the spectrum specially showing this effect. The H lines also
brightened up, but hardly so much in proportion as the N.


_Carbonic-Acid tube._

[Sidenote: Tube marked C A; lighting-up described.]

A Geissler tube marked C A was examined. Capillary stream a brilliant
bluish white; bulbs grey-blue, with a slight tint of green; slight
stratification in positive bulb; stream diffuse, not quite filling
the bulbs, and changing in volume as the coil-break was touched; glow
round the violet-pole considerable, but markedly white in tint, rather
than violet; stratification strong in capillary. With magnet excited,
the capillary stream diminished in volume, but greatly increased in
brightness. It “tailed over” into the negative bulb, and the stream
through both bulbs curved towards the sides. A slight pattering noise
was heard in the tube. In the positive bulb bright, imperfectly formed,
saddle-shaped rings of light, with a tendency to spiral formation, were
seen, somewhat similar to the effects in the Plücker tube after described
(see Plate XVII. fig. 11).

[Sidenote: Effects when magnet was excited.]

The whole spectrum, under influence of the magnet, became much brightened
up. Faint bands in the red came out bright, as also did some in the
violet. The violet-glow was examined (without the magnet), and the light
was found condensed into four prominent shaded bands, one red, one
yellow-green, one green, and one blue, with fainter bands seen between.


_Chlorine-tubes._

[Sidenote: Chlorine-tube No. 1 lighted-up. Action of magnet upon the tube
and spectrum.]

A chlorine-tube (No. 1) was lighted-up with the small-coil. Capillary
stream of a pale green tint. Bulbs with very little glow in them;
spectrum pale, and not very distinct. Under action of the magnet this
tube brightened up throughout, and the glow became more condensed, and
ran to the sides of the tube. The spectrum also brightened, the faint
lines becoming stronger, but the general character was preserved.

[Sidenote: Chlorine-tube No. 2 lighted-up. Effect on glow when magnet was
put on.]

A second chlorine-tube (No. 2) was then tried. Both bulbs were
completely filled with a dense white (very slightly rosy-tinted) opaque
light, and capillary the same, but brighter. A very slight violet tinge
was seen at the negative pole. When the magnet was put on, both bulbs
were at once filled with flickering bright streams of light, running
towards the side of the tube, according to the direction of the current.

The capillary stream at the same time changed from white to an intense
bright green. The spectrum without the magnet consisted of sets of
lines, with two well-marked absorption-spaces between, all seen somewhat
faintly, as if through a mist.

[Sidenote: Changes in spectrum when magnet was excited.]

When the magnet was put on, the marked character of the absorption spaces
was lost. The sets of lines in the yellow-green and green started up
intensely bright, while those in the blue only slightly brightened.

The misty appearance was altogether lost, and the bright lines all shone
up upon a perfectly dark background, with a strikingly metallic look; we
could not, however, trace change of position or actually new lines. It
seemed as if lines which had been faint in the yellow-green and green
region suddenly increased in intensity, the other parts of the spectrum
not being similarly influenced. They quite flashed up when sudden contact
was made with the magnet commutator.


_Iodine-tubes._

[Sidenote: Iodine-tube No. 1.]

This tube (No. 1) had been used for photographic purposes, and the bulbs
were partly obscured by a white deposit.

[Sidenote: Lighting-up described. Effect of touching one wire with the
finger.]

On lighting it up, both bulbs were filled with a violet-grey diffused
light, with much coarse well-marked lenticular stratification. This
stratification was mainly lost on changing the direction of the current,
but made its reappearance when one conducting-wire was touched with a
finger. This effect was still more marked when one finger of each hand
was applied to the wire. The capillary stream was of a pale lemon-yellow.
On putting on the magnet the light in the whole tube was nearly
extinguished, a faint thin stream of condensed light running through the
centre of the tube alone remaining.

[Sidenote: Effect of the magnet.]

On placing the bulbs between the magnet-poles, effects were produced
similar to those in the case of the tube, after described (p. 144, and
marked Si Fl₆), but in a less marked degree.

[Sidenote: Tube again tested.]

The iodine-tube was subsequently again tested, and it lighted-up better
than on the last occasion, showing nearly the same effects in bulbs and
capillary, the former having somewhat of a rosy tint and the latter an
amber.

[Sidenote: Magnet effects. The spectrum described. Change when magnet was
excited.]

On exciting the magnet, the capillary part of the tube changed from
amber to a decided light green. The spectrum, without the magnet,
gave one very bright line, and several less bright ones near, in
the blue-green. The rest of the spectrum, with the exception of the
absorption-spaces, was misty and continuous, with lines showing faintly
through. The red and yellow portions of the spectrum were quite bright.
When the magnet was excited, the spectrum entirely changed. The red
and yellow portions of the spectrum, and the misty continuous light,
all quite disappeared; while a set of sharp lines on the yellow-green
and green flashed up bright and clear, and stood out alone upon a dark
background, in which the absorption-spaces were lost. The effect was very
strongly marked, and gave a totally different character to the appearance
of the spectrum. The change seemed to arise from the suppression of one
part of the spectrum, and the increase in intensity of the lines in the
other part.

[Sidenote: No change in line-position.]

The principal lines could not be traced to change in actual position.

This tube differing somewhat from a second one we examined (No. 2) in
tint of glow and spectrum, it suggested itself to us that there might be
a partial mixture of N or H (or both) with the iodine vapour, giving rise
to some of the brighter parts of the spectrum which were extinguished
under the action of the magnet.

[Sidenote: Comparison of iodine-tubes No. 1 and No. 2. Comparison of the
spectra.]

We therefore compared these two tubes, viz. the old one (No. 1) and the
new one (No. 2), and also their spectra, by means of a comparison-prism
on the slit of the spectroscope. To the eye, the tubes differed much
in appearance. No. 1 had a distinct transparent rosy tint throughout,
with considerable coarse flickering stratification; and this contrasted
strongly with the dense whitish light of tube No. 2, which showed neither
movement nor stratification. The spectra were also found different in
general look. That of tube No. 1 was strongly tinged in the red and
yellow, and showed a bright continuous spectrum, crossed by many sharp
lines, with little trace of absorption-spaces. The spectrum of No. 2
was much whiter in tint, showed very little of the red and yellow, and
the absorption-spaces were very dark. A few bright lines, mainly in the
yellow-green and green, were faintly seen.

[Sidenote: The two tubes examined in detail. No. 2.]

The two tubes were then examined separately in detail. No. 2, excited
by the magnet, showed curious effects. The glow was rendered weak and
intermittent, and the rosy tint almost disappeared. The capillary changed
to a decided green colour, and the positive electrode was surrounded
by a yellow glow. The changes in the spectrum were no less decided.
Without the magnet, the spectrum was found to be a bright continuous one
of H (with a full set of principal and intermediate lines) and N—the N
spectrum being rather faint and misty, with very slight, if any, traces
of the iodine-spectrum. On the magnet being excited, the spectrum changed
as if by magic; the H and N spectra disappeared (except hydrogen F, which
still faintly remained), and the iodine lines, mostly in the yellow-green
and green, shone out wonderfully sharp and bright on quite a dark ground.
No. 1, upon examination, showed between the magnet-poles only the same
changes as on last occasion. The spectrum seemed to be one of iodine,
with the addition of slight traces of the H spectrum.

[Sidenote: Effects discussed.]

On excitation of the magnet, the misty continuous part of the spectrum
nearly disappeared, and the bright lines shone up sharply upon the dark
background as before. The effects in the case of both tubes were strongly
marked. The impression as to tube No. 2 was that, without the magnet,
the slight iodine-spectrum was overpowered and masked by the N and H
spectra; while under the influence of the magnet the N and H spectra
were almost altogether suppressed, the iodine-spectrum being at the same
time intensified. The disappearance of the continuous spectrum under the
action of the magnet in No. 1 (with the supposition it was mainly H)
would be accounted for in the same way.


_Bromine-tubes._

[Sidenote: Bromine-tube No. 1. Lighting-up described. Effect of the
magnet. Bromine-tube No. 2. Effect of magnet.]

This tube (No. 1) had been previously worked for photographic purposes.
Excited by the small coil, the whole tube was filled with a faint
flickering light. The positive bulb contained a faint purple glow, with
a yellow-green tinge at the electrode, a curious flickering stream of
light flashing from the electrode to the side of the tube. The negative
pole showed pretty much the same effect as the positive. The capillary
stream expanded at the opening into the positive bulb, but ran in a
condensed stream into the negative bulb. In colour it was of a rather
bright lilac. Upon putting the magnet on, the light-glow in the tube was
at once and permanently extinguished, the coil still working as if the
current passed. The same effect happened repeatedly; but now and then the
tube lighted-up for a second, showing spiral arrangement in the bulb.
We tried another bromine-tube (No. 2): it lighted-up easily; both bulbs
were filled with a purple stream of light; capillary stream bright grey.
The glass of the tube was strongly fluorescent and of a yellow tinge.
When the magnet was excited the stream of light was somewhat condensed in
the bulbs, and flew to the side of the tube; while the capillary stream
at the same time brightened. The spectrum without the magnet was fairly
bright; it increased in brightness under the influence of the magnet,
and additional lines appeared; but we considered them to be only faint
existing ones brightened up. No change in the position of the principal
lines was traced.


_Silicic-Fluoride tubes._

[Sidenote: Si Fl₆ tube. Lighting-up described. Effect of magnet.]

(1) A tube marked Si Fl₆ had been worked for photographic purposes; it
lighted-up easily. Both bulbs were filled with a brown-pink diffused
light, inclined to condense into a stream in the positive bulb. The
violet glow was very bright, and nearly filled the space round the
electrode. The capillary stream was of a bright violet tint. The effect
of the magnet was to decrease the intensity of the light throughout the
whole tube.

In the positive bulb the stream broke up into a number of vibrating
streamlets, with little bright threads of light intermixed, which flew
towards the side of the tube at right angles to the magnetic poles. There
was an inclination to spiral arrangement in the streamlets. This stream
changed from side to side of the tube coincidently with change in the
magnetic poles. At the negative pole the violet glow formed an arc in the
direction of the magnetic curves, while a spiral of fainter (positive?)
light was formed in the upper part of the bulb. A slight ringing sound
was heard in the tube.

[Sidenote: Comparison of Si F₄ and Si Fl₆ tubes.]

(2) We compared two tubes (Si F₄ and the one marked Si Fl₆). The Si Fl₆
tube in general effect, and in its spectrum, when lighted-up, resembled
Si F₄. We compared the one tube under the influence of the magnet with
the other not so, by means of a comparison-prism on the slit. As the
spectroscope and second tube were necessarily removed some distance from
the magnet, the spectrum of the tube between the poles was not bright.
We could not trace a change of position in any of the principal lines.
The tube between the poles was brightened up when the magnet was in
action[15].


_Sulphuric-Acid (SO₃) tubes._

[Sidenote: SO₃ tube No. 1; lighting-up described. Effect of the magnet.
Changes in the spectrum.]

(1) Excited by the small coil, both bulbs of this tube (No. 1) lighted-up
brightly, with a misty light-blue tinted stream of opaque light, a
yellow glow appearing at the negative pole. The capillary stream
partook of the same blue tint, but was whiter and brighter. Under the
magnet’s influence, the glow in the bulbs flew to the side of the tube
in flickering streams of light, the capillary at the same time changing
to a distinctly green tint. The spectrum without the magnet consisted
of four fairly bright bands of light in the yellow, green, blue, and
violet, connected by a faint misty continuous spectrum (O or possibly the
hydrocarbon spectrum found in O tubes by way of impurity).

When the magnet was excited, this spectrum entirely disappeared;
and a set of bright metallic-looking lines upon a dark background
(line-spectrum of S) took its place. This effect was produced whenever
the magnet was excited, and we tried it several times, to make sure of
the complete change. After a time, when the magnet, battery, and the
coil-power were all weaker, with the magnet on, we obtained a compound of
both spectra, the bright lines being seen upon the continuous spectrum in
which the bands appeared. When the magnet was taken off, the bright lines
disappeared, and the O spectrum alone remained.

[Sidenote: SO₃ tube No. 2 examined.]

(2) We also tried another SO₃ tube (No. 2) which had been worked for
photographic purposes, and was suspected of a carbon impurity. Without
the magnet, the spectrum was very like that of the first tube; but when
the magnet was excited, the spectrum only brightened, and no bright
metallic-looking lines appeared.


_Sulphur-tube._

[Sidenote: Sulphur-tube. Lighting-up (without heating) described.]

(1) A small bent vacuum-tube containing some solid sulphur, excited by
the smaller coil, and without being heated, gave a narrow stream of
bright blue-green light running straightly through it. With the magnet
on, this stream was deflected in the bulbs, and the capillary changed
from a blue-green to a distinct rosy tint.

[Sidenote: Effect of magnet. Changes in the spectrum.]

Without the magnet, the spectrum consisted of four bright bands, with a
continuous spectrum between, resembling that of SO₃ tube No. 1. With the
magnet on, the spectrum brightened, especially in the yellow and red,
which were dull before; and a set of lines appeared upon it (a line or
band in the yellow especially showing) which were not seen before. The
lines were distinct, but not very bright. The action on the capillary was
noticed to be strongest just between the conical points of the armatures;
and, in accordance with this, the central part of the spectrum-band in
the red and yellow showed an increased brightness.

[Sidenote: Effects when one of the bulbs of the tube was heated. Changes
in the spectrum under influence of magnet.]

(2) One of the bulbs of the tube was then gradually heated with a
small gas-flame. The single stream in the heated bulb became somewhat
deflected and broken up into a number of smaller streams; and these,
when placed under the magnetic influence, had small spark-like threads
of light running among them. The capillary, as the tube was heated,
and the sulphur rose in it, changed somewhat in tint, and, under the
magnetic influence, became of yellow-rose hue. As the heat was applied
to the bulb the bands of sulphur gradually appeared in the field of
the spectroscope, until at last the band-spectrum of sulphur entirely
took the place of the spectrum seen in the cool tube. The magnet being
excited, the spectrum changed at once, a set of bright sharp lines
(line-spectrum of S) appearing upon a faint and dull image of the
band-spectrum.

This effect was constantly repeated upon the magnet being excited. The
magnet being taken off, the band-spectrum alone was to be seen.




CHAPTER XV.

EFFECT OF MAGNET ON A CAPILLARY GLASS TUBE.


[Sidenote: Capillary portion of a Geissler tube tested in three ways.]

The capillary portion of a Geissler tube was cut away from the bulbs,
cleaned, and connected by a small vulcanite tube with the gas-pipe in
the room conveying coal-gas at ordinary pressure. The flame was small
and oval in shape, 8 millims. high, by 4 millims. wide, and burnt quite
steadily. (Plate XVII. fig. 13.)

[Sidenote: No effect on flame.]

(1) The capillary tube was placed between the poles of the excited
magnet, almost, but not quite, touching them; no effect at all was
produced on the flame.

(2) The tube was placed so that the conical ends of the armatures were
allowed to compress the centre of it between them; still no effect was
produced on the flame.

(3) The tube was placed so that the straight sides of the armatures
compressed it between them; still no effect took place on the flame.

[Sidenote: Flame between poles of magnet.]

(4) The flame itself was placed between the poles of the magnet. It was
slightly drawn towards one pole with an inclination to form the magnetic
curve.

[Sidenote: Quill glass tubing tested. No effect on the flame.]

(5) A piece of quill glass tubing was selected, 5 millims. in diameter
and 1 millim. thick, and drawn out to a point, the end of which was
snapped off and the tubing connected as before. The flame was 20 millims.
high, and 5 millims. across, and somewhat lambent. On being placed (1)
between the conical ends and (2) between the flat ends of the armatures,
no effect could be seen on the flame.

[Sidenote: Effect on taper and spirit-lamp flames.]

(6) A small taper-flame was placed between the poles of the magnet: no
effect was produced, except that the flame gave a slight “jump” each time
the magnet was excited. A spirit-lamp flame was tried with a similar
result.


_Action of Magnet on a bar of heavy glass._

[Sidenote: Heavy glass bar and mounting described.]

A piece of heavy yellow-tinted glass was selected, being a bar 10
centimetres in length, and 8 millimetres square. This was mounted in a
frame with a Nicol prism at one end, and a double-image prism (next the
eye) at the other.

[Sidenote: Placed along poles of magnet. Effect of magnet on
candle-images.]

(1) The glass bar and mounting were placed upon and along the poles
of the magnet (in the direction of the magnetic curves), and the
double-image prism and Nicol were so adjusted that two images of a candle
were seen—the one below bright and normal, the one above, by rotation of
the prism, as nearly as possible extinguished (Plate XVII. fig. 4). On
exciting the magnet the faint image at once conspicuously brightened,
at the same time assuming a slightly green tinge. To get full effect of
brightening, it seemed necessary to have good pressure-contact between
the battery-wires and the binding-screws.

[Sidenote: Effect on using a tourmaline as analyzer.]

(2) Using a tourmaline as analyzer in lieu of the double-image prism,
the candle-flame was seen alternately brightened and darkened, as the
tourmaline was rotated; and when the image was obscured by rotation,
excitation of the magnet caused it to brighten strongly. This effect was
accompanied by the apparent removal of a dusky red patch or spot, which
occupied the centre of the field when the flame was obscured.

[Sidenote: Bar placed at right angles to the poles: no effect produced.]

(3) The bar of glass and double-image prism being placed between the
conical ends of the armatures, but at right angles to, instead of along,
the poles, upon excitation of the magnet no effect at all was produced.

(4) The bar and prism being placed in the same position between the flat
ends of the armatures, no effect at all was produced.

[Sidenote: Slight effect on second experiment.]

(4_a_) Experiment No. 4 was repeated. It was thought that on excitation
of the magnet the secondary image slightly brightened; but there was a
doubt about it, and the effect (if any) was slight.

[Sidenote: Effects produced when a biquartz was introduced.]

(5) The apparatus was now changed for one of the following
arrangement:—1, a rotating Nicol prism next the eye; 2, the glass bar;
3, a biquartz with the halves horizontal; 4, another Nicol prism. The
neutral-passage tint of the biquartz was found to be rather green (from
mixture with the yellow of the glass).

[Sidenote: Change in colour of the halves.]

(i.) Placed _along_ the poles of the magnet and the magnet excited, a
change of tint was seen in both halves of the biquartz, the slightly
purple-reddish tint of the upper half passing into a full purple. Effect
not so marked as with the double-image prism.

(ii.) Placed _across_ flat ends of the armatures (as in experiment No. 4)
no effect was seen.




CHAPTER XVI.

EFFECT OF MAGNET ON WIDE AIR (AURORA) TUBE.


[Sidenote: Wide air-tube described.]

A large, wide air-tube was tried; it was 14½ inches long by 1 inch
in diameter, of the same bore throughout, and with straight platinum
electrodes.

[Sidenote: Magnet effect when tube placed vertically between conical
armatures.]

(1) To excite it the larger coil was used. The tube was filled with
bright, steady, rosy light, and beautiful stratification, which, as
it flickered, seemed to incline to a continuous spiral (Plate X. fig.
8). This stratification was very close and fine, and extended nearly
throughout the tube. On excitation of the magnet (the tube having
been placed _vertically_ between the conical armatures), the glow was
condensed into a bright solid line or stream of light at the point
which lay directly between the poles. This line or stream expanded into
an elongated funnel-shape as it retreated from this centre towards
the extremities of the tube, the stratification showing itself more
distinctly as the glow of light became less dense (Plate XVIII. fig. 3).
The stream of light was driven away at right angles to the poles, and
changed from side to side of the tube with the direction of the current.

[With the small coil this tube showed only a flickering stream of light,
with very slight indications of stratification.]

[Sidenote: Effect when tube placed horizontally between the armatures.]

(2) The tube was placed _horizontally_ between the conical ends of
the armatures. The condensed stratified stream of light flew upwards
and downwards (according to direction of current) instead of to the
respective sides of the tube.

[Sidenote: Tube placed along the poles of the magnet.]

(3) The tube was placed along the poles of the magnet. In the interval
between these the stream was driven upwards, but at either end sideways,
right or left according to whether the pole was N. or S. (Plate XVIII.
fig. 4). The result gave a complete spiral of stratified condensed light
within the tube.


_Note on Stratification._

[Sidenote: Stratification in small tubes arranged in series.]

The current from the large coil was sent through a set of five small
French vacuum-tubes, of equal calibre, containing salts of strontium and
calcium, and showing phosphorescent effects. These tubes were arranged in
single series; and, from the colour of the glow-discharge, were presumed
to contain rarefied air in contact with the salts.

A strong coarse stratification was seen in the central (No. 3) tube.
Tubes Nos. 2 and 4 also showed stratification, but in a less degree;
while the outside tubes, Nos. 1 and 5, showed no stratification at all.
The current was steady, and these effects did not fluctuate.


_Effect of Magnet on Plücker (Air-) Tube._

[Sidenote: Plücker air-tube. Lighting-up described. Effect of magnet on
the positive-pole stream.]

(1) A Plücker air-tube was selected of the form shown on Plate V. fig.
1, and was excited by the small coil. The ring was used for the positive
pole, the straight electrode for the negative. When lighted up, the
tube glowed with a perfectly steady and quiescent light. The negative
electrode was surrounded by the usual bright violet glow, extending
itself and being gradually lost at a short distance from the wire, while
the ring let fall a faint, tubular, salmon-coloured, diffused stream of
light, which met the violet glow as it approached the negative pole. The
tube was then placed vertically between the poles of the electro-magnet,
the armatures being almost in contact with the sides of the tube around
the negative pole. On excitation of the magnet, an instantaneous change
took place. The stream of light from the positive pole contracted itself,
so that it became of a long funnel-shape (the ring forming the mouth of
the funnel), while it tapered almost to a point where it met the violet
glow.

[Sidenote: Effect on negative violet glow.]

The stream also became very brilliant (the sides of the tube being left
proportionately free from light), and crossing it were a set of bands,
or striæ, having a waving or vibratory motion. The whole of the negative
violet glow was simultaneously gathered into a brilliant narrow arc,
which stretched across between the poles of the magnet. These effects are
shown on Plate V. fig. 1. The edges of the arc were remarkably sharp and
well defined, and with no surrounding aura or shading off.

[Sidenote: Arc of light followed the magnetic curves.]

By moving the tube between the armatures it was seen that the arc of
light followed the magnetic curves. If the tube was moved upwards, the
arc curved towards the zenith, if downwards, contrariwise; and a middle
position could be selected, in which the edges of the arc were nearly
parallel. Moving the tube a short distance from the pole had the effect
of rendering the arc more diffuse, but not of otherwise altering its
character.

[Sidenote: Direction of the current changed. Effects on glow described.]

(2) The direction of the current in the tube was then changed; and,
without the magnet, the ring electrode was surrounded by a diffused
violet glow; while the straight wire gave forth a faint salmon-coloured
stream of light, spreading up to the ring.

[Sidenote: Magnet effects described. On negative pole. Rings from
positive pole described. Effects on rings of making and breaking contact
with magnet. Shape of rings described.]

On excitation by the magnet (the positive pole being now placed between
the armatures), the violet glow of the negative pole contracted into a
compact mass round the ring electrode. At the same time from the positive
pole sprang a set of bright saddle-shaped rings, which increased in size
as they advanced; and spreading upwards with a rapid but smooth motion
towards the negative pole, closely approached to, but never actually
came in contact with, the violet glow. The positive end of the tube
was otherwise but slightly lighted, and the sudden appearance of this
brilliant stream of rings of light was very striking. A single bright ray
was also seen running from the positive wire, in a somewhat transverse
course, along one side of the tube. When wire-contact with the magnet
ceased, so that it was not excited, the rings ran back in succession to
the positive pole and disappeared, and by making and breaking contact
they were caused to advance and retire at will. They were accompanied by
a waving or vibratory motion, and were evidently of the same character
as the smaller striæ or bands mentioned as seen when the ring formed
the positive pole. The general appearance was that of a hollow cone of
light (the base towards the negative pole), composed of brilliant rings
with dark spaces between, which appeared and expanded under the magnetic
influence, and contracted and disappeared on its removal. The rings did
not appear to be flat disks, but were somewhat curved or saddle-shaped.
They reminded one much of the diatom _Campylodiscus spiralis_; that is
to say, they were apparently flat if looked at from above, but like a
figure of 8 when viewed sideways, the peak of the saddle forming a kind
of brilliant point or apex.

All this is difficult to describe; but an illustration from a sketch made
of the tube is given on Plate XVII. fig. 2.

[Sidenote: Negative pole placed vertically on the magnet.]

(3) The negative pole (straight electrode) was then placed vertically on
one of the poles of the electro-magnet. On excitation, the violet glow
was contracted into a small upright brush or column of bright light, with
a slight inclination to curvature.

[Sidenote: Tube laid horizontally across poles of magnet.]

(4) The same Plücker tube was laid horizontally across the poles of the
electro-magnet (without armatures), the respective electrodes being above
each pole.

[Sidenote: Effects produced.]

From the negative (straight electrode) pole sprang a dense and compact
arc of violet light, in the direction of the magnetic curves, which
terminated at the upper circumference of the tube, but which, if
prolonged, would have followed the curves to the opposite pole. The
stream from the positive pole was very considerably brightened, as in the
other experiments, but did not appear in the form of rings or waves. It
assumed that of a bright steady continuous glow, which formed round the
tube a not perfectly continuous, but distinct and well-marked, spiral.
This form of discharge seems connected with the peculiar contour of
the rings mentioned in experiment 2. One might, indeed, conjecture the
spiral-shaped glow to be a ring of light extended or drawn out towards
the negative pole.

[Sidenote: Effects like those obtained by Gassiot.]

Experiment No. 2 seems in result very like that of Gassiot’s with his
grand battery and the Royal Institution magnet, the effects (though of
course upon a smaller scale) being similar to those obtained by him.


_Effect of Magnet on Plücker Tube (Tin Chloride)._

[Sidenote: Plücker tube (tin chloride). Lighting-up described.]

A large Plücker tube was examined, which had a bulb attached at each end,
communicating with the central portion by a narrow neck or constriction.
On connexion with the small coil, a narrow stream of pale diffused
cobalt-blue light ran along the whole tube, from point to point of the
electrodes, the positive wire at the same time glowing with an aura of
amber-yellow light. (See Plate XVII. fig. 3, where the narrow stream of
light is shown by dotted lines.) At the two necks or constrictions the
stream of light was perceptibly brightened.

[Sidenote: Effects of magnet upon the stream.]

When the magnet was connected, the stream in the positive bulb was not
much changed, but only slightly bent. In the central partition of the
tube and in the negative bulb, the stream of light was broken and split
into a number of smaller streams, and at the same time bent or forced
against the sides of the tube. (See Plate XVII. fig. 3.)

[Sidenote: Peculiar noise within the tube.]

In the central partition, the blue streamlets were accompanied by a
number of spark-like threads of golden light, which shone out among them
as the whole vibrated against the side of the tube; at the same time a
peculiar pattering, as of a miniature hail-storm within the tube, made it
ring with a slightly metallic tinkle.

The direction of the bending or deflection of the stream was at right
angles to the axis of the poles of the magnet, and changed from side to
side of the tube as the direction of the current from the coil was varied.

[Sidenote: Spectrum described. Without magnet. With the magnet excited.]

In the positive bulb the stream, instead of joining the point of the
electrode, left this and ran along one side of the whole length of the
wire. (See effect, Plate XVII. fig. 3.) The spectroscope was applied to
the neck of one of the bulbs where the stream was bright. Without the
magnet a faint continuous spectrum, mainly of the blue and green, with
very slight traces of the yellow and red, was seen. Upon this, five or
six faint but sharp and metallic-looking lines were seen. On the magnet
being excited, the continuous spectrum was not changed; but the sharp
lines shone out brighter and clearer, one in the blue being especially
conspicuous. These lines were measured with a micrometer; and their
places being compared with Lecoq de Boisbaudran’s “Spectres lumineux,”
they were easily recognized to be those of tin. On each excitation of the
magnet the same brightening of the lines took place.


_Effect of Magnet on Tin-Chloride Geissler Tube._

[Sidenote: Geissler tube, Sn Cl₄, examined. Glow described. Effect of the
magnet.]

We then examined a Geissler tube, marked Sn Cl₄. When first excited
by the small coil, the spark passed freely. The glow in the bulbs was
of a diffused, light purple tint; the positive electrode had a bright
yellow glow around it. The capillary stream was of a sharp green-yellow,
at times brightening up to a metallic-looking green. When the magnet
was first employed, the tube distinctly and permanently brightened up
throughout.

[Sidenote: Spiral formed in positive bulb. Glow in tube extinguished.]

The negative bulb was not much changed in appearance; but in the positive
bulb a curious permanent and steady cloud-like spiral, of a purple
colour, made its appearance, and lasted while the tube was under the
magnetic influence. (See Plate XVII. fig. 12.) After a short time the
tube seemed to lose a great deal of its conducting-power, and to light
up in a feeble and intermittent manner, brightening only when the coil
was made to work its best. While in this condition, the magnet (which had
been previously disconnected) was excited, and at once what moderate glow
was still shining in the tube was totally extinguished. At first it was
thought some accident might have happened to the conducting-coil wires;
but repeated trials satisfied us that the effect was due to the magnetic
influence alone. Efforts were made, by looking to the coil and battery,
to brighten up the tube as at first, but they quite failed; and it was
evident some change had taken place in its conducting qualities. This
tube was accidentally broken, so that we had no opportunity to renew the
experiments.

[Sidenote: Another tin-chloride tube tried. Glow described.]

We subsequently tried another tin-chloride tube, purchased of Mr.
Browning. This lighted up like the former tube, but brighter. There was
an amber glow at the junction of the negative bulb which adjoined the
capillary part. This was lost on putting on the magnet. At the same
time a perceptible pattering ringing noise was heard in the tube, and
metallic-looking threads of light ran through the bulbs.

[Sidenote: Spectrum described.]

Without the magnet, the spectrum was a continuous faint misty one,
with bright lines of tin occasionally flashing up. With the magnet,
the tin lines at once shone out bright, strong, and clear upon a black
background, the change in effect being very marked.




CHAPTER XVII.

EFFECT OF MAGNET ON BULBED PHOSPHORESCENT TUBE.


[Sidenote: Large phosphorescent bulbed tube. Lighting-up described.
Spectrum described. Glow when discharge stopped, described.]

Mr. John Browning kindly lent me a large phosphorescent tube with five
bulbs, said to be filled with anhydrous sulphurous-acid gas (SO₂). (See
Plate XVIII. fig. 1.) This tube lighted up beautifully with the large
coil. The connecting tubular parts of it were filled with a bright,
beaded, transparent, rosy light; while the bulbs glowed with a more
opaque blue-tinted effect. The spectrum of the tubular part was found to
agree exactly with the principal bright band seen in a SO₃ Geissler tube.
The spectrum of the bulb-glow was a faint green-blue continuous one, with
bright bands or lines faintly flashing up at times. When the discharge
was stopped, the tube still glowed with a moderately bright, opaque,
grey-green light. This glow gradually faded out, always commencing with
the bulb forming the negative or violet pole, and so dying out, bulb by
bulb, towards the positive pole. The negative-pole bulb at times was, on
suddenly stopping the current, hardly lighted at all, the other bulbs
being luminous.

[Sidenote: Comparison with SO₃ Geissler tube.]

(1) We compared the large tube with a SO₃ Geissler tube, by means of
a comparison-prism on the slit, with the result before detailed. The
Geissler tube, however, showed no after-glow.

[Sidenote: Effects in bulbs on lighting-up the tube described. Effects of
reversal of the current. After-glow restored by passing of current.]

(2) We lighted up the Browning tube with the large coil. The negative
bulb was always the least filled with the blue opaque vapour, and the
other bulbs increased in vapour-density in the order they approached
towards the positive bulb. When the current was reversed, so that the
negative and positive glow changed places, the negative bulb still
remained transparent, although the positive opaque glow had (presumably)
been thrown into it. When the after-glow had quite disappeared in the
bulbs, it was again strongly restored, by the passing of the current for
a few seconds only through the tube.

[Sidenote: Effect of reversal of current on positive-pole glow.]

(3) The tube was well excited, and the four bulbs (other than the
negative one), upon stopping the current, glowed strongly. The current
was then sent through reversed, so as to throw the negative glow for a
few seconds into the positive bulb. The after-glow in the positive bulb
was at once extinguished. On once more reversing the current, it was only
restored after a certain amount of continuance of the positive stream.

[Illustration: Plate XVIII.]

[Sidenote: Effect of change of current on the three central bulbs.]

The time during which the negative glow was thrown into the positive bulb
did not appear sufficient to have heated it. After rapidly changing the
direction of the current several times and then stopping it, the three
central bulbs alone had an after-glow, the two extreme ones having none,
being both equally transparent.

[Sidenote: Effect of heat on the bulbs. Effect of cooling by ether-spray.]

(4) A moderate heat from a spirit-lamp was applied to the centre bulb
(_a_) while the current was on; and also (_b_) when this was stopped,
and the bulb glowed. In the first case the bulb was found to get more
transparent; and in the second case the after-glow disappeared in a
proportionately shorter time in the heated bulb than in the others. To
test the result of cooling the bulbs, the negative-pole bulb and also the
central one were each subjected to the action of ether-spray, and also
of ether and water-spray mixed. This was done, (_a_) when the current
was passing, and (_b_) when it was stopped and the glow only was in the
bulb. The bulbs were cooled until a marked cold effect to the touch was
produced. We did not notice any difference in the behaviour of the bulbs
so treated as compared with the others, either when the current was
passing or in the case of the after-glow.

[Sidenote: Negative-pole bulb between the armatures of magnet. Effects on
negative and positive glow.]

(5) We placed the negative-pole bulb between the conical points of the
armatures, and excited the magnet. The negative glow contracted itself
into a condensed violet-tinted crescent, in accord with the magnetic
curves. The positive glow of the same bulb lost its beaded (stratified)
character, and was condensed into a bright stream of light, which latter
protruded from the small inner tube and formed a spreading spiral set of
cloud-rings within the bulb (see Plate XVIII. fig. 2). The action of the
magnet seemed to be exercised in subduing the stratification, condensing
the glow into a bright stream of light, and forcing the latter to “tail
over” at each extremity of the tubular joints into the bulbs—this effect
extending even so far as the second bulb.

When the positive bulb was placed between the poles of the magnet, the
glow was simply condensed into a bright stratified stream, which flew to
either side of the bulb.

[Sidenote: Effect of magnet on glow in bulb No. 4.]

(6) _a._ Bulb No. 4 (see Plate XVIII. fig. 1) was placed between the
poles of the excited magnet, and the current was passed and then stopped.
The glow in that bulb faded away out of its order, and earlier than in
ordinary cases (nearly as soon as No. 2).

[Sidenote: Other bulbs tested in similar manner.]

_b._ The same and other bulbs were tested in a similar manner. In all
cases the bulb influenced by the magnet, when the current was stopped,
was found perceptibly fainter in after-glow.

[Sidenote: Effect of magnet upon the after-glow itself.]

_c._ The tube was arranged with one of the bulbs between the poles
of the unexcited magnet; the current was passed and stopped, and
the after-glow obtained. The magnet being then quickly excited, the
after-glow in the bulb, under its influence, faded out; and the bulb
became transparent, perceptibly sooner than under ordinary circumstances.
We tried this several times, with the same result in each case.

[Sidenote: Mr. Thompson’s experiments on action of magnets upon liquid
rings.]

_Note._—In relation to these experiments, it may be mentioned that Mr.
S. P. Thompson, of Bristol, is reported to have studied the action of
magnetism upon rings of coloured liquid projected through water, and
to have observed their retardation and partial destruction in passing
through a powerful magnetic field.

[Sidenote: Mr. Ladd’s explanation of some of the phenomena observed.]

Mr. Ladd has suggested to me that some of the phenomena produced
indicate a driving of the gas in the direction from the negative to the
positive pole—a theory which is supported by the action of the magnet
on the bulbs, if this be considered a repulsive one as regards the gas
influenced.


_Effect of Magnet on small Phosphorescent (powder) Tubes._

[Sidenote: Tubes containing phosphorescent powders described.]

We examined six vacuum-tubes containing phosphorescent powders, which,
upon exposure to sunlight and removal to the dark, or after passing of
the electric current over them, continued to glow in the tubes after the
exciting cause had ceased. They were of thin glass, and of equal calibre
throughout.

One was 6½ inches long and ⅝ inch in diameter, and had no label; the
other five were 7½ inches long and ½ inch in diameter, and were labelled
respectively:—

  Strontium vert,
    ”       jaune,
  Calcium   violet,
    ”       orange,
    ”       vert-bleuâtre.

[Sidenote: Lighting-up of the tubes described. Effect of magnet on
⅝-diameter tube. Spectrum without magnet.]

The powders in tubes of this description are said to contain either
sulphide of strontium, or calcium, or sulphate of quinine. The
first-mentioned tube shone with a white and bright light, and probably
contained the latter substance. The general effect of the current on the
tubes was similar in all cases. Under a sufficiently strong current,
they lighted up with a brilliant, slightly green-white glow; in which,
however, by looking sideways, it was possible to detect a delicate rosy
tint. Any colours beyond these in the tubes seemed to depend on the
powders enclosed in them. When the current was stopped, the powders
alone glowed in accordance with the colours mentioned on the labels, the
rarefied gas or air in the tubes not giving any after-glow, as in the
case of the sulphurous-acid tube. When the ⅝-diameter tube was excited
by the small coil, the effect of the magnet was to entirely suppress and
extinguish the glow. When this and the other tubes were worked with the
larger coil, the spectrum, without the magnet, was bright and continuous,
either showing no lines or else very faint traces of them, and, extending
through the whole range of colours was brightest in and about the green.

[Sidenote: Magnet effect on glow. Same on spectrum.]

With the magnet excited, a bright line of pink light was condensed
against the upper side of the tube; while the glow in the tube generally
became very decidedly fainter, except at the electrodes, which still
preserved a certain amount of brilliancy. The spectrum also was much
changed. The bright continuous glow became much fainter, and many sharp
and fairly bright lines were seen upon it. These lines were, as to
character, not easy to recognize. Hydrogen (F) was, however, plainly
distinguished; and other lines, which we considered to be N, were common
to all the tubes. Some lines were also remarked as being, without the
magnet, not so constant.

[Sidenote: Tubes examined and compared for spectra.]

Calcium orange and calcium violet, compared for spectra, were identical;
the two strontium tubes hardly so, but with strontium vert a bright
continuous spectrum mainly hid the lines.

Strontium jaune and calcium orange were not alike; strontium vert and
calcium violet differed. Calcium orange and calcium vert-bleuâtre were
considered alike; but the comparison was not easy, as the calcium vert
was bright, and the lines were only seen faintly upon the continuous
spectrum.

In order not to shift the powders, the tubes were laid horizontally, and
two spectra simultaneously examined across the tubes.


_Lighting-up Tubes with One Wire only (Marquis of Salisbury’s
Observations)._

[Sidenote: One wire only connected with an electrode.]

The vacuum-tubes employed were examined in the usual way, but one wire
only was connected with an electrode. The other wire was attached to the
end of a glass rod, and circuit was from time to time completed while the
tube was before the spectroscope.

The large coil was used. In all cases, with the one wire, the glow was
very faint as compared with that of the closed circuit.

[Sidenote: Ether vapour.]

(1) _Ether Vapour._—With both wires, in company with the usual bright
bands of the carbon spectrum, shading-off towards the violet, the H lines
were very sharp and brilliant. With the one wire only, the carbon bands
were left faintly shining, with both sides nebulous alike, and with no
shading-off towards the violet. (We were not quite sure whether this
was not the effect of the reduction of the light.) The H lines, though
originally stronger than the carbon bands, quite disappeared from the
spectrum.

[Sidenote: Coal-gas.]

(2) _Coal-gas._—The same effects were produced; but we thought we could
detect very faint traces of the H lines.

[Sidenote: Nitrogen.]

(3) _Nitrogen._—The N lines, as well as those of H (also seen in the
tube), were much fainter with one wire, but the H lines more so in
proportion.

[Sidenote: Hydrogen.]

(4) _Hydrogen._—Only a marked reduction in brilliancy of the whole
spectrum.

[Sidenote: Oxygen, N and H.]

(5) _Oxygen._—An impure tube, showing O (some of the lines hydrocarbon?),
N, and H spectra simultaneously. With one wire the O lines still remained
fairly bright, the N and H being only faintly seen.

[Sidenote: Water-gas.]

(6) _Water-gas._—Same effect.

[Sidenote: Turpentine vapour.]

(7) _Turpentine Vapour._—Same effect as ether, but the H lines could be
faintly seen.




CHAPTER XVIII.

ACTION OF THE MAGNET ON THE ELECTRIC SPARK.


[Sidenote: Apparatus employed.]

The magnet was excited with two plates of the large battery, and the
larger coil with the other two plates, the action in both cases being
strong.

1. A spark from the coil was passed between two platinum wire electrodes,
about three centimetres apart.

[Sidenote: Spark and aura described.]

It consisted centrally of a thin stream of bluish-white light, vividly
bright, around which was seen a narrow, uniform, diffuse, yellow-tinted
aura, which accompanied the spark in all its movements. The spark always
struck across from the extreme points of the electrodes (see Plate XVII.
fig. 5).

[Sidenote: Effect of magnet upon the aura.]

2. On being placed between the conical poles of the excited magnet
the bright thread of the spark did not change; but instead of the
inconsiderable yellow-tinted aura which accompanied the unmagnetized
spark, there now struck out, at right angles to the magnet-poles, a
thin rosy-tinted half-disk of aura-like flame. This extended aura ran
considerably along each electrode, though the spark proper still struck
from the points.

[Sidenote: Extended aura described.]

The aura was somewhat larger in extent upon one electrode than on the
other. In the first case, it sprang from a considerable number of minute
illuminated points; on the other electrode, these illuminated points
were fewer in number, and the flame was more purple in tint. Reversing
the current these effects were reversed. The aura was uniformly thin and
disk-like, and the curved edge remarkably true in shape (see Plate XVII.
fig. 6).

The lateral direction of the aura was changed when the current was
reversed.

[Sidenote: Aura not proportionate to length of spark.]

3. The aura was found not proportionate to the length of the spark. When
the electrodes were approached, so as to very much shorten the spark, the
aura still sprang out to a distance and extent quite out of proportion
to the length of the spark. Even when the electrodes were approached so
close that the spark was very short indeed, still, under the magnetic
influence, a very considerable aura made its appearance.

[Sidenote: Effect of working coil-break upon the aura.]

4. Upon working the coil-break, it was found that in proportion as the
contact screw was drawn apart from the break, so the aura gradually
diminished in extent, until at last, by continuing to increase the
distance between the screw and the break, a point was reached when thin
bright sparks, without any aura, passed. Upon the screw being worked up
closer, thicker sparks passed, and the aura again made its appearance. As
the aura diminished in size it gradually changed in tint from yellowish
rose-pink to purple.

[Sidenote: Spark taken in glass bulb.]

5. The spark was taken in a glass bulb, the tube in which it was blown
being open at both ends, with the same effect as in the open air.

6. A plate of glass was laid on the poles of the magnet, and the spark
was passed _along_ the poles (in the same direction as the heavy glass
was laid in the Faraday experiment). No aura was formed. The points were
then moved round, so as to carry the spark at right angles to the poles,
and the aura was formed as before.

[Sidenote: Aura could be blown away from the spark.]

7. The aura, it was found, could be blown away at right angles to the
spark. When strongly urged, it assumed the shape of a flickering tongued
curtain of flame, flying away in the contrary direction to that from
which the current of air proceeded, and again returning to its original
shape as the impulse was removed. The spark proper was not influenced
(see Plate XVII. fig. 8).

[Sidenote: Effect of withdrawing spark from central position between the
poles.]

8. As the spark was withdrawn from its central position between the poles
of the magnet, the convex edge of the aura became gradually less perfect,
and assumed a ragged and broken-up appearance, the inequality at times
amounting almost to jets or flickering sprays of light. The spark was
also slightly curved away from the electrodes (see Plate XVII. fig. 7).

[Sidenote: Magnet had no effect upon condensed spark.]

9. A condenser of four coated plates was introduced into the circuit,
causing a sharp brilliant blue-white spark, apparently divided into
streams and with no aura. The magnet had no effect whatever upon this
form of spark.




CHAPTER XIX.

THE DISCHARGE IN VACUO IN LARGER VESSELS, AND MAGNETIC EFFECTS THEREON.


A Tate’s air-pump was used, and the spark from the larger coil. The
exhaustion could not be carried very far.

[Sidenote: Globular receiver described. Discharge described.]

(1) A globular receiver was used, having brass caps for exhaustion,
and platinum wires passing through the opposite sides for electrodes
(see Plate XVIII. fig. 6). With partial exhaustion, from the positive
electrode proceeded long, sharp, bright, rosy sparks, striking in zigzags
across the receiver. From the negative terminal sprang a larger number of
bluer and more diffuse streams of light, like spiders’ webs; and these
were enveloped, for a short distance from the terminal, in a slight misty
aura. Both sets played round the sides of the glass as well as across.

[Sidenote: Bell-shaped receiver described. Discharge described.]

(2) A bell-shaped receiver, with terminals inserted at the sides and one
also at the top, was next used (see Plate XVIII. fig. 9). When the side
terminals were employed, the effect was much the same as in the last
case. When the top terminal was used for one wire (the other wire being
connected with the pump-plate) a single stream of bright rosy light ran
from the upper terminal to the plate. First striking the central part of
the plate, the stream then glided towards one of the lateral terminals,
and so to the edge of the receiver. After partly discharging itself by
contact with the terminal, the stream as rapidly retreated to the centre
of the plate again—this effect being from time to time repeated while the
current was passing. The current being reversed, a number of bright, but
weaker and more diffused, streams of light had the appearance of shooting
from the upper electrode, and of striking upon the plate below; with a
tendency to fly off from where they struck, in a similar manner to the
single stream before described. Where each stream touched the plate a
brilliant point of light appeared, and a strong pattering noise was heard
in the receiver.

[Sidenote: Bell-shaped receiver without electrodes. Induction discharge
described.]

(3) Another bell-shaped receiver of similar shape was used. This had no
electrodes forming a direct communication with the interior; but, in
lieu of these, two wafers of thin sheet brass were cemented, one inside
and one outside the glass, opposite to one another. On connexion being
made with the outside wafer, the effects produced by induction were
similar to, and very nearly as strong as, those in the cases where direct
communication with the interior of the receiver was made.

[Sidenote: Long large tube exhausted and illuminated. Spiral form of
discharge.]

(4) A large tube, 24 inches long and 2 inches in diameter, with ball and
point electrodes respectively, was exhausted, and the current passed
through it. The effects were similar in most respects to those produced
in the globular and bell receivers, but the streams of light assumed a
distinctly spiral form in their passage (see Plate XVIII. fig. 5). This
tube when placed between the poles of the magnet showed no effect, except
a slight condensation of the streams of light towards the sides of the
glass.

[Sidenote: Globular receiver again used.]

(5) The globular receiver first described was again used (the Tate pump
having been cleaned and working easier).

[Sidenote: Phosphorescent after-glow succeeding the spark.]

(_a_) When exhaustion was as good as it could be got, the spark struck
across in a single, slightly expanded, stream of rosy light, having a
tendency to curve upwards (see Plate XVIII. fig. 6). The electrodes had
but little glow round them, only just enough to distinguish the poles
apart. When the flow of the stream was interrupted by breaking contact
with one terminal, so that sparks passed in succession, we thought we
detected a faint blue phosphorescent after-glow succeeding each spark.

[Sidenote: Positive wire only attached.]

(_b_) The positive wire only was attached to one electrode, the negative
being unconnected. A set of faint whity-blue cobweb-looking streams of
light spread from the electrode all over the receiver, having a vibratory
motion. The spaces between these were dark, and there was no aura—the
effect being similar, but not quite so bright and pronounced, as when
both wires were attached (see Plate XVIII. fig. 7).

[Sidenote: Negative wire only attached.]

(_c_) The negative wire only was attached. The cobweb streams were
absent, or only shot out very occasionally. The main effect was a
straight nebulous stream of violet light, which commenced at the
electrode and spread out in a fan-shape towards the lower brass cap of
the receiver; while, at the same time, an aura or glow of similar light,
but fainter in quality, spread from the electrode over at least one half
of the receiver. This aura would no doubt have filled a small flask (see
Plate XVIII. fig. 8).

[Sidenote: Effect of gradual exhaustion on the discharge.]

(_d_) When exhaustion was first commenced, both electrodes of the
receiver (both wires being connected) threw out spider-web-like streams,
as in Experiment 1, pale blue from the one pole and somewhat rosy from
the other.

As the exhaustion progressed the pale-blue streams disappeared, while the
rosy flickering ones diminished in quantity and extent until ultimately
a single rosy stream of light crossed the receiver as in Experiment
5_a_. Upon admitting the air, these effects took place in an inverse
order—the single stream being gradually broken up, and the spider-webs
taking its place.

[Sidenote: Globular receiver placed on poles of the magnet. Magnet
effect.]

(_e_) The exhausted globular receiver was placed upon the poles of the
excited magnet, with the stream at right angles to them. Looking across
the S. pole of the magnet, the negative electrode was on the left hand,
and the positive on the right. The effect of the magnet on the stream
was apparently to split it up into several; but this appearance must
have been due to vibration only, as a revolving mirror showed the stream
as single. When the current was reversed, the stream which, without the
magnet, was somewhat flickering and vibrating, slightly straightened at
the positive pole, and the whole stream became steadier.

[Sidenote: Single wires attached.]

(_f_) Single wires were successively attached to the negative and
positive poles, and the cobweb streamers and glow before described
obtained. The magnet was found to have no decided effect on either of
these.

[Sidenote: Plücker tube placed between poles of magnet with negative wire
only attached.]

(6) The Plücker air-tube (Plate XVII. fig. 2) was placed between the
poles of the magnet, and the negative wire only was connected with the
straight electrode. A pale violet glow was seen round this electrode, and
another, but rather fainter, glow of a similar description at the ring
electrode, the intermediate space being filled with a salmon-coloured
light. This violet glow was condensed into an arc by the action of the
magnet. Reversing the current, the violet glow still remained at each
electrode, and that between the poles of the magnet was still influenced
into an arc.

[Sidenote: Geissler tube substituted, with similar results.]

(7) An air Geissler tube was substituted for the Plücker tube, with very
much the same result. Whichever wire was attached, a violet glow appeared
at the connected electrode, and a fainter one of the same character at
the other; and the magnet influenced both. The connecting salmon-coloured
glow was faint.

[Sidenote: Globular receiver treated with phosphoric anhydride.]

(8_a_) The globular receiver had some phosphoric anhydride shaken into
it; and it was then exhausted. The cobweb streamers and violet glow each
appeared according to which wire was connected. There was no marked
difference between the receiver with the anhydride and without; except
that in the former case the streamers and glow were reduced in extent and
strength, and were comparatively faint.

[Sidenote: Discharge in water-vapour described.]

(_b_) The anhydride having been washed out, first with plain and
afterwards with distilled water, some drops of the latter were allowed
to remain in the receiver. On exhaustion a vapour-cloud was formed, and
the discharge passed (both terminals being connected with the coil)
through this. The rosy stream of light was formed as usual, but was
more flickering and unsteady. As the exhaustion was lessened, the rosy
stream disappeared, and the cobweb streams began to fill the receiver.
These were, however, not so bright and sharp as in a dry receiver, but
were faint and broad; while some diffused and nebulous streams of light,
running (slightly bent) from pole to pole, and from ¼ to ⅜ of an inch
broad, were intermixed with them. When one wire only was connected, the
glow and streamers from the electrode were very faint.

[Sidenote: Large bell-receiver and plate described. Receiver exhausted
and stream of light formed.]

(9) A large bell-shaped receiver, 11 × 8 inches, was next used. It was
open at the bottom, which was ground as usual; and had a small opening
at the top, also carrying a ground edge. A solid brass plate was
prepared, ground only round the edge (in order to take the receiver),
and in the centre of this brass plate were inserted two disks of soft
iron, corresponding in position and size with the poles of the Ladd
electro-magnet (see Plate XVIII. fig. 11). When this plate was placed on
the magnet, the poles and disks were in contact; and the disks became
N. and S. poles within the receiver. A small brass plate carrying a tap
and exhaust-tube, and a binding-screw for attaching an electrode within,
closed the receiver at top. The receiver and plate being placed on the
magnet-poles, the former was exhausted until the discharge became a rosy
slightly-diffused stream of light; with a small unilluminated space
between it and the negative pole, where the usual violet glow appeared
round the wire.

This stream of light was used for the experiments after detailed. In some
cases the conical armatures were placed within the receiver, in others
the disks alone were used as the magnetic poles.

[Sidenote: Effect on same when magnet excited.]

(_a_) With the apparatus arranged as shown on Plate XVIII. fig. 10, and
the magnet excited, a violet glow appeared round the end of the wire
which was negative. A small unilluminated space then intervening, the
stream ran in a curve between the wire and the armature, which latter was
positive. The stream was not steady and had a tendency to rotate; but as
this was better observed with the disks only, it is described further on.

[Sidenote: Experiments with the conical armature removed. Vibrating
stream.]

(_b_) The conical armature within the receiver was removed, and the
stream allowed to connect with the centre of the pump-plate. When the
magnet was excited, the stream was violently projected at right angles
to the poles, with a vibrating movement to either side according to the
direction of the current. When the wire was positive the movement was
towards the left, with a slight inclination towards the N. pole. When
the wire was negative the movement was to the right, but in a rather
strong curve towards the N. pole. The vibrating motion was very distinct,
and gave the appearance of six or seven streams running off at regular
intervals (see Plate XVIII. fig. 12).

[Sidenote: Rotating wire over S. pole.]

(_c_) The wire was next placed over the centre of the disk forming the
S. pole. With the wire negative and the pole positive, rotation of the
stream was decidedly, but not very strongly, from right to left from the
centre of the plate (as the hands of a watch). With the wire positive
and the pole negative, rotation was strongly left to right, with a
disposition to spiral twist in the stream (see Plate XVIII. fig. 13).

[Sidenote: Same over N. pole.]

(_d_) The wire was placed over the disk forming the N. pole. With the
wire negative and the pole positive, rotation of the stream was left to
right. With the wire positive and the pole negative, rotation was right
to left (see Plate XVIII. fig. 14).

[Sidenote: Stream thrown across the receiver above the magnet-poles.]

(_e_) The stream was thrown across the receiver from the lateral
binding-screws above, and at right angles to, the disks, and afterwards
in the opposite direction, _i. e._ along them. In neither case was there
any marked change when the magnet was excited.

(_f_) The conical armatures were placed with the pointed ends upon the
disks in the receiver, and the stream thrown above and along them. It
diverged—one part running straight across between the electrodes, whilst
another stream and some cobwebs ran from each electrode to its nearest
pole. The streams and cobwebs flickered a good deal. There was no marked
change when the magnet was excited.


_Some of Baron Reichenbach’s Magnetic Researches tested._

[Sidenote: Baron Reichenbach’s researches.]

In 1846 Dr. W. Gregory published an abstract of Baron Reichenbach’s
‘Researches on Magnetism and on certain allied subjects, including a
supposed new Imponderable.’

[Sidenote: Auroræ considered to be magnetic lights. Flames seen by
“sensitive” persons.]

From a paragraph in this work, it would seem that the Baron considered
his observations as tending to an explanation of the Aurora Borealis;
and, since it was generally admitted that these phenomena occur within
our atmosphere, that there appeared a great probability of Auroræ being
visible magnetic lights. The Baron, in the original work, fully describes
the Aurora Borealis; and concludes it must be similar in its nature
to the flames of light seen streaming from the magnet-poles by Mdlle.
Reichel and other sensitive patients of the Baron’s. It is unfortunate
that these flames were only seen by certain “sensitive” persons. The
drawings given of them, too, show no analogy to the magnetic curves.

[Sidenote: Magnet tested for such flames.]

Having the opportunity of a powerful magnet in that used during our
tube-experiments, we made an attempt to detect the Baron’s magnetic
flames, on or around the poles of our magnet, in a perfectly dark room.
Arrangements were made to silently connect and disconnect the battery
with the magnet, without the knowledge of any one except the operator.
The experiment proved a complete failure; no flames or discharges of
light of any kind were to be seen. The observers were five in number, two
gentlemen and three ladies, but not one of the party proved “sensitive.”

[Sidenote: Mr. Brooks’s experiments on action of the magnet on a
sensitive photographic plate.]

Some experiments made by Mr. W. Brooks, and detailed in a paper read by
him before the South London Photographic Society, seem to corroborate
(to a certain extent) the statements made by the Baron in regard to the
influence of the magnet on a sensitive photographic plate.

Remembering, however, how it has been demonstrated that light may be
“bottled up” as an actinic source for a considerable period of time, it
seems a question whether the images obtained were not due to some such
source rather than to any magnetic aura.


SUMMARY OF THE FOREGOING EXPERIMENTS AND THEIR RESULTS.

[Sidenote: Summary of the experiments.]

Chapter XIV. Action of magnet on glow and spectrum of Geissler gas
vacuum-tubes demonstrated.

Chapter XV. Action of magnet on glass capillary tube negatived. Faraday’s
experiment with heavy-glass bar repeated.

Chapter XVI. Action of magnet on glow in wide air-tube demonstrated. Note
on stratification. In Plücker tube, action of magnet on negative pole
(arc formed) and positive pole (Gassiot’s rings produced) demonstrated.
Effects of magnet upon glow and spectrum of tin-chloride vacuum-tubes
demonstrated.

Chapter XVII. Effect of magnet upon after-glow in a bulbed phosphorescent
tube demonstrated. Effect of magnet upon glow in small phosphorescent
(powder) tubes examined. Marquis of Salisbury’s experiments (lighting-up
with one wire only) tested, and confirmatory results arrived at.

Chapter XVIII. Action of magnet on aura of electric spark demonstrated.

Chapter XIX. Effects of magnet on discharges _in vacuo_ in larger vessels
demonstrated. Ångström’s flask experiment tested; same results not
obtained unless one wire only was connected. Experiments demonstrating
the action of a magnet on an electric stream, viz. vibration between, and
rotation round, poles. Baron Reichenbach’s magnetic flames tested without
result.




CHAPTER XX.

SOME CONCLUDING REMARKS.


It is usual, in concluding a work on a special subject, to sum up its
contents, and to examine the general results arrived at. This, however,
it is not easy to do in the present case. The contents of our volume
comprise a short history of the Aurora, its qualities and spectrum; and
a statement has been given of the several conclusions at which various
observers have arrived as to its character and causes. In the present
state of our knowledge of the subject, to add an opinion to these might
seem to savour of presumption; and the questions involved may perhaps
be better treated as still _sub judice_, and as requiring further and
fuller evidence before arriving at a verdict. The following observations
must therefore be taken rather as further notes and memoranda, than as
conclusions. Apart from the spectroscopic questions involved, the oldest
and most received theory of the Aurora—that of its being some form of
electric discharge in the more rarefied regions of the atmosphere,—seems
to hold its own: and if, as is probable, some form of phosphorescence is
involved in the discharge, M. Lecoq de Boisbaudran’s observations on the
brightening of the red line under the influence of cold, and the falling
of the yellow-green line within a band of phosphoretted hydrogen, come
into play; and a connexion, though slight and imperfect, may be in this
respect traced between the discharge and its spectrum. The experiments
detailed in Part II. seem to have an important bearing, as showing the
very marked effect of the magnet on the rarefied glow, as well as on the
spark in air at ordinary pressure. The well-defined arc formed by the
aura of the spark, the flickering jets which replace the even edge of
the arc when partially withdrawn from the magnetic influence, and the
streamers formed when the aura is blown away from the spark (Plate XVII.
figs. 6, 7, and 8), are certainly highly suggestive of frequent forms
of Auroral discharge; and, but for trial and failure, might lead one to
expect results from a comparison of the line air-spectrum with that of
the Aurora. The experiments with a wire attached to one electrode only,
show how the glow may be affected and varied in colour and character when
the discharge is interrupted and incomplete. Differences in electric
tension may also considerably vary the character of the discharge.

The influence of the magnet in exciting and brightening the glow and
spectrum of one gas, while it depresses and extinguishes the glow and
spectrum of another gas in the same tube, suggests an explanation of
the observed variation in intensity, and difference in number, of the
Aurora-lines. Intensity of lines depending on temperature, and this
again on resistance, and it appearing that resistance is influenced by
the magnetic action, the same effects of brightening or depressing of
the spectrum are probably produced in the Aurora, as in the vacuum-tubes
placed between the magnet-poles.

In the Marquis of Salisbury’s observations, paraffin-vapour gave C
and H lines when connected with both poles of the battery, but C
lines only when connected with one pole; and in that case the lines
were equally sharp on both sides. These observations (repeated in our
experiments) may afford an explanation why the hydrogen-lines are not
seen in the Aurora-spectrum; although there can be hardly any doubt
that the phenomenon usually takes place in air more or less moist.
Professor Ångström’s researches on the violet-pole glow are not entirely
corroborated by our experiments; and it seems doubtful whether his
results in the exhausted flask were not obtained from the negative pole
only. One great difficulty in the comparison of the Aurora-spectrum
with the violet pole of air-tubes and some other spectra (including
oxygen), arises from the presence in the latter of broad bands; and it
is difficult to understand how these bands can be aptly compared with
the definite, though faint, lines observed by Dr. Vogel and others in
the Aurora-spectrum. It must, too, be borne in mind that the conditions
under which we may consider the Aurora to obtain, are such as can be
only very imperfectly imitated in the laboratory. Auroræ also no doubt
differ in density and thickness of layer; and Kirchhoff’s observation
must be remembered:—“That if thickness of a film of vapour be increased,
the lines are increased in intensity, the bright lines more slowly than
the fainter; and it may happen that the spectrum appears to be totally
changed when the mass of the vapour is altered.” Were it possible to test
with the spectroscope a cloud or film of gaseous vapour corresponding
in some degree in density and thickness with an Auroral discharge, we
might perhaps get nearer the truth. Mr. Procter also remarks (as we
proved in our magnet experiments):—“That frequently very small traces
appropriate to themselves the whole of electrical discharges at low
pressures, and completely mask the spectra of any other gases present.”
The oxygen-spectrum, with its possible variation by the conversion of
that gas into the allotropic condition termed ozone, seemed at first
to afford a prospect of close relation to the Aurora-spectrum; which,
however, disappeared on closer examination. If nitrogen could be modified
in some such way as oxygen is converted into ozone, it might perhaps
afford another opportunity for investigation; but we have no evidence
at present of such a change. The spectrum of nitrogen is usually found
singularly distinct and persistent; and, except as varied from band to
line by intensity of the discharge, not liable to alteration[16].

Colours of lines are functions of wave-length, subject, however, to the
observation that in a weak spectrum the colours lose their intensity. The
red line in the Aurora has sometimes been found brighter than the green.
It has been suggested that the red and green may be independent spectra;
but the variations of tint observed in the capillary of hydrogen and
other tubes according to resistance of the current, demonstrate that the
varying colours of the Aurora may be connected with the lighting-up of
particular parts of the spectrum, and do not necessarily indicate that
different gases and spectra are excited.

Absorption may also play an important part in the nature of the
Aurora-spectrum (Zöllner’s theory that the lines are really spaces
between absorption bands). Most gases will give a continuous spectrum
under certain circumstances, even at a low pressure.

The question of cosmic dust is inviting, but the facts collated hardly
warrant at present its probable connexion with the Aurora.

If Auroræ were composed of incandescent glowing meteors, it would be
reasonable to expect to find in the spectrum the lines of iron, a metal
constituting so prominently the composition of meteorites. No connexion
between the iron and the Aurora-spectra is, however, proved; though it
may be suspected. The iron-spectrum, as remarked elsewhere, contains so
many lines that some may, as a mere accidental circumstance, closely
agree with the Aurora-lines.

The iron-lines are, it may be remarked, as a rule, sharper and finer than
the Auroral lines, though it is possible that these characteristics might
vary if the spectrum were obtained in a rarefied medium. Tubes with iron
terminals are said to evolve a compound gas of H and Fe. I have not had
an opportunity to verify this.

It may be added that the comparative faintness of the more refrangible
lines of the Aurora-spectrum suggests a feeble resistance to the exciting
current, and a low temperature inconsistent with a meteoric theory; and
this is not contradicted by the brightness of the red and green lines,
if these are due to a phosphorescent origin. Expansion of a line is
recognized to be dependent on pressure, and consequently the breadth
of the green or red lines might indicate the height of the Aurora;
while their brightness or otherwise might also give some idea as to its
density. No observations in this direction have, as far as I am aware,
been recorded.

As the general result of spectrum work on the Aurora up to the present
time, we seem to have quite failed in finding any spectrum which, as to
position, intensity, and general character of lines, well coincides with
that of the Aurora. Indeed, we may say we do not find any spectrum so
nearly allied to portions even of the Aurora-spectrum, as to lead us to
conclude that we have discovered the true nature of one spectrum of the
Aurora (supposing it to comprise, as some consider, two or more). The
whole subject may be characterized as still a scientific mystery—which,
however, we may hope some future observers, armed with spectroscopes
of large aperture and low dispersion, but with sufficient means of
measurement of line positions, and possibly aided by photography, may
help to solve. The singular absence of Auroræ has, for some time past,
given no opportunity in that direction. May some of my readers be more
fortunate in obtaining opportunities of viewing the glorious sky-fires,
and assist to unravel so interesting a paradox!




APPENDICES.




APPENDIX A.

REFERENCES TO SOME WORKS AND ESSAYS ON THE AURORA.

(Most of these are cited in the ‘Edinburgh Encyclopædia’ and the
‘Encyclopædia Britannica.’)


Musschenbroek, Instit. Phys. c. 41.

‘Trai. Phys. et Hist. de l’Aurore Boréale,’ par M. de Mairan. Paris, 1754.

Beccaria, ‘Dell’Elettricismo Artif. e Nat.’ p. 221.

Smith’s ‘Optics,’ p. 69.

D’Alembert’s ‘Opuscules Mathématiques,’ vol. vi. p. 334.

‘Philosophical Transactions’ as under:—

  Vol.         Pages

  1716         406
  1717         584, 586
  1719         1099, 1101, 1104, 1107
  1720         21
  1721         180, 186
  1723         300
  1724         175
  1726         128, 132, 150
  1727         245, 301
  1728         453
  1729         137
  1730         279
  1731         53-55
  1734         243, 291
  1736         241
  1740         368
  1741         744, 839, 840, 843
  1750         319, 345, 346, 499
  1751         39, 126
  1752         169
  1753         85
  1762         474, 479
  1764         326, 332
  1767         108
  1769         86, 307
  1770         532
  1774         128
  1781         228
  1790         32, 47, 101

‘Miscell. Berolinens.’ 1710, vol. i. p. 131.

‘Comment. Petrop.’ tom. i. p. 351, tom. iv. p. 121.

‘Acta Petrop.’ 1780, vol. iv. p. 1.

‘Mem. Acad. Paris,’ 1747, pp. 363, 423; 1731; 1751.

‘Mem. Acad. Berl.’ 1710, vol. i. p. 131; 1747, p. 117.

Schwed. ‘Abhandlungen,’ 1752, p. 169; 1753, p. 85; 1764, pp. 200, 251.

Bergman, ‘Opusc.’ vol. v. p. 272.

‘Americ. Trans.’ vol. i. p. 404.

‘Mém. de Mathémat. et Phys.’ tom. viii. p. 180.

Rozier, vol. xiii. p. 409; vol. xv. p. 128; vol. xxxiii. p. 153.

Franklin’s Works, vol. ii.

Weidler, ‘De Aurora Boreale.’ 4to.

Nocetus, ‘De Iride et Aurora Boreale, cum Notis Boscovisch.’ Rome, 1747.

Chiminello, ‘Mem. Soc. Ital.’ vol. vii. p. 153.

Gilbert’s ‘Journal,’ vol. xv. p. 206; and (particularly) Dr. T. Young’s
‘Nat. Phil.’ vol. i. pp. 687, 716, and vol. ii. p. 488.

Wiedeburg, ‘Ueber die Nordlichter.’ Jena, 1771.

Hüpsch, ‘Untersuchung des Nordlichts.’ Cologne, 1778.

Van Swinden, ‘Recueil de Mémoires.’ Hague, 1784.

Wilke, ‘Von den neuesten Erklärungen des Nordlichts,’ Schwed. Mus.
Wismar, 1783.

Dalton’s ‘Meteor. Observ.’ 1793, pp. 54, 153.

Loomis, ‘Sill. Journal,’ 2nd series, xxxii. p. 324; xxxiv. p. 34. The
same, 3rd series, v. p. 245; B. V. Marsh, 3rd series, xxxi. p. 311.

Oettingen and Vogel, Pogg. Ann. cxlvi. pp. 284, 569.

Galle and Sirks, _ibid._ cxlvi. p. 133; cxlix. p. 112.

Silbermann, ‘Comptes Rendus,’ lxviii. pp. 1049, 1120, 1140, 1164.

Prof. Fritz, “Geog. Distrib.,” Petermann’s Mitth., Oct. 1874.

Zehfuss, ‘Physikalische Theorie.’ Adelman, Frankfort.

‘Nature,’ iii. pp. 6, 7, 28, 104, 126, 346, 348, 510; iv. pp. 209, 213,
345, 497, 505; x. 211 (Ångström).

‘Edinburgh Astronomical Observations,’ vol. xiv. 1870-1877.

‘English Mechanic,’ No. 461 (January 23, 1874), pp. 445-447; and No. 462,
pp. 475, 476.




APPENDIX B.

EXTRACTS FROM THE MANUAL AND INSTRUCTIONS FOR THE (ENGLISH) ARCTIC
EXPEDITION, 1875.


_Note on Auroral Observations. By Prof. STOKES, Sec. R.S._

The frequency of the Aurora in Arctic regions affords peculiar facilities
for the study of the general features of the phenomenon, as in case the
observer thinks he has perceived any law he will probably soon, and
repeatedly, have opportunities of confronting it with observation. The
following points are worthy of attention:—

_Streamers._—It is well known that, at least as a rule, the streamers
are parallel to the dipping-needle, as is inferred from the observation
that they form arcs of great circles passing through the magnetic zenith.
It has been stated, however, that they have sometimes been seen curved.
Should any thing of this kind be noticed, the observer ought to note the
circumstances most carefully. He should notice particularly whether it
is one and the same streamer that is curved, or whether the curvature is
apparent only, and arises from the circumstance that a number of short,
straight streamers start from bases so arranged that the luminosity as a
whole presents the form of a curved band.

Have the streamers any lateral motion? and if so, is it from right
to left or left to right, or sometimes one and sometimes the other,
according to the quarter of the heavens in which the streamer is seen, or
other circumstances? Again, if there be lateral motion, is it that the
individual streamers move sideways, or that fresh streamers arise to one
side of the former, or partly the one and partly the other? Do streamers,
or does some portion of a system of streamers, appear to have any uniform
relation to clouds, as if they sprang from them? Can stars be seen
immediately under the base of streamers? Do streamers appear to have any
definite relation to mountains? Are they ever seen between the observer
and a mountain, so as to appear to be projected on it? This or any other
indication of a low origin ought to be most carefully described.

When streamers form a corona, the character of it should be described.

_Auroral Arches._—Are arches always perpendicular to the magnetic
meridian? If incomplete, do they grow laterally? and if so, in what
manner, and towards which side? Do they always move from north (magnetic)
to south? and if so, is it by a southerly motion of the individual
streamers, or by new streamers springing up to the south of the old ones?
What (by estimation, or by reference to known stars) may be the breadth
of the arch in different positions in its progress? Do arches appear
to be nothing but congeries of streamers, or to have an independent
existence? What relations, if any, have they to clouds? and if related,
to what kind of clouds are they related?

_Pulsations._—Do pulsations travel in any invariable direction? What
time do they take to get from one part of the heavens to another? Are
they running sheets of continuous light, or fixed patches which become
luminous, or more luminous, in rapid succession? and if patches, do these
appear to be foreshortened streamers? Are the same patches luminous in
successive pulsations?

_Sounds_ (?).—As some have suspected the Aurora to be accompanied by
sound, the observer’s attention should be directed to this question when
an Aurora is seen during a calm. If sound be suspected, the observer
should endeavour, by changing his position, brushing off spicules of
ice from the neighbourhood of the ears, his whiskers, &c., to ascertain
whether it can be referred to the action of such wind as there is on
some part of his dress or person. If it should clearly appear that it
is not referable to the wind, then the circumstance of its occurrence,
its character, its relation (if any) to bursts of light, should be most
carefully noted.

These questions are prepared merely to lead the observer to direct
his attention to various features of the phenomenon. Answers are not
demanded, except in such cases as definite answers can be given; and
the observer should keep his attention alive to observe and regard any
other features which may appear to be of interest. It is desirable that
drawings should be made of remarkable displays.

Observations with Sir William Thomson’s electrometer would be very
interesting in connexion with the Aurora, especially a comparison of the
readings before, during, and after a passage of the Aurora across the
zenith.


_Spectroscopic Observations. By Prof. G. G. STOKES, Sec. R.S._

_Spectrum of the Aurora._

The spectrum of the Aurora contains a well-known conspicuous bright line
in the yellowish green, which has been accurately observed. There are
also other bright lines of greater refrangibility, the determination of
the positions of which is more difficult on account of their faintness,
and there are also one or more lines in the red, in red auroras.

Advantage should be taken of an unusually bright display to determine the
positions of the fainter lines. That of the brightest lines, though well
known, should be measured at the same time to control the observations.
The character of the lines (_i. e._ whether they are strictly lines,
showing images of the apparent breadth of the slit, or narrow bands,
sharply defined or shaded-off) should also be stated.

Sometimes a faint gleam of light is seen at night in the sky, the
origin of which (supposed from the presence of clouds) is doubtful. A
spectroscope of the roughest description may in such cases be usefully
employed to determine whether the light is auroral or not, as in the
former case the auroral origin is detected by the chief bright line.
The observer may thus be led to be on the look-out for a display which
otherwise might have been missed.

It has been said, however, that the auroral light does not in all cases
exhibit bright lines, but sometimes, at least in the eastern and western
arch of the Aurora, shows a continuous spectrum. This statement should be
confronted with observation, special care being taken that the auroral
light be not confounded with light which, though seen in the same
direction, is of a different origin, such, for example, as light from a
bank of haze illuminated by the moon.

Sir Edward Sabine once observed an auroral arch to one side (say north)
of the ship, which was in darkness. Presently the arch could no longer
be seen, but there was a general diffuse light, so that a man at the
mast-head could be seen. Later still, the ship was again in darkness, and
an auroral arch was seen to the south.

Should any thing of the kind be observed, the whole of the circumstances
ought to be carefully noted, and the spectroscope applied to the diffuse
light.


_Polarization of Light. By W. SPOTTISWOODE, M.A., LL.D., Treas. R.S._

It has been suggested that the Aurora, inasmuch as it presents a
structural character, may afford traces of polarization. Having reference
to the fact that the striæ of the electric discharge in vacuum-tubes
present no such feature, the probability of the suggestion may be
doubted. But it will still be worth while to put the question to an
experimental test.

If traces of polarization be detected, it must not at once be concluded
that the light of the Aurora is polarized; for the Aurora may be seen on
the background of a sky illuminated by the moon, or by the sun, if not
too far below the horizon, and the light from either of these sources is,
in general, more or less polarized; therefore, if the light of the Aurora
is suspected to be polarized, the polariscope should be directed to an
adjacent portion of clear sky, free from Aurora, but illuminated by the
moon or sun as nearly as possible similar, and similarly situated to the
former portion; and the observer must then judge whether the polarization
first observed be merely due to the illumination of the sky.

The presence of polarization is to be determined:—

(1) With a Nicol’s prism, by observing the light through it by turning
the prism round on its axis, and by examining whether the light appears
brightest in some positions and least bright in others. If such be the
case, the positions will be found to be at right angles to one another.
The direction of “the plane of polarization” will be determined by
that of the Nicol at either of these critical positions. The plane of
polarization of the light transmitted by a Nicol, is parallel to the
longer diagonal of the face; and, accordingly, the plane of polarization,
or partial polarization, of the observed light is parallel to the longer
diameter of the Nicol when the transmitted light is at its greatest
intensity, or to the shorter when it is at its least.

(2) The observation with a double-image prism is similar to that with a
Nicol. This instrument, as its name implies, gives the images which would
be seen through the Nicol in two rectangular positions, both at once,
so that they can be directly compared; and when in observing polarized
light the instrument is turned so that one image is at a maximum, the
other is simultaneously at a minimum. Both these methods of observation,
(1) and (2), are especially suitable for faint light; because in such a
case the eye is better able to appreciate differences of intensity than
differences of colour.

(3) The observation with a biquartz differs from (1) only by holding a
biquartz (a right-handed and a left-handed quartz cemented side by side)
at a convenient distance beyond the Nicol, and by observing whether
colour is or is not produced. If the Nicol be so turned that the two
parts of the biquartz give the same colour (choose the neutral tint,
_teint de passage_, rather than the yellow), we can detect a change in
the position of the plane of polarization by a change in colour, one
half verging towards red, the other towards blue. This observation is
obviously applicable to a change in the plane, either at different parts
of the phenomenon at the same time, or at the same parts at different
times.

(4) We may use a Savart’s polariscope, which shows a series of coloured
bands in the field of view. For two positions at right angles to one
another corresponding to the two critical positions of a Nicol, these
bands are most strongly developed; for two positions midway between the
former the bands vanish. In the instruments here furnished, the plane of
polarization of the observed light will be parallel to the bands when the
central one is light, perpendicular to them when the central band is dark.


_Instructions in the use of the Spectroscopes supplied to the Arctic
Expedition._

_By J. NORMAN LOCKYER, F.R.S._

_Spectroscopic Work._

Scales prepared on Mr. Capron’s plan, together with forms for recording
positions, also accompany the instrument.

_A._ In using these, carefully insert the principal solar lines in their
places on the forms, as taken from a fine slit, and keep copies of this
scale for use. If the slit opens _only on one_ side, note on scale in
which direction the lines widen out, whether towards red or violet. Also
fill up some of these forms with gas and other spectra, as taken at
leisure _with the same instrument_ and scale.

When observing, close the slit (after first wide opening it) as much as
light will permit, and then with pen or pencil record the lines as seen
upon the micrometer-scale on the corresponding part of the form, and note
_at once relative intensities_ with Greek letters, α, β, &c. (or numbers).

Reduce at leisure line-places on scale to wave-lengths, and note as to
each line the _probable limits of instrumental error_. _B._

In case the auroral spectrum is so faint that the needle-point or
micrometer-scale is invisible, half of the field of view may be covered
with tinfoil, with a perfectly straight smooth edge running along the
diameter of the field, in perfect focus, and parallel to the lines of the
spectra. The reading-screw being set to 10, the bending-screw should then
be adjusted so that the green line of the Aurora is just eclipsed behind
the blackened edge of the tinfoil. A similar eclipse of other lines will
give their positions.

In this instrument the reference-prism is brought into action by turning
the slipping piece to which is fixed the two terminals. Care should be
taken that the prism itself is adjusted before commencing observations,
as it may be shaken out of position on the voyage. The tubes provided
for the reference-spectra may be either fastened to the terminals or
arranged in some other manner. The air-spectrum may also be used as a
reference-spectrum. To get this, two wires should be screwed into the
insulators, their ends being at such a distance apart and in such a
position that the spectrum is well seen.


_General Observations regarding the Spectrum of the Aurora[17]._

_C._ Note appearance, colour, &c. of _arc_, _streamers_, _corona_, and
_patches_ of light.

Get compass positions of principal features, and _note any change of
magnetic intensity_. If corona forms, take its position and apparent
height.

Look out for _phosphorescence_ of Aurora and adjacent clouds. Listen
for reported sounds. Note any peculiarity of cloud scenery, prior to or
pending the Aurora.

Sketch principal features of the display, and indicate on this sketch the
parts spectroscopically examined.

Examine line in _red_ specially in reference to its assumed connexion
with _telluric_ lines (little _a_ group), and note _as to its brightening
in sympathy with any of the other lines_.

Examine line in yellow-green (Ångström’s) as to _brightness_, _width_,
and _sharpness_ (_or nebulosity_) at the edges. Notice as to a peculiar
_flickering_ in this line sometimes seen; note also whether this line is
_brighter_ (or the reverse) _with a fall of temperature_. Note _ozone_
papers at the time of Aurora.

Note whether the Auroræ can by their spectra be classed into distinct
types or forms, and examine for _different spectra_ as under:—

    α. The auroral _glow_, pure and simple.

    β. The white arc.

    γ. The streamers and corona.

    δ. Any phosphorescent or other patches of light, or light cloud
    in or near the Auroræ. _D._

The information collected together in the ‘Manual’ should be carefully
consulted, and the line of observations suggested by Ångström’s later
work followed out. To do this, not only record the positions of any
features you may observe in the spectrum, but endeavour to determine,
if any, and if so which, of the features vary together. Compare, for
instance, the two spectra of nitrogen in the Geissler tube supplied, by
observing first the narrow and then the wider parts of the tube. It will
be seen that the difference in colour and spectrum results simply from an
addition to the spectrum in the shape of a series of channelled spaces
in the more refrangible end in the case of the spectrum of the narrow
portion.

Try to determine whether the difference between red and green Auroras may
arise from such a cause as this, and which class has the simpler spectrum.

See whether indications of great auroral activity are associated with the
widening or increased brilliancy of any of the auroral lines.

Remember that if auroral displays are due to gaseous particles thrown
into vibration of electric disturbance, increased electric tension
may either (1) dissociate those particles and thus give rise to a new
spectrum, the one previously observed becoming dimmer; or (2) throw the
particles into more intense vibration without dissociation, and thus give
rise to new lines, those previously observed becoming brighter.

Careful records of auroral phenomena from both ships may enable the
height of some, observed from both, to be determined. It will be very
important that those the heights of which are determined by such means
should be carefully observed by the spectroscope, in order to observe
whether certain characteristics of the spectrum can be associated with
the height of the Aurora.




APPENDIX C.

EXTRACTS FROM PARLIAMENTARY BLUE BOOK, CONTAINING THE “RESULTS DERIVED
FROM THE ARCTIC EXPEDITION 1875-76.” (Eyre and Spottiswoode, 1878.)


_Auroras observed 1875-1876, at Floebery Beach and Discovery Bay._

_By Lieutenant A. C. PARR, R.N._

Though the auroral glow was often present, and served in some degree to
lighten the darkness of the sky during the long winter, when the moon was
absent, the actual appearances of the Aurora itself were few, and the
nimbus worthy of any particular remark extremely small. Those which were
stationary assumed the form of low arches, with streamers flashing up to
them from the horizon, and usually to the eastward. But the more common
form was for an arch to appear low down in some part of the sky where the
glow was brightest; at first it was very faint and narrow, but as it rose
gradually in the heavens it would increase both in size and intensity,
till on arriving near the zenith, with its ends extending nearly to the
horizon, it would be about the breadth of three or four rainbows, and its
colour that of white fleecy clouds lit up by the rays of the full moon.
On reaching this point, however, its course was nearly run; for after
appearing to remain stationary, as little white gaps would suddenly rend
the arch asunder, the portions thus detached seemed to roll together and
concentrate all their brightness in the smaller space, and then gradually
fade away and become extinct. Sometimes a very pale green would show
itself in the more luminous patches, and once or twice there was a slight
suspicion of red; but never was the whole sky illuminated by streams
running in all directions, and forming coronæ, while these colours varied
every moment.

When instead of the arch rising up from the horizon a streamer appeared,
its origin was in the north. From the northern horizon it would stretch
out towards the zenith, passing nearly overhead, and reaching to within
a few degrees of the land to the south. In appearance they would be the
same as the arches, but sometimes a second would grow out of the first,
and on one occasion three were visible at the same time. They had lateral
motion either from east to west, or west to east, but there was no
flashing to brighten them, and they gradually faded away.

The time at which Auroras usually occurred was between 9 P.M. and
midnight, the last display being on February 19th, commencing at 11
P.M. It was a beautifully clear night, without mist or haze of any
description, and small stars visible close down to the horizon. At
the above-named hour two arches made their appearance, and remained
stationary; the lower one was the brighter, being of a pale green colour,
its centre bearing E.S.E. (true), and having an altitude of about 5°,
with a breadth of about twice that of a rainbow. The second arch was
concentric with the first, and about 7° above it, but rather broader and
fainter. These arches maintained their altitude, the upper one at about
the same intensity, but that of the lower one varied considerably. It
would gradually lighten up, then send flashes to the upper one, then
break up and fade away; before, however, it had quite disappeared,
flashes would come up to it from the horizon which seemed to endue it
with new life, for the arch would be reformed, brighten up, and the same
performance would be again repeated. This occurred three or four times in
the course of three quarters of an hour; but the flashes from the horizon
never extended beyond the lower arch, and those from the lower never went
beyond the upper. During this display the citron-line was obtained very
clearly with the spectroscope, but no other lines were visible.

On six or seven occasions Auroras were visible at the same time on board
both the ‘Alert’ and ‘Discovery;’ but the absence of characteristic
features makes it impossible to determine whether they were the same
display, or merely two distinct ones which happened to occur at the same
time. But as by far the larger number of those recorded in the one ship
were not visible at the other, it was certainly only under exceptional
conditions that they could be simultaneously observed at both stations,
if, indeed, they ever were. Auroras seemed to appear indifferently both
when there was wind and when it was calm, with either a high or low
barometer, and seemed quite unconnected with the temperature, although on
an occasion the thermometer was observed to fall 3° during the display,
and to rise 2° almost immediately afterwards. But it was never seen
illuminating the edges of clouds, as we saw it on the passage home, nor
playing about the outline of the land, and never was there the slightest
suspicion of sound being produced by it.

The opportunities for observing the spectrum of the Aurora in this
position have been most unsatisfactory, as the displays were small in
number and deficient in brilliancy.

The form they generally assumed was to rise like an arch from a portion
of the horizon where there was a luminous glow, at first very faint, but
gradually increasing in brilliancy till near the zenith, where it would
remain stationary for a short time and then break up and disappear.
Sometimes they would rise up as streamers, but only occasionally was more
than one visible at a time, and they lasted for such a short time, that
even if they had been bright it would have been very difficult to make
satisfactory observations.

Very few showed any signs of colour, and those only the slightest tinge.
Nearly all that were observed gave the citron-line with the small pocket
spectroscope with more or less distinctness, though no signs of any other
lines were ever seen; but on only two occasions was it bright enough to
get the line with Nury’s spectroscope, and then only for such a short
time that a satisfactory measure could not be obtained.

       *       *       *       *       *

Then follows a descriptive list of the Auroræ seen, from which I have
selected three of the finest, viz. January 2nd, February 14th, and
February 19th, 1876.

January 2nd, 1876. Lieut. Parr. _Floeberg Beach._—9 P.M. Streams of
Aurora. Stars shining brightly.

Register. _Discovery Bay._—9 P.M. Observed an Aurora like a pale band of
light in the form of an arch whose centre was on the true meridian and
15° from the zenith. It shortly afterwards broke up into feathered edges,
their direction being a little to the eastward of the zenith. The arch
grew fainter, and shifted to the eastward of the meridian four points;
the left extremity of the arch faded away, and the right assumed the
shape of the folds of a curtain doubled over. The weather was clear and
calm. The display lasted upwards of 30 minutes.

A spectroscope, one of Browning’s 8-in. direct-vision, was directed
towards the Aurora, but the light was not sufficient to give any spectrum.

The temperature was -39°. Barometer 29·56 inches. No wind. Clouds stratus
2. Eight meteors were observed during the time the Aurora was visible.

February 14th. Register. _Discovery Bay._—At 2 A.M. a faint Aurora
passing across the heavens from S.E. to S.W. was observed, like an arch
of a pale colour. It lasted only a short time, and was very indistinct.
Temperature -47°. Barometer 30·44 inches. No wind or clouds.

Lieut. Aldrich. _Floeberg Beach._—2 A.M. A faint Aurora towards the S.W.
Weather calm. Cumulus-stratus clouds 3. Temperature -46°. 8 P.M. Faint
flashes of Aurora in the E. and S.W.

Lieut. Aldrich and Lieut. Parr. _Floeberg Beach._—11.50 P.M. A moderately
bright arch of Aurora extended from due N. to about S.S.W., where it
terminated close down to the horizon in a crook turned to the eastward.
In a few moments a streamer flashed from the end of the crook parallel
to the first and right across the heavens, its edges being quite sharp
and parallel to each other. A third streamer shot up a minute afterwards,
but did not extend more than 80° upwards. The streamers were visible for
a very short time, the first remaining longest. The second-named arch
gradually faded away till within a few degrees of the S.S.W. horizon, and
(still being a continuation of the crook) bent round to the eastward,
and towards the horizon, going on to what was left of the stump of the
third arc. A lateral motion to the eastward now began, the whole body
gradually turning round until it disappeared about due south. Stars were
visible through it at its brightest, but not very distinctly. This is
the most intense and variegated Aurora we have experienced, but scarcely
any colours were to be seen. Temperature -51°. Barometer 30·43 inches,
stationary. Calm weather. Clouds cumulus 1. Preceded and followed by calm
weather.

Meteorological Register. _Discovery Bay._—9.15 P.M. An Aurora was
observed to the southward, spreading out like a fan in separate ways.
It was faint. A few cirro-stratus clouds were visible, apparently
between the observer and the Aurora. It lasted about 40 minutes, and
then gradually faded away. Temperature -47°. Barometer 30·51 inches,
stationary. No wind. Clouds cirro-stratus 4.

February 19th. Meteorological Report. _Discovery Bay._—9.45 P.M. An
Aurora like a fluted arch, with rays flashing towards the Pole, was
observed spanning the hills from the south to the east. The direction of
the lines of light from all parts of the arch was towards the zenith.
Above the arch a pale band of colour appeared, like a secondary arch
above the other. It appeared very much as if it was caused by the
reflected light of the Aurora. The Aurora was bright for a few seconds,
and then gradually died away. It lasted altogether about 30 minutes.
The centre of the arch bore S.E., having an altitude of about 30°. The
secondary arch was about 15° above the former. Both arches were of a pale
light colour, the upper one very faint. Temperature -34°. Barometer
29·87 inches, rising rapidly. Weather calm. Misty. No clouds.

Lieut. Parr. _Floeberg Beach._—An Aurora appeared shortly after 11
P.M., consisting of bright arch, whose centre bore about E.S.E., and
had an altitude of about 5°, with a second broader and fainter arch
about 7° above the first. These arches maintained their altitudes, the
upper one at about the same intensity, but that of the lower one varied
considerably. It would gradually brighten up, then send streamers up to
the second, then break up into light patches, and gradually fade away.
This happened three or four times during the 40 minutes that the display
lasted. At times streamers would come up from the horizon to the lower
arch, for it was a splendidly clear night, and seemed to brighten it up,
but none of them extended beyond it. Neither did the streamers from the
lower arch extend beyond the upper one. It was slightly green in colour
when brightest, and the citron-line was well defined, but no others were
visible. Temperature -46°. Barometer 29·95 inches, steady. Weather calm.
Cumulus clouds 4. Misty.


TABLE of DATES when AURORAS were observed by the ARCTIC EXPEDITION,
1875-76.

  ----------------+--------------------------+-----------------------
                  |     H.M.S. ‘Alert,’      |  H.M.S. ‘Discovery,’
       Date.      |     Floeberg Beach.      |    Discovery Bay.
  ----------------+--------------------------+-----------------------
  1875, October 25| 11.45 P.M. Faint.        | Cloudy.
   ”    ”       26| 10 P.M. Very faint.      | 10 P.M.
   ”    ”       30| Sky obscured. Faint.     | Ditto.
   ”   November  1| Ditto. Faint, but well   |
                  |  marked.                 | Ditto.
   ”    ”        2| 9 to 10 P.M. Arches and  |
                  |  streamers.              | A few clouds.
   ”    ”       21| Ditto. Bright            | 9 to 10 P.M.
                  |  streamer.               |  and 10 to 11 P.M.
   ”    ”       22| 2 P.M. and 8 P.M. Slight,|
                  |  red.                    | Clear sky.
   ”    ”       25| 9.30 A.M. Character not  |
                  |  recorded.               | Ditto.
   ”    ”       26| 10 A.M. Stream of light.}| A few clouds.
   ”    ”       26| Cloudy to 10 P.M.,      }|
                  |  bright afterwards.     }| 10 P.M.
   ”    ”       27| Midnight. Slight.        | 11.40 P.M.
   ”    ”       28| 1 A.M. Bright streak.    | Clear sky.
   ”    ”       29| Cloudy, brighter at 11   |
                  |  A.M. Faint glow.        | 9.30 A.M.
   ”    ”       30| A few clouds. Very faint.| 4.30 A.M.
   ”    ”       30| 5 P.M., 8 P.M., and 10   |
                  |  P.M. Flashes.           | 5 P.M.
   ” December    2| Evening. Streamers.      | Clear sky.
   ”    ”        3| 1 A.M. Flashes.          | Ditto.
   ”    ”        3| Bright sky. Faint Aurora.| 2.30 P.M.
   ”    ”       16| 10 P.M. Slight; showed   |
                  |  citron-line.            | 11 P.M.
   ”    ”       19| 3 P.M. to 5 P.M., faint; |
                  |  and 9 to 10 P.M.,       | Very clear sky.
                  |  moderately bright arc.  |
   ”    ”       22| 10 P.M. Slight.          | Ditto.
   ”    ”       23| 6 P.M. Ditto.            | Ditto.
   ”    ”       24| Misty, a few stars       |
                  |  visible. Arch.          | 9 A.M.
   ”    ”       26| Very bright sky. Faint.  | 6 P.M.
   ”    ”       29| Ditto. Very faint.       | 6.15 P.M.
   ”    ”       31| 4 P.M. Same.             | Sky obscured.
  1876, January  1| 5 P.M. and 11 P.M.       |
                  |  Slight.                 | A few clouds.
   ”    ”        2| 9 P.M. Described and     |
                  |  figured.                | 9 P.M.
   ”    ”       17| Very bright sky. Very    |
                  |  faint streamers.        | 9.25 A.M.
   ”    ”       18| 9.45 P.M. and 10.5 P.M.  |
                  |  Character not recorded. | 10.15 P.M.
   ”    ”       19| Very bright sky. Faint.  | 9.45 P.M.
   ”    ”       20| 2 A.M. Slight.           | 2.30 A.M.
   ”    ”       23| 7.55 A.M. and 2 P.M.     |
                  |  Slight.                 | 8.45 P.M.
   ”    ”       24| Bright sky. Slight flash.| 2 A.M.
   ”    ”       24| 5 P.M. and 11.15 P.M.    |
                  |  Faint Aurora.           | Very clear sky.
   ”    ”       27| 2 A.M. to 3.45 A.M.      |
                  |  Faint.                  | 1 A.M. to 4 A.M.
   ”    ”       27| Very bright sky. Faint   |
                  |  double arch.            | 8.30 P.M.
   ”    ”       28| 6 P.M. and 7 to 9 P.M.   |
                  |  Faint flashes.          | 7.20 P.M.
   ”    ”       30| 8 P.M. Streak.           | 7.50 to 9 P.M.
   ”    ”       31| 8.30 A.M. and 7.30       |
                  |  P.M. Very faint.        | 8.25 A.M., 5.30 P.M.
   ” February    3| 10 P.M. Slight flash.    | Very clear sky.
   ”    ”       11| Sky obscured. Very faint.| 11 P.M.
   ”    ”       13| 11 P.M. Flashes.         | Clear sky.
   ”    ”       14| 2 A.M., 9.15 to 10 P.M.  |
                  |  Described and figured.  | 2 A.M. and 11.50 P.M.
   ”    ”       19| 9.45 P.M.                | 11 P.M.
   ”    ”       20| 2 A.M. Very faint.       | 2.30 A.M.
   ”    ”       22| 2 A.M. Character not     |
                  |  recorded.               | Very clear sky.
   ”    ”       24| Bright sky. Very faint.  | Midnight.
   ”    ”       26| 10 P.M. and 11 P.M.      |
                  |  Faint flashes.          | Sky obscured.
  ----------------+--------------------------+-----------------------

I have added to the above Table the character of the Aurora in each
instance as taken from the fuller descriptions given.—J. R. C.


_Auroras and Magnetic Disturbances._

The appearances of Auroras and the synchronous movements of the
declinometer-magnet were subjects of special observation during the stay
of the ‘Alert’ and ‘Discovery’ at their winter-quarters. The Table on
page 187 gives the dates and hours when Auroras were visible. On all
occasions they were observed to be faint, with none of those brilliant
manifestations which are described by our own officers as seen at Point
Barrow, and by the Austro-Hungarian Expedition at Franz-Josef Land, where
the magnetical instruments were so sensibly disturbed.

These phenomena were not observed either in the ‘Alert’ or the
‘Discovery,’ especially no connexion between magnetical disturbances and
the appearances of Auroras could be traced.

This is quite in accordance with the remarks of previous observers within
the region comprehended between the meridians of 60° and 90° west, and
north of the parallel of 73° north. For example:—

In the Phil. Trans. 1826, Part IV. p. 76, Capt. Parry and Lieut. Foster
remark, in the discussion of their magnetical observations at Port
Bowen:—“As far, however, as our own observations extended, we have
reason to believe that on no occasion were the needles in the slightest
degree affected by Aurora, meteors, or any other perceptible atmospheric
phenomenon.”

Again, in the Smithsonian Contributions, vol. x., 1858, Mr. A. Schott,
in his discussion of Dr. Kane’s observations at Van Rensselaer Harbour,
in 1854, remarks—“In conformity with the supposed periodicity of this
phenomenon as recognized by Professor Olmstead, no brilliant and complete
Auroras have been seen; with an exception of very few, they may all be
placed in his fourth class, to which the most simple forms of appearances
have been referred.” The following statement is given in the same page
as a footnote:—“The processes have no apparent connexion with the
magnetic dip, and in _no_ case did the needle of our unifilar indicate
disturbance.”

       *       *       *       *       *

The following description of the Aurora observed on 21st November, 1875,
is given by Commander Markham and Lieut. Giffard, in their abstract of
observations at Floeberg Beach:—

“Between 10 and 11 P.M. bright broad streamers of the Aurora appeared
10° or 15° above the north horizon, stretching through the zenith, and
terminating in an irregular curve about 25° above the south horizon,
bearing S.S.W. During the Aurora’s greatest brilliancy the magnet was
observed during five minutes to be undisturbed.”

    [_Note._—I applied for a loan of the lithographic stones
    to enable me to give copies of the three diagrams of
    Auroræ referred to in the Arctic “Results;” but the Lords
    Commissioners of H.M. Treasury refused this, except on the
    terms of my paying one third of the original cost of production
    of such diagrams. I did not think it worth while to accept
    these conditions. Only one of the drawings has any special
    interest; and this is a “curtain” Aurora, similar to that
    figured on Plate II. of this work.—J. R. C.]




APPENDIX D.

THE AURORA AND OZONE.


[Sidenote: Aurora and ozone. Dr. Allnatt’s notes and conclusions deduced
therefrom.]

While Part I. was in the press, Dr. Allnatt, formerly of Frant, and for
many years the well-known meteorological contributor to ‘The Times’
newspaper, kindly placed at my disposal his large series of notes. Upon
an examination of these we came to the following conclusions:—

1. That Auroral periods are also periods of comparative abundance of
ozone.

2. That instances are by no means wanting in which an abnormal
development of ozone appears to be coincident with the manifestation of
an Aurora.

[Sidenote: Year 1870 remarkable for sun-spots, auroræ, and ozone.]

In reference to the first point, it is found, as the result of an
examination of Dr. Allnatt’s notes, that particular years and months are
notable at once for Auroræ and for ozone in abundance. 1870 was one of
these years, and was specially recorded by Dr. Allnatt, in his ‘Summary
for the Year,’ as remarkable for sun-spots, Auroræ, and ozone.

[Sidenote: Particulars of some of the monthly records.]

The month of February in that year was marked by intense cold and
brilliant Auroræ. Atmospheric electricity was feeble, but ozone was,
throughout the month, well developed; and there was no tangible period of
antozone.

In the month of April of the same year, eight days consecutively (19th to
26th) were marked for ozone 10, the maximum of Dr. Allnatt’s scale.

In May of the same year there were magnificent Auroræ, and atmospheric
electricity was intense. Ozone was scanty; but this was accounted for by
the wind being generally E.N.E., ozone being mostly developed with a W.
or S.W. wind, and a moist state of the atmosphere.

In August 1870 the unusually large number of 22 days were recorded for a
maximum of ozone.

September 1870 was hardly less remarkable, with 19 days of maximum. It
was recorded that there were splendid Auroræ during this month, and the
solar spots were very large.

October 1870 had 20 days of maximum ozone, and November had several
fine Auroræ and maxima of ozone noted. In fact, nearly every month in
that year was referred to by Dr. Allnatt for displays of Aurora (of
both Arctic and Antarctic forms) and for a development of ozone very
considerably above the average.

[Sidenote: Year 1871.]

The year 1871 had more or less of the same character. In the month
of October of that year, fine Auroræ were prevalent, and ozone was
registered as at its maximum during 22 days.

There seems reason to conclude that if a systematic comparison of annual
or other periods of Aurora and ozone development were made, it would
result in disclosing a connexion (probably an intimate one) between the
two phenomena.

[Sidenote: Instances showing a connexion between a specific Aurora and an
ozone maximum.]

With reference to the second point, the following (among other) instances
may be quoted, for the purpose of showing a connexion between a specific
Aurora and an ozone maximum.

The Aurora of 24th September, 1870, was splendid and universal, being
seen in Europe, Asia, Africa, America, and Australia. Ozone reached, on
the morning of the 24th, 8 of the scale (the scale running from 1 to 10),
and, on the morning of the 25th, 10, the maximum.

In October 1870 there were grand displays on the 14th, 20th, 22nd, 24th,
and 25th, and ozone was correspondingly abundant, as is seen by the
following Table:—

  +-------------------+----------+------+------------------------+
  |       Date.       | Aurora.  |Ozone.|                        |
  +-------------------+----------+------+------------------------+
  |1870, October 14th.|Aurora.   |   8  |The display of the 24th |
  | ”       ”    20th.|Aurora.   |  10  | was accompanied by     |
  | ”       ”    21st.|None seen.|   5  | the formation of a     |
  | ”       ”    22nd.|Aurora.   |  10  | corona, and that of    |
  | ”       ”    23rd.|None seen.|   8  | the 25th was splendidly|
  | ”       ”    24th.|Aurora.   |  10  | seen in Edinburgh.     |
  | ”       ”    25th.|Aurora.   |   8  |                        |
  +-------------------+----------+------+------------------------+

The foregoing figures somewhat point to the conclusion that ozone
quantity rises and falls coincidently with the Aurora displays.

The following seems, however, a case still more strongly in point.

  +-------------------+-------+---------------------+------+
  |       Date.       | Wind. |       Aurora.       |Ozone.|
  +-------------------+-------+---------------------+------+
  |1871, January 25th.| E.S.E.| None seen.          |   0  |
  | ”       ”    26th.| N.N.W.| None seen.          |   2  |
  | ”       ”    27th.| E.S.E.| Aurora at night in  |      |
  |                   |       |  N. and S. horizons.|  10  |
  | ”       ”    28th.|   E.  | None seen.          |   8  |
  | ”       ”    29th.| S.E.  | None seen.          |   2  |
  +-------------------+-------+---------------------+------+

It is curious, in examining the above Table, to note how the ozone rose,
notwithstanding an east wind, from 0 on the 25th, and 2 on the 26th, to
10 on the 27th, when the Aurora appeared, and 8 on the 28th, when it
might have lingered; and how it again descended to 2 on the 29th.

The case of the Aurora of 6th of October, 1869, when a broad belt of
Aurora was in the north, is also an illustrative one, as will be seen by
the following data:—

  +------------------+------+------+-------+
  |      Date.       |Wind. |Ozone.|Aurora.|
  +------------------+------+------+-------+
  |1869, October 5th.|S.S.W.|   1  |  —    |
  | ”       ”    6th.|S.S.E.|   5  |Aurora.|
  | ”       ”    7th.|S.S.W.|  10  |  —    |
  | ”       ”    8th.|  S.  |  10  |  —    |
  | ”       ”    9th.| S.E. |   5  |  —    |
  +------------------+------+------+-------+

The Aurora of the night of the 6th was here represented by the
ozone-paper of the morning of the 7th with a maximum of 10, which lasted
till the 8th.

[It should be borne in mind, in examining these Tables, that the Aurora
is of the night of the given date, while the ozone-papers are taken and
recorded in the morning of the date quoted.]

[Sidenote: Other instances.]

We will now take instances where the ozone has not reached its maximum;
but even in these cases a certain amount of rise and fall of the ozone
development towards and from the Aurora is traceable.

  +-------------------+--------+------+-------------------------+
  |       Date.       | Wind.  |Ozone.|        Aurora.          |
  +-------------------+--------+------+-------------------------+
  |1871, April     8th| S.S.E. |   5  |Aurora on 9th, but       |
  |  ”      ”      9th| S.S.E. |   8  | wind E. and unfavourable|
  |  ”      ”     10th| S.E.   |   5  | to ozone.               |
  +-------------------+--------+------+-------------------------+
  |1871, November  9th|   N.   |   5  |Aurora on all three      |
  |  ”      ”     10th|  N.W.  |   8  | nights.                 |
  |  ”      ”     11th|   N.   |   5  |                         |
  +-------------------+--------+------+-------------------------+
  |1872, February  3rd|  S.W.  |   4  |Aurora on night of the   |
  |  ”       ”     4th| S.S.W. |   5  | 4th represented by      |
  |  ”       ”     5th|  S.W.  |   8  | ozone-paper of morning  |
  |  ”       ”     6th|  S.W.  |   5  | of the 5th.             |
  +-------------------+--------+------+-------------------------+

Other cases are, we are bound to say, found, in which ozone was either
not remarkable for quantity, or positively fell during the Aurora, as,
for instance, this:—

  +-----------------+------+------+------------------------------+
  |      Date.      | Wind.|Ozone.|          Aurora.             |
  +-----------------+------+------+------------------------------+
  |1874, March 16th |W.N.W.|   6  |Aurora on the 18th represented|
  |  ”     ”   17th | S.W. |   6  |  by test-paper of            |
  |  ”     ”   18th |  W.  |   5  |  the 19th with only two      |
  |  ”     ”   19th |S.S.W.|   2  |  degrees of discoloration.   |
  +-----------------+------+------+------------------------------+

It is, however, possible that such instances may be accounted for, either
by some reaction on the test-papers after they have been coloured, or
by some accidental antagonistic circumstance affecting the tests. The
following is a case well illustrating this:—

  +------------------+------+------+---------------------------------+
  |       Date.      | Wind.|Ozone.|           Aurora.               |
  +------------------+------+------+---------------------------------+
  |1874, January 31st|N.N.W.|   6  |There was an Aurora on           |
  | ”    February 1st| N.W. |   8  | the night of the 2nd represented|
  | ”       ”     2nd| N.W. |   2  | by the ozone-paper              |
  | ”       ”     3rd|N.N.W.|   4  | (4 only) on the                 |
  | ”       ”     4th|E.N.E.|   8  | morning of the 3rd.             |
  +------------------+------+------+---------------------------------+

This instance would seem strongly opposed to the theory of a connexion
between Aurora and ozone but for the fact that on the 2nd, when the
Aurora was seen at night, and on other days in the same month, Dr.
Allnatt has recorded a strong wave of antozone to have swept over the
whole of England, and blanched the ozone-papers, however deep their
coloration might have previously been. Indeed, it is easy to understand
that some antozonic influence may, at times, disturb the evidence of the
test-papers, even in so elevated and apparently pure an atmosphere as
that of Frant.

It may not be considered that the foregoing instances are enough to
establish a case of ozone=Aurora; but there seems, at least, sufficient
to base a requisition for further inquiry upon.

It would, too, be interesting to investigate whether Auroræ and ozone
development are respectively localized. Mr. Ingall’s fine Aurora, seen
at Champion Hill, S.E., July 18th, 1874 (_antè_, pp. 22 and 23), was not
observed at Frant, and the ozonoscopes there were described as blanched
by antozone.




APPENDIX E.

INQUIRIES INTO THE SPECTRUM OF THE AURORA.

BY H. C. VOGEL.[18]


The frequent appearance of the Aurora in the past winter, as well as
this spring, has given me opportunity to institute exact inquiries into
the spectrum of the Aurora. It is known that the nature of Auroræ is as
yet but little explored. It has been considered necessary to abandon
the former view—that they are discharges of the electricity collected
at the poles—because it has been hitherto found impossible to bring the
chief lines of the Aurora-spectrum into coincidence with the spectra of
the atmospheric gases. Theoretical considerations, based on the great
alterations to which the spectrum of the same gas is subject under
varying conditions of temperature and density, have very recently led
Zöllner to the opinion that probably the spectrum of the _Aurora does not
coincide with any known spectrum_ of the atmospheric gases, only because
it is a spectrum of another form of our atmosphere hitherto incapable of
artificial demonstration[19].

The following article will show how far I have succeeded, in conjunction
with Dr. Lohse, in supporting this view by exact observations of the
Aurora-spectrum itself, as well as by comparison with the spectra of the
gases constituting the air.

The star-spectrum apparatus belonging to the 11-inch equatorial of the
Bothkamp Observatory was used for these observations. It consists of
a set of prisms _à vision directe_, five prisms with refracting angle
90°, slit, collimator, and observing telescope. The lowest eyepiece
(magnifying four times) of this telescope was employed. The telescope is
capable of being moved in such a way, by the aid of a micrometer-screw,
that different portions of the spectrum can be brought into the centre
of the field of vision. As fractions of the rotation of this screw are
marked, the distances of the spectral lines can be readily found.

Repeated measurements of 100 lines of the solar spectrum have enabled
me, upon the basis of Ångström’s Atlas (‘Spectre normal de Soleil’), to
express the indications of the screw directly in wave-lengths.

In place of the cross wires originally introduced into the focus of the
observing telescope, I have inserted a tiny polished steel cone, the very
fine point of which reaches to the centre of the field of vision. The
axis of this cone stands perpendicular to the length of the spectrum,
therefore parallel with the spectral lines, and the setting of the point
of the cone on the latter is accomplished with great sharpness. If the
spectrum is very faint, or consists only of bright lines, the cone is
lighted by a small lamp. For this purpose, opposite to the point of the
cone, there is an opening in the telescope, through which, regulated by
a blind, light can be thrown on the point. As the latter is polished, a
fine line of light thus appears, which extends to the centre of the field
of vision, and the brilliancy of which can be altered by withdrawing
the lamp to a greater distance or lowering the blind, so that even the
faintest lines of a spectrum can be brought with facility and certainty
into coincidence with this line of light.

The head of the micrometer-screw is divided into 100 parts, and each
part, in the neighbourhood of the Fraunhofer line F, answers to about
·00016 wave-length. The probable error of position on one of the
well-marked lines in the sun’s spectrum amounts to about 0·008 of a turn
of the screw with the lowest eyepiece of the telescope. I have subjected
the screw itself to a thorough examination with reference to such range,
as well as to periodical inequalities in the single worms of the screw,
but could discover no error exceeding 0·01 of a turn of the screw. I
have to mention, further, that after each observation in the position in
which the instrument was used, readings followed on the sodium-lines,
or on some of the hydrogen-lines, in order to eliminate errors which
might arise in the unavoidable disturbance of any particular part of the
spectral apparatus.


1. _Observations of the Aurora._

1870, Oct. 25th.—A very bright Aurora. In the brightest parts, besides a
very bright line between D and E, several other fainter lines were to be
discerned, situated further towards the blue end of the spectrum. They
appeared on a dimly-lighted ground, and stretched out over the Fraunhofer
lines E and _b_ to about midway between _b_ and F. Towards the red end
the spectrum was terminated by the bright line first mentioned. No
measurements could be taken, as the apparatus had not yet undergone the
above-mentioned alterations, and even the brightest line of the spectrum
did not diffuse sufficient light to be able to perceive the fine cross
wires. The red rays of the Aurora were not examined.

1871, Feb. 11th.—Towards ten o’clock appeared in the north-west a very
bright light-bow of greenish colour as the edge of a dark segment.
Even with a very narrow slit, the line between D and E could be well
recognized and measured. The average of six readings gave 7·11 turns,
equal to 5572 wave-length. In a small spectroscope of low dispersion
which is arranged on Browning’s plan, a few more lines placed further
towards the blue could be recognized (as in October). Towards the red end
of the spectrum no lines were observable. The greatest development of the
Aurora was about midnight. Magnificent rays rose to about 60° elevation;
they had the same greenish colouring as the bow of light, and the
appearance of the spectrum also was exactly the same. I again obtained
two sets of measurements: the average of six readings in the first set
gave 7·10 turns, 5572 wave-length; in another part of the heavens at the
same time 7·10 was the result of four readings.

On Feb. 12, towards eight o’clock, the intensity of the Aurora was
already great enough to allow measurements of the brightest line. The
average of six readings gave 7·09 turns, or 5576 wave-length. Dr. Lohse
took observations later, with the same apparatus, and found from six
readings 7·12 turns, or 5569 wave-length.

Yet the appearance of the spectrum in the spectroscope of low dispersion
was essentially distinct from that of February 11th. The green continuous
spectrum was present; it extended from the bright Aurora-line to the
lines _b_ of the solar spectrum, and was traversed by some bright lines.
Between band _b_ and F, was another line standing alone, out beyond F, in
the blue part of the spectrum, a clear bright stripe; and just before G a
very faint broad band of light was perceived.

Amongst the rays which, later on, shot upwards, and were coloured red
at their ends, another very intense red line appeared in the spectrum
between C and D, yet placed nearer to C[20].

April 9th.—An exceedingly brilliant Aurora, of which the greater
development took place in the early morning hours. Magnificent red
sheaths rose up to the zenith. The spectrum was like that observed on
February 12th, only much more intense, so that the lines could be seen
and measured with the larger spectral apparatus. In the brightest part of
the Aurora was the dark segment; the spectrum consisted of five lines in
the green, and a somewhat indistinct broad line or band in the blue.

The red rays, on the other hand, allowed us to recognize seven lines,
whilst the bright line again appeared in the red part of the spectrum.
I could not again perceive the faint stripe observed on February 12th,
in the vicinity of line G. The mean measurements of four readings on an
average, for each line, gave:—

  ------+-------+-------+-------+------------------------------------------
  Turns | Prob- |       | Prob- |
    of  | able  | Wave- | able  |
  screw.|errors.|length.|errors.|                Remarks.
  ------+-------+-------+-------+------------------------------------------
   4·62 | ·0037 | 6297  |·00014 |Very bright stripe.              } On
   7·12 |     9 | 5569  |     2 |Brightest line of the spectrum;  } a
        |       |       |       | becomes noticeably fainter at   } faintly
        |       |       |       | appearance of the red line.     } lighted
   7·92 |     — | 5390  |     — |Extremely faint line; unreliable } ground.
        |       |       |       | observation.                    }
   8·71 |    21 | 5233  |     4 |Moderately bright.
   8·95 |    49 | 5189  |     9 |This line is very bright when the red line
        |       |       |       | appears at the same time, otherwise equal
        |       |       |       | in brilliancy with the preceding one.
  10·06 |    20 | 5004  |     3 |Very bright line.
  12·33 |     — | 4694  |     — |}Broad band of light, somewhat less
  12·59 |    22 | 4663  |     — |} brilliant in the middle.
  12·88 |     — | 4629  |     3 |Very faint in those parts of the Aurora
        |       |       |       | in which the red line appears.
  ------+-------+-------+-------+------------------------------------------

April 14th.—Faint Aurora; only the bright line in the green could be
recognized in its spectrum. The mean of two readings gave 7·12 turns, or
5569 wave-length.

I append a table of the wave-lengths of the brightest line, as exactly
measured on four evenings:—

  1871, February 11    5573
           ”     12    5573
        April     9    5569
           ”     14    5569

Therefore the average result (if only half-weight is allowed to the
last observation, because it only depends upon two readings) gives for
the wave-length of the brightest line 5571·3, with a probable error of
·000·92. According to Ångström[21], the wave-length of this line is 5567;
according to Winlock[22], on the other hand, 5570.


2. _On the Spectra of some Gases in Geissler’s Tubes, as well as on the
Spectrum of the Atmospheric Air._

Numerous experiments have been made in order to find out some admitted
connexion between the spectrum of the Aurora and the spectra of the
principal gases composing the atmosphere. I limit myself to noticing some
of the often-repeated observations in Plücker’s tubes, which contained
oxygen, hydrogen, and nitrogen, as well as the observations of the
spectrum of the air under different conditions. The experiments were
made with a small inductive apparatus, in which the length of the spark
between platinum points in ordinary air was 15 millims. at the most. As
Zöllner (in the pamphlet mentioned) comes to the conclusion, that if
the development of the light in the Aurora, according to the analogy of
gases brought to glow in rarefied spaces, is of an electric nature, it
must belong to very low temperature—in order to bring the gases enclosed
in the tubes to glow at the lowest possible temperature, I have always
employed such weak currents that the gas was only just steadily alight.

The following observations have been repeated often and at various times.
The figures are averages of the indications of the micrometer-screw, so
that the uncertainty of the figures will, in the rarest cases, amount
to no more than 0·015 turn of the screw, and must be reckoned somewhat
more highly only in the case of completely faint misty lines. The
spectrum apparatus was that described above, and the slit was nearly
the same in every experiment, and so narrow that the sodium-lines could
be seen separated. The measurements, for the most part, extend only to
the Fraunhofer line G, as I feared lest, through further turning the
telescope by means of the micrometer-screw, too great a pressure might be
exercised on the worms of the latter.


I. _Oxygen._

_a._ In the narrow part of the Plücker tube.

  +------+------------+--------------------------------------------------+
  |Screw.|Wave-length.|                   Remarks.                       |
  +------+------------+--------------------------------------------------+
  | 3·97 |   6562     |Moderately bright.                                |
  | 5·04 |   6146     |Very bright.                                      |
  | 6·98 |   5603     |Very bright, misty towards the violet.            |
  | 8·19 |   5332     |Faint.                                            |
  | 8·95 |   5189     |Moderately bright.                                |
  |10·97 |   4870     |   ”         ”                                    |
  |11·02 |   4863     |Faint.                                            |
  |11·26 |   4829     |Bright; misty towards the red end of the spectrum.|
  |13·30 |   4583     |Very faint.                                       |
  |14·05 |   4506     |Moderately bright.                                |
  |15·55 |   4372     |   ”         ”                                    |
  +------+------------+--------------------------------------------------+

_b._ In the wide part of the Plücker tube.

  +------+------------+------------------+
  |Screw.|Wave-length.|    Remarks.      |
  +------+------------+------------------+
  | 6·98 |   5603     |Very faint.       |
  | 8·95 |   5189     |Very bright.      |
  |11·26 |   4829     |Moderately bright.|
  +------+------------+------------------+

The lines near 3·97 and 11·02 belong to hydrogen. Probably traces of
aqueous vapour were present in the tube, which were decomposed by
the galvanic current. These two lines are not to be found in a lower
temperature in the broad part of the tube. It is striking that the red
nitrogen-line near 5·04 is also missing there. In the narrow part of the
tube the lines stand out in the green on a very dimly-lighted ground,
whilst in the wider part they appear on a perfectly dark ground.


II. _Hydrogen._

_a._ In the narrow part of the tube.

  +------+------------+--------------------------------------------------+
  |Screw.|Wave-length.|                  Remarks.                        |
  +------+------------+--------------------------------------------------+
  | 3·98 |    6558    |Very bright.                                      |
  | 6·16 |    5813    |Moderately bright, on both sides very faint lines.|
  | 7·01 |    5596    |Moderately bright.} On a dimly lighted ground,    |
  | 7·18 |    5555    |Moderately bright.}  which becomes fainter towards|
  | 7·77 |    5422    |Faint.            }  the violet.                  |
  |      |            |                                                  |
  | 8·95 |    5189    |Moderately bright.}                               |
  |10·03 |    5008    |Faint.            } On a faint steadily bright    |
  |10·55 |    4929    |Moderately bright.}  ground.                      |
  |      |            |                                                  |
  |11·04 |    4861    |Very bright.      } From 11·5 to 12·9 a bright    |
  |12·86 |    4632    |Moderately bright.}  ground, which towards the    |
  |      |            |                  }  violet becomes very bright.  |
  |      |            |                                                  |
  |13·32 |    4581    |Very faint.       }                               |
  |14·05 |    4506    |    ”             } On a dull ground.             |
  |15·90 |    4342    |Very bright.      }                               |
  +------+------------+--------------------------------------------------+

_b._ In the broad part of the tube.

  +-------+------------+------------------+
  | Screw.|Wave-length.|     Remarks.     |
  +-------+------------+------------------+
  |  5·30 |    6063    |Faint.            |
  |  7·00 |    5598    |Bright.           |
  |  8·96 |    5187    |Very bright.      |
  | 11·28 |    4828    |     ”            |
  | 14·04 |    4507    |Moderately bright.|
  +-------+------------+------------------+

The lines appeared on a perfectly dark ground.

The tube shows in the narrow part the hydrogen-spectrum of the first
order; the lines in the green do not coincide with the lines of the
nitrogen, though some lines belonging to nitrogen are found. Here, too,
most probably small particles of aqueous vapour have been enclosed in
the tube and are decomposed. Very striking is the spectrum in the broad
part of the tube; nothing is to be seen of the bright shining lines Hα
3·98, Hβ 11·04, Hγ 15·90; on the other hand, four very bright lines and
one quite faint one are in the red end of the spectrum, which appear,
in opposition to the spectrum of the narrow part, not on a partially
lighted, but on an entirely dark ground. The appearance is very striking
if we bring the tube in front of the slit; and so, by degrees, at first
the light in the narrow part, then the light at the connecting-point of
the narrow and wide parts, and, finally, the light in the latter fall
upon the slit. At the connecting-point of the wide ends of the tube the
three well-known hydrogen-lines decrease in intensity, the continuous
ground of some parts of the spectrum disappears, and a new line appears
near 11·28, which has about the same brilliancy as Hβ.

A comparison with the spectrum of oxygen shows the bright lines which are
in the spectrum in the wide end of the tube as belonging to that element.
The heat evolved by the current appears insufficient to bring the
hydrogen to glow, whilst by it the oxygen, which is of a more rarefied
character, becomes incandescent. An alteration of the direction of the
current has no influence on the appearance.


III. _Nitrogen._

_a._ In the narrow part of the tube.

  +------+------------+--------------------------------------------------+
  |Screw.|Wave-length.|                    Remarks.                      |
  +------+------------+--------------------------------------------------+
  | 3·84 |   6620     |}Several faint, broad, close lines, increasing in |
  | 4·85 |   6213     |} brilliancy as they approach the violet end.     |
  |      |            |                                                  |
  | 5·30 |   6063     |}Broad bright lines,                              |
  | 5·51 |   6000     |} so close together                               |
  | 5·69 |   5948     |} that the intervening spaces appear              |
  | 5·87 |   5896     |} like fine dark lines. This part of              |
  | 6·04 |   5846     |} the spectrum is very bright, but                |
  | 6·20 |   5802     |} not uniform, being brighter towards             |
  | 6·43 |   5741     |} the violet end.                                 |
  |      |            |                                                  |
  | 6·96 |   5607     |}Group of faint but at the same time              |
  | 7·13 |   5567     |} very broad lines. The                           |
  | 7·28 |   5532     |} last is the brightest.                          |
  |      |            |                                                  |
  | 7·55 |   5470     |}The dark intervening spaces are somewhat broader,|
  | 7·74 |   5428     |} the bright lines somewhat more intense than     |
  | 7·92 |   5389     |} in the preceding group, and all of almost equal |
  | 8·09 |   5353     |} brilliancy.                                     |
  |      |            |                                                  |
  | 8·32 |   5306     |}                                                 |
  | 8·50 |   5272     |}Very faint fine lines.                           |
  | 8·69 |   5237     |}                                                 |
  |      |            |                                                  |
  | 9·01 |   5178     |Very bright broad misty line.                     |
  |      |            |                                                  |
  | 9·67 |   5066     |Very bright line.          }                      |
  |10·25 |   4975     |”       ”                  }                      |
  |10·66 |   4913     |”       ”                  } The bright lines     |
  |11·03 |   4862     |Very faint line.           }  are sharply         |
  |11·41 |   4811     |Bright line.               }  defined towards     |
  |12·11 |   4721     |”       ”                  }  the red end of      |
  |12·57 |   4666     |Faint line.                }  the spectrum,       |
  |12·57 |   4644     |Bright, broad, misty line. }  fading away towards |
  |13·42 |   4570     |Very bright line.          }  the other end of    |
  |14·24 |   4487     |”       ”                  }  the spectrum.       |
  |15·02 |   4417     |Bright line.               }                      |
  |      |            |                                                  |
  |15·66 |   4363     |Bright lines.   }   Bright lines sharply defined  |
  |15·72 |   4357     |     ”          }    towards red end, indistinct  |
  |15·87 |   4345     |Bright line.    }    towards other end of         |
  |16·72 |   4273     |     ”          }    spectrum.                    |
  +------+------------+--------------------------------------------------+
                    Here follow several lines.

_b._ In the wide part of the tube.

  +-------+------------+------------------------------+
  | Screw.|Wave-length.|          Remarks.            |
  +-------+------------+------------------------------+
  |  6·20 |    5802    |Faint, indistinct, broad line.|
  |  7·72 |    5433    |Dull stripe.                  |
  |  8·20 |    5330    |Faint line.                   |
  |  8·94 |    5191    |Very faint line.              |
  |  9·03 |    5175    |Broad band of light.          |
  |  9·90 |    5029    |Dull band of light.           |
  | 10·68 |    4911    |Moderately bright line.       |
  | 11·42 |    4809    |Faint line.                   |
  | 12·59 |    4663    |Bright line.                  |
  | 13·43 |    4569    |    ”                         |
  | 14·07 |    4504    |Moderately bright line.       |
  | 14·25 |    4486    |Very bright line.             |
  | 15·85 |    4347    | ”      ”                     |
  | 16·76 |    4273    |Moderately bright line.       |
  +-------+------------+------------------------------+

_c._ At the aura of the negative pole.

  +-------+------------+------------------------------------------------+
  | Screw.|Wave-length.|                 Remarks.                       |
  +-------+------------+------------------------------------------------+
  |  5·18 |    6100    |} Broad, moderately bright stripe, indistinct   |
  |  5·70 |    5945    |}  at the edges.                                |
  |       |            |                                                |
  |  7·60 |    5159    |} Broad, moderately bright stripe.              |
  |  8·41 |    5289    |}                                               |
  |       |            |                                                |
  |  8·76 |    5224    |{ Very bright line, somewhat indistinct towards |
  |       |            |{  the violet.                                  |
  |       |            |                                                |
  |  9·19 |    5147    |Faint line.                                     |
  | 10·00 |    5004    |Bright line, indistinct towards the red.        |
  |       |            |                                                |
  | 10·67 |    4912    |{ Somewhat fainter than the last, indistinct    |
  |       |            |{  towards the red.                             |
  |       |            |                                                |
  | 11·43 |    4808    |Very faint line.                                |
  | 12·25 |    4704    |Very intense, broad, indistinct towards         |
  |       |            |  the violet.                                   |
  | 12·73 |    4646    |Very faint line.                                |
  | 13·43 |    4569    |Moderately bright, indistinct towards the       |
  |       |            |  violet.                                       |
  | 14·25 |    4486    |Like the last.                                  |
  | 15·03 |    4417    |Quite a faint line.                             |
  | 15·86 |    4346    |Moderately bright line.                         |
  | 16·76 |    4275    |Very bright line.                               |
  +-------+------------+------------------------------------------------+

Here follow several other lines.

The observations in the different parts of the tube show plainly the
dependence of the spectrum on the temperature. The aura of the negative
pole gives the line near 10·07 so characteristic of the air-spectrum.
This is the same line which is met with in the spectra of most of the
nebulæ. The very striking groups of lines in the red and yellow in the
spectrum of the narrow part of the tube disappear entirely in the wide
part. If we compare the spectra with those above quoted, of oxygen and
hydrogen, we find line Hβ very faint in the spectrum of the narrow part
of the tube near 11·03; on the other hand, oxygen-lines appear in the
broad part near 8·20, 8·94, and 14·07. Thence I would conjecture that
the tube was not filled with pure nitrogen, the appearance of which is
precise, but with dry rarefied air, since Wüllner’s researches have
proved that dry air yields the same spectrum as nitrogen gas. Perhaps
the air in the tube examined by me had not been thoroughly dried, and
thus the appearance of some lines of the elements before named is to be
explained.

I must further mention that the electrodes of the tubes consisted of
aluminium; yet a comparison of the spectra observed and the aluminium
spectrum has shown no connexion between them.


IV. _Atmospheric Air._

  +-----------+------------+------------------------------------------+
  |  Screw.   |Wave-length.|               Remarks.                   |
  +-----------+------------+------------------------------------------+
  |      5·88 |    5892    |Very bright double line (Na).             |
  |      6·67 |    5680    |Very bright line.                         |
  |      7·20 |    5550    |Faint line.                               |
  |      9·00 |    5180    |Very bright line.                         |
  |      9·79 |    5047    |Fine faint line.                          |
  |     10·03 |    5008    |} Very bright double line.                |
  |     10·07 |    5002    |}                                         |
  |     11·43 |    4803    |Faint confused line.                      |
  |     12·69 |    4651    |}                                         |
  |     12·84 |    4633    |} Faint line not sharply defined.         |
  |     13·04 |    4612    |}                                         |
  |           |            |                                          |
  |From 14·61 |    4453    |} Confused band of light, which ends with |
  |to   15·88 |    4444    |}  a broad washy line.                    |
  +-----------+------------+------------------------------------------+

Here follow several other lines.

Rarefied air saturated with aqueous vapour.

  +----------+------------+----------------------------------------------+
  |  Screw.  |Wave-length.|                    Remarks.                  |
  +----------+------------+----------------------------------------------+
  |     3·97 |    6562    |Moderately bright line.                       |
  |    (5·88)|    5892    |Bright double line (Na).                      |
  |    (6·25)|    5789    |Bright line (H).                              |
  |From 7·03 |    5591    |}Broad dull band of light; near 7·03 a        |
  |to   7·55 |    5470    |} somewhat brighter line.                     |
  |     7·59 |    5461    |Bright line (H).                              |
  |     8·72 |    5231    |Dull stripe.                                  |
  |     8·96 |    5187    |Broad misty stripe.} On a dull steady ground. |
  |    10·07 |    5002    |Faint line.        }                          |
  |    11·05 |    4859    |Very bright line.                             |
  |    12·21 |    4709    |Moderately bright line.} On dimly lighted     |
  |    12·75 |    4644    |{Line fainter than the }  ground, becoming    |
  |          |            |{ preceding.           }  fainter towards     |
  |    13·28 |    5585    |Very faint line (H).   }  the violet.         |
  |   (15·71)|    4358    |Very bright line (H).                         |
  |    15·90 |    4341    | ”         ”                                  |
  +----------+------------+----------------------------------------------+

Here follow several more lines.

In the first observations, the electric spark, about 1 centim. in length,
was allowed to pass between platinum points in ordinary air.

The sodium-line near 5·88 appeared continually. The bright double line at
10·03 and 10·07, with a weaker current or longer spark, was no longer to
be recognized as a double line, but appeared as a broad somewhat confused
line, of which the brightest part was near 10·05. No lines belonging to
the platinum spectrum appeared. Ordinary rarefied air, under a pressure
of 25 to 30 millims., and which was enclosed by mercury in a tube 8
millims. wide, showed exactly the same lines as Plücker’s nitrogen-tube
(_b_), except that some lines belonging to the spectrum of mercury also
appeared.

This observation may be regarded as a confirmation of the conjecture
above expressed as to the condition of Plücker’s tube III. (nitrogen).
In the observations described under _b_, the air saturated with aqueous
vapour was under a pressure of 22 millims. Besides the sodium-lines,
lines of the mercury-spectrum appeared at 6·25, 7·59, and 15·71. The
spectrum of rarefied air under similar pressure was found to accord
completely with the spectrum of the light in the broad part of Plücker’s
tube.

III. (Nitrogen _b._)—A comparison of the spectrum of rarefied air
saturated with aqueous vapour with the former shows the striking
alterations in the spectrum which are brought about by the presence of
the aqueous vapour.


3. _Comparison of the Aurora-Spectrum with the Spectra of Atmospheric
Gases and of Inorganic Substances._

In the next place, I turn to the comparison of the observed spectra
of different gases and of the air with the spectrum of the Aurora.
The first band of light in the red part of the Aurora-spectrum most
probably coincides with the first system of lines in the spectrum of
nitrogen (_a_). Probably only the bright part of this group of lines
is perceptible, on account of the extreme faintness of the Aurora; and
as in nitrogen the increase of the brilliancy of the spectrum takes
place towards the violet end, the absence of the intermediate spectrum
towards this direction would find its explanation. The most intense line
of the Aurora-spectrum at 7·12 is to be also found in the spectrum of
nitrogen (_a_)—as a very faint line, however. That this line appears
in the Aurora by itself, and with intensity relatively great, need not
appear strange, considering the great alteration of the gas-spectra under
different conditions of pressure and temperature. The third line of the
Aurora-spectrum, very vaguely defined on account of its great faintness,
coincides in the same way with a nitrogen-line.

The line at 8·71 is met with in the nitrogen-spectrum (_c_), as well
as in the air-spectrum (_b_). The third line of the oxygen-spectrum at
8·95, which _seems to appear under very different conditions_, is found
again, as the fifth line in the spectrum of the Aurora. _Moreover, the
sixth line in the Aurora at 10·06 coincides very exactly_ with the known
nitrogen-line appearing in the spectra of some of the nebulæ. Lastly, as
to the broad band of light in the Aurora-spectrum from 12·33 to 12·88,
several lines are found in this place in the spectrum of nitrogen as well
as the air-spectrum (_a, b_); so that here, too, a coincidence between
the spectra may be regarded as probable.

The observations show with some certainty that at least one line at 10·06
of the Aurora-spectrum coincides with the maximum brilliancy of the
air-spectrum, and that the other lines appear with great probability in
the spectra of atmospheric gases.

In the very great difference of the gas-spectra under varying conditions
of pressure and temperature, it would indeed be difficult to succeed
in producing artificially a spectrum which should resemble that of the
Aurora in all parts. Moreover, it must be admitted, under the hypothesis
that the Auroræ are electric discharges in rarefied air-strata, that
these strata, qualified for the transmitting of electricity, will have a
very considerable thickness.

In this case the conditions of pressure on these air-strata are
themselves so different that, within certain limits, each will yield its
own peculiar spectrum; but we shall see the sum of collective spectra, so
to speak, spread out behind each other; and therefore the impossibility
of attaining a perfect agreement between the Aurora-spectrum and the
artificially exhibited spectra of mixed gases is evident.

A comparison of the Aurora-spectrum with the spectra of inorganic
substances may be easily worked out by the help of the above-quoted
wave-lengths of the single lines of the former, with due regard to
probable errors, and with the aid of Ångström’s Atlas of the Solar
Spectrum. Here the perfect harmony of the brightest Aurora-line (which
was fixed with an exactitude of about one seventh of the separation of
the sodium-lines) with the lines of the iron-spectrum is especially
striking. The wave-lengths in the above-cited observations of the bright
Aurora-line vary between 556·9 and 557·3, whilst, according to Ångström,
two lines of the iron-spectrum are situated at 556·85 and 557·17.

Iron-lines corresponding to the other Aurora-lines, within certain limits
of accuracy, are also to be found, as will be seen from the following
comparison:—

  +--------------+--------------------+----------------------+
  |Aurora-lines. |    Lines of the    |      Remarks.        |
  |              |   iron-spectrum.   |                      |
  +--------------+--------------------+----------------------+
  |              |    { 630·08 }      | Moderately bright.   |
  |    629·7     |    { 629·85 }      |                      |
  |              |                    |                      |
  |              |    { 539·60 }      |                      |
  |              |    { 539·92 }      | Mostly very faint.   |
  |    539·0     |    { 539·05 }      |                      |
  |              |    { 538·85 }      |                      |
  |              |                    |                      |
  |              |    { 523·43        | Very faint.          |
  |    523·3     |    { 523·21        | Moderately bright.   |
  |              |    { 522·90        | Very faint.          |
  |              |                    |                      |
  |              |    { 519·79        |    ”                 |
  |              |    { 519·40        |    ”                 |
  |    518·9     |    { 519·16        | Moderately bright.   |
  |              |    { 519·06        |      ”       ”       |
  |              |    { 518·51        | Very faint.          |
  |              |                    |                      |
  |              |    { 500·65 }      |                      |
  |              |    { 500·52 }      |                      |
  |    500·4     |    { 500·49 }      | Very faint.          |
  |              |    { 500·30 }      |                      |
  |              |    { 500·20 }      |                      |
  |              |                    |                      |
  |    From  469·4 }                                         |
  |    to    462·9 } 3 stronger and 4 very faint iron-lines. |
  +----------------------------------------------------------+

Yet this agreement, though remarkable, can only be considered as
complete proof of the presence of iron-vapour in the atmosphere when we
shall have succeeded in showing by observation analogous modifications of
the relative conditions of brilliancy in the iron-spectrum by alterations
of temperature and density; and in this way explain the appearance of
relatively very faint iron-lines in the Aurora-spectrum, or, on the other
hand, the absence of the most intense lines.

It will meanwhile remain far more in accordance with probability to
regard the _Aurora-spectrum as a modification of the air-spectrum_;
since we are already aware, in the case of gases, of the alteration of
the spectra by conditions of temperature and pressure; and an agreement,
at any rate, quite as certain between the spectrum in question and the
spectra of atmospheric gases has been proved above.

    [I am indebted to Miss Annie Ludlam for a translation from the
    German of the above Memoir.—J. R. C.]




FOOTNOTES


[1] βόθυνος, a hollow.

[2] πίθος, a cask.

[3] χάσμς, a chasm.

[4] M. Lemström (Swedish Expedition, 1868) concludes that the corona of
the Aurora Borealis is not entirely a phenomenon of perspective, but that
the rays have a true curvature, that they are currents flowing in the
same direction and attract each other. There is also an account [_antè_,
p. 16] of an Aurora at Melville Island (Parry’s first voyage), in which
two arches were seen curving towards each other.

[5] A brilliant display in December 1870, on the east coast of Sicily,
was followed by very violent storms, with the overflow of the Tiber and
the flooding of Rome.

[6] Some curious instances have been recently (January 1879) given in
the ‘Times’ of such electric phenomena, comprising, amongst others, gas
lighted by the finger in Canada, points of flame seen on the ironwork of
Teignmouth Bridge, and similar points seen on the alpenstocks and axes of
a party making a mountain ascent in Switzerland.

[7] On the occasion of the Aurora of September 24, 1870, Dr. Allnatt
says, “the air seemed literally alive with the unwonted phosphorescence.”

[8] See, however, Dr. Schuster’s article “On the Spectra of Lightning,”
Phil. Mag. May 1879, p. 316.

[9] The proof from occulted stars merely goes to the fact that the moon
possesses no atmosphere _appreciable in that way_. It may still be a
question whether there does not exist something of the kind, lying low
and close to the surface, and possibly of a rarefied character, which
would scarcely make itself visible by its effects in occultations.
Cloud-vapour might form in an atmosphere of inconsiderable density.

[10] This observation is not without a certain amount of confirmation by
more recent ones, in which certain lunar objects and regions have been
suspected of mist or vapour. Mr. Birt (‘English Mechanic,’ vol. xxviii.
no. 725) mentions two—the cloud-like appearance of the white patch west
of Picard, and the interior of Tycho, which at one time always misty and
ill-defined, is now become perfectly distinct and sharply defined.

December 4, 1878, 4h 45m. I observed Klein’s crater as a dull dark spot,
larger than the true object; and while definition was good and other
objects were well defined, “the floor of Klein’s object, the oval spot
near, and also Agrippa (especially), all had _an odd misty look as if
vapour were in or about them_” (‘English Mechanic,’ vol. xxviii. no.
727). The mystery of different observers seeing and not seeing Klein’s
object on the same night is hardly to be accounted for by the angle of
illumination.

[11] Some doubt has been cast on this observation, on the ground that
nothing unusual was seen, and that the appearances were only those
ordinarily presented by the moon at its then phase. I simply give the
account as it appears in the scientific journal in which it was published.

[12] The question of a connexion between the waxing and waning of the
solar corona and the prevalence of sun-spots is now being mooted, and
may have an important bearing on the subject of the constitution of the
corona. It would seem that when the corona has been examined about the
time of minimum of sun-spots, it has proved fainter though more extended,
while the bright lines of the spectrum have been absent, indicating a
change or variance in the gaseous part of it at those periods.

[13] There seems to be some confusion as to the W.L. here given; 5567 is
usually accepted as Ångström’s line, while Prof. Smyth refers to it as
5579. The position, too, when examined with a spectroscope of greater
dispersion, is not exactly over the citron-line of acetylene, both the
above referred to lines lying somewhat more towards the violet end of the
spectrum (see Plate V. fig. 7).

[14] Ångström’s drawing, in giving this character to the two Aurora-bands
which are said to correspond with violet-pole bands about 47 and 43, is
incorrect, and calculated to mislead by giving the Aurora-bands a feature
corresponding to the violet-pole bands which they do not possess. I am
not aware of any Aurora-line or band which is described as distinguished
by degrading towards the violet.

[15] The tubes generally seem marked Si Fl instead of the ordinary
notation Si F. Si Fl₆ is probably, in fact, Si F₄.

[16] Dr. Schuster has found that while the line-spectrum of lightning is
attributable to N, it has also a band-spectrum, which he considers due to
O and a slight trace of CO₂ (Phil. Mag. 5th ser. vol. vii. p. 321).

[17] _In these observations some suggestions made by Mr. Capron have been
incorporated._

[This was Mr. Lockyer’s note. In point of fact, the Author was
responsible for the verbatim paragraphs comprised between the letters _A_
and _B_, and _C_ and _D_, in the instructions as now reprinted.]

[18] Communicated by the author to the Royal Saxon Academy of Science,
1871.

[19] Reports of the Royal Saxon Academy of Science, Oct. 31, 1871.

[20] This red line was first noticed by Zöllner.

[21] Recherches sur le Spectre Solaire, p. 42.

[22] American Journal of Science, lxviii. 123.