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                          A Century’s Progress
                                   IN
                               Astronomy


                                   BY
                        HECTOR MACPHERSON, Jun.
             MEMBER OF THE SOCIÉTÉ ASTRONOMIQUE DE FRANCE;
               MEMBER OF THE SOCIÉTÉ BELGE D’ASTRONOMIE;
                   AUTHOR OF ‘ASTRONOMERS OF TO-DAY’


                       WILLIAM BLACKWOOD AND SONS
                          EDINBURGH AND LONDON
                                 MCMVI
                         _All Rights reserved_




                                PREFACE.


The present volume originated in a desire to present, in small compass,
a record of the marvellous progress in astronomy during the past hundred
years. Indebtedness should be acknowledged to the valuable works of
Professor Newcomb, Professor Schiaparelli, Professor Lowell, Professor
Young, Sir Robert Ball, Mr Gore, M. Flammarion, and Miss Clerke, who, as
the historian of modern astronomy, occupies a place at once
authoritative and unique.

Portions of Chapters II. and XII. have already appeared in the form of
an article on the Construction of the Heavens, contributed by the writer
to the American periodical, ‘Popular Astronomy.’

  Balerno, Mid-Lothian,
      _October 1906_.




                               CONTENTS.


                                                                    PAGE
  CHAPTER I.                                                           1
                           HERSCHEL THE PIONEER.
      Influence of Herschel’s work—His characteristics—Birth and
          early years—Emigration to England—Caroline
          Herschel—Discovery of Uranus—King’s Astronomer—Latter
          years and death—Death of Caroline Herschel
  CHAPTER II.                                                         15
                          HERSCHEL THE DISCOVERER.
      Solar researches—Study of Venus—Of Mars—The
          Asteroids—Jupiter—Saturn—Discovery of satellites—Uranian
          satellites—Cometary researches—Motion of the Solar
          System—Discovery of binary stars—Clusters and
          nebulæ—Nebulous stars—The Nebular
          Hypothesis—Star-gauging—The disc-theory—Subordinate
          clusters—Abandonment of the disc-theory—Second method of
          star-gauging—Estimate of Herschel’s work
  CHAPTER III.                                                        43
                                  THE SUN.
      Schwabe and the sun-spot period—Researches of Wolf, Lamont,
          Sabine, Gautier—Observations of Carrington and
          Spörer—Career and work of Fraunhofer—Spectrum
          analysis—Work of Kirchhoff—Solar eclipse work—The Solar
          prominences—Janssen and Lockyer—Huggins and Zöllner—Work
          of Young—The Italian spectroscopists, Secchi, Respighi,
          Tacchini—Career of Tacchini—The reversing layer—The
          Corona—Doppler’s principle—Rotation of the Sun—Work of
          Dunér—Janssen’s solar atlas—Maunder and magnetism—Solar
          theories—Distance of the Sun—Summary
  CHAPTER IV.                                                         65
                                 THE MOON.
      Life and work of Schröter—Of Mädler—Of Schmidt—Changes on the
          Moon—Selenography in England—Lunar atmosphere—Lunar
          photography—Work of W. H. Pickering—The new
          Selenography—The Moon’s heat—Motion of the
          Moon—Acceleration of the Moon’s mean motion—Work of
          Laplace, Adams, Delaunay
  CHAPTER V.                                                          80
                             THE INNER PLANETS.
      The problem of Vulcan—Mercury—Work of Schröter—Schiaparelli,
          his life and work—Work of Lowell—Spectrum of
          Mercury—Venus—Rotation period: work of Schröter, Di Vico,
          Schiaparelli, Tacchini, Lowell—Atmosphere and surface of
          Venus—The Earth: variation of latitude—Mars—Rotation of
          Mars—Surface—Discovery of canals—Work of Schiaparelli and
          Lowell—Interpretation of the canals—The theory of
          intelligent life—Spectrum of Mars—Satellites—The
          Asteroids—Bode’s law—Work of Piazzi and Olbers—Application
          of photography by Wolf—Discovery of Eros
  CHAPTER VI.                                                        103
                             THE OUTER PLANETS.
      Physical condition of Jupiter—Work of Zöllner and Proctor—The
          red spot—Satellites—Discovery of fifth satellite—Sixth and
          seventh satellites—Rings of Saturn: Bond, Maxwell,
          Keeler—Struve’s theory—Globe of Saturn—New
          satellites—Uranus and its satellites—Discovery of
          Neptune—Adams and Le Verrier—Satellite—Trans-Neptunian
          planets
  CHAPTER VII.                                                       123
                                  COMETS.
      Life and work of Olbers—His repulsion theory—Life and work of
          Encke—His comet—Biela’s comet—Faye’s comet—Return of
          Halley’s comet—Donati’s comet—Comet of 1861—Spectroscopic
          study of comets—Theory of Brédikhine—Spectra of
          comets—Comets of 1880 and 1882—The capture theory—Cometary
          photography
  CHAPTER VIII.                                                      138
                                  METEORS.
      Meteoric shower of 1833—Work of Olmsted—Work of Erman and
          Kirkwood—Of H. A. Newton—Adams and the meteoric
          orbit—Shower of 1866—Connection of comets and meteors—Work
          of Schiaparelli—Shower of meteors in 1872—‘Le Stelle
          Cadenti’—Meteoric observation—A. S. Herschel—Work of
          Denning—Stationary radiants—Bolides and aerolites—Origin
          of aerolites
  CHAPTER IX.                                                        150
                                 THE STARS.
      Distance of the stars—Life and work of Bessel—Studies of
          Struve—Life and work of Henderson—Work of Peters, Otto
          Struve, Brünnow, and Ball—Measures of Gill—Parallax of
          first-magnitude stars—Relative and absolute parallax—Work
          of Kapteyn—Application of
          photography—Star-catalogues—Argelander’s
          ‘Durchmusterung’—Work of Schönfeld—Work of Gould—The ‘Cape
          Photographic Durchmusterung’—Work of Gill and
          Kapteyn—International chart of the heavens—Work of
          Peck—Proper motions of the stars—Star-drift—Discoveries of
          Proctor and Flammarion—Radial motion—Work of Huggins,
          Vogel, and Campbell—Solar motion
  CHAPTER X.                                                         169
                          THE LIGHT OF THE STARS.
      Work of Fraunhofer and Donati—Life and work of Secchi—His
          types of spectra—Life and work of Huggins—Photography of
          spectra—Life and work of Vogel—His classification of
          spectra—Work of Dunér—Of Pickering—Spectroscopic
          catalogues—Analysis of spectra—Stellar photometry—Life and
          work of E. C. Pickering—Variable stars—Work of
          Goodricke—Of Argelander, Schmidt, Heis, Schönfeld—Studies
          of Dunér—Of Gore—Photographic
          discoveries—Classification—Algol variables: their
          explanation—Explanation of other variables—η
          Argus—Temporary stars—Of 1848—Of 1866—Of 1876—Of 1885—Of
          1892—Photographic discoveries—Nova Persei, 1901—New star
          of 1903—Theories of temporary stars
  CHAPTER XI.                                                        197
                        STELLAR SYSTEMS AND NEBULÆ.
      Life and work of John Herschel—Binary stars—Computation of
          orbits—Work of Wilhelm Struve—Of Otto Struve—Of
          Burnham—Satellites of Sirius and Procyon—Astronomy of the
          invisible—Work of Pickering and Vogel—Spectroscopic
          binaries—Work of Bélopolsky and
          Campbell—Star-clusters—Nature of nebulæ—Spectroscopic work
          of Huggins—Of Copeland—Nebular photography—Work of
          Roberts, Barnard, Wolf—Of Keeler
  CHAPTER XII.                                                       214
          STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE.
      Work of John Herschel—Researches of Wilhelm Struve—Extinction
          of light—Mädler’s “central sun”—Distribution of
          nebulæ—Work of Proctor—Aggregation of stars on the
          Galaxy—Work of Gore and Schiaparelli—Studies of
          Gould—Researches of Kapteyn—Of Newcomb—Is the Universe
          limited? Newcomb’s argument—Observations of
          Celoria—Researches of Seeliger—External Universes—Gore’s
          speculations
  CHAPTER XIII.                                                      227
                            CELESTIAL EVOLUTION.
      Laplace’s nebular hypothesis—Helmholtz and solar
          contraction—Theories of solar heat—Objections to Laplace’s
          theory—Faye’s hypothesis—Ball’s exposition—The meteoritic
          theory of Proctor—Its extension by Lockyer—Evolution of
          the stars—Vogel’s order of evolution—Tidal friction: work
          of Darwin—See’s explanation of double stars—Future of the
          Universe




                   A CENTURY’S PROGRESS IN ASTRONOMY.




                               CHAPTER I.
                         HERSCHEL THE PIONEER.


In astronomy, as in other sciences, the past hundred years has been a
period of unparalleled progress. New methods have been devised, fresh
discoveries have been made, new theories have been propounded; the field
of work has widened enormously. In fact, the science of the heavens has
become not only boundless in its possibilities, but more awe-inspiring
and marvellous.

To whom in the main is this great advance due? To the great pioneer of
what may be called modern astronomy—William Herschel. Not only did
Herschel reconstruct the science and widen its bounds, but his powerful
genius directed the course of nineteenth century research. As an
astronomical observer he has never been surpassed. In the breadth of his
views he was equalled only by Newton; and indeed he excelled Newton in
his unwearied observations and his sweeping conceptions of the Universe.
To quote his own remark to the poet Campbell, he “looked farther into
space than ever human being did before him.”

Herschel studied astronomy in all its aspects. In all the branches of
modern astronomy he was a pioneer. He observed the Sun, Moon, and
planets, devoting special attention to Mars and Saturn. He doubled the
diameter of the Solar System by the discovery of Uranus. He discovered
several satellites and studied comets. He was pre-eminently the founder
of sidereal astronomy. He discovered binary stars, thus tracing the law
of gravitation in the distant star-depths; while to him is due the
credit of the discovery of the motion of the Solar System. He founded
the study of star-clusters and nebulæ, propounded the nebular
hypothesis, and devised two methods of star-gauging. Above all, he was
the first to attempt the solution of one of the noblest problems ever
attacked by man—the structure of the Universe. In fact, the latter
problem was the end and aim of his observations. As Miss Clerke remarks,
“The magnificence of the idea, which was rooted in his mind from the
start, places him apart from and above all preceding observers.” Most of
the departments of modern astronomy find a meeting-place in Herschel, as
the branches run to the root of the tree. He discussed astronomy from
every point of view. Before, however, proceeding to examine the work of
this great man, it is well to note a few of his characteristics. These
characteristics, once understood, give us the key to his researches.
Before we can master Herschel the astronomer we must understand Herschel
the man.

Notwithstanding the fact that Herschel spent most of his life in
England, and that he is included in the ‘Dictionary of National
Biography,’ he was pre-eminently a German. Like most Germans his style
of writing was somewhat obscure, and this was emphasised when he wrote
in English, owing to his imperfect command of the language. Had he
written in German as well as in English, he would probably have been
better understood in his native country, where erroneous views of his
theories were long entertained. Even so distinguished an astronomer as
Wilhelm Struve, when translating Herschel’s papers into German, made a
mistake when translating a certain passage, which leaves the erroneous
impression that Herschel believed the Universe to be infinite—a mistake
which would not have arisen had he written in German.

The student of Herschel should also be careful in quoting the views of
the great astronomer. Had Herschel at the close of his life written a
volume containing his final views on the construction of the heavens,
this would not have been necessary; but Herschel did not write such a
volume. His researches were embodied in a series of papers communicated
to the Royal Society from 1780 to 1818. As he observed the heavens his
opinions progressed, so that a statement of his views at any given time
was by no means a statement of his final opinions. The late R. A.
Proctor, who was the first great exponent of Herschel in England, has
well said: “It seems to have been supposed that his papers could be
treated as we might treat such a work as Sir J. Herschel’s ‘Outlines of
Astronomy’; that extracts might be made from any part of any paper
without reference to the position which the paper chanced to occupy in
the entire series.”

Herschel, like the true student of nature, held theories very lightly.
They were to him but roads to the truth. Unlike many scientists, he did
not interpret observations by hypothesis: he framed his theories to fit
his observations. If he found that a certain theory did not agree with
what he actually saw in the heavens, he abandoned it: he did not
hesitate to change his views as his investigations proceeded. “No fear
of ‘committing himself,’” says Miss Clerke in her admirable work on ‘The
Herschels,’ “deterred him from imparting the thoughts that accompanied
his multitudinous observations. He felt committed to nothing but truth.”

In the mind of Herschel imagination and observation were marvellously
blended. He was a philosophical astronomer. Although his imagination was
a very vivid one he did not allow his fancies to run away with him, as
Kepler sometimes did: on the other hand, he did not, like Flamsteed,
refrain from speculating altogether. “We ought,” he wrote in 1785, “to
avoid two opposite extremes. If we indulge a fanciful imagination, and
build worlds of our own, we must not wonder at our going wide from the
path of truth and nature. On the other hand, if we add observation to
observation, without attempting to draw not only certain conclusions but
also conjectural views from them, we offend against the very end for
which only observations ought to be made.”

These characteristics—the lightness with which he held his theories, his
vivid imagination, and his philosophical reasoning—are the secrets of
Herschel’s success as an astronomer. Nearly all his ideas and
speculations have been confirmed. As Arago has said, “We cannot but feel
a deep reverence for that powerful genius that has scarcely ever erred.”
Herschel, like all other great students of Nature, was deeply religious.
He could not observe the heavens without feeling awed at the marvels
which his telescopes revealed. In his own words, “It is surely a very
laudable thing to receive instruction from the Great Workmaster of
Nature.”

Friedrich Wilhelm Herschel, born in Hanover on November 15, 1738, was
the fourth child of Isaac Herschel, an oboist in the band of the
Hanoverian Guard. Isaac Herschel, a native of Dresden, was an
accomplished musician, and all his children, ten in number, inherited
his talent. Of these ten, six survived, and only two became famous.
These were William, the great astronomer, and his sister Caroline (born
on March 16, 1750), who became a student of the heavens only second to
her brother.

At the garrison school in Hanover, where the Herschels were educated,
William Herschel showed intense love and aptitude for learning, and was
more diligent and persevering than his brother Jacob, his senior by four
years. In 1753 he became oboist in the band of the Hanoverian Guard in
which his father was now bandmaster. In her valuable memoirs, his sister
relates that her father was very interested in astronomy, and that he
taught his children the names of the constellations. William became
devoted to the science, and constructed a small celestial globe on which
equator and ecliptic were engraved. But his studies were much hampered.
His mother had a great dislike to learning: she had no sympathy with
aspirations, and tried to prevent her children becoming well educated.
Above all, the Hanoverian Guard was ordered to England in 1755, when a
French invasion was feared, and to that country Herschel proceeded,
along with his father and brother.

Returning to Germany in 1756, the Hanoverian Guard was employed the
following year in the Seven Years’ War. Hanover was invaded by the
French, and, conscription being the rule, the musicians were not
exempted from service. Under the command of the Duke of Cumberland the
Guard suffered a terrible defeat at Hastenbeck. William Herschel spent
the night after the battle in a ditch, and decided that soldiering would
not be his profession. He deserted, and, with the consent of his
parents, he sailed for England. After his arrival at Dover, he wandered
through the country in search of musical employment. At length, in 1760,
he was appointed to train the band of the Durham Militia, and four years
later paid a secret visit to Hanover, where he was welcomed by his
father, whose health was now failing, and by his sister Caroline. In the
following year he was promoted to the post of organist at Halifax, and
in 1766 he removed to Bath as oboist in Linley’s Orchestra. Finally, in
1767, he became organist in the new Octagon Chapel at Bath. Herschel was
now twenty-nine years old, and known as a famous musician. As Miss
Clerke remarks: “The Octagon Chapel soon became a centre of fashionable
attraction, and he soon found himself lifted on the wave of public
favour. Pupils of high rank thronged to him, and his lessons often
mounted to thirty-five a-week.”

In the year of his appointment his father died, aged sixty, after a life
of trouble and hardship. His death was a great blow to his daughter
Caroline, whom he had educated when her mother was from home. Caroline
Herschel was naturally possessed of musical ability, but her mother and
elder brother had determined that she should be a housemaid,—a
determination which William, who was devotedly attached to his sister,
opposed. Finally, in 1772, he visited Hanover, and took his sister to
England with him to act as his housekeeper. But for her unwearied
devotion it is doubtful whether William Herschel would have become the
great astronomer.

About the time of his appointment in Bath Herschel commenced the study
of languages and mathematics, reading Maclaurin’s ‘Fluxions’ and
Ferguson’s ‘Astronomy.’ The perusal of the latter volume revived his
love for astronomy. After fourteen or sixteen hours’ teaching he would
retire to his bedroom and read of the wonders of the heavens. His
interest increased as he proceeded, until, in his own words, “I resolved
to take nothing upon trust, but to see with my own eyes all that other
men had seen before me.” Accordingly he hired a small reflector.
Inquiring the price of a larger instrument, he found it to be quite
beyond his means. Then in 1772, when his sister came to keep his house
for him, he resolved to make his own telescope. First he tried the
fitting of lenses into pasteboard tubes, but this being a total failure,
he bought the apparatus of a Quaker optician who had constructed, or
attempted to construct, reflecting telescopes. In June 1773, assisted by
his sister and by his brother Alexander, then in Bath, he commenced
work. His first speculum mirror was five inches in diameter; and, while
it was in process of construction, he was obliged to hold his hands on
it for sixteen hours at a stretch, while his sister supplied his food
and read ‘The Arabian Nights,’ ‘Don Quixote,’ and other tales aloud to
him to pass the time. At last, after two hundred failures, he finished a
5-inch reflector, and on March 4, 1774, he observed the Orion nebula. No
sooner had Herschel commenced his celestial explorations than he
resolved to survey the entire heavens, leaving no spot unvisited.

In 1775 he commenced his review of the heavens, but finding his
telescope inadequate he began the work of telescope-making afresh.
Meanwhile he had much to distract him from astronomy. In 1776 he became
director of the Public Concerts at Bath. Yet his enthusiasm was
unbounded: he would run to his house between the acts at the theatre to
observe the heavens. In 1779, when observing the Moon from the street in
front of his house, a gentleman asked permission to see the celestial
wonders, a request which Herschel granted. The gentleman, Dr Watson of
Bath, introduced Herschel to the Literary Society, and we find him in
1780 contributing two papers to the Royal Society on Mira Ceti and the
Moon. In the same year he commenced his second review of the heavens,
and during its progress he made his first great discovery. On March 13,
1781, while surveying the constellation Gemini, he discovered a faint
object distinguished by a disc, which he concluded to be a tailless
comet, but which was soon shown to be a new planet beyond the orbit of
Saturn. This was the first planetary discovery made within the memory of
man. King George III. summoned Herschel to London, and gave him a
pension of £200 a-year, with the title of King’s Astronomer, pardoning
him also for his desertion from the army more than twenty years
previously. Herschel then named the new planet the “Georgium Sidus,” a
title now abandoned and replaced by Uranus.

William and Caroline Herschel now moved to Datchet, near Windsor, in
1785 to Clay Hall, and finally, in 1786, to Slough,—“the spot of all the
world,” said Arago, “where the greatest number of discoveries have been
made.” Here Herschel and his sister worked for nearly forty years. He
communicated to the Royal Society paper after paper on astronomy in all
its aspects. He also continued the work of telescope-making, and
constructed, in 1789, his 40-foot reflector, the wonder of the age. In
1787 his sister was appointed his assistant, and together the Herschels
worked from dusk to dawn. Caroline Herschel herself detected eight
comets and numerous nebulæ. She relates in her memoirs that on one
occasion, while she was acting as assistant, the ink froze in her pen.
But such inconveniences mattered not to the Herschels. As Miss Clerke
has well remarked, “Every serene dark night was to him a precious
opportunity, availed of to the last minute. The thermometer might
descend below zero, ink might freeze, mirrors might crack; but, provided
the stars shone, he and his sister worked on from dusk to dawn.... On
one occasion he is said to have worked without intermission at the
telescope and the desk for seventy-two hours.”

Honours were showered on Herschel. He was knighted in 1816, and became
President of the Royal Astronomical Society in 1820, besides receiving
several honorary degrees. But honours in no way elated him. Advancing
years in no way affected his wonderful mind. But his duties as King’s
Astronomer necessitated his acting as “showman of the heavens” on the
visits of royalties to Windsor, often after a whole day’s work, when
rest was absolutely necessary. This tremendous strain, which reflects
little credit on the Court, proved too much for the old man. His health
began to give way, although his mind was as vigorous as ever.

Herschel contributed his last paper to the Royal Society in 1818, and
three years later sent a list of double stars to the new Astronomical
Society. He made his last observation on June 1, 1821. His strength had
now left him, and to this he could not reconcile himself. As Miss Clerke
puts it, “All his old instincts were still alive, only the bodily power
to carry out their behests was gone. An unparalleled career of
achievement left him unsatisfied with what he had done.... His strong
nerves were at last shattered.” After a prolonged period of failing
health he died at Slough, at the age of eighty-three, on August 25,
1822. On September 7 he was buried in the church-yard of St Laurence at
Upton. On his tombstone are engraved the words—“Cœlorum perrupit
claustra”—he broke through the barriers of the skies.

The death of her brother was a terrible blow to Caroline Herschel.
Expecting to live only a twelvemonth, she returned to Hanover to the
home of her brother, Dietrich Herschel. But she lived twenty-five years
among people who cared nothing for astronomy. She was delighted at Sir
John Herschel’s continuation of his father’s work. She compiled a
catalogue of all the clusters and nebulæ observed by her brother, for
which she received the gold medal of the Astronomical Society, and she
was created an honorary member. In 1846 she received from the King of
Prussia the gold medal of science. But no honours made her in any way
elated. She always held that whoever said much of her said too little of
her brother. After a prolonged decline of health, she died on January 9,
1848, aged ninety-seven years, and was buried beside her father in the
churchyard of the Gartengemeinde at Hanover, leaving behind her a noble
example of self-sacrifice and devotion.




                              CHAPTER II.
                        HERSCHEL THE DISCOVERER.


One result of Herschel’s discoveries among the stars and nebulæ is that
his studies of the Sun and planets, with the exception of the discovery
of Uranus, have been completely thrown into the shade. Nevertheless, his
work in solar and planetary astronomy alone would have gained for him a
higher position in astronomy than his contemporaries. The planets,
satellites, and comets were all attentively studied by the great
astronomer; indeed, the scientific investigation of the surfaces of Mars
and Saturn began with Herschel.

“His attention to the Sun,” Miss Clerke truly remarks, “might have been
exclusive, so diligent was his scrutiny of its shining surface.”
Sunspots were specially investigated by Herschel, who closely studied
their peculiarities, regarding them as depressions in the solar
atmosphere. He also paid much attention to the faculæ, but could not
observe them to the north and south of the Sun, thus proving their
connection with the spots which are confined to the regions north and
south of the equator. “There is all over the Sun a great unevenness,”
said Herschel, “which has the appearance of a mixture of small points of
an unequal light; but they are evidently a roughness of high and low
parts.”

Herschel’s solar observations were very valuable, and did much for our
knowledge of the orb of day. His theory of the Sun’s constitution—a
development of the hypothesis put forward by _Alexander Wilson_
(1714-1786), Professor of Astronomy in Glasgow—was, however, very far
from the truth. This was almost the only instance in which Herschel was
mistaken. He regarded the Sun as a cool, dark globe, “a very eminent,
large, and lucid planet, evidently the first, or, in strictness of
speaking, the only primary one of our system.” In his opinion an
extensive atmosphere surrounded the Sun, the upper stratum forming what
Schröter named the “photosphere.” This atmosphere, estimated as two or
three thousand miles in depth, was regarded as giving out light and
heat. Below this shining atmosphere there existed, Herschel believed, a
region of clouds protecting the globe of the Sun from the glowing
atmosphere, and reflecting much of the light intercepted by them. The
spots were believed to be openings in these atmospheres, caused by the
action of winds, the umbra or dark portion of the spot thus representing
the globe of the Sun, which Herschel believed to be “richly stored with
inhabitants.” This theory held its ground for many years. Newton, it is
true, believed the Sun to be gaseous, but he propounded no hypothesis of
its constitution. Herschel’s theory, on the other hand, was fully
developed, plausible, and attractive. It was held by eminent men of
science until 1860, when the revelations of the spectroscope showed it
to be quite untenable. The theory was supported for many years by Sir
John Herschel, who, however, abandoned it in 1864. Herschel made several
attempts to ascertain whether any connection existed between the state
of the Sun and the condition of the Earth. In 1801 he was inclined to
believe that “some temporary defect of vegetation” resulted from the
absence of sun-spots, which, he thought, “may lead us to expect a
copious emission of heat, and, therefore, mild seasons.” Herschel
believed, in fact, that food became dear at the times of spot-minima. It
may be remarked that Herschel never noted the spot-period of eleven
years, the discovery of which was afterwards made by Schwabe.

Herschel closely scrutinised the surfaces of the planets. Mercury alone
was neglected by him. From 1777 to 1793 he observed Venus, with the
object of determining the rotation period, but he was unable to observe
any markings on the surface of the planet. He did not place reliance on
Schröter’s value of the rotation period (about twenty-three hours).
Meanwhile, Schröter announced the existence on Venus of mountains which
rose to five or six times the height of Chimborazo. As to these, said
Herschel, “I may venture to say that no eye which is not considerably
better than mine, or assisted by much better instruments, will ever get
a sight of them.” Herschel demonstrated the existence of an extensive
atmosphere round Venus.

“The analogy between Mars and the Earth,” Herschel wrote in 1783, “is
perhaps by far the greatest in the whole Solar System.” In 1777 he
began, in his house at Bath, a series of observations on the red planet,
which yielded results of the utmost importance. Fixing his attention on
the white spots at the north and south poles,—discovered by Maraldi,
nephew of Cassini,—he soon ascertained the fact that they waxed and
waned in size, the north polar cap shrinking during the summer of the
northern hemisphere, increasing in winter, and _vice versa_ in the
southern hemisphere. He regarded the caps as masses of snow and ice
deposited from “a considerable, though moderate, atmosphere,” a theory
now generally accepted. Herschel gave an immense impetus to the study of
Mars. He carefully examined the planet’s surface, and the dark markings
were regarded by him as oceans.

During Herschel’s lifetime the four small planets, Ceres, Pallas, Juno,
and Vesta, were discovered by Piazzi, Olbers, and Harding. The great
astronomer was much interested in these small worlds. He commenced a
search through the Zodiacal constellations for new planets, but failed.
He was of opinion that many minor planets would be discovered. Accepting
Olbers’ theory of the disruption of a primitive planet, Herschel
calculated that Mercury might be broken up into 35,000 globes equal to
Pallas. Meanwhile Herschel named the four new planets “Asteroids,” owing
to their minute size. He estimated the diameter of Ceres at 162 miles
and Pallas at 147 miles, but Professor Barnard’s measures have shown
them to be larger.

In connection with the discovery of the Asteroids, Herschel showed a
very fine spirit. In ‘The Edinburgh Review’ Brougham declared that
Herschel had devised the word “asteroid,” so that the discoveries of
Piazzi and Olbers might be kept on a lower level than his own discovery
of Uranus. Many scientists would have been much offended at this
contemptible insult, but Herschel merely remarked that he had incurred
“the illiberal criticism of ‘The Edinburgh Review,’” and that the
discovery of the Asteroids “added more to the ornament of our system
than the discovery of another planet could have done.”

In Herschel’s time astronomers were acquainted with three of the outer
planets,—Jupiter, Saturn, and Uranus,—all of which were closely studied
by the great astronomer. The belts of Jupiter were supposed by him to be
analogous to the “trade-winds” in the atmosphere of the Earth; while the
drifting-spots on Jupiter’s disc and their irregular movements were
carefully noted. His observations on the four satellites of Jupiter led
him to believe that, like our Moon, they rotated on their axes in a
period equal to that of their revolution round their primary—an opinion
shared by Laplace, and by many modern astronomers.

Herschel’s researches regarding Saturn were, however, much more
important than those on Jupiter. The globe of the planet, the rings and
the satellites, were favourite objects of study at Bath and Slough. In
1794 he perceived a spot on the surface of Saturn, and made the first
determination of the rotation of the planet, which he fixed as 10 hours
16 minutes,—a result confirmed by modern astronomers. The rings were
subjected to the closest scrutiny. Herschel believed them to be solid,
and he also considered them to revolve round Saturn in about 10 hours.
It appears that he observed the famous “dusky ring,” but supposed it to
be a belt on the surface of the planet. He also studied Cassini’s
division in the ring, ascertaining its reality.

On completing his famous 40-foot reflector, Herschel, on August 28,
1789, turned it on Saturn and its five known satellites. Near the
planet, and in the plane of the ring, was seen another object, which
Herschel believed to be a sixth satellite. To settle the question, he
watched the planet for several hours to see if the object would partake
in the planet’s motion. Finding that it did, he announced it as a new
satellite, which he found to revolve round Saturn in 1 day 8 hours.
About three weeks later, on September 17, Herschel discovered another
satellite yet closer to Saturn, revolving round the planet in about 22
hours. These two satellites were not seen by any astronomers except
Herschel; and after his death they could not be observed. His son,
however, rediscovered them.

The eighth satellite, Japetus, was shown by Herschel to rotate on its
axis in a period equal to that of its revolution, and his observations
were confirmed by modern observers. “I cannot,” Herschel said, “help
reflecting with some pleasure on the discovery of an analogy which shows
that a certain uniform plan is carried on among the secondaries of our
Solar System; and we may conjecture that probably most of the satellites
are governed by the same law.” In April 1805 Herschel observed the globe
of Saturn to present not a spherical but a “square-shouldered” aspect.
It was for long believed that this was an optical illusion; but Proctor
and others have shown that it is quite possible for storms in Saturn’s
atmosphere to cause the planet’s apparent distortion in shape.

Herschel paid much attention to the planet Uranus, which he discovered
on March 13, 1781. The discovery of Uranus, which was mentioned in a
previous chapter, was in a sense the most striking of Herschel’s
achievements. Uranus was the first planet discovered within the memory
of man: besides, the discovery enlarged the diameter of the Solar System
from 886 to 1772 millions of miles. Throughout his lifetime Herschel
referred to the planet as the “Georgium Sidus,” out of gratitude to
George III. for appointing him King’s Astronomer; but the astronomers of
France and Germany, who, as Sir Robert Ball remarks, “saw no reason why
the King of England should be associated with Jupiter and Saturn,”
opposed this term. Lalande called the planet “Herschel,” but Herschel’s
countrymen, the Germans, named it Uranus, in keeping with the custom of
designating the planets from the Greek mythology. The name of Uranus
ultimately prevailed.

In January 1787 Herschel discovered two satellites to Uranus, with the
aid of his 20-foot telescope. These satellites he believed to revolve
round Uranus in 8 days and 13 days respectively, and accordingly he made
a drawing of what their positions should be on February 10. On that day
he found them in their predicted places. In 1797 he announced that the
satellites revolved round Uranus in orbits at right angles to the
ecliptic, and in a retrograde direction. In subsequent years Herschel
believed that he had discovered other four satellites to Uranus, but he
was unable to confirm his belief. As Mr Gore says, some of the
satellites “must, therefore, have been either optical ‘ghosts’ or else
small fixed stars which happened to be near the planet’s path at the
time of observation. Herschel also suspected that he could see traces of
rings round Uranus like those round Saturn, but his observation was
never confirmed, either by himself or other observers.”

Although Herschel made several important observations on the Moon, and
measured the heights of the lunar mountains, he was not a devoted
student of our satellite. Caroline Herschel remarks in her memoirs that
if it had not been for clouds or moonlight, neither her brother nor
herself would have got any sleep; adding that Herschel on the moonlight
nights prepared his papers or made visits to London. However, he did
make some investigations, and in 1783 and 1787 believed himself to have
witnessed the eruption of three lunar volcanoes. He afterwards
concluded, however, that what he believed to be eruptions was really the
reflexion of earth-shine from the white peaks of the lunar mountains.
Herschel never discovered a comet, leaving that branch of astronomy to
his sister, who discovered eight of these objects. He was, however, much
interested in comets, and attentively studied them, introducing the
terms of “head,” “nucleus,” and “coma.” Herschel anticipated the view
that comets are not lasting, but are partly disintegrated at their
perihelion passages. He was of opinion that they travelled from star to
star. The extent of their tails and appendages he thought to be a test
of their age.

We have now completed our sketch of Herschel’s important labours
regarding our Solar System. As Miss Clerke says, “A whole cycle of
discoveries and successful investigations began and ended with him.” But
through observing the stars he made a further discovery in connection
with the Solar System; indeed, one of the greatest discoveries in the
history of astronomy—the movement through space of the Sun, carrying
with it planets and comets.

“If the proper motion of the stars be admitted,” said Herschel, “who can
deny that of our Sun?” Of course it was plain that the motion of the Sun
could only be detected through the resulting apparent motion of the
stars. Thus, if the Sun is moving in a certain direction, the stars in
front will appear to open out, while those behind will close up. But the
problem is by no means so easy as this. The stars are also in motion,
and, before the solar motion can be discovered, the proper motions of
the stars—themselves very minute—have to be decomposed into two parts,
the real motion of the star, and the apparent motion, resulting from the
movement of the Solar System. To any astronomer but Herschel the problem
would have been insoluble. Only sixty years had elapsed since Halley had
announced the proper motions of the brighter stars which had been
previously supposed to be immovable—hence the name of “fixed stars.”
Herschel did not deal with the motions of many stars. Only a few proper
motions were known with accuracy when he attacked the problem in 1783.
Making use of the proper motions of seven stars, and separating the real
from the apparent motion, he found that the Solar System was moving
towards a point in the constellation Hercules, the “apex” being marked
by the star λ Herculis. The rate of the solar motion, Herschel thought,
was “certainly not less than that which the Earth has in her annual
orbit.” This extraordinary discovery was one of Herschel’s greatest
works. “Its directness and apparent artlessness,” Miss Clerke remarks,
“strike us dumb with wonder.” In 1805 Herschel again attacked the
subject, utilising the proper motions of thirty-six stars. His second
inquiry, on the whole, confirmed his previous result, the “apex” being
again situated in Hercules; but the determination of 1783 was probably
the more accurate of the two.

Herschel was far in advance of his time regarding the solar motion. The
two greatest astronomers of the next generation, Bessel and Sir John
Herschel, rejected the results reached by Sir William Herschel. But in
1837 Argelander, after a profound mathematical discussion, confirmed
Herschel’s views, and proved the solar motion to be a reality. Since
that date the problem has been attacked by various methods by Otto
Struve, Gauss, Mädler, Airy, Dunkin, Ludwig Struve, Newcomb, Kapteyn,
Campbell, and others, with the result that the reality of the solar
motion and of the direction fixed by Herschel has been proved beyond a
doubt. As Sir Robert Ball well remarks, mathematicians have exhausted
every refinement, “but only to confirm the truth of that splendid theory
which seems to have been one of the flashes of Herschel’s genius.”

In his volume ‘Herschel and his Work,’ Mr James Sime writes: “To
Herschel belongs the credit not merely of having suspected the
revolution of sun around sun in the far-distant realms of space, but
also of actually detecting that this was going on among the stars.”
Throughout his career double stars were favourite objects of
observation. The study of double stars was commenced by Herschel while a
musician in Bath. Before his day, of course, double stars had been
discovered and studied, but it was believed that the proximity of two
stars was merely an optical accident, the brighter star being much
nearer to us than the other. Herschel, at first sharing the general
view, observed double stars in the hope of measuring their relative
parallaxes; assuming one star to be much farther away from the Solar
System than another, he attempted to measure the parallactic
displacement of the brighter star relatively to the position of the
fainter. “This,” he afterwards wrote, “introduced a new series of
observations. I resolved to examine every star in the heavens with the
utmost attention, that I might fix my observations upon those that would
best answer my end. I took some pains to find out what double stars had
been recorded by astronomers; but my situation permitted me not to
consult extensive libraries, nor, indeed, was it very material; for as I
intended to view the heavens myself, Nature, that great volume, appeared
to me to contain the best catalogue.”

Herschel, on January 10, 1782, submitted to the Royal Society a
catalogue of 269 double stars: of these he himself discovered 227. In
December 1784 he forwarded another catalogue, containing 434 stars. He
soon found that he was unable to measure stellar parallax, and the idea
dawned on him that the double stars were physically connected by the law
of gravitation, though he made no announcement to that effect for many
years. On July 1, 1802, Herschel informed the Royal Society that “casual
situations will not account for the multiplied phenomena of double
stars.... I shall soon communicate a series of observations, proving
that many of them have already changed their situation in a progressive
course, denoting a periodical revolution round each other.” In 1803 he
showed that many stars were revolving round their centres of gravity,
proving them, in his own words, to be “intimately held together by the
bond of mutual attraction.” In other words, Herschel discovered that the
law of gravitation prevailed in the Stellar Universe, as well as in our
Solar System—that the law which Newton ascertained to prevail in the
Solar System extended throughout the depth of space.

Herschel did not merely prove the revolution of the binary stars; he
assigned periods to those which he had particularly studied. He believed
the period of Castor to be 342 years; γ Leonis 1200 years; δ Serpentis
375 years; and ε Böotis 1681 years. Herschel did not compute the orbits
mathematically. This was not done for nearly thirty years, when the
calculation of binary star-orbits was commenced by Savary, Sir John
Herschel, and Encke.

In 1782 the French astronomer, _Charles Messier_ (1730-1817), published
a list of 103 nebulæ. In the following year Herschel commenced his
famous sweeps of the heavens with his large reflectors, and during these
he made many remarkable discoveries. In 1786 he published in the
‘Philosophical Transactions’ of the Royal Society a catalogue of a
thousand new nebulæ and star-clusters, in which he gave the position of
each object with a short description of its appearance, written by
Caroline Herschel while her brother actually had the object before his
eyes. In 1786 Herschel published a catalogue of another thousand
clusters and nebulæ, followed in 1802 by a list of 500; making a total
of 2500 clusters and nebulæ discovered by the great astronomer. This
alone would have gained a great name for William Herschel in this branch
of astronomy. In the space of only twenty years 2500 nebulæ and clusters
had been discovered. The various nebulæ and clusters were divided into
eight classes, as follows: the first class being “bright nebulæ,” the
second “faint nebulæ,” the third “very faint nebulæ,” the fourth
“planetary nebulæ,” so named by Herschel from their resemblance to
planetary discs, the fifth class contained “very large nebulæ,” the
sixth “very compressed and rich clusters of stars,” the seventh “pretty
much compressed clusters of large or small stars,” and the eighth
“coarsely scattered clusters of stars.”

At first Herschel believed all nebulæ to be clusters of stars, the
irresolvable nebulæ being supposed to be farther from our system than
the resolvable nebulæ. As many of the nebulæ which Messier could not
resolve had yielded to Herschel’s instruments, Herschel believed that
increase of telescopic power would resolve the hazy spots of light which
remained nebulous. In the paper of 1785, in which Herschel dealt with
the construction of the heavens, he stated his belief that many of the
nebulæ were external galaxies—universes beyond the Milky Way; and in
1786 he remarked that he had discovered fifteen hundred universes!

Arago, Mitchel, Nichol, Chambers, and other writers quite misinterpreted
Herschel’s views on the nebulæ when they said that he believed them to
be all external galaxies. In 1785 Herschel believed many to be connected
with the sidereal system; considering that in some parts of the Galaxy
“the stars are now drawing towards various secondary centres, and will
in time separate into different clusters.” He was coming to the view
that the star-clusters were secondary aggregations within the Galaxy,
probably the true theory. He pointed out that in Scorpio, the cluster
Messier 80 is bounded by a black chasm, four degrees wide, from which he
believed the stars had been drawn in the course of time to form the
cluster. His sister records that one night, after a “long, awful
silence,” he exclaimed on coming on this chasm—“Hier ist wahrhaftig ein
Loch im Himmel!” (Here, truly, is a hole in the heavens.)

Herschel was now gradually giving up his theory of external galaxies and
his “disc-theory” of the Universe; but he still believed even the
nebulous objects to be irresolvable only through immensity of distance.
In 1791, however, he drew attention to a remarkable star in Taurus,
surrounded by a nebulous atmosphere, regarding which he wrote, “View,
for instance, the nineteenth cluster of my sixth class, and afterwards
cast your eye on this cloudy star. Our judgment, I will venture to say,
will be that _the nebulosity about the star is not of a starry_
_nature_. We therefore either have a central body which is not a star,
or have a star which is involved in a shining fluid, of a nature totally
unknown to us.” And with caution he added that “the envelope of a cloudy
star is more fit to produce a star by its condensation than to depend
upon the star for its existence.”

This was written in 1791, five years before Laplace propounded his
nebular theory. Meanwhile Herschel, believing that “these nebulous stars
may serve as a clue to unravel other mysterious phenomena,” found that
the theory of a “shining fluid” would suit the appearance of the
irresolvable planetary nebulæ and the great nebula in Orion much better
than the extravagant idea of “external universes.” Herschel now
considered the Orion nebula to be much nearer to the Solar System than
he formerly did, and ceased to regard it as external to the Galaxy. For
twenty years Herschel patiently observed the nebulæ, and it was not
until 1811 that he propounded his nebular hypothesis of the evolution of
the Sun and stars. He found the gaseous matter in all stages of
condensation, from the diffused cloudy nebulæ like that in Orion,
through the planetary nebula and the regular nebula, to the perfect
stars, like Sirius and the Sun. Herschel’s nebular theory was a grand
conception, and a magnificent attack on the secrets of nature.

Sir Robert Ball says: “Not from abstract speculation like Kant, nor from
mathematical suggestion like Laplace, but from accurate and laborious
study of the heavens, was the great William Herschel led to the
conception of the nebular theory of evolution.” Herschel’s nebular
theory was wider and less rigorous than that of Laplace. Laplace reached
his theory by reasoning backwards; Herschel by observing the nebulæ in
process of condensation. Consequently, while Laplace’s theory has
required modification, Herschel’s, from its width, is universally
accepted, because there is nothing mathematically rigorous in it. The
great German did not go into details like his French contemporary. He
sketched the evolution of the stars in a wider sense.

The astronomer’s “1500 universes,” Miss Clerke remarks, “had now
logically ceased to exist.” Herschel had gathered much evidence about
nebular distribution which shattered his belief in external universes,
although he still thought in 1818 that some galaxies were included among
the non-gaseous nebulæ. In 1784 Herschel pointed out that the clusters
and nebulæ “are arranged to run in strata”; and some time later he found
that the nebulæ were aggregated near the galactic poles; in other words,
where nebulæ are numerous, stars are scarce, and _vice versa_. So
rigorously did this rule hold, that when dictating his observations to
his sister Caroline, he would, on noting a paucity of stars, warn her to
“prepare for nebulæ.”

“A knowledge of the construction of the heavens has always been the
ultimate object of my observations.” So Herschel wrote in 1811. All his
investigations were secondary to the problem which was constantly before
his mind—the extent and structure of the Universe. He aspired to be the
Copernicus of the Sidereal System. Although Bruno, Kepler, Wright, Kant,
and Lambert had speculated regarding the construction of the heavens,
they had not the slightest evidence on which to base their ideas. There
was no science of sidereal astronomy. The stars were observed only to
assist navigation, and the primary object of star-catalogues was to
further knowledge of the motions of the planets. In Herschel’s day,
also, the distances of the stars had not been measured, and he had to
base his views on the distribution of the stars. In 1784, therefore, he
commenced a survey of the heavens, in order to ascertain the number of
stars in various parts of the sky. This method, which he named
“star-gauging,” consisted in counting the number of stars in the
telescopic field. Totally he secured 3400 gauges. His studies showed
that in the region of the Galaxy the stars were much more numerous than
near the galactic poles. Sometimes he saw as many as 588 stars in a
telescopic field, at other times only 2. He remarked that he had “often
known more than 50,000 pass before his sight within an hour.” Assuming
that the stars were, on the average, of about the same size, and
scattered through space with some approach to uniformity, Herschel was
able to compute the extent to which his telescope penetrated into space;
and, assuming that the Universe was finite and that his
“gauging-telescope” was sufficiently powerful to completely resolve the
Milky Way, he was enabled to sketch the shape and extent of the
Universe.

Thus Herschel concluded that the Universe extended in the direction of
the Galaxy to 850 times the mean distance of stars of the first
magnitude. In the direction of the galactic poles the thickness was only
155 times the distance of stars of the same magnitude. Herschel was thus
enabled to sketch the probable form of the Universe, which he regarded
as cloven at one of its extremities, the cleft being represented by the
famous gap in the Milky Way. The Universe was, in fact, supposed to be a
cloven disc, and the Milky Way was merely a vastly extended portion of
it and not a region of actual clustering. On this theory the clusters
and nebulæ were supposed to be galaxies external to the Universe. Even
in 1785, however, Herschel believed that there were regions in the Milky
Way where the stars were more closely clustered than others. “It would
not be difficult,” he wrote in 1785, “to point out two or three hundred
gathering clusters in our system.”

Strange to say, Herschel’s original ideas regarding the Universe were
accepted for many years by astronomical writers. Arago accepted
Herschel’s original theory, unaware that he had in reality abandoned it,
and he was followed by a host of French and English writers who did not
take the trouble to read each of Herschel’s papers, merely quoting that
of 1785, and believing that it represented his final ideas on the
subject. Even Sir John Herschel seems to have been unaware that his
father gave up the disc theory of the Universe. The famous German
astronomer, Wilhelm Struve, after an exhaustive study of Herschel’s
papers, was enabled to prove in 1847 that the theory had been abandoned
by Herschel; and in England the late R. A. Proctor independently
demonstrated the same thing. Meanwhile, supposing Herschel had not given
up his theory, it would be quite untenable. After considering the fact
that the brighter stars, down to the ninth magnitude, aggregate on the
Milky Way, Mr Gore says: “As the stars are by hypothesis supposed to be
uniformly distributed throughout every part of the disc, and as the
limiting circles for stars to the eighth and ninth magnitudes fall well
within the thickness of the disc, there is no reason why stars of these
magnitudes should not be quite as numerous in the direction of the
galactic poles as in that of the Milky Way itself. We see, therefore,
that the disc theory fails to represent the observed facts, and that
Struve and Proctor were amply justified in their opinion that the theory
is wholly untenable, and should be abandoned.”

The observations made by Herschel himself eventually proved fatal to the
disc theory—a hypothesis which he had all along held very lightly. His
ideas about subordinate clusters within the Milky Way were soon
confirmed, and though in 1799 he still adhered to the disc theory, he
wrote in 1802, “I am now convinced, by a long inspection and continued
examination of it, that the Milky Way itself consists of stars very
differently scattered from those which are immediately about us. This
immense starry aggregation is by no means uniform. The stars of which it
is composed are very unequally scattered”—a conclusion quite opposed to
the disc theory, where the Milky Way was supposed to be merely an
extended portion of the Universe.

In 1811 Herschel wrote as follows: “I must freely confess that by
continuing my sweeps of the heavens, my opinion of the arrangement of
the stars, and their magnitudes, and some other particulars, has
undergone a gradual change; and, indeed, when the novelty of the subject
is considered we cannot be surprised that many things formerly taken for
granted should on examination prove to be different from what they were
generally but incautiously supposed to be. For instance, an equal
scattering of the stars may be admitted in certain calculations; but
when we examine the Milky Way, or the closely compressed clusters of
stars, of which my catalogues have recorded so many instances, this
supposed equality of scattering must be given up.”

This was the virtual abandonment of the disc theory. Six years later
Herschel announced that in six cases he had failed to resolve the Milky
Way, stating that his telescopes could not fathom it. This was the
abandonment of his second assumption—namely, that his telescope was
sufficiently powerful to penetrate to the limits of the Universe. Yet he
still thought that some of the star-clusters might be external galaxies,
although he could not even dogmatically assert our Universe to be
limited. In an error of translation, Struve left the impression that
Herschel believed our Universe to be unfathomable or infinite, and was
obliged to devise a most artificial theory of the extinction of light to
account for the fact that the sky did not shine with the brilliance of
the Sun, which it would do were the stars infinite in number. Of course,
Herschel did not actually believe the Universe to be infinite, and, had
he lived, he would probably have shown that all the star-clusters which
we see are included within the bounds of our finite Galaxy.

In 1814 Herschel was “still engaged in a series of observations for
ascertaining a scale whereby the extent of the Universe, as far as it is
possible for us to penetrate into space, may be fathomed.” In 1817 he
described another method of star-gauging, which Arago and other writers
have confused with that which he devised in 1785. The two methods,
however, were quite distinct from each other. In the first system, one
telescope was used on different regions of the heavens; whereas in the
second method, various telescopes were used on identical regions. The
principle was that the telescopic power necessary to resolve groups of
stars indicates the distance at which the stars of the groups lie. This,
however, also assumed an equal distribution of stars, and as the late Mr
Proctor says, “I conceive that no question can exist that the principle
is unsound, and that Herschel would himself have abandoned it had he
tested it earlier in his observing career.... In applying it, Sir W.
Herschel found regions of the heavens very limited in extent, where the
brighter stars (clustered like the fainter) were easily resolved with
low powers, but where his largest telescopes could not resolve the
faintest. These regions, if the principle were true, must be long,
spike-shaped star groups, whose length is directed exactly towards the
astronomer on Earth,—an utterly incredible arrangement.”

Herschel, at the time of his death, left unsolved the problem of the
construction of the heavens. It is still unsolved, and will doubtless
remain so until astronomers know more about the distances and motions of
the stars. His last observation of the Galaxy showed that even with his
40-foot reflector he could not fathom it. Consequently, as we have
mentioned, Struve and his successors regarded the Universe as infinite—a
theory which has now received its death-blow. Herschel was undoubtedly
correct when he stated his belief in a limited Universe.

Herschel’s star-gauges, and those of his son, still remain of immense
value to astronomers in any discussion of the construction of the
heavens. Thus, although they failed to reveal to Herschel the structure
of the Universe, they have been of much use to his successors.
Herschel’s discussion of the supreme problem—the ultimate object of his
observations—constitutes one of the most interesting chapters in the
history of science, and marks a new era in human thought. In the words
of Miss Clerke: “One cannot reflect without amazement that the special
life-task set himself by this struggling musician—originally a penniless
deserter from the Hanoverian Guard—was nothing less than to search out
the ‘construction of the heavens.’ He did not accomplish it, for that
was impossible; but he never relinquished, and, in grappling with it,
laid deep and sure the foundations of sidereal science.”




                              CHAPTER III.
                                THE SUN.


Four years after the death of Herschel, an apothecary in the little
German town of Dessau procured a small telescope, with which he began to
observe the Sun. The name of this apothecary was _Samuel Heinrich
Schwabe_ (1789-1875). In 1826 he commenced to observe the spots on the
Sun’s disc, counting them from day to day, more for self-amusement than
from any hope of discovery; for previous astronomers had agreed that no
law regulated the number of the sun-spots. Every clear day Schwabe
pointed his telescope at the Sun and took his record of the spots; this
he continued for forty-three years, until within a few years of his
death on April 11, 1875. As early as 1843 Schwabe hinted that a possible
period of ten years regulated the distribution of the spots on the Sun,
but no attention was given to his idea. In 1851, however, the result of
his twenty-six years of observation was published in Humboldt’s
‘Cosmos,’ and Schwabe was able to show that the spots increased and
decreased in a period of about ten years. Astronomers at once recognised
the importance of Schwabe’s work, and in 1857 he was rewarded by the
Gold Medal of the Royal Astronomical Society of London.

_Rudolf Wolf_ (1813-1892) of the Zürich Observatory now undertook to
search through the records of sun-spot observation, from the days of
Galileo and Scheiner, to find traces of the solar cycle discovered by
Schwabe. He was successful, and was enabled to correct Schwabe’s
estimate of the length of the period, fixing it as on the average 11·11
years. Additional interest, however, was given to Schwabe’s and Wolf’s
investigations by the remarkable discoveries which followed. In
September 1851 _John Lamont_ (1805-1879), a Scottish astronomer,—born at
Braemar in Aberdeenshire, but employed as director of the Munich
Observatory,—after searching through the magnetic records collected at
Göttingen and Munich, discovered that the magnetic variations indicated
a period of 10⅓ years. Soon after this Sir _Edward Sabine_ (1788-1883),
the English physicist, from a discussion of an entirely different set of
observations, independently demonstrated the same thing, proving
conclusively that once in about ten years magnetic disturbances reached
their height of violence; and Sabine was not slow to notice the
correspondence between the magnetic period and the sun-spot period. In
the same year (1852) Wolf and _Alfred Gautier_ (1793-1881) independently
made the same discovery, which had thus been made by four separate
investigators.

In the same year an English amateur astronomer, _Richard Christopher
Carrington_ (1826-1875), commenced a series of solar observations which
led to some remarkable discoveries. From observations on the spots,
Carrington discovered that while the Sun’s rotation was performed in 25
days at the equator, it was protracted to 27½ days midway between the
equator and the poles. In 1858 Carrington demonstrated the fact that
spots are scarce in the vicinity of the solar equator, but are confined
to two zones on either side, becoming scarce again at thirty-five
degrees north or south of the equator. Contemporary with Carrington was
_Friedrich Wilhelm Gustav Spörer_ (1822-1895), who was born in Berlin in
1822 and died at Giessen, July 7, 1895. He commenced his solar
observations about the same time as Carrington, and independently
discovered the Sun’s equatorial acceleration. From observations at his
little private observatory at Anclam in Pomerania, continued at the
Astrophysical Observatory in Potsdam, Spörer demonstrated a remarkable
law regarding sun-spots. This law is thus described by a well-known
astronomer: “The disturbance which produces the spots of a given
sun-spot period first manifests itself in two belts about thirty degrees
north and south of the Sun’s equator. These belts then draw in toward
the equator, and the sun-spot maximum occurs when their latitude is
about sixteen degrees; while the disturbance gradually and finally dies
out at a latitude of eight or ten degrees. Two or three years before
this disappearance, however, two new zones of disturbance show
themselves. Thus, at the sun-spot minimum there are four well-marked
spot-belts,—two near the equator, due to the expiring disturbance, and
two in high latitudes, due to the newly beginning outbreak.” These
remarkable discoveries, which resulted from the investigations of
Schwabe, Carrington, and Sporer, are a brilliant example of what may be
done by amateurs in astronomy.

At the time when Carrington and Spörer were pursuing these researches,
the spectroscope came into use as an astronomical instrument, and since
1859 solar astronomy has been almost entirely spectroscopic. Before we
can rightly understand the principles of spectroscopic astronomy, we
must go back to the life and work of its founder—Joseph von Fraunhofer.

The son of a poor glazier, _Joseph von Fraunhofer_ was born on March 6,
1787, at Straubing, in Bavaria. His father and mother having died when
their son was quite young, the boy, on account of his poverty, was
apprenticed to a looking-glass manufacturer in Munich named
Weichselberger, who acted tyrannically, keeping him all day at hard
work. Still the lad borrowed some old books, and spent his nights in
study. Young Fraunhofer lodged in an old tenement in Munich, which on
July 21, 1801, collapsed, burying in its ruins its occupants. All were
killed but Fraunhofer, who, though seriously injured, was dug out from
the ruins four hours later. The distressing accident was witnessed by
Prince Maximilian Joseph, Elector of Bavaria. He became interested in
Fraunhofer, and presented him with a sum of money. Of this he made good
use. He was already interested in optics, and he bought some books on
that subject, as well as a glass-polishing machine. The remainder of the
money served to procure his release from his tyrannical master,
Weichselberger.

Fraunhofer became acquainted with prominent scientists at Munich, who
provided him with books on optics and mathematics. Meanwhile the young
optician occupied his time in shaping and finishing lenses. In 1806 he
entered the optical department of the Optical and Physical Institute of
Munich, and the following year, when only twenty years of age, was
appointed to the chief post in that department. In 1814 he commenced his
investigations with the prism, which have made his name famous.

Newton had found that, in passing through a prism, white light is
dispersed into its primary colours, making up the band of coloured light
known as the solar spectrum. But he failed to recognise the existence of
dark lines in the spectrum. Casually seen in 1802 by _William Hyde
Wollaston_ (1786-1828), an English physicist, these lines were first
thoroughly examined by Fraunhofer. Allowing light from the Sun to pass
through a prism attached to the telescope, he was amazed to find several
dark lines in the spectrum. By the year 1814 he had detected no less
than 300 or 400 of these lines. Fraunhofer named the more prominent
lines by the letters of the alphabet, from A in the red to H in the
violet. They are now known as the Fraunhofer lines. At first he was much
perplexed regarding the nature of the dark lines. He suspected that they
might be an optical effect, depending on the quality of the glass used,
and he tried different prisms, but the lines were still to be seen. Then
he turned his prism to bright clouds to see if they were visible in
reflected sunlight, and he found that they were. He examined the Moon
and again perceived them, as moonlight is merely reflected sunlight; and
they were also conspicuous in the spectra of the planets. It was thus
proved that these lines were characteristic of sunlight, whether direct
or reflected. It was, however, still possible that they might be caused
by the passage of the rays of light from the celestial bodies through
the Earth’s atmosphere. In order to test this theory, Fraunhofer
examined the spectra of the brighter stars. He found that the lines
visible in the solar spectrum were not to be seen in the spectra of the
stars, thus proving that the lines were not merely an atmospheric
effect. Each star, Fraunhofer observed, had a different spectrum from
both the Sun and from other stars. These spectra were also characterised
by numerous dark lines, much fainter than those in the solar spectrum.

Although he ascertained the existence of the dark lines in the Sun’s
spectrum, Fraunhofer never really found out what they represented. As
Miss Giberne expresses it, “Although he now saw the lines he could not
understand them: he could not read what they said. They spoke to him
indeed about the Sun, but they spoke to him in a foreign language, the
key to which he did not possess.” However, he expressed the belief that
the pair of lines in the solar spectrum, which he marked D, coincided
with the pair of bright lines emitted by incandescent sodium. Although
he doubtless suspected that the lines conveyed intelligence regarding
the elements in the Sun, he never was able properly to decipher their
meaning. Had he lived, he would probably have made the great discovery;
but these investigations were cut short by his sudden and untimely death
on June 7, 1826.

After the death of Fraunhofer, very little was done to forward the study
of spectrum analysis. Investigations in this branch of research were
made, however, by Sir _John Herschel_ (1792-1871), _William Allen
Miller_ (1817-1870), Sir _David Brewster_ (1781-1868), and others. Two
famous men of science had partly discovered the secret. These were Sir
_George Stokes_ (1819-1903), of Cambridge, and _Anders John Angström_
(1812-1872) of Upsala. Of Angström’s work, published in 1853, it has
been said that it would “have obtained a high celebrity if it had
appeared in French, English, or German, instead of Swedish.”

It was not until 1859 that the principles of spectrum analysis were
fully enunciated by _Gustav Robert Kirchhoff_ (1824-1887), and his
colleague in the University of Heidelberg, _Robert Wilhelm Bunsen_
(1811-1899). Kirchhoff demonstrated that a luminous solid or liquid
gives a continuous spectrum, and a gaseous substance a spectrum of
bright lines. In the words of Miss Clerke, “Substances of every kind are
opaque to the precise rays which they emit at the same temperature. That
is to say, they stop the kinds of light or heat which they are then
actually in a condition to radiate.... This principle is fundamental to
solar chemistry. It gives the key to the hieroglyphics of the Fraunhofer
lines. The identical characters which are written bright in terrestrial
spectra are written dark in the unrolled sheaf of sun-rays.” Kirchhoff
made several determinations of the substances in the Sun, proving the
existence of sodium, iron, calcium, magnesium, nickel, barium, copper,
and zinc. His great map of the solar spectrum was published by the
Berlin Academy in 1860, and represented an enormous amount of labour. It
was succeeded by another map by Angström, published in 1868. But both of
these maps have been recently superseded by the investigations of Sir
_Joseph Norman Lockyer_ (born 1836), and of the American physicist,
_Henry Augustus Rowland_ (1848-1901). Rowland largely increased our
knowledge of the elements in the solar atmosphere.

The spectroscope had become, by 1868, a recognised instrument of
astronomical research, and in that year it was applied during the famous
total eclipse, visible in India. There were many eclipse problems,
arising from the observations made by the eclipse expeditions of 1842,
1851, and 1860. The eclipse of 1851 had finally proved that the red
flames seen surrounding the Sun during total eclipses belonged to the
Sun, and not to the Moon, as many astronomers had believed. At the
eclipse of 1860, visible in Spain, the Italian astronomer, _Angelo
Secchi_ (1818-1878), and the Englishman, _Warren De la Rue_ (1815-1889),
secured photographs of the solar prominences. The problem of 1868 was
the constitution of these prominences.

_Pierre Jules César Janssen_, born in Paris in 1824, was stationed at
Guntoor, in India, to observe the eclipse. He succeeded in observing the
spectrum of the prominences during the progress of totality, and found
it to be one of bright lines, proving the gaseous nature of the
sun-flames. During the progress of the eclipse, Janssen was specially
struck by the brilliancy of the bright lines, and it occurred to him
that the prominence-spectrum could be observed in full daylight, if
sufficient dispersive power was used to enfeeble the ordinary continuous
spectrum. At ten o’clock on the following morning, August 19, 1868,
Janssen applied his spectroscope to the sun, and observed the
prominence-spectrum. After a month’s observation in India, he sent to
the French Academy an account of his success. A short time, however,
before his report arrived, the Academy had received a similar one from
Lockyer, who had independently made the same discovery. Two years
previously, in 1866, the new method had occurred to him, but his
spectroscope was not powerful enough; and although he ordered a more
powerful one at once, it was not until October 16, 1868, that he had the
instrument in his hands. Four days later he observed the
prominence-spectrum in full daylight.

The next advance in the study of the prominences was announced in 1869.
Janssen and Lockyer had shown astronomers how to observe the spectrum of
the prominences; but the researches of other two famous astronomers
enabled observers to see the forms of the prominences. These two men
were _William Huggins_ (born 1824) and _Johann Carl Friedrich Zöllner_.
The latter astronomer, born in Leipzig in 1834, was one of the most
successful students of the solar prominences. He was Professor of
Astrophysics in the University of Leipzig, a position which he filled
with success until his untimely death on April 25, 1882. Independently
of Huggins, he found that by opening the slit of the spectroscope wider,
the forms of the prominences themselves could be seen. The study of the
prominences was at once taken up by the most famous solar observers:
these were Huggins and Lockyer in England, Spörer and Zöllner in
Germany, Janssen in France, Secchi, Respighi, and Tacchini in Italy,
Young in America. To _Charles Augustus Young_ (born 1834) we owe the
careful study of individual prominences. On September 7, 1871, he
observed the most gigantic outburst on the sun ever witnessed, fragments
of an exploded prominence reaching a height of 100,000 miles: Young,
also, made the first attempt to photograph the prominences.

To the Italian school of astronomers, however, we owe the persistent and
systematic study of the prominences. Among them the three greatest names
are _Angelo Secchi_ (1818-1878), _Lorenzo Respighi_ (1824-1889), and
_Pietro Tacchini_ (1838-1905). After the death of Secchi, the recognised
head of spectroscopy in Italy was Pietro Tacchini. Born at Modena in
1838, he was appointed director at Modena in 1859, assistant at Palermo
in 1863, and director at Rome in 1879. In 1870 he commenced to take
daily observations of the prominences, noting their sizes, forms, and
distribution, and these observations were continued for thirty-one
years, until within four years of Tacchini’s death, which took place on
March 24, 1905. Tacchini did for the study of the prominences what
Schwabe did for the spots. The Italian spectroscopists found that the
prominences increased and decreased every eleven years in harmony with
the spots. Tacchini demonstrated that the streamers of the solar corona
originate in regions where the prominences are most numerous, and that
the shape of the corona, on the whole, varies in sympathy with the
prominences.

The researches of Lockyer indicated that the prominences originated in a
shallow gaseous atmosphere which he termed the chromosphere. Formerly
astronomers had to observe only isolated prominences, but in 1892 an
American astronomer, _George Ellery Hale_ (born 1868), formerly director
of the Yerkes Observatory, and now director of the Solar Observatory in
California, succeeded in photographing, by an ingenious process, the
whole of the chromosphere, prominences, and faculæ visible on the solar
surface.

Another solar envelope was discovered in 1870 by Dr Charles Augustus
Young, who from 1866 to 1877 directed the Observatory at Dartmouth, New
Hampshire, and from 1877 to 1905, that at Princeton, New Jersey. During
the eclipse of December 22, 1870, Young was stationed at Tenez de
Frontena, Spain. As the solar crescent grew apparently thinner before
the disc of the Moon, “the dark lines of the spectrum,” he says, “and
the spectrum itself gradually faded away, until all at once, as suddenly
as a bursting rocket shoots out its stars, the whole field of view was
filled with bright lines, more numerous than one could count. The
phenomenon was so sudden, so unexpected, and so wonderfully beautiful,
as to force an involuntary exclamation.” The phenomenon was observed for
two seconds, and the impression was left on the astronomer that a bright
line had taken the place of every dark one in the solar spectrum, the
spectrum being completely reversed. Hence the name which was given to
the hypothetical envelope—“the reversing layer.” For long the existence
of the reversing layer was disputed by numerous astronomers. In 1896
photographs taken during the solar eclipse of that year finally
demonstrated the existence of the “flash spectrum” as seen by Young.

The last of the solar appendages, the corona, can only be seen during
total eclipses. The researches of Young and Janssen indicate that it is
partly gaseous and partly meteoric in its constitution; and various
photographs, taken at the eclipses since 1870, have demonstrated its
variation in shape, which is in harmony with the eleven-year period.
Several attempts have been made to observe the corona without an
eclipse. In 1882 Huggins made the attempt, but failed, and Hale, with
his photographic process, had no better success. More recently, in 1904,
a Russian astronomer, _Alexis Hansky_, observing from the top of Mont
Blanc, secured some photographs on which he believes the corona is
represented, but so far his observations have not been confirmed by
other astronomers.

The application of the spectroscope to the motions on the solar surface
is perhaps one of the most wonderful triumphs in astronomical science.
In 1842 _Christian Doppler_ (1803-1853), Professor of Mathematics at
Prague, had expressed the view that the colour of a luminous body must
be changed by its motion of approach or recession. It was obvious to
Doppler that if the body was approaching, a larger number of light waves
must be entering the eye of the observer than if it were retreating.
Miss Clerke thus illustrates Doppler’s principle: “Suppose shots to be
fired at a target at fixed intervals of time. If the marksman advances,
say, twenty paces between each discharge of his rifle, it is evident
that the shots will fall faster on the target than if he stood still;
if, on the contrary, he retires by the same amount, they will strike at
correspondingly longer intervals.” It occurred to various astronomers
that it would be possible to measure cyclones and hurricanes in the Sun,
not by change of colour in the spectrum, but by the shifting of the
lines; and in 1870 this was successfully done by Lockyer. In the next
few years efforts to measure the solar rotation were made by Young,
Zöllner, and others, who succeeded in measuring the displacement of the
lines, but not the time of rotation. This was reserved for the famous
Swedish astronomer, Dunér.

_Nils Christopher Dunér_, born in 1839 in Scania, was employed as an
assistant at Lund Observatory from 1858 to 1888, when he was appointed
director of the Observatory at Upsala. In that year he commenced a study
of the solar rotation, measuring it by means of Doppler’s principle. He
confirmed the telescopic work of Carrington and Spörer on the equatorial
acceleration, and measured the displacement up to within fifteen degrees
of the poles. He brought out the surprising fact that the rotation
period of the Sun is there protracted to 38½ days. These remarkable
researches were published in 1891.

In 1873 the Astronomer-Royal of England commenced at Greenwich
Observatory to photograph the Sun daily. This work has been carried on
there by _Edward Walter Maunder_ (born 1851), and Greenwich Observatory
possesses a photographic record of sun-spots. At the Meudon
Astrophysical Observatory, near Paris, Janssen has, since 1876, secured
photographs of the solar surface, which were comprised in a great atlas,
published by him in January 1904. These photographs have revealed a
remarkable phenomenon—the “réseau photospherique,” the distribution over
the solar surface of blurred patches of light, which Janssen considers
are inherent in the Sun. The Greenwich records of sun-spots and of
magnetic disturbances have been made use of by Maunder in his remarkable
studies, promulgated in 1904, of the connection between sun-spots and
terrestrial magnetism. Maunder finds that on the average magnetic storms
are dependent on the presence of sun-spots, and on the size of the spot.
The magnetic action, he finds, does not radiate equally in all
directions from the sun-spots, but along definite and restricted lines.

Herschel’s hypothesis of a dark and cool globe beneath the solar
photosphere was seen to be untenable after the introduction of the
spectroscope. The first important theory as to the solar constitution
was that advanced in 1865 by the French astronomer, _Hervé Faye_
(1814-1902). Numerous other theories were afterwards advanced by Secchi,
Zöllner, Young, and others, but a complete description of the various
developments in solar theorising cannot be given here. There is no
complete “theory” of the exact constitution of every part of the Sun,
but the unpretentious “Views of Professor Young on the Constitution of
the Sun,” which appeared in April 1904 in ‘Popular Astronomy,’ represent
the latest ideas of the foremost solar investigator. Professor Young
regards the reversing layer and the chromosphere as “simply the
uncondensed vapours and gases which form the atmosphere in which the
clouds of the photosphere are suspended.” He says that the contraction
theory of Helmholtz,—explained in another chapter,—advanced to explain
the maintenance of the Sun’s heat, is true so far as it goes; but that
it is all the truth is now made doubtful by the discovery of radium,
which “suggests that other powerful sources of energy may co-operate
with the mechanical in maintaining the Sun’s heat.”

The important question of the distance of the Sun was thoroughly
investigated in 1824 by _Johann Franz Encke_ (1791-1865), then of
Seeberg, near Gotha, who, from a discussion of the transits of Venus in
1761 and 1769, found a parallax of 8″·571, corresponding to a mean
distance of 95,000,000 miles. This value was accepted for thirty years,
until _Peter Andreas Hansen_ (1795-1874), in 1854, and _Urban Jean
Joseph Le Verrier_ (1811-1877), in 1858, found from mathematical
investigations that the distance indicated was too great. Preparations
were accordingly made for the observation of the transits of Venus,
which took place respectively on December 8, 1874, and December 6, 1882.
On the first occasion many expeditions were sent to view the transit,
consisting of French, German, American, English, Scottish, Italian,
Russian, and Dutch astronomers, and it was hoped that the solar parallax
would be accurately measured once for all. However, the transit,
although favoured with good weather, was not successful, owing to the
difficulty of making exact measurements, by reason of the illumination
and refraction in the atmosphere of Venus. Accordingly the values
deduced for the parallax were far from unanimous. The transit of 1882
was not observed so extensively, as astronomers had found the transit of
Venus to be by no means the best method. In 1877 Sir _David Gill_ (born
1843), the great Scottish astronomer, determined the solar parallax
successfully from measures of the parallax of Mars in opposition. His
value was 8″·78, corresponding to 93,080,000 miles. Some years previous
to this _Johann Gottfried Galle_ (born 1812), the German astronomer,
had, from measurements of the parallax of the asteroid Flora, deduced a
solar parallax of 8″·87. Gill’s work at the Cape in 1888, on the
Asteroids, was successful in giving a more accurate value than the
transit of Venus: in 1900 and 1901 measures of the parallax of the
asteroid Eros, the nearest minor planet, were made by many different
observatories, and agree with the other results. The values which have
been derived from the velocity of light, and from the constant of
aberration, are fairly in agreement with those derived from direct
measurement. On the whole, the most probable value of the parallax is
about 8″·8, indicating a mean distance of about 92,700,000 miles, with a
“probable error” of about 150,000 miles.

What a different picture the sun presents to us at the beginning of the
twentieth century from that which it presented to Herschel and his
contemporaries at the beginning of the nineteenth! To Herschel, the Sun
was a cool dark globe, surrounded by a luminous atmosphere. As the
outcome of the researches and discoveries outlined in this chapter, the
Sun is now seen to be a vast central world, which is over a million
times larger than the Earth. In the words of an able writer, “It is most
probably a world of gases, where most of the metals and metallic gases
that we know exist only as vapours, even at the Sun’s surface, hotter
than any furnace on earth, and getting a still fiercer heat for every
mile of descent lower. Of that heat in the Sun’s interior we can form no
conception. The pressure within the Sun is equally inconceivable. A
cannon-ball weighing 100 lb. on earth would weigh 2700 on the Sun. Thus
a mighty conflict goes on unceasingly between imprisoned and expanding
gases and vapours struggling to burst out, and massive pressures holding
them down. For reasons we cannot fully understand, no equilibrium is
reached. For millions of years up-rushes and down-rushes of the
white-hot materials have been proceeding on that bright photosphere
which gives us light, and looks a picture of calm and quiescence. Above
that is a comparatively thin rose-coloured layer, the chromosphere,
agitated with fiery ‘prominences,’ and outside all these the coronal
glory—all alike pointing to immeasurable activities.”

The following remark of Professor Newcomb shows our inability to realise
the solar activity. “Suppose,” he says, “every foot of space in a whole
country covered with 13-inch cannon, all pointed upward, and all
discharged at once. The result would compare with what is going on
inside the photosphere about as much as a boy’s popgun compares with the
cannon.”




                              CHAPTER IV.
                               THE MOON.


It is somewhat remarkable that the one celestial body which Herschel
neglected was our satellite, the Moon; and it is also remarkable that
the Moon was for many years the chief object of study of his
contemporary astronomer, _Johann Hieronymus Schröter_ (1745-1816). Born
at Erfurt, near Hanover, on August 30, 1745, Johann Hieronymus Schröter
was originally intended for the study of law, for which he was sent to
the University of Göttingen. At the same time he studied mathematics,
and particularly astronomy, under the mathematician, Kaestner of
Göttingen. Deeply interested in music, he became acquainted with the
Herschel family, and, inspired by William Herschel’s example, determined
to study the heavens. In 1779 he became the possessor of a small
achromatic refractor, and commenced to observe the Sun and Moon. In 1778
he entered the legal profession at Hanover, and four years later he was
appointed “oberamtmann” or Chief Magistrate of Lilienthal—“the Vale of
Lilies”—in the Duchy of Bremen. At Lilienthal Schröter erected a small
observatory, and acquired in 1785 one of Herschel’s 7-foot reflectors.
In 1792 the astronomer superintended the construction of a 13-foot
reflector, made by Schrader of Kiel, who transferred his workshop to
Lilienthal. With these instruments the great work of Schröter was
accomplished.

Schröter directed his powers of observation to the study of the Moon. He
originated the study of the surface of the Moon, and founded the branch
of astronomy known as selenography, or the study of the Moon’s surface.
The foundations of this branch were laid in 1791 with the publication of
Schröter’s ‘Seleno-topographische Fragmente’. The astronomer determined
to make a comparative study of the surface of our satellite, and before
1801 discovered eleven “rills” or clefts on the Moon’s surface, and
recognised a large number of craters. He likewise believed that he had
seen a lunar atmosphere, an observation of which was made by him in
February 1792. Schröter seems never to have doubted what Herschel and
his contemporaries believed—that the Moon was a living world with
volcanoes in active eruption, surrounded by an atmosphere, and inhabited
by beings like ourselves. Unfortunately, Schröter was not good at making
drawings of what he saw; nevertheless, he accomplished a vast amount of
work. In the little observatory at Lilienthal the foundations were laid
of the comparative study of the surface of the Moon.

But these observations were destined to be rudely interrupted. In 1810
Hanover was occupied by the invading troops of Napoleon, and Schröter
lost his appointment as Chief Magistrate of Lilienthal, and also his
income. But there was worse to follow. On April 20, 1813, three years
after, the French, under Vandamme, with that cruelty which seems to
belong to warfare, occupied Lilienthal, and set fire to the little
village. A few days later the French soldiers entered the observatory
and burned it to the ground. All Schröter’s precious observations,
accumulated after thirty-four years’ labour, were destroyed with a few
exceptions, the observations on Mars narrowly escaping the
conflagration. Unable to forget the destruction of his observatory, and
without the means to repair the loss, he lived only three years after
the disaster. He died on August 29, 1816, “leaving behind him,” says Mr
Arthur Mee, “an imperishable record, and a noble example to observers of
all time.”

_Wilhelm Gotthelf Lohrmann_, a land-surveyor of Dresden, continued the
observations of Schröter, and in 1824 published four of the twenty-five
proposed sections of a large lunar chart. In 1827, however, his sight
began to fail, and he was obliged to abandon his intention. But a
successor had already appeared on the scene. _Johann Heinrich von
Mädler_ (1794-1874) was born in Berlin in 1794, and, after a severe
struggle to earn a living, entered the University of Berlin in 1817. In
1824 he became acquainted with _Wilhelm Beer_ (1797-1850), a wealthy
banker, who had come to him for instruction in astronomy, and who
erected in 1829 an observatory near his villa in Berlin, where pupil and
tutor pursued their studies.

In 1830 Mädler, with Beer’s assistance, commenced a great
trigonometrical survey of the surface of the Moon. The observations of
Beer and Mädler were made with no larger instrument than a 3¾-inch
refractor. They ascertained the positions of 919 lunar spots, and
measured the height of 1095 mountains. Their great chart of the
Moon—which was afterwards followed by a smaller one—was issued in four
parts during 1834-36. “The amount of detail,” wrote Proctor, “is
remarkable, and the labour actually bestowed upon the work will appear
incredible.” The chart has neither been revised nor superseded, and it
remains to this day one of the standard works on the subject.

The chart was succeeded in 1837 by a descriptive volume entitled ‘Der
Mond.’ In this work Beer and Mädler did much for the progress of lunar
astronomy. Their observations led to a change of opinion regarding our
satellite’s physical condition. Herschel, Schröter, Olbers, and other
astronomers seem to have considered the Moon a living world. Mädler
declared that it was a dead world. He believed it to be destitute of
life of any kind, and the changes observed by Schröter and other
observers were put down as illusions. ‘Der Mond’ was the end of Mädler’s
work in lunar astronomy, for, receiving an appointment at Dorpat, he
went there in 1846, and retained his post until within a few years of
his death, which took place at Hanover on March 14, 1874.

Mädler’s successor in the field of lunar astronomy was _Johann Friedrich
Julius Schmidt_ (1825-1884), who was born at Eutin in Lübeck in 1825. At
a very early age he gave indications of a taste for astronomy.
Fortunately his father possessed a small hand telescope, with which
young Schmidt commenced his lunar studies. Appointed assistant at Bonn
and Olmütz and director at Athens successively, he kept up his
persistent study of the surface of the Moon for over forty years. In
1839, when fourteen years of age, he began the valuable series of
observations which were destined to form the basis of his great chart of
the surface of the Moon. Between 1853 and 1858, when employed at Olmütz,
Schmidt made and calculated no fewer than 4000 micrometrical measures of
the altitudes of lunar mountains. Before 1866 Schmidt had found no fewer
than 278 “rills,” and his discoveries were the means of augmenting the
number of these curious objects to nearly a thousand.

In a word, it may be said that Schmidt drew out a lunar geography, and
the result of his labours, together with those of Schröter and Mädler,
is that in a sense we now know the features of the Moon better than
those of the Earth. For instance, astronomers see the whole surface of
the Moon spread before their eyes, while geographers can never have a
similar view of the terrestrial features: we have never seen the poles
of the Earth, while the lunar poles are well known to astronomers. For
twenty years after his appointment at Athens, Schmidt worked at fixing
the positions of lunar objects, measuring the heights of mountains and
the depths of craters. An idea of his enthusiasm in constructing his
great chart may be gained from the fact that he made almost a thousand
original sketches.

Mädler’s dogmatic assertion that the Moon was entirely a dead world was
generally believed until Schmidt made observations to the contrary. From
1837 to 1866 the popular opinion was that our satellite was an
absolutely dead world. Consequently there was little progress in lunar
astronomy during those thirty years. Although Mädler’s view was much
nearer the truth than the opinions of his predecessors, it was also too
positive. His confident assertion, which was received without
hesitation, was never questioned until Schmidt came upon the scene. To
Schmidt the Moon was not entirely dead, and it was he who brought
forward indisputable evidence as to the existence of changes on its
surface. In October 1866 he announced that the crater Linné had lost all
appearance of such, and that it had become entirely effaced. Lohrmann
and Mädler had observed it under a totally different aspect, as also had
Schmidt himself from 1840 to 1843. There was great excitement in the
astronomical world on Schmidt’s announcement, and many astronomers
denied the change, although Schmidt’s observation was confirmed by
Secchi and Webb. The evidence in favour of it preponderated, and very
few observers now consider the Moon’s surface to be absolutely
changeless.

In 1865 Schmidt had begun to arrange his observations on the Moon into
the form of a chart. At first he decided to have a chart of six feet
diameter, divided, like that of Mädler, into four sections. But in April
1868, on making an estimate of the value of such a chart, he was
dissatisfied, and determined to construct a map of the same size divided
into twenty-five sections instead of four. He began the work in 1868,
and after six years the great map was completed. After some delay the
German Government undertook to issue the chart at their expense, and it
was published in 1879, after fourteen years of preparation. It contained
no fewer than 30,000 objects, and its completed diameter was six feet
three inches—more than double the size of any previous map of the Moon.
Indeed, it was probably the greatest contribution ever made to lunar
astronomy. Schmidt lived only a few years after the publication of his
great chart. He died at Athens, in his fifty-ninth year, February 8,
1884.

Schmidt’s announcement of the change in the appearance of Linné was
followed in 1878 by a statement by _Hermann Joseph Klein_ (born 1842) of
Cologne, to the effect that a new crater had been formed to the north of
the well-known lunar crater, Hyginus. The change in this case, however,
is by no means so certain as in that of Linné. It will be observed that
the majority of the students of the Moon were Germans. In England the
study was not taken up until 1864, when a Lunar Committee of the British
Association was appointed. Some good lunar work was done by the
well-known astronomer, _Thomas William Webb_ (1807-1885), while the
study was popularised by _James Nasmyth_ (1808-1890), the famous
engineer, who published, in 1874, in conjunction with _James Carpenter_
of Greenwich Observatory, a beautifully-illustrated volume entitled ‘The
Moon.’ This was succeeded, in 1876, by the larger work of _Edmund
Neison_ (now Nevill), Government Astronomer of Natal. About this time
several English astronomers, devoted to the study of the Moon, formed
themselves into the Selenographical Society. After a few years, however,
the society came to an end, and the enthusiasts formed themselves into
the lunar section of the British Astronomical Association, on the
foundation of that society in 1890. Chief among those English
selenographers was _Thomas Gwyn Elger_ (1837-1897), whose observations
of the Moon and drawings of the various craters were of the utmost
value. Two years before his death, in 1895, Elger published his
important work, ‘The Moon,’ along with an exhaustive chart of the
visible face of our satellite.

Herschel and Schröter firmly believed in the existence of a lunar
atmosphere, the latter believing that he had actually observed the
Moon’s atmospheric envelope. Early in the nineteenth century it was soon
observed, however, that on the Moon passing over and occulting stars,
these stars disappeared suddenly behind the Moon’s limb, instead of
gradually, as they should have done, had an atmosphere of any density
existed. Accordingly astronomers gave up believing in a lunar
atmosphere. On January 4, 1865, Huggins observed with his spectroscope
the occultation of a small star in Pisces. There was not the slightest
sign of absorption in a lunar atmosphere; the entire spectrum vanished
at once.

Lunar photography was introduced as long ago as 1858 by _Lewis Morris
Rutherfurd_ (1816-1892), the well-known American astronomer; but for
years very little was done in this matter, although Rutherfurd secured
fairly good photographs. Rutherfurd, De la Rue, and the older
astronomical photographers took photographs of the entire Moon, but this
plan was abandoned in favour of what Miss Clerke calls “bit by bit
photography.” About 1890 this method was introduced, and has been
followed with success by _Maurice Loewy_ (born 1833), and his assistant,
Pusiex, at the Paris Observatory; by _Ladislas Weinek_ at Prague; by the
astronomers of the Lick Observatory; and by _William Henry Pickering_
(born 1858), the distinguished astronomer of Harvard, whose discoveries
and investigations have created quite a new interest in lunar astronomy.
These investigations were commenced in 1891 at Arequipa, on the slope of
the Andes, in Peru. An occultation of Jupiter, witnessed by W. H.
Pickering on October 12, 1892, gave support to the view that a very
tenuous lunar atmosphere does exist. In 1900 he established, near
Mandeville, Jamaica, a temporary astronomical station, where he obtained
many excellent photographs. Totally he secured eighty plates. These
appeared, as the first complete photographic lunar atlas ever published,
in his work ‘The Moon’ (1903), in which he sums up all his observations
since 1891, and concludes that “the evidence in favour of the idea that
volcanic activity upon the Moon has not yet ceased is pretty strong, if
not fairly conclusive.”

Pickering points out that the density of the lunar atmosphere is not
greater than one ten-thousandth of that at the Earth’s surface, and,
under these circumstances, water cannot exist above freezing-point,
which of course brings us to the subject of snow. He considers that snow
is observed on the mountain peaks and near the poles of the Moon, and he
believes his conclusion to be verified by observations on the well-known
crater, Linné. He brings forward evidence of the probable existence on
the Moon of organic life, pointing out that the difference between the
conditions of the Earth and the Moon is not so great as that above and
below the ocean on our own planet. He has collected evidence of the
existence of something resembling vegetation on the Moon “coming up,
flourishing, and dying, just as vegetation springs and withers on the
Earth.”

The first successful attempt to measure the heating power of moonlight
was made in 1846 on Mount Vesuvius by _Melloni_, an Italian physicist,
whose results were confirmed four years later by _Zantedeschi_, another
Italian. The most important work in this direction was accomplished by
the present _Earl of Rosse_ (born in 1840), who in the years 1869-72
believed himself to have measured the lunar heat; but these conclusions
were not altogether confirmed by the observations of Dr _Otto
Boeddicker_ (Lord Rosse’s astronomer), during the total lunar eclipse of
October 4, 1884. Further investigations on this subject were afterwards
made by _Samuel Pierpont Langley_ (1834-1906), of Alleghany, and by his
assistant, _Frank Very_.

The motion of the Moon and its perturbations were made the subject of
deep study by the famous _Pierre Simon Laplace_ (1749-1827), the
contemporary of Herschel, and the worthy successor of Newton. He devoted
much attention to the secular acceleration of the Moon’s mean motion, a
problem which had baffled the greatest mathematicians. After a profound
discussion he found, in 1787, that the average distance of the Earth and
Moon from the Sun had been slowly increasing for several centuries, the
result being an increase in the Moon’s velocity. In the third volume of
the ‘Mécanique Céleste’ Laplace worked out the lunar theory in great
detail, although he calculated no lunar tables. After his death the
subject was taken up by _Charles Theodore Damoiseau_ (1768-1846), and
the most important advance was made by _Giovanni Antonio Amadeo Plana_
(1781-1864), the director of the Turin Observatory, who published in
1832 a very complete lunar theory. The work of Plana was followed by
that of _Peter Andreas Hansen_ (1795-1874), whose lunar tables were used
for the Nautical Almanac, and whom Professor Simon Newcomb considers to
be the greatest master of celestial mechanics since Laplace. The theory
of the Moon’s motion was worked out in detail by the famous astronomer
_Charles Eugene Delaunay_ (1816-1872), who from 1870 till 1872 occupied
the post of director of the Paris Observatory. Delaunay was about to
work out the lunar tables when, in 1872, he was accidentally drowned by
the capsizing of a pleasure-boat at Cherbourg. The work accomplished in
this direction by _Simon Newcomb_ (born 1835) is of great importance,
particularly in his correction of Hansen’s tables. _John Couch Adams_
(1819-1892), one of the discoverers of Neptune, while at work on the
lunar theory, had occasion to correct Laplace’s supposed solution of the
acceleration of the lunar motion. On going over the calculation Adams
found that several quantities, omitted by Laplace as unimportant, showed
that the Moon has a minute increase of speed for which the theory of
gravitation will not account,—a conclusion opposed by Plana, Hansen, and
Pontécoulant, but fully confirmed by Delaunay. Delaunay suggested in
1865 that the minute apparent increase was due to the retardation of the
Earth’s rotation by tidal friction. This brings us to the subject of
celestial evolution, which is discussed in another chapter.




                               CHAPTER V.
                           THE INNER PLANETS.


Much progress has been made during the last hundred years in our
knowledge of the planets. In fact, the study of Mercury only dates from
the commencement of the nineteenth century. Our knowledge of the
vicinity of the Sun is very limited, and Mercury is difficult of
observation. So limited, in fact, is our knowledge of the Sun’s
surroundings, that it is not yet known for certain whether there is a
planet, or planets, between Mercury and the Sun. Perturbations in the
motion of the perihelion of Mercury’s orbit led Le Verrier in 1859 to
the belief that a planet of about the size of Mercury, or else a zone of
asteroids, existed between Mercury and the Sun. It was, however, obvious
that such a planet could only be seen when in transit across the Sun’s
disc, or during a total eclipse. Meanwhile a French doctor, Lescarbault,
informed Le Verrier that he had seen a round object in transit over the
Sun’s disc. Le Verrier, certain that this was the missing planet, named
it “Vulcan,” and calculated its orbit, assigning it a revolution period
of twenty days. But it was never seen again. Transits of “Vulcan” were
fixed for 1877 and 1882, but nothing was seen on these dates. During the
total eclipse of July 29, 1878, two observers—_James Watson_
(1838-1880), the well-known astronomer, and _Lewis Swift_ (born
1820)—believed themselves to have discovered two separate planets, and
ultimately claimed two planets each, which were never heard of again.
During the total eclipse of 1883 an active watch for “suspicious
objects” was kept, but with no result. At the eclipses of 1900 and 1901
respectively, photographs were exposed by the American astronomers, W.
H. Pickering and _Charles Dillon Perrine_ (born 1867), but on none of
these plates could any trace of “Vulcan” be found. At the total eclipse
of August 30, 1905, plates were again exposed, but no announcement has
been made of an intra-Mercurial planet; and the prevalent opinion among
astronomers is that no planet comparable with Mercury in size exists
between that planet and the Sun.

The study of the physical appearance of Mercury was inaugurated by
Schröter, who in 1800 noticed that the southern horn of the crescent
presented a blunted appearance, which he attributed to the existence of
a mountain eleven miles in height. From observations of this mountain he
came to the conclusion that the planet rotated in 24 hours 4 minutes.
This was afterwards reduced by _Friedrich Wilhelm Bessel_ (1784-1846) to
24 hours 53 seconds.

After the time of Schröter there was no astronomer who paid much
attention to either Mercury or Venus until the arrival on the scene of
the most persistent planetary observer and one of the foremost
astronomers of the nineteenth century. _Giovanni Virginio Schiaparelli_
was born at Savigliano, in Piedmont, in 1835, and graduated at Turin in
1854. Called to Milan as assistant in the Brera Observatory in 1860, he
became director in 1862, and there for thirty-eight years he studied
astronomy in all its aspects, making a great name for himself in various
branches of the science. In 1900 he retired from the post of director,
and pursues his astronomical researches in his retirement.

In 1882 Schiaparelli took up the study of Mercury in the clear air of
Milan. Instead of observing the planet through the evening haze, like
Schröter and others, he examined it by day, and was enabled to follow it
hourly instead of looking at it for a short period when near the
horizon. At length, after seven years’ observation, he announced, on
December 8, 1889, that Mercury performs only one rotation during its
revolution round the Sun—in fact, that its day and year coincide. As a
consequence, the planet keeps the same face towards the Sun, one side
having everlasting day and the other perpetual night; but owing to the
libratory movement of Mercury—the result of uniform motion on its axis
and irregular motion in its orbit—the Sun rises and sets on a small zone
of the planet’s surface. Schiaparelli’s observations indicated that
Mercury is a much spotted globe, with a moderately dense atmosphere, and
he was enabled to form a chart of its surface-markings.

Schiaparelli’s conclusions remained until 1896 unconfirmed and yet not
denied, although most astronomers were sceptical on the subject. In 1896
the subject was taken up by the American astronomer, _Percival Lowell_
(born 1855), who, in the clear air of Arizona, confirmed Schiaparelli’s
conclusions, fixing 88 days as the period of rotation. He remarked,
however, that no signs of an atmosphere or clouds were visible to him.
The surface of Mercury, he says, is colourless,—“a geography in black
and white.” The determination of the rotation period by Schiaparelli and
Lowell is now generally accepted, and is confirmed by the theory of
tidal friction. It is only right to add that _William Frederick Denning_
(born 1848) in 1881 suspected a rotation period of 25 hours, but this
remains unconfirmed. In April 1871 the spectrum of Mercury was examined
by _Hermann Carl Vogel_ (born 1842) at Bothkamp. He suspected traces of
an atmosphere similar to ours, but was not certain. Of more interest are
the photometric observations of Zöllner in 1874. These observations
indicated that the surface of Mercury is rugged and mountainous, and
comparable with the Moon,—a conclusion supported by Lowell’s
observations in 1896.

Venus, the nearest planet to the Earth, has been attentively studied for
three centuries, and still comparatively little is known regarding it.
This is due to its remarkable brilliancy, combined with its proximity to
the Sun. The great problem at the beginning of the nineteenth century
was the rotation of the planet. In 1779 the subject was taken up by
Schröter at Lilienthal. Nine years later, from a faint streak visible on
the disc, he concluded that rotation was performed in 23 hours 28
minutes, and in 1811 this was reduced by seven minutes; but as Herschel
was unable to observe the markings seen by Schröter, many astronomers
were inclined to be sceptical regarding the accuracy of the Lilienthal
observers results. Schröter also observed the southern horn of Venus
when in the crescent form to be blunted, and he ascribed this to the
existence of a great mountain, five or six times the elevation of
Chimborazo; while he observed irregularities along the terminator, which
he considered to be more strongly marked than those on the Moon.
Schröter’s opinion on this point, although rejected by Herschel, was
confirmed by Mädler, Zenger, Ertborn, Denning, and by the Italian
astronomer _Francesco Di Vico_ (1805-1848), director of the Observatory
of the Collegio Romano. In 1839 Di Vico attacked the problem of the
rotation, and his results were confirmatory of those of Schröter. He
estimated that the axis of Venus was inclined at an angle of 53° to the
plane of its orbit. Meanwhile a series of important observations had
been made on Venus by the Scottish astronomer and theologian, _Thomas
Dick_ (1772-1857), who suggested daylight observations on Venus to solve
the problem of the rotation.

In 1877 the question was attacked by Schiaparelli, who commenced a
series of observations on Venus at Milan in that year. The results of
his studies were summed up in 1890 in five papers contributed to the
Milan Academy. He came to the conclusion that the markings observed by
Schröter, Di Vico, and others were not really permanent, and
concentrated his attention on round white spots, which remained fixed in
position. Instead of observing Venus in the evening, Schiaparelli
followed it by day, watching it continuously on one occasion for eight
hours. But the markings remained fixed. Schiaparelli accordingly
concluded that the planet’s rotation was performed in probably 225 days,
equal to the time of revolution. One face is turned towards the Sun
continually, while the other is perpetually in darkness.

The announcement was so startling that, as Miss Clerke says, “a clamour
of contradiction was immediately raised, and a large amount of evidence
on both sides of the question has since been collected.” Perrotin at
Nice, Tacchini at Rome, Cerulli at Teramo, Mascari at Catania and Mount
Etna, and Lowell in Arizona, all in favourable climates, confirmed
Schiaparelli’s results, as also did a second series of observations by
the Milan astronomer himself in 1895. On the other hand, Neisten,
Trouvelot, _Camille_ _Flammarion_ (born 1842), and others, under less
favourable climatic conditions, arrived at a period of 24 hours.
_Aristarch Bélopolsky_ (born 1854), from spectroscopic observations at
Pulkowa, by means of Doppler’s principle, found a period of 12 hours.
Lowell, by the same principle, found, in 1901-03, a period of 225 days,
in agreement with Schiaparelli’s results. This is the last word on the
subject. Schiaparelli’s rotation period, confirmed by the theory of
tidal friction, is generally accepted.

That Venus has an atmosphere was one of the conclusions reached by
Schröter in 1792; and in this at least he was correct, as the atmosphere
of Venus, illuminated by the solar rays, has been seen extending round
the entire disc of the planet. Spectroscopic observations by Tacchini,
Ricco, and Young, during the transits of 1874 and 1882, indicated the
existence of water-vapour in the planet’s atmosphere. Very little has
been discovered regarding the “geography” of Venus. White patches at the
supposed “poles” of the planet were observed in 1813 by _Franz von
Gruithuisen_, and in 1878 by the French astronomer _Trouvelot_
(1827-1895). The secondary light of Venus, similar to the “old Moon in
the new Moon’s arms,” was repeatedly observed since the time of Schröter
by Vogel, Lohse, Zenger, and others. Vogel attributed it to twilight,
and Lamp, a German observer, to electrical processes analogous to our
auroræ. In 1887 a Belgian astronomer, _Paul Stroobant_, submitted to a
searching examination all the supposed observations of a satellite of
Venus, and was enabled to explain nearly all the supposed satellites as
small stars which happened to lie near the planet’s path in the sky at
the time of observation.

The study of our own planet can hardly be said to belong to the realm of
astronomy. Nevertheless, it is through astronomical observation that the
motion of the North Pole has been discovered. For many years it has been
a problem whether there is a variation of latitude resulting from the
motion of the pole. Euler had declared, from theoretical investigation,
that, were there such a motion, the period must be 10 months. The
question was revived in 1885 by the observations of _Seth Carlo
Chandler_ (born 1846) at Cambridge, Mass., with his newly-invented
instrument, the “almucantar,” which indicated an appreciable variation
of latitude. This was confirmed by _Friedrich Küstner_ (born 1856), now
director of the Observatory at Bonn. The idea now occurred to Chandler
to search through the older records to discover if there was any trace
of the variation of latitude, with the result that he brought out a
period of 14 months instead of 10. This aroused much interest, and many
prominent astronomers denied Chandler’s results, which were announced in
1891. As a well-known astronomer has expressed it, “Euler’s work had
shown what period the motion must have, and any appearance of another
period must be due to some error in the observations. Chandler replied
to the effect that he did not care for Euler’s mathematics: the
observations plainly showed 14 months, and if Euler said 10, _he_ must
have made the mistake. I do not exaggerate the situation in the least;
it was a deadlock: Chandler and observation against the whole weight of
observation and theory.” It was now shown by Newcomb that Euler had
assumed the Earth to be an absolutely rigid body, while modern
investigations show that it is not so. Chandler’s discovery is now
accepted, and proves that the North Pole is not fixed in position, but
has a small periodic motion, though never twelve yards from its mean
position. That the small resulting variation in the position of the
stars has been noticed at all is a striking illustration of the accuracy
of astronomical observation.

Of all the planets Mars has been most studied during the nineteenth
century. Many illustrious astronomers have devoted years to the study of
the red planet, with the result that more is known of the surface of
Mars than of any other celestial body, with the exception of the Moon.
After the time of Herschel, the leading students of Mars were Beer and
Mädler, who carefully studied the planet from 1828 to 1839. They
identified at each opposition the same dark spots, frequently obscured
by mists, and they also made the most accurate determination of the
rotation period, which they fixed at 24 hours 37 minutes 23 seconds.
This estimate was confirmed in 1862 by _Friedrich Kaiser_ (1808-1872) of
Leyden, in 1869 by _Richard Anthony Proctor_ (1837-1888), and in 1892 by
_Henricius Gerardus van de Sande Bakhuyzen_ (born 1838), director of the
Leyden Observatory. In 1862 Lockyer identified the various markings seen
by Beer and Madler in 1830. The other great names in Martian study prior
to 1877 are Angelo Secchi and _William Rutter Dawes_ (1799-1868), who
studied Mars from 1852 to 1865 and secured a very valuable series of
drawings. These drawings were used by Proctor for the construction of
the first reliable map of Mars, which was published in 1870 in his work,
‘Other Worlds than Ours.’ Proctor gave names to the various Martian
features, the reddish-ochre portions of the disc being named continents
and the bluish-green portions seas; and Proctor’s views on Mars found
favour for many years. In 1877, however, Schiaparelli opened a new era
in the study of Mars. In September of that year, during the very
favourable opposition of the planet, Schiaparelli, while executing a
trigonometrical survey of the disc, discovered that the continents were
cut up by numerous long dark streaks, which he called _canali_. In 1879,
to his surprise, he found that some of the canals had become double; and
he confirmed this in 1881 and at subsequent oppositions. Meanwhile, as
Schiaparelli was the only observer who had hitherto seen the canals,
there was much scepticism as to their reality. In 1886, however, they
were seen at the Nice Observatory by _Henri Perrotin_ (1845-1904), who
also observed their duplication. Since 1886 they have been observed by
many astronomers, including Camille Flammarion in France, _William
Frederick Denning_ (born 1848) in England, _Vincenzo Cerulli_ (born
1859) in Italy, Percival Lowell and W. H. Pickering in the United
States. In 1892 W. H. Pickering successfully observed the canals, and
discovered at the junctions of two or more canals round black spots, to
which he gave the name of “lakes,” in keeping with the view that the
dark regions of the planet were seas.

In 1894 Percival Lowell erected at Flagstaff, Arizona, an observatory
for the specific purpose of observing Mars and its canals in good and
steady air. He was assisted by W. H. Pickering and by _Andrew Ellicott
Douglass_ (born 1867). During a year’s study Douglass measured the
Martian atmosphere and discovered canals crossing the dark regions of
the planet, finally disproving the idea of their aqueous character.
Lowell recognised all Schiaparelli’s canals, and discovered many more.
He also attentively studied the south polar cap of Mars, which
disappeared entirely on October 12, 1894. Lowell noticed, also, that as
the cap melted the canals became darker, as if water was being conveyed
down; and accordingly he adopted the view put forward by Schiaparelli,
that the canals are waterways lined on either side by banks of
vegetation. His observations were published in the end of 1895 in his
work ‘Mars.’ He is of opinion that the reddish-ochre regions or
“continents” are deserts, and the greenish areas marshy tracts of
vegetation. The lakes are named by him “oases,” and, as Miss Clerke
observes, he “does not shrink from the full implication of the term.” He
regards the canals as strips of vegetation fertilised by a small canal,
much too small to be seen, an idea which originated with W. H.
Pickering. The canals are believed by Lowell to be waterways down which
the water from the melting polar cap is conveyed to the various oases.
He considers, in fact, that the canals are constructed by intelligent
beings with the express purpose of fertilising the oases, regarded by
him as centres of population. He remarks that water is scarce on the
planet, owing to its small size, and as a consequence the inhabitants
are forced to utilise every drop. The canal system is the result.

Lowell’s theory has not been cordially received—although it is now
gradually gaining popularity,—and several other hypotheses have been
propounded to explain the canals. Proctor, who died some years before
Lowell’s theory was given to the world, regarded them as rivers, but
this view may now be looked upon as abandoned. It was suggested that the
canals might be cracks in the surface of Mars or meteors ploughing
tracks above it: and Professor _John Martin Schaeberle_ (born 1853) of
the Lick Observatory put forward the view that the canals were chains of
mountains running over the light and dark regions. None of these
theories, however, gained popularity, and had to give way to a more
popular theory, the “illusion” hypothesis, put forward by the Italian
astronomer Cerulli, and supported by Newcomb and Maunder. On the basis
of the illusion theory, Newcomb explains that the “canaliform”
appearance “is not to be regarded as a pure illusion on the one hand or
an exact representation of objects on the other. It grows out of the
spontaneous action of the eye in shaping slight and irregular
combinations of light and shade, too minute to be separately made out
into regular forms.” Experiments were made by Maunder in 1902, and the
results pointed to the truth of the theory that the canals were really
illusions. But the studies of Lowell at the oppositions of 1903 and 1905
have seriously weakened the hypothesis of Cerulli and Maunder, and
strongly confirm the theory of the artificial origin of the canals. In
1903 Lowell was enabled, from a study of the development of the canals,
to show the probability of their artificial nature, and his study of the
double canals showed a distinct plan in their distribution. Finally, on
May 11, 1905, several photographs of Mars were secured at the Lowell
Observatory, on which the canals appeared, not as dots of light and
shade, as on the illusion theory, but as straight dark lines. This goes
far to prove the reality of the canals,—in spite of the ridicule cast on
them and their observers,—and consequently the truth of the theory of
intelligent life in Mars.

Meanwhile the old-fashioned Martian observations have been continued in
less favourable climates than Arizona and Italy by various astronomers,
among them the famous Camille Flammarion, the American astronomers
_James Edward Keeler_ (1857-1900), _Edward Emerson Barnard_ (born 1857),
the English astronomer W. F. Denning, and others. These conscientious
and painstaking observers have done much for Martian study in increasing
the number of accurate delineations of the Martian surface.

The spectrum of Mars was first examined by Huggins in 1867. He found
distinct traces of water-vapour, and this was confirmed by Vogel in
1872, and by Maunder some years later. In 1894, however, _William
Wallace Campbell_ (born 1862), the American astronomer, observing from
the Lick Observatory, California, was unable to detect the slightest
difference between the spectra of Mars and the Moon, indicating that
Mars had no appreciable atmosphere; and from this he deduced that the
Martian polar caps could not be composed of snow and ice, but of frozen
carbonic acid gas. In 1895, however, Vogel confirmed his previous
observations, and reaffirmed the presence of water-vapour in the Martian
atmosphere.

During the opposition of 1830, Mädler undertook an extensive search for
a Martian satellite, but was unsuccessful. In 1862 the search was
resumed by _Heinrich Louis D’Arrest_ (1822-1875), the famous German
observer, who was also unsuccessful. Accordingly the red planet was
referred to by Tennyson as the “moonless Mars.” In 1877 the search was
taken up by _Asaph Hall_, the self-made American astronomer, born at
Goshen, Connecticut, in 1829, and employed from 1862 to 1891 at the
Naval Observatory, Washington. During the famous opposition of August
1877, favoured by the great 26-inch refractor, he succeeded in
discovering two very small satellites of Mars, to which he gave the
names of Phobos and Deimos. He determined the time of revolution of
Phobos at 7 hours 39 minutes, and that of Deimos at 30 hours 17
minutes,—Phobos revolving round Mars more than three times for one
rotation of the planet on its axis. These two satellites are very small,
not more than thirty miles in diameter. After Hall’s successful search,
photographs were exposed at the Paris Observatory for other Martian
satellites, but none was discovered. No further moons have been found
belonging to the red planet, nor is it likely that any further
satellites of Mars are in existence.

The discovery of a zone of small planets in the space between Mars and
Jupiter belongs completely to the nineteenth century, although the
existence of a planet in the vacant space was suspected three centuries
ago. In 1772 the subject was taken up by _Johann Elert Bode_
(1747-1826), afterwards director of the Berlin Observatory, who
investigated a curious numerical relationship, since known as Bode’s
Law, connecting the distances of the planets. If four is added to each
of the numbers—0, 3, 6, 12, 24, 48, 96, and 192, the resulting series
represents pretty accurately the distances of the planets from the Sun,
thus—4 (Mercury), 7 (Venus), 10 (The Earth), 16 (Mars), 28, 52,
(Jupiter), and 100 (Saturn). After the discovery of Uranus, in 1781, it
was found that it filled up the number 196. Bode, however, saw that the
number 28, between Mars and Jupiter, was vacant, and predicted the
discovery of the planet. Aided by _Franz Xavier von Zach_ (1754-1832),
he called a congress of astronomers, which assembled in 1800 at
Schröter’s observatory at Lilienthal, when, for the purpose of searching
for the missing planet, the zodiac was divided into twenty-four zones,
each of which was given to a separate astronomer. One of them was
reserved for _Giuseppe Piazzi_ (1746-1826), director of the Observatory
of Palermo.

Born in 1746 at Ponte, in Lombardy, Giuseppe Piazzi, after entering the
Theatine Order of monks, became in 1780 Professor of Mathematics at
Palermo, where an observatory was erected in 1791; and at that
observatory Piazzi worked till his death in 1826. In 1792 he commenced a
great star-catalogue, and while making his nightly observations he
discovered, on January 1, 1801—the first night of the nineteenth
century,—what he took to be a tailless comet, but which proved to be a
small planet revolving round the sun in the vacant space. The discovery
was hailed by Bode and Von Zach with much enthusiasm, and Piazzi named
the planet Ceres. The little planet was, however, soon lost in the rays
of the sun before sufficient observations had been made; but the great
mathematician, _Friedrich Gauss_ (1777-1855), came to the rescue, and
pointed out the spot where the planet was to be rediscovered. In that
spot it was found on December 31, 1801, by Von Zach at Gotha, and on the
following evening by _Heinrich Olbers_ (1758-1840) at Bremen.

On March 28, 1802, while observing Ceres from his house at Bremen,
Olbers was struck by the presence of a strange object near the path of
the planet. At first he supposed it to be a variable star at maximum
brilliance, but a few hours showed him that it was in motion, and was
therefore another planet. He named it Pallas, and propounded the theory
that the two “Asteroids”—so named by Herschel—were fragments of a
trans-Martian planet, which, through some accident, had been shattered
to pieces in the remote past. Olbers urged the necessity of searching
for more small planets. His advice was taken. In 1804 _Karl Ludwig
Harding_ (1765-1834), Schröter’s assistant, discovered Juno, and Olbers
himself detected Vesta, March 29, 1807.

After 1816 the search was relinquished, as no more planets were
discovered. In 1830, however, a German amateur, _Karl Ludwig Hencke_
(1793-1866), ex-postmaster of Driessen, commenced a search for new
planets, which was rewarded, after fifteen years, by the discovery of
Astræa, December 8, 1845. On July 1, 1847, he made another discovery,
that of Hebe. A few weeks later, _John Russell Hind_ (1823-1895), the
English astronomer, discovered Iris. Since 1847 not a year has passed
without one or more planets being found, sometimes as many as twenty
being discovered in a single year. Some astronomers have made the search
for asteroids their chief business. The principal asteroid discoverers
have been _Christian H. F. Peters_ (1813-1890), Henri Perrotin, _Paul
Henry_ (1848-1905), _Prosper Henry_ (1849-1903), James Watson, _Robert
Luther_ (1822-1900), _Johann Palisa_ (born 1848), and _Max Wolf_ (born
1863).

In 1891 a new impulse was given to asteroid study by the application of
photography by Max Wolf to the discovery of the minor planets. It
occurred to Wolf that the asteroid would be represented on the plate by
a trail, caused by its motion during the time of exposure; and assisted
by _Arnold Schwassmann_ (born 1870), _Luigi Carnera_ (born 1875), and
others, Wolf has discovered over a hundred asteroids, and he has the
whole field of asteroid hunting to himself. Few minor planets are now
discovered by the older method. In 1901 Wolf invented his new instrument
of research, the stereo-comparator, which, on the principle of the
old-fashioned stereoscope, represents the planetary bodies as suspended
in space far in front of the stars. In this way this ingenious
astronomer has been enabled to discover asteroids at the first glance:
year by year fresh discoveries are announced from the Heidelberg
Observatory, until more than five hundred asteroids are now known.

Waning interest in the ever-increasing family of asteroids was revived
in 1898 by the discovery by _Karl Gustav Witt_ (born 1866) of a small
planet, to which he gave the name of Eros, which comes nearer to the
Earth than Mars, and which is of great assistance to astronomers in the
determination of the solar parallax. For some time prior to 1898
astronomers had considered it a waste of time to search for new
asteroids; but this idea is not now so popular, in view of the benefit
conferred on astronomy by the discovery of Eros.

Of the physical nature of the asteroids astronomers know nothing. Only
the four largest have been measured. For many years it was supposed that
Vesta, the brightest of the asteroids, was also the largest. The
measures of Barnard with the great Lick refractor in 1895, however,
showed that Ceres is the largest, with a diameter of 477 miles. Pallas
comes next, with a diameter of 304 miles; while the diameters of Vesta
and Juno are respectively 239 and 120 miles. Barnard saw no traces of
atmosphere round any of the asteroids. It should be stated that in 1872
Vogel thought he could detect an “air-line” in the spectrum of Vesta: he
admitted that the observation required confirmation, but it has not been
corroborated either by himself or any other observer.




                              CHAPTER VI.
                           THE OUTER PLANETS.


Jupiter, the greatest planet of the Solar System, has perhaps been more
persistently studied by astronomers than any other. In the early
nineteenth century the prevalent idea was that Jupiter was a world
similar to the Earth, only much larger,—a view held by Herschel and
other famous astronomers, and put forward by Brewster in ‘More Worlds
than One.’ This view prevailed for many years, although Buffon in 1778,
and Kant in 1785, had stated their belief in the idea that Jupiter was
still in a state of great heat—in fact, that the great planet was a
semi-sun. This idea, however, was long in being adopted by astronomers,
and very little attention was paid to Nasmyth’s expression of the same
opinion in 1853. The older view still held the field—namely, that the
belts of Jupiter represented trade-winds, and that a world similar to
the terrestrial lay below the Jovian clouds. In 1860 _George Philip
Bond_ (1826-1865), director of the Harvard Observatory, found from
experiments that Jupiter seemed to give out more light than it received,
but he did not dare to suggest that Jupiter was self-luminous,
considering that the inherent light might result from Jovian auroras.

In 1865 Zöllner showed that the rapid motions of the cloud-belts on both
Jupiter and Saturn indicated a high internal temperature. At the
distance of Jupiter sun-heat is only one twenty-seventh as great as on
the Earth, and would be quite incapable of forming clouds many times
denser than those on the Earth. In 1871 Zöllner drew attention to the
equatorial acceleration of Jupiter, analogous to the same phenomenon on
the Sun. In 1870 these opinions of Zöllner’s were adopted and supported
by Proctor in his ‘Other Worlds than Ours.’ In his subsequent volumes
Proctor did much to popularise the idea, which is now accepted all over
the astronomical world.

During the century many valuable observations on Jupiter were made by
numerous observers, among them Airy, Mädler, Webb, Schmidt, and others.
Much time was devoted to the accurate determination of the rotation
period, which was fixed at 9 hours 55 minutes 36·56 seconds by Denning
in observations from 1880 to 1903. No really important discovery was
made till 1878, when Niesten at Brussels discovered the “great red
spot,” a ruddy object 25,000 miles long by 7000 broad, attached to a
white zone beneath the southern equatorial belt. This remarkable object
has been observed ever since. In 1879 its colour was brick-red and very
conspicuous, but it soon began to fade, and Riccó’s observation at
Palermo in 1883 was thought to be the last. After some months, however,
it brightened up, and, notwithstanding changes of form and colour, it is
still visible, a permanent feature of the Jovian disc. In 1879 a group
of “faculæ,” similar to those on the Sun, was observed at Moscow by
_Theodor Alexandrovitch Brédikhine_ (1831-1904), and at Potsdam by
_Wilhelm Oswald Lohse_ (born 1845). It was soon observed that the
rotation period, as determined from the great red spot, was not
constant, but continually increasing. A white spot in the vicinity
completed its rotation in 5½ minutes less, indicating the differences of
rotation on Jupiter.

The great red spot has been observed since its discovery by Denning at
Bristol and _George Hough_ (born 1836) at Chicago. Twenty-eight years of
observation have not solved the mystery of its nature. The researches
made on it, in the words of Miss Clerke, “afforded grounds only for
negative conclusions as to its nature. It certainly did not represent
the outpourings of a Jovian volcano; it was in no sense attached to the
Jovian soil—if the phrase have any application to the planet; it was not
a mere disclosure of a glowing mass elsewhere seethed over by rolling
vapours.”

In 1870 _Arthur Cowper Ranyard_ (1845-1894), the well-known English
astronomer, began to collect records of unusual phenomena on the Jovian
disc to see if any period regulated their appearance. He came to the
conclusion that, on the whole, there was harmony between the markings on
Jupiter and the eleven-year period on the Sun. The theory of inherent
light in Jupiter, however, has not been confirmed. The great planet was
examined spectroscopically by Huggins from 1862 to 1864, and by Vogel
from 1871 to 1873. The spectrum showed, in addition to the lines of
reflected sunlight, some lines indicating aqueous vapour, and others
which have not been identified with any terrestrial substance. A
photographic study of the spectrum of Jupiter was made at the Lowell
Observatory by Slipher in 1904, probably the most exhaustive
investigation on the subject. The spectroscope has, however, given
little support to the theory of inherent light, and “we are driven to
conclude that native emissions from Jupiter’s visible surface are local
and fitful, not permanent and general.”

Herschel’s idea, that the rotations of the four satellites of Jupiter
were coincident with their revolutions, has on the whole been confirmed
by recent researches, although in the case of the two near satellites
(Io and Europa) W. H. Pickering’s observations in 1893 indicated shorter
rotation periods. There is much to learn regarding the geography of the
satellites, although in 1891 Schaeberle and Campbell at the Lick
Observatory observed belts on the surface of Ganymede, the third
satellite analogous to those on Jupiter. Surface-markings on the
satellites have also been seen by Barnard at the Lick Observatory, and
by Douglass at Flagstaff.

Since the time of Galileo no addition had been made to the system of
satellites revolving round Jupiter. Profound surprise was created,
therefore, by the announcement of the discovery of a fifth satellite by
Barnard at the Lick Observatory, on September 9, 1892. The satellite,
one of the faintest of telescopic objects, was discovered with the great
36-inch telescope, and its existence was soon confirmed by _Andrew
Anslie Common_ (1841-1903), with his great 5-foot reflector at Ealing,
near London. The new satellite was found by Barnard to revolve round
Jupiter in 11 hours 57 minutes at a mean distance of 112,000 miles.

Although the existence of other satellites of Jupiter was predicted by
Sir _Robert Stawell Ball_ (born 1840) soon after the discovery of the
fifth, much surprise was created by the announcement, in January 1905,
that a sixth satellite had been discovered by Perrine, who, in the
following month, announced the discovery of a seventh. These discoveries
were made by photography, the objects being very faint. The periods of
revolution were found to be 242 days and 200 days for the sixth and
seventh satellites respectively, the mean distances being 6,968,000 and
6,136,000 miles. It is possible that they may belong to a zone of
asteroidal satellites. In fact, the fifth moon may belong to a similar
zone, so that Jupiter may have two asteroidal zones; but this is
anticipating future discovery.

A particular charm has always attached itself to the study of Saturn,
the ringed planet. The magnificent system of rings has for two and a
half centuries been the object of wonder and admiration in the Solar
System, and accordingly they have been exhaustively studied by many
eminent observers. While observing the two bright rings of Saturn on
June 10, 1838, Galle noticed what Miss Clerke calls “a veil-like
extension of the lucid ring across half the dark space separating it
from the planet.” No attention, however, was paid to Galle’s
observation. On November 15, 1850, _William Cranch Bond_ (1789-1859), of
the Harvard Observatory in Massachusetts, discovered the same phenomenon
under its true form—that of a dusky ring interior to the more brilliant
one. A fortnight later, before the news of Bond’s observation, Dawes
made the same discovery independently at Wateringbury in England. This
ring is known as the dusky or “crape” ring.

The discovery of the dusky ring brought to the front the problem of the
composition of the ring-system. Laplace and Herschel considered the
rings to be solid, but this was denied in 1848 by _Edouard Roche_
(1820-1880), who believed them to consist of small particles, and in
1851 by G. P. Bond, who asserted that the variations in the appearance
of the system were sufficient to negative the idea of their solidity;
but he suggested that the rings were fluid. In 1857 the question was
taken up by the Scottish physicist, _James Clerk-Maxwell_ (1831-1879),
who proved by mathematical calculation that the rings could be neither
solid nor fluid, but were due to an aggregation of small particles, so
closely crowded together as to present the appearance of a continuous
whole. Clerk-Maxwell’s explanation—which had been suggested by the
younger Cassini in 1715, and by Thomas Wright in 1750—was at once
adopted, and has since been proved by observation. In 1888 _Hugo
Seeliger_ (born 1849), director of the Munich Observatory, showed from
photometric observations the correctness of the satellite-theory; while
Barnard in 1889 witnessed an eclipse of the satellite Japetus by the
dusky ring. The satellite did not disappear, but was seen with perfect
distinctness. The final demonstration of the meteoric nature of the
rings was made by Keeler at the Alleghany Observatory in 1895, with the
aid of the spectroscope. By means of Doppler’s principle, he found that
the inner edge of the ring revolved in a much shorter time than the
outer, proving conclusively that they could not be solid. This was
confirmed by the observations of Campbell at Mount Hamilton, _Henri
Deslandres_ at Meudon, and Bélopolsky at Pulkowa.

In 1851 a startling theory regarding Saturn’s rings was put forward by
the famous _Otto Wilhelm von Struve_ (1819-1905). Comparing his
measurements on the rings made at Pulkowa in 1850 and 1851 with those of
other astronomers for the past two hundred years, he reached the
conclusion that the inner diameter of the ring was decreasing at the
rate of sixty miles a-year, and that the bodies composing the rings were
being drawn closer to the planet. Accordingly, Struve calculated that
only three centuries would be required to bring about the precipitation
of the ring-system on to the globe of Saturn. In 1881 and 1882 Struve,
expecting a further decrease, made another series of measures, but these
did not confirm his theory, which was accordingly abandoned.

The study of the globe of Saturn has made less progress than that of the
rings. The surface of the planet had been known since before the time of
Herschel to be covered with belts, but as spots seldom appear on Saturn,
only one determination of the rotation period had been made, that by
Herschel. Much interest was aroused, therefore, by the discovery, by
Hall, at Washington, on December 7, 1876, of a bright equatorial spot.
Hall studied this spot during sixty rotations of the planet, determining
the period as 10 hours 14 minutes 24 seconds. This was confirmed by
Denning in 1891, and by _Stanley Williams_, an English observer, in the
same year. On June 16, 1903, Barnard, at the Yerkes Observatory,
discovered a bright spot, from which he deduced a rotation period of 10
hours 39 minutes,—a period considerably longer than that found by Hall.
In the same year various spots on Saturn were observed by Denning, who
found a period of 10 hours 37 minutes 56·4 seconds, and at Barcelona by
_José Comas Sola_, now director of the Observatory there, who may be
considered Spain’s leading astronomer. The result of these observations
has been to show that the spots on Saturn have probably a proper motion
of their own, apart from the rotation of the planet. As to the spectrum
of Saturn, little has been learned. It closely resembles that of
Jupiter. In 1867 Janssen, observing from the summit of Mount Etna, found
traces of aqueous vapour in the planet’s atmosphere.

In the chapters on Herschel we have seen that he discovered the sixth
and seventh satellites of Saturn. The next discovery was made on
September 19, 1848, by W. C. Bond, at Harvard, Massachusetts, and
independently by _William Lassell_ (1799-1880), at Starfield, near
Liverpool. The new satellite received the name of Hyperion, and was
found to be situated at a distance of about 946,000 miles from Saturn.
Its small size led Sir John Herschel to the idea that it might be an
asteroidal satellite. Fifty years elapsed before another satellite of
Saturn was discovered. In 1888 W. H. Pickering commenced a photographic
search for new satellites of the planet. At last, on developing some
photographs of Saturn, taken on August 16, 17, and 18, 1898, he found
traces of a new satellite which he named “Phœbe.” But, as the satellite
was not seen or photographed again for some years, many astronomers were
sceptical as to its existence. However, photographs taken in 1900, 1901,
and 1902 revealed the satellite, which was again photographed in 1904,
and seen visually by Barnard in the same year with the 40-inch Yerkes
telescope. At that time the discoverer brought out the amazing fact that
the motion of the satellite is retrograde—a fact which he attempts to
explain by a new theory of the former rotation of Saturn. He likewise
demonstrated that its distance from Saturn varied from 6,120,000 to
9,740,000 miles. Early in 1905 Pickering announced the discovery of a
tenth satellite of Saturn, which received the name of Themis, with a
period and mean distance nearly similar to Hyperion, so that Sir John
Herschel’s idea of Hyperion being an asteroidal satellite is being
confirmed after a lapse of half a century.

If little is known of the globe of Saturn, still less is known regarding
Uranus. Dusky bands resembling those of Jupiter were observed by Young
at Princeton in 1883. In the following year Paul and Prosper Henry
discerned at Paris two grey parallel lines on the disc of the planet.
This was confirmed by the observations of Perrotin at Nice, which also
indicated rotation in a period of ten hours. In 1890 Perrotin again took
up the study and re-observed the dark bands. On the other hand, no
definite results regarding the planet were obtained by the Lick
observers in 1889 and 1890. Measurements of the planet by Young,
Schiaparelli, Perrotin, and others indicate a considerable polar
compression. The spectrum of the planet has been studied by Secchi,
Huggins, Vogel, Keeler, Slipher, and others. The spectrum shows six
bands of original absorption, a line of hydrogen, which, says Miss
Clerke, “implies accordingly the presence of free hydrogen in the
Uranian atmosphere, where a temperature must thus prevail sufficiently
high to reduce water to its constituent elements.” From a photographic
study of the spectrum at the Lowell Observatory in 1904, Slipher
observed a line corresponding to that of helium, indicating the presence
of that element in the planet’s atmosphere.

Herschel left our knowledge of the Uranian satellites in a very
uncertain state. The two outer satellites, Titania and Oberon, were
rediscovered in 1828 by his son, but the other four, which he was
believed to have discovered, were never seen again. In 1847 two inner
satellites, Ariel and Umbriel, were discovered by Lassell and Otto
Struve respectively, their existence being finally confirmed by
Lassell’s observations in 1851.

After the discovery of Uranus by Herschel, mathematical astronomers
determined its orbit and calculated its position in the future. _Alexis
Bouvard_, the calculating partner of Laplace, published tables of the
planet’s motions, founded on observations made by various astronomers
who had considered it a star before its discovery by Herschel; but as
the planet was not in the exact position which Bouvard predicted, he
rejected the earlier observations altogether. For a few years the planet
conformed to the Frenchman’s predictions, but shortly afterwards it was
again observed to move in an irregular manner, and the discrepancy
between observation and the calculations of mathematicians became
intolerable. Did the law of gravitation not hold good for the frontiers
of the Solar System? Gradually astronomers arrived at the conclusion
that Uranus was being attracted off its course by the influence of an
unseen body, an exterior planet. Bouvard himself was one of the first to
make the suggestion, but died before the planet was discovered. An
English amateur, the Rev. _T. J. Hussey_, resolved to make, in 1834, a
determination of the place of the unseen body, but found his powers
inadequate; and in 1840 Bessel laid his plans for an investigation of
the problem, but failing health prevented him carrying out his design.

In 1841 a student at the University of Cambridge resolved to grapple
with the problem. John Couch Adams, born at Lidcot in Cornwall in 1819,
entered in 1839 the University of Cambridge, where he graduated in 1843.
From 1858 Professor of Astronomy at Cambridge, and from 1861 director of
the Observatory, he died on January 21, 1892, after a life spent in
devotion to mathematical astronomy. In 1843, on taking his degree, he
commenced the investigation of the orbit of Uranus. For two years he
worked at the difficult question, and by September 1845 came to the
conclusion that a planet revolving at a certain distance beyond Uranus
would produce the observed irregularities. He handed to _James Challis_
(1803-1882), the director of the Cambridge Observatory, a paper
containing the elements of what was named by Adams “the new planet.” On
October 21 of the same year he visited Greenwich Observatory, and left a
paper containing the elements of the planet, and approximately fixing
its position in the heavens. But the Astronomer-Royal of England, Sir
_George Biddell Airy_ (1801-1892), had little faith in the calculations
of the young mathematician. He always considered the correctness of a
distant mathematical result to be a subject rather of moral than of
mathematical evidence: in fact, regarding Uranus, the Astronomer-Royal
almost called in question the correctness of the law of gravitation.
Besides, the novelty of the investigations aroused scepticism, and the
fact that Adams was a young man, and inexperienced, went against Airy’s
acceptance of the theory. However, he wrote to Adams questioning him on
the soundness of his idea. Adams thought the matter trivial, and did not
reply. Airy, therefore, took no interest in the investigations, and no
steps were taken to search for the unseen planet. Meanwhile the Rev. W.
R. Dawes happened to see Adams’ papers lying at Greenwich, and wrote to
his friend, the well-known astronomer Lassell, who was in possession of
a very fine reflector, erected at his residence near Liverpool, asking
him to search for the planet. But Lassell was suffering from a sprained
ankle, and Dawes’ letter was accidentally destroyed by a housemaid. So
Adams’ theory remained in obscurity.

The question now came under the notice of _François Jean Dominique
Arago_ (1786-1853), the director of the Paris Observatory. He recognised
in a young friend of his a rising genius, who was competent to solve the
problem. Urban Jean Joseph Le Verrier, born at Saint Lo, in Normandy, in
1811, became in 1837 astronomical teacher in the École Polytechnique,
and in 1853 director of the Paris Observatory. In consequence of
differences with his staff he was obliged, in 1870, to resign from this
position, but two years later was restored to the post, which he held
till his death on September 23, 1877.

In 1845, ignorant of the fact that Adams had already solved the problem,
Le Verrier began his investigations of the irregular motions of Uranus.
In a memoir communicated to the Academy of Sciences in November of that
year, he demonstrated that no known causes could produce these
disturbances. In a second memoir, dated June 1, 1846, he announced that
an exterior planet alone could produce these effects. But Le Verrier had
now before him the difficult task of assigning an approximate position
to the unseen body, so that it might be telescopically discovered. After
much calculation Le Verrier, in his third memoir (August 31, 1846),
assigned to the planet a position in the constellation Aquarius.

Meanwhile one of Le Verrier’s papers happened to reach Airy. Seeing its
resemblance to Adams’ papers, which had been lying on his desk for
months, his scepticism vanished, and he suggested to Challis that the
planet should be searched for with the Cambridge equatorial. In July
1846 the search was commenced. The planet was actually observed on
August 4 and 12, but, owing to the absence of star maps, it was not
recognised. “After four days of observing,” he wrote to Airy, “the
planet was in my grasp if I had only examined or mapped the
observations.”

Le Verrier wrote to Encke, the illustrious director of the Berlin
Observatory, desiring him to make a telescopic search for a planetary
object situated in the constellation Aquarius, as bright as a star of
the eighth magnitude and possessed of a visible disc. “Look where I tell
you,” wrote the French astronomer, “and you will see an object such as I
describe.” Encke ordered his two assistants, Galle and D’Arrest, to make
a search on the night of September 23, 1846. In a few hours Galle
observed an object not marked in the star-maps of the Berlin
Observatory, which had been recently published. The following night
sufficed to show that the object was in motion, and was therefore a new
planet. On September 29 Challis found the planet at Cambridge, but he
was too late, as the priority of the discovery was now lost to Adams.
The planet received the name of “Neptune.”

For some time, indeed, it appeared as if the French astronomer alone was
to receive the honour of the discovery. But on October 3, 1846, a letter
from Sir John Herschel appeared in the ‘Athenæum’ in which he referred
to the discovery made by Adams. The French scientists were extremely
jealous. Indeed, Arago actually declared that, when Neptune was under
discussion, the entire honour should go to Le Verrier, and the name of
Adams should not even be mentioned,—Arago’s line of reasoning being that
it was not the man who first made a discovery who should receive the
credit, but he who first made it public. However, the credit of the
discovery is now given equally to Adams and Le Verrier, both of whom are
regarded as among the greatest of astronomers.

Only a fortnight after the discovery of Neptune, the astronomer Lassell
observed a satellite to the distant planet on October 10, 1846. This
discovery was confirmed in July 1847 by the discoverer himself, and
shortly afterwards by Bond and Otto Struve. Regarding the globe of
Neptune, we know practically nothing. No markings of any kind have been
observed on its surface. However, in 1883 and 1884, _Maxwell Hall_, an
astronomer in Jamaica, noticed certain variations of brilliance which
suggested a rotation-period of eight hours, but this was not confirmed
by any other astronomer. The spectrum of Neptune has been investigated
by various observers, who have found it to be similar to that of Uranus.

The existence of a trans-Neptunian planet has been suspected by many
astronomers. In November 1879 the first idea of its existence was thrown
out by Flammarion in his ‘Popular Astronomy.’ Flammarion noticed that
all the periodical comets in the Solar System have their aphelion near
the orbit of a planet. Thus Jupiter owns about eighteen comets; Saturn
owns one, and probably two; Uranus two or three; and Neptune six. The
third comet of 1862, however, along with the August meteors, goes
farther out than the orbit of Neptune. Accordingly, Flammarion suggested
the existence of a great planet, assigning it a period of 330 years and
a distance of 4000 millions of miles.

Two independent investigators, _David Peck Todd_ (born 1855) in America
and _George Forbes_ in Scotland, have since undertaken to find the
planet. Todd, utilising the “residual perturbations” of Uranus, assigned
a period of 375 years for his planet. Forbes, on the other hand, working
from the comet theory, stated his belief in the existence of two planets
with periods of 1000 and 5000 years respectively. In October 1901 he
computed the position of the new planet on the celestial sphere, fixing
its position in the constellation Libra, and computing its size to be
greater than Jupiter. A search was made by means of photography, in
1902, but without success. Nevertheless, astronomers are pretty
confident of the existence of one or more trans-Neptunian planets.
Lowell is very definite on this subject when he says in regard to meteor
groups, “The Perseids and the Lyrids go out to meet the unknown planet,
which circles at a distance of about forty-five astronomical units from
the Sun. It may seem strange to speak thus confidently of what no mortal
eye has seen, but the finger of the sign-board of phenomena points so
clearly as to justify the definite article. The eye of analysis has
already suspected the invisible.”




                              CHAPTER VII.
                                COMETS.


At the time of Herschel the ancient superstitions in regard to comets
had to a great extent vanished, thanks mainly to the return of Halley’s
comet in 1758. Yet, although comets had ceased to be objects of terror,
no explanation or rational theory of their nature was put forward until
the appearance of the great comet of 1811. This comet was visible from
March 26, 1811, to August 17, 1812, a period of 510 days. It was one of
the most magnificent comets ever seen, its tail being 100 millions of
miles in length and its head 127,000 miles in diameter. This wonderful
phenomenon was the subject of much investigation, particularly by
Olbers, the great German astronomer.

Heinrich Wilhelm Matthias Olbers was born at Arbergen, a village near
Bremen, October 11, 1758. His father was a clergyman who, in addition to
considerable mathematical powers, was an enthusiastic lover of
astronomy. At the age of thirteen young Olbers became deeply interested
in that science. While taking an evening walk in the month of August, he
observed the Pleiades, and determined to find out to which constellation
they belonged. He therefore bought some books on astronomy, along with a
few charts of the sky, and he began to study the science with much
enthusiasm. He read every book he could lay his hands on, and a few
months sufficed to make him acquainted with all the constellations.

In 1777, when in his nineteenth year, Olbers entered the University of
Göttingen to study medicine, and at the same time he learned much
regarding mathematics and astronomy from the mathematician Kaestner.
When twenty-one years of age he observed the stars at Göttingen, and
devised a method of calculating the orbits of comets, the idea coming to
him while he was attending at the bedside of a fellow-student who had
taken ill. “Although not made public until 1797,” writes Miss Clerke,
“‘Olbers’ method’ was then universally adopted, and is still regarded as
the most expeditious and convenient in cases where absolute rigour is
not required. By its introduction, not only many a toilsome and
thankless hour was spared, but workers were multiplied and encouraged in
the pursuit of labours more useful than attractive.”

Towards the end of 1781 he returned to Bremen, settled as a medical
doctor, and continued in practice for about forty-one years. But
although he had adopted perhaps the most toilsome profession, his love
of science prevailed, and night after night he explored the heavens with
untiring zeal. He never slept more than four hours, and the upper part
of his house in the Sandgasse, in Bremen, was fitted up with
astronomical instruments. The largest telescope which he possessed was a
refractor 3¾ inches in aperture. He remained in active practice till
1823, when he retired, and was enabled to devote more attention to his
beloved science. He died on March 2, 1840, at the advanced age of
eighty-one.

Miss Clerke says of Olbers, “Night after night, during half a century
and upwards, he discovered, calculated, or observed the cometary
visitants of northern skies.” He was the discoverer of the comet of
1815, known as Olbers’ comet. It moves round the Sun in a period of over
seventy years, and returned to perihelion in 1887, forty-seven years
after the death of its discoverer. The great comet of 1811 was the
subject of a memoir which Olbers published the following year, and in
which he originated the “electrical repulsion” theory of comets’ tails.
Even after the fulfilment of Halley’s great prediction, comets were
still looked upon with profound awe, and the popular fear regarding them
was still prevalent. Olbers, however, showed that the tails of comets
resulted from purely natural causes. He regarded the Sun as possessed of
a repulsive as well as an attractive force, and considered the tails to
be vapours repelled from the nucleus of the comet by the Sun. He
calculated that in the comet of 1811 the particles of matter expelled
from the head reached the tail in eleven minutes, with a velocity
comparable to that of light. The theory of electrical repulsion, since
elaborated by other observers, is now generally accepted among
astronomers. No other hypothesis represents in such a complete manner
the formation and growth of the luminous appendages of the celestial
bodies so picturesquely called “pale-winged messengers” as that put
forward by the physician of Bremen.

Some years after Olbers’ famous theory was given to the world, a great
advance was made in cometary astronomy by another great German
astronomer, his friend and pupil Encke. The son of a Hamburg clergyman,
Johann Franz Encke was born in that city in 1791, and died in 1865 at
Spandau. After taking part in the war against Napoleon, he was in 1822
appointed director of the Gotha Observatory, being called to Berlin in
1825. In early life he was the pupil of Olbers and Gauss, and his
investigations and discoveries formed an epoch in astronomy. His most
famous discovery related to the little comet which bears his name. The
comet was discovered by _J. L. Pons_ (1761-1831) at Marseilles, although
it had previously been seen by Méchain and Caroline Herschel. In 1819
Encke computed the orbit of the comet, and boldly announced that it
would reappear in 1822, its period being about 3¼ years, or 1208 days.
In 1822 the comet, true to Encke’s prediction, returned to perihelion,
and was observed at Paramatta in Australia, the perihelion passage
taking place within three hours of the time predicted by Encke. As Miss
Clerke remarks, “The importance of this event will be better understood
when it is remembered that it was only the second instance of the
recognised return of a comet; and that it, moreover, established the
existence of a new class of celestial bodies, distinguished as comets of
short period.”

In 1825 the comet was again observed by Valz, passing perihelion on
September 16, and in 1828 it was seen by Struve. Encke now made a very
remarkable discovery. Determining its period with great accuracy, in
1832 he found that his comet returned to perihelion two and a half hours
before the predicted time. As this repeatedly happened, Encke put
forward the theory that the acceleration was due to the existence of a
resisting medium in the neighbourhood of the Sun, too rarefied to retard
the planetary motions, but quite dense enough to make the comet’s path
smaller, and to eventually precipitate it on the Sun. The theory was
widely accepted, but after 1868 the acceleration began to decrease,
diminishing by one-half; besides, no other comet is thus accelerated,
and the hypothesis has accordingly been abandoned.

The second comet recognised as periodic was that discovered on February
27, 1826, by an Austrian officer, _Wilhelm von Biela_ (1782-1856), and
ten days later by the French observer, _Gambart_ (1800-1836), both of
whom, in computing its orbit, noticed a remarkable similarity to the
orbits of comets which appeared in 1772 and 1805. Accordingly, they
concluded it to be periodic, with a period of between six and seven
years. The comet returned in 1832. In 1828 Olbers had published certain
calculations showing that portions of the comet would sweep over the
part of the Earth’s orbit a month later than the Earth itself. This gave
rise to a panic that the comet would destroy the Earth, which did not
subside till it was announced by Arago that the Earth and the comet
would at no time approach to within fifty million miles of each other.
The comet returned again in the end of 1845. It was kept well in view by
astronomers in Europe and America. On December 19, 1846, Hind noticed
that the comet was pear-shaped, and ten days later it had divided in
two. The two comets returned again in 1852 and were well observed; but
they were never seen again, at least as comets. Their subsequent history
belongs to meteoric astronomy.

A comet discovered by Faye at Paris in 1843 was found to have a period
of seven and a half years. It has returned regularly since its
discovery, true to astronomical prediction. Its motion was particularly
investigated for traces of a resisting medium, by _Didrik Magnus Axel
Möller_ (1830-1896), director of the Lund Observatory, who reached a
negative conclusion.

In 1835 Halley’s comet returned to perihelion, and was attentively
studied by the most famous astronomers of the age. It was particularly
studied by Sir John Herschel and by Bessel, who assisted in developing
Olbers’ theory of electrical repulsion. But the most brilliant comet of
the century was that which suddenly appeared on February 28, 1843, in
the vicinity of the Sun. This great comet, whose centre approached the
Sun within 78,000 miles, rushed past its perihelion at the speed of 366
miles a second. The comet’s tail reached the length of 200 millions of
miles. The comet of 1843 was however outshone, not in brilliance but as
a celestial spectacle, by the great comet discovered on June 2, 1858, by
_Giovanni Battista Donati_ (1826-1873) at Florence, and since known by
his name. It became visible to the naked eye on August 19, and was
telescopically observed until March 4, 1859. There was abundance of
time, therefore, to study the comet, which was exhaustively observed by
G. P. Bond at Harvard. His observations convinced him that the light
from Donati’s comet was merely reflected sunshine, and this was
generally accepted. Another great comet appeared in 1861. Like that of
1843, its appearance was sudden, being observed after sunset on June 30,
1861, when, says Miss Clerke, “a golden yellow planetary disc, wrapt in
dense nebulosity, shone out while the June twilight in these latitudes
was still in its first strength.” On the same evening the Earth and the
Moon passed through the tail of the great comet. The vast majority of
people never knew that such a phenomenon had taken place, and even the
astronomers only noticed a singular phosphorescence in the sky—a proof
of the extreme tenuity of comets.

The first application of the spectroscope to the light of comets was
made by Donati in 1864. The spectrum was found to consist of three
bright bands, but Donati was unable to identify them. However, his
observation gave the death-blow to the theory that comets shone by
reflected light alone, for it implied the existence of glowing gas in
them. On the appearance in 1868 of the periodic comet discovered by
_Friedrich August Theodor Winnecke_ (1835-1897), the spectrum was
examined by Huggins, who identified the bright bands with the spectrum
of hydrocarbon. This was confirmed in regard to Coggia’s comet of 1874
by Huggins himself, and also Brédikhine and Vogel. The hydrocarbon
spectrum is characteristic of comets, and has been recognised in all
those spectroscopically studied.

The time had now come for a more complete theory of comets than that of
Olbers. The theory of electrical repulsion was developed in 1871 by
Zöllner, whose principle of investigation is thus described by Miss
Clerke: “The efficacy of solar electrical repulsion relatively to solar
attraction grows as the size of the particle diminishes.” If the
particle is small enough, it will obey the repulsive, and not the
attractive, power of the Sun. Zöllner considered that the smallest
particles of comets obeyed the repulsive power, and thus formed the
tails of comets. The development of a complete cometary theory is due,
however, to the genius of a Russian astronomer. Theodor Alexandrovitch
Brédikhine, born in 1831 at Nicolaieff, was employed at Moscow
Observatory from 1857 to 1890, when he was promoted to the position of
director at Pulkowa. He resigned in 1895, and spent his last years in St
Petersburg, where he died on May 14, 1904. From the beginning of his
astronomical career he was devoted to the study of comets and their
tails, but it was the appearance of Coggia’s comet in 1874 which marked
the commencement of his most important observations. In that year, on
making certain calculations regarding the hypothetical repulsive force
exerted by the Sun on various comets, he reached the conclusion that the
values representing the intensity of the repulsion fell into three
classes. This was the first hint of a classification of cometary tails.
Meanwhile he carefully studied the tails of comets both from direct
observation and from drawings.

In 1877 he wrote: “I suspect that comets are divisible into groups, for
each of which the repulsive force is perhaps the same.” Subsequent
investigations led Brédikhine to divide the tails of comets into three
types. The first type consists of long, straight tails, pointed directly
away from the Sun, represented by the tails of the great comets of 1811,
1843, and 1861. In the second type, represented by Donati’s and Coggia’s
comets, the tails, although pointed away from the Sun, appear
considerably curved. In the third type the tails are, to quote Miss
Clerke, “short, strongly-bent, brushlike emanations, and in bright
comets seem to be only found in combination with tails of the higher
classes.”

In 1879 Brédikhine fully developed his cometary theory. Assuming the
reality of the repulsive force, he concluded that to produce tails of
the first type, the repulsion requires to be twelve times greater than
the solar attraction; the production of tails of the second type
necessitates a repulsive force about equal to gravity; while the force
producing third-type tails has only one-fourth the power of gravitation.
It was concluded that the tails are formed by particles of matter
repelled from the comet by the repulsive force of the Sun, and in tails
of the first type the velocity with which these particles leave the body
of the comet is four or five miles a second. Brédikhine reached the
conclusion that the Sun’s repulsive force is invariable, and that the
different types of tails are formed by the same force acting on
different elements. The numbers 12, 1, and ¼, are inversely proportional
to the atomic weights of hydrogen, hydrocarbon gas, and iron vapour.
Here, then, was the key to the mystery. Brédikhine pointed out that in
all probability the first-type tails are formed of hydrogen, the second
of hydrocarbon, and the third of iron, with a mixture of sodium and
other elements.

Within a few years of the publication of Brédikhine’s theory, five
bright comets made their appearance, and there was abundant chance of
testing the theory spectroscopically. In 1882 Well’s comet was
particularly studied at Greenwich by Maunder, who discerned a
sodium-line in its spectrum. The magnificent comet which appeared in
1882 was spectroscopically studied at Dunecht in Aberdeenshire by _Ralph
Copeland_ (1837-1905), Astronomer-Royal of Scotland, who identified in
its spectrum the prominent iron-lines as well as the sodium-line. These
observations were certainly confirmatory of Brédikhine’s theory. It
should also be stated, however, that several comets have shown, in
addition to the hydrocarbon spectrum, that of reflected sunlight, which
proves that the light we receive from comets is of a compound nature.

The comet which appeared in 1880 was announced by _Benjamin Apthorp
Gould_ (1824-1896) to be a return of the great comet of 1843.
Calculations by Gould, Copeland, and Hind revealed a close similarity
between the elements of the two orbits. Eventually it had to be admitted
that the comets were separate bodies travelling in the same orbit. Then,
two years later, the great September comet of 1882 was found to revolve
in the same orbit as those of 1668, 1843, and 1880. Four years later,
another comet, discovered in 1887, was found to move in the same path.

Closely allied to this subject is the existence of “comet families,”
demonstrated by Hoek of Utrecht in 1865, and mentioned in our chapter on
the Outer Planets. These comets are found to be dependent on the
planets, Jupiter, Saturn, Uranus, and Neptune, each possessing a
comet-group. Various theories have been advanced to account for the
existence of these groups. One of these theories is that the comets have
been captured by the various planets, who have forced them into their
present orbits. A mathematical study by _Jean Pierre Octave Callandrean_
(1852-1904) shows that the large number of comets possessed by the
various planets may be explained by the disintegration of large comets
into small ones. The capture theory, it must be remembered, is purely
hypothetical, and must not be regarded as anything but a theory. All
that we really know is the existence of comet-families, and of comets
moving in the same orbits.

The first photograph of a comet was that of Donati’s, taken in 1858 by
Bond. In 1881 Tebbutt’s comet was photographed in England by Huggins,
and in America by _Henry Draper_ (1837-1882), while in 1882 Gill secured
excellent photographs of the great September comet. The first
photographic discovery of a comet was made by Barnard in 1892. Since
then photography has been much used in cometary astronomy. No bright
comets have appeared since 1882,—if we except the comet of 1901, only
seen in the southern hemisphere,—although several have been just visible
to the naked eye, among them Swift’s comet of 1892 and Perrine’s in the
autumn of 1902. Telescopic comets, however, are very numerous, and a
year never passes without one or more being discovered. The ordinary
periodic comets, such as Encke’s, Faye’s, and others, are very faint,
and are becoming fainter at each return—a clear proof that comets die,
as Kepler said three centuries ago. This brings us to the subject of the
next chapter, Meteoric Astronomy.




                             CHAPTER VIII.
                                METEORS.


There is no more interesting chapter in the history of astronomy than
that relating to meteors. A hundred years ago shooting-stars were not
considered to be astronomical phenomena. They were supposed to be merely
inflammable vapours which caught fire in the upper regions of our
atmosphere, although both Halley and the scientist _Ernst Chladni_
(1756-1827) had notions of their celestial origin. For thirty-three
years after the beginning of the century, however, nothing was heard of
meteoric astronomy, nor was the subject considered as part of the
astronomer’s labours.

A great meteoric shower took place on the night of November 12 and
morning of November 13, 1833. The shower was probably the grandest ever
witnessed, the shooting-stars being literally innumerable. The display
was best observed in America, and was attentively watched by _Denison
Olmsted_ (1791-1859), Professor of Mathematics at Yale, and by the
American physicist, _A. C. Twining_ (1801-1884). These investigators
discovered that all the meteors which fell during the great shower
seemed to come from the same part of the celestial vault. In other
words, their paths, when traced back, were found to converge to a point
near the star γ Leonis. This observation gave the death-blow to the
theory of their terrestrial origin. The point known as the “radiant” was
clearly a point independent of the Earth. Olmsted also recognised the
fact that the shower had taken place in the previous year, and he
regarded it as produced by a swarm of particles moving round the Sun in
a period of 182 days. Soon after this it was noticed that the phenomenon
took place in 1834 and subsequent years with gradually decreasing
intensity. It was then remembered that Humboldt had observed in November
1799 a very brilliant shower, and accordingly Olbers suggested that
another shower might be seen in 1867.

The falling stars of August were next proved by _Adolphe Quetelet_
(1791-1874) to form another meteoric system; and accordingly the theory
of Olmsted that the November meteors moved round the Sun in 182 days had
to be abandoned, for, says Miss Clerke, “If it would be a violation of
probability to attribute to _one_ such agglomeration a period of an
exact year or sub-multiple of a year, it would be plainly absurd to
suppose the movements of _two_ or more regulated by such highly
artificial conditions.” Accordingly Erman suggested in 1839 the theory
that meteors revolved in closed rings, intersecting the terrestrial
orbit; and that when the Earth crossed through the point of
intersection, it met some members of the swarm. The subject now remained
in abeyance for thirty-four years, if we except some wonderful ideas put
forward in 1861 by _Daniel Kirkwood_ (1813-1896), an American
astronomer, who stated his belief in the disintegration of comets into
meteors; but little attention was paid to his opinions. In 1864 the
subject was taken up by _Hubert Anson Newton_ (1830-1896), Professor at
Yale, who undertook a search through ancient records for the
thirty-three-year period of the Leonids or November meteors. His search
was highly successful, and having demonstrated the existence of the
period, Newton set himself to determine the orbit. He indicated five
possible orbits for the swarm, ranging from 33 years to 354½ days.
Newton was unable to solve the question mathematically; but here Adams,
the discoverer of Neptune, came to the rescue, and demonstrated that the
period of 33¼ years was alone possible, and that the others were
untenable. These investigations, completed in March 1867, proved the
existence of a great meteoric orbit extending to the orbit of Uranus.

Meanwhile Newton had predicted a meteoric shower on the evening of
November 13 and morning of November 14, 1866. His prediction was
fulfilled. The shower was inferior to that of 1833, but was still a
magnificent spectacle. Sir Robert Ball, then employed at Lord Rosse’s
Observatory, observed the shower, and records the impossibility of
counting the meteors. This great shower attracted the attention of
astronomers all over the world to the study of meteors. Meanwhile
Schiaparelli had been working at the subject for some time, and in four
letters addressed to Secchi, towards the end of 1866, he showed that
meteors were members of the Solar System, possessed of a greater
velocity than that of the Earth, and travelling in orbits resembling
those of comets, in the fact that they moved in no particular plane, and
that their motion was both direct and retrograde. Schiaparelli computed
the orbit of the Perseids or August meteors, and was astonished to find
it identical with the comet of August 1862. This was a proof of the
connection between these two apparently widely different types of
celestial bodies. Early in 1867 Schiaparelli found that Le Verrier’s
elements for the orbit of the Leonids were identical with those of the
comet of 1866, discovered by _Ernst Tempel_ (1821-1889). Peters of
Altona had meanwhile reached the same conclusion; while _Edmund Weiss_
(born 1837) of Vienna pointed out the similarity of the orbit of a
star-shower on April 20 and that of the comet of 1861. He also drew
attention, independently of Galle and D’Arrest, to the close connection
between the orbits of the lost Biela’s comet and the Andromedid meteors
of November.

All doubt as to the connection of comets and meteors was removed by the
great shower on November 27, 1872. Biela’s lost comet was due at
perihelion in 1872, and although searched for was not observed; but when
the Earth crossed its orbit, a great meteoric shower took place. “It
became evident,” says Miss Clerke, “that Biela’s comet was shedding over
us the pulverised products of its disintegration.” The shower was little
inferior to that of 1866. Meanwhile _Ernst Klinkerfues_ (1827-1884),
Professor at Göttingen, believing that Biela’s comet itself had
encountered the Earth, telegraphed to _Norman Robert Pogson_
(1829-1891), Government astronomer at Madras, to search for the comet in
the opposite region of the sky. Pogson did observe a comet, but
certainly not Biela’s, although probably another fragment of the missing
body.

The theory of the actual disintegration of comets was enunciated by
Schiaparelli in 1873, and developed in his work ‘Le Stelle Cadenti.’ He
was led to regard comets as cosmical clouds formed in space by “the
local concentration of celestial matter.” He then remarks that a
cosmical cloud seldom penetrates to the interior of the Solar System,
“unless it has been transformed into a parabolic current,” which may
occupy years, or centuries, in passing its perihelion, “forming in space
a river, whose transverse dimensions are very small with respect to its
length: of such currents, those which are encountered by the earth in
its annual motion are rendered visible to us under the form of showers
of meteors diverging from a certain radiant.”

Schiaparelli next pointed out that when the current of meteors
encounters a planet, the resulting perturbations cause some of the
meteoric bodies to move in separate orbits, forming the bolides and
aerolites which fall from the sky at intervals. “The term _falling
stars_” he says, “expresses simply and precisely the truth respecting
them. These bodies have the same relation to comets that the small
planets between Mars and Jupiter have to the larger planets.” In the
third chapter of his ‘Le Stelle Cadenti’ he explicitly states that “the
meteoric currents are the products of the dissolution of comets, and
consist of minute particles which certain comets have abandoned along
their orbits, by reason of the disintegrating force which the Sun and
planets exert on the rare materials of which they are composed.”

In 1878 _Alexander Stewart Herschel_ (born 1836), son of Sir John
Herschel, and a famous meteoric observer, published a list of known or
suspected coincidences of meteoric and cometary orbits, amounting to
seventy-six. Meanwhile much progress has since been made in the
observation of meteoric showers and the determination of their radiant
points. In this branch of astronomy, by far the greatest name is that of
William Frederick Denning, the self-made English astronomer. Born at
Redpost, in Somerset, in 1848, his career of meteoric observation
commenced in 1866. For the past forty years he has attentively devoted
himself to the observation of meteors. From 1872 to 1903 he determined
the radiant points of no fewer than 1179 meteoric showers. In addition
to this, he published, in 1899, a catalogue of meteoric radiants,
containing 4367; and he has carefully studied the remarkable objects
known as fireballs or “sporadic meteors.” He has occasionally been able
to trace a connection between fireballs and weak meteoric showers, but
he concludes that they “must either be merely single sporadic bodies, or
else the survivors of some meteor group, nearly exhausted by the waste
of its material during many past ages.” All of Denning’s meteoric work
has been done in his spare time, for it must be borne in mind that he
pursues the profession of accountant in Bristol, and that only his
leisure hours have been devoted to the science of astronomy. His
researches have been entirely conducted with the unaided eye. His only
instrument is a perfectly straight wand, which he uses as a help and
corrective to the eye in ascribing the paths of the meteors. Thanks to
the laborious work of this able English astronomer, the observation of
meteors is now a _scientific_ branch of astronomy. In the words of
Maunder, “for six thousand years men stared at meteors and learned
nothing, for sixty years they have studied them and learned much, and
half of what we know has been taught us in half that time by the efforts
of a single observer.”

Further meteoric showers from Biela’s comet were seen in 1885 and 1892.
The Leonid shower was confidently predicted for 1899, in accordance with
the thirty-three-year period, but the great display did not come off,
either in 1899 or 1900. In 1901 there was a certain weak shower observed
in America; and similar displays took place in 1903 and 1904. Many
explanations have been given as to the failure of the shower, the most
probable idea being that the attraction of Jupiter diverted the meteors
from their course.

Denning’s observations on meteors resulted, as early as 1877, in the
discovery of so-called “stationary radiants.” The radiant-point of a
long enduring shower usually exhibits an apparent motion, resulting from
the combined orbital motions of the Earth and the meteors; but Denning
found that in some cases the shower, though lasting for months,
persistently exhibited the same radiant-point, implying that the motion
of the Earth must be insignificant compared with that of the meteors,
computed by Ranyard at 880 miles per second. The difficulty of admitting
so great a velocity led the French astronomer, _François Felix
Tisserand_ (1845-1896), to doubt the existence of these stationary
radiants; but the fact of their existence cannot be doubted, although no
really satisfactory explanation has been offered.

Another type of meteors comprises the bodies termed respectively as
bolides, uranoliths, and aerolites,—stones which fall to the Earth from
the sky. In 1800 the French Academy declared the accounts of stones
having fallen from the heavens to be absolutely untrue. Three years
later an aerolite fell at Laigle, in the Department of Orne, on April
26, 1803, attended by a terrific explosion. In the words of Flammarion,
“Numerous witnesses affirmed that some minutes after the appearance of a
great bolide, moving from south-east to north-east, and which had been
perceived at Alençon, Caen, and Falaise, a fearful explosion, followed
by detonations like the report of cannon and the fire of musketry,
proceeded from an isolated black cloud in a very clear sky. A great
number of meteoric stones were then precipitated on the surface of the
ground, where they were collected, still smoking, over an extent of
country which measured no less than seven miles in length.”

Some aerolites, instead of being shattered into fragments, have been
observed to fall to the Earth intact, and bury themselves in the ground.
Numerous instances have been observed during the last century, and
masses of meteoric stones have been found in positions which clearly
indicate that they must have fallen from the sky. Chemists have made
analyses of the elements in these remarkable bodies, and have found them
to contain iron, magnesium, silicon, oxygen, nickel, cobalt, tin,
copper, &c. The spectrum of these aerolites, raised to incandescence,
has been studied by Vogel and by the Swedish observer, _Bernhardt
Hasselberg_ (born 1848), who detected the presence of hydrocarbons,
which are also present in cometary spectra.

When the existence of aerolites as celestial bodies was first
recognised, Laplace suggested that they had been ejected from volcanoes
on the Moon. This theory, although supported by Olbers and other
astronomers, was soon rejected. Next, it was suggested that they were
ejected from the Sun, and Proctor believed them to come from the giant
planets. A very detailed discussion of the subject is to be found in
Ball’s ‘Story of the Heavens’ (1886), in which he expresses views in
harmony with those of the Austrian physicist Tschermak. Ball
demonstrated that the meteors which fall to the Earth cannot have come
from any other planet, nor from the Sun. Accordingly, he concluded that
they were originally ejected by the volcanoes of the Earth many ages
ago, when they were active enough to throw up pieces of matter with a
velocity great enough to carry them away from the Earth altogether. Such
meteors would, however, intersect the terrestrial orbit at each
revolution.

The alternative theory to this, supported by Schiaparelli and Lockyer,
is that the aerolites are merely larger members of the meteor-swarms,
which have been deflected from their paths. The chief objection to this
theory is the absence of connection between the meteoric showers and the
falls of aerolites and bolides. Only on one occasion was a meteoric
stone observed to fall during a shower. On November 27, 1885, during the
shower of Andromedid meteors from Biela’s comet, a large bolide,
weighing more than eight pounds, fell at Mazapil, in Mexico. This,
however, was the only case hitherto observed; and it may have been
merely a coincidence.




                              CHAPTER IX.
                               THE STARS.


The most remarkable progress in astronomy during the past century has
been in the department of sidereal science, or the study of the Suns of
space, observed for their own sakes, and not merely for the purpose of
determining the positions of the Sun and Moon, and to assist navigation.
Thanks to Herschel, the nineteenth century witnessed the steady
development of stellar astronomy, combined with many important
discoveries and investigations.

The one pre-Herschelian problem in sidereal astronomy was the distance
of the stars. Owing to its bearing on the Copernican theory, the problem
was attacked by the astronomers of the seventeenth and eighteenth
centuries. Herschel made numerous attempts to detect the parallax of the
brighter stars, but failed. Meanwhile there had been many illusions.
Piazzi believed that his instruments—which in reality were worn out and
unfit for use—had revealed parallaxes in Sirius, Aldebaran, Procyon, and
Vega; Calandrelli, another Italian, and _John Brinkley_ (1763-1835),
Astronomer-Royal of Ireland, were similarly deluded; and in 1821 it was
shown by _Friedrich Georg Wilhelm Struve_ (1793-1864), the great German
astronomer, that no instruments then in use could possibly be successful
in measuring the stellar parallax. A few years later, however,
Fraunhofer brought the refractor to a degree of perfection surpassing
all previous efforts. In 1829 he mounted for the observatory at
Königsberg a heliometer, the object-glass of which was divided in two,
and capable of very accurate measurements. This heliometer eventually
revealed the parallax of the stars in the able hands of Friedrich
Wilhelm Bessel.

Friedrich Wilhelm Bessel was born at Minden, on the Weser, south-west of
Hanover, on July 22, 1784. His father was an obscure Government
official, unable to provide a university education for his son. Bessel’s
love of figures, together with an aversion to Latin, led him to pursue a
commercial career. At the age of fourteen, therefore, he entered as an
apprenticed clerk the business of Kuhlenkamp & Sons, in Bremen. He was
not content, however, to remain in that humble position. His great
ambition was to become supercargo on one of the trading expeditions sent
to China; and so he learned English, Spanish, and geography. But he
never became a supercargo. In order to be fully equipped for such a
position, he determined to learn how to take observations at sea, and
his acquaintance with observation aroused a desire to study astronomy.
He constructed for himself a sextant, and by means of this, along with a
common clock, he determined the longitude of Bremen.

Such enthusiasm could not be long without its reward. For several years
Bessel remained a clerk, and the hours devoted to study were those
spared from sleep. He studied the works of Bode, Von Zach, Lalande, and
Laplace, and in two years was able to compute the orbits of comets by
means of mathematics. From some observations of Halley’s comet at its
appearance in 1607, Bessel calculated its orbit, and forwarded the
calculation to Olbers, then the greatest authority on cometary
astronomy. Olbers was delighted at this work, and he sent the results to
Von Zach, who published them. The self-taught young astronomer had
accomplished a piece of work which fifteen years before had taxed the
skill and patience of the French Academy of Sciences.

In 1805, Harding, Schröter’s assistant at Lilienthal, resigned his
position for a more promising one at Göttingen. Olbers procured for
Bessel the offer of the vacant post, which the latter accepted. At
Lilienthal Bessel received his training as a practical astronomer. He
remained in Schröter’s observatory until 1809. Although only twenty-five
years of age, he had become so well known in Germany that in that year
he was appointed Professor of Astronomy in the University of Königsberg,
and was chosen to superintend the erection of the new observatory there.
Within a few years a clerk in a commercial office had worked his way
from obscurity to fame.

In 1813 the Königsberg Observatory was completed, and here Bessel worked
for thirty-three years, until his death, on March 17, 1846. It was only
about ten years before his death that he commenced his search for the
stellar parallax, with the aid of Fraunhofer’s magnificent heliometer.
He determined to make a series of measures on a small double star of the
fifth magnitude in the constellation Cygnus, named 61 Cygni, the large
proper motion of which led him to suspect its proximity to the Solar
System. From August 1837 to September 1838 he made observations on 61
Cygni, and he found that there was an annual displacement which could
only be attributed to parallax. In order to have no mistake, he made
another year’s observations, which confirmed the results he arrived at
previously, and all doubt was removed by a third series. The resulting
parallax was 0·3483″, corresponding to a distance of 600,000 times the
Earth’s distance from the Sun. This was confirmed some years later by C.
A. F. Peters at Pulkowa, and still later by Otto Struve, who estimated
the distance at forty billions of miles. Meanwhile, F. G. W. Struve,
working at Pulkowa, found a parallax of 0·2613″ for Vega, but this was
afterwards found to be considerably in error. Accordingly, Struve does
not rank with Bessel as a successful measurer of star-distance. But
independently of Bessel, another accurate measure had been made by
_Thomas Henderson_, the great Scottish astronomer.

Born in Dundee in 1798, Thomas Henderson was the youngest of five
children of a hard-working tradesman. After education in his native town
he went to Edinburgh, where he worked for years as an advocate’s clerk,
pursuing studies in astronomy as a recreation from his boyhood. In 1831
he had become so well known, that he received the appointment of
Astronomer-Royal at the new observatory at the Cape of Good Hope. But
the climate of South Africa did not suit his health, and after a year he
returned to Scotland. In 1834 he became Professor of Astronomy in the
University of Edinburgh, and Astronomer-Royal of Scotland, which
position he held till his death on November 23, 1844, at the early age
of forty-six.

During a year’s work at the Cape, Henderson undertook a series of
observations on the bright southern star, α Centauri, with a view to
determining its parallax. These observations were made in 1832 and 1833,
but were not reduced until Henderson’s return to Scotland. At length, on
January 3, 1839, he announced to the Royal Astronomical Society that he
had succeeded in measuring the parallax of α Centauri, which he
determined as about one second of arc, corresponding to a distance of
about twenty billions of miles. This result was confirmed by the
observations of _Thomas Maclear_ (1794-1879), his successor at the Cape,
and by those of later observers, notably Sir David Gill, who has reduced
the parallax to 0·75″.

Other determinations of stellar parallax, some genuine and others
illusory, were made soon after these successful observations. C. A. F.
Peters and Otto Struve at Pulkowa were among the most famous
parallax-hunters in the middle of the century. One of the most
successful searchers after parallax was the German astronomer _Friedrich
Brünnow_ (1821-1891), who was employed from 1865 to 1874 as
Astronomer-Royal of Ireland. He determined the parallax of Vega as
0·13″, and this was confirmed in 1886 by Hall at Washington: while he
measured the parallax of the star Groombridge 1830, which turned out to
be 0·09″. He resigned his post in 1874, and his successor at Dublin
Observatory proved to be his successor also in this branch of astronomy.
_Robert Stawell Ball_, born in Dublin in 1840, was astronomer to Lord
Rosse in 1865 and 1866, and became in 1874 Astronomer-Royal of Ireland
in succession to Brünnow, a position which he filled until his
appointment in 1892 as Professor of Astronomy at Cambridge, and director
of the observatory there. During his term of office in Dublin he
undertook, in 1881, a “sweeping search” for large parallaxes, thereby
disproving certain ideas as to the proximity to the Earth of red and
temporary stars; while he also determined the parallax of the star 1618
Groombridge.

But the greatest extension of our knowledge of stellar distances, in
recent years, is due to a Scottish astronomer, who has maintained the
reputation of Scotland, and also of the Cape Observatory, in this line
of research. Born in Aberdeen in 1843, _David Gill_ directed Lord
Lindsay’s private observatory at Dunecht, in Aberdeenshire, from 1876 to
1879. In the latter year he succeeded _Edward James Stone_ (1831-1897)
as Astronomer-Royal at the Cape, a position which he has since filled
with conspicuous ability. From 1881 he has been engaged in the hunt for
parallax. In conjunction with _William Lewis Elkin_ (born 1855), now
director of Yale College Observatory, he determined the parallaxes of
nine stars with the aid of Lord Lindsay’s heliometer. In 1887, with a
larger instrument, he resumed the search, while Elkin worked in
co-operation with him, but at Yale Observatory, where he undertook the
measurement of the parallaxes of northern stars. He fixed in 1888 an
average parallax for first-magnitude stars, which was determined at
0·089″, corresponding to a journey for light of thirty-six years.

Most of the successful determinations of parallax have been made by the
“relative” method—that is, the determination of the displacement of a
star in reference to another star, assumed to be situated at an
immeasurable distance. The method of absolute parallax, on the other
hand,—the star’s displacement in right ascension and declination,—has
been seldom used, owing to the laborious reduction which has to be gone
through before the result can be reached. In 1885, however, a series of
observations were undertaken at Leyden by _Jacobus Cornelius Kapteyn_
(born 1851), who determined by the absolute method the parallaxes of
fifteen northern stars.

The first application of photography to the problem was due to the zeal
and energy of _Charles Pritchard_ (1808-1893), Professor of Astronomy at
Oxford, who determined by this method the parallax of 61 Cygni, which he
announced in 1886 to be 0·438″, in agreement with Ball’s determination.
He also determined the average parallax of second-magnitude stars, which
came out as 0·056″. Since the time of Pritchard’s observations various
other more or less satisfactory determinations of parallax have been
made. Few of the parallax determinations are probably very accurate, and
none exact; but an idea of the difficulty of the measurement may be
gathered from the remark of an American writer, Mr G. P. Serviss, that
the displacement “is about equal to the apparent distance between the
heads of two pins, placed an inch apart, and viewed from a distance of a
hundred and eighty miles.”

Closely allied to the question of parallax is the determination of the
exact positions of the stars and the formation of star-catalogues. In
this branch, too, much is due to the genius of Bessel. The observations
of Bradley at Greenwich from 1750 to 1762 were reduced by Bessel into
the form of a catalogue, which was published in 1818, with the title of
‘Fundamenta Astronomiæ.’ During the years 1821 to 1823 Bessel took
75,011 observations, by which he brought up the number of accurately
known stars to 50,000. At the same time notable catalogues had been
constructed, particularly by the English astronomer, _Francis Baily_
(1774-1844), and by _Giovanni Santini_ (1786-1877), director of the
observatory at Padua; but Bessel’s successor in this branch of research
was _Friedrich Wilhelm August Argelander_ (1799-1875). In 1821 he became
assistant to Bessel at Königsberg, in 1823 director of the Observatory
at Abo, in Finland, and in 1837 of that at Bonn. Here he commenced in
1852 the great ‘Bonn Durchmusterung,’ a catalogue and atlas of 324,198
stars visible in the northern hemisphere. The great catalogue was
published in 1863. After Argelander’s death it was extended so as to
include 133,659 stars in the southern hemisphere, by his assistant
_Eduard Schönfeld_ (1828-1891), who succeeded him in 1875 as director of
Bonn Observatory, where he died in 1891. Meanwhile a greater undertaking
was commenced in 1865 by the Astronomische Gesellschaft. This was the
co-operation of thirteen observatories in Europe and America for the
exact determination of the places of 100,000 of Argelander’s stars.

In the southern hemisphere, working at Cordova in Argentina, was the
great American astronomer, _Gould_, whose ‘Uranometria Argentina,’
published in 1879, gives the magnitudes of 8198 stars, and whose
Argentine General Catalogue, containing reference of 32,448 stars, was
published in 1886. The late Radcliffe observer, Stone, published a
useful catalogue in 1880 from his observations at the Cape.

The application of photography to the work of star-charting dates from
1882, when Gill photographed the comet of 1882, and was struck with the
distinctness of the stars on the background. For some time he had
contemplated the extension of the ‘Durchmusterung,’ from the point where
Schönfeld left it, to the southern pole, and the idea struck him to
utilise photography for the purpose. In 1885, accordingly, Gill
commenced work, and in four years all the photographs were taken. The
reduction of the observations into the form of a catalogue was
spontaneously undertaken by the great Dutch astronomer, Kapteyn, who was
occupied with the work for fourteen years, until in 1900 the great
catalogue, known as the ‘Cape Photographic Durchmusterung,’ was
completed. Half a million stars are represented on the plates taken at
the Cape.

By the time the ‘Durchmusterung’ was completed, a greater undertaking
was in progress. Paul and Prosper Henry, astronomers at the Paris
Observatory, when engaged in continuing Chacornac’s ecliptic charts,
applied photography to their work, and found it very successful.
Accordingly Gill’s proposal, on June 4, 1886, of an International
Congress of Astronomers, to undertake a photographic survey of the
heavens, was enthusiastically received by the French astronomers. The
Congress met at Paris in 1887, under the presidentship of _Amédée
Mouchez_ (1821-1892), director of the Paris Observatory, fifty-six
astronomers of all nations being present. The Congress resolved to
construct a Photographic Chart, and a Catalogue, the former containing
twenty million stars, the latter a million and a quarter. Meetings were
held in Paris in 1891, 1893, 1896, and 1900 to superintend the progress
of the work, which is now (1906) well advanced towards completion.

A unique star catalogue is in course of preparation by the Scottish
astronomer, _William Peck_ (born 1862), astronomer to the City of
Edinburgh since 1889. Mr Peck’s catalogue is accompanied by a series of
charts. His star-magnitudes are those of all famous catalogues reduced
to a standard scale. This catalogue, the result of more than fifteen
years’ work, will be an important addition to the many valuable works of
the kind already in existence, and will further increase the already
great reputation of Scotsmen in practical astronomy.

The determination of the proper motions of the stars is another
important branch of practical astronomy in which much progress has been
made since the time of Herschel. Stars with much larger proper motions
than those of the first magnitude have been discovered. For many years
the small sixth-magnitude star in Ursa Major, 1830 Groombridge, was
supposed to be the swiftest of the stars, and was named by Newcomb the
“runaway star.” But in 1897, on examining the plates of the ‘Cape
Durchmusterung,’ Kapteyn discovered a still swifter star of the eighth
magnitude, situated in the southern constellation, Pictor. The rate of
its motion is over eight seconds of arc yearly; and an idea of the vast
distance of the stars may be obtained by the statement that it would
take 200 years for the star—known as Gould’s Cordova Zones, V Hour
243—to move over a space equal to the moon’s diameter. Important
observations have been made on the stellar motions, and on their bearing
on the structure of the Universe, by various astronomers, including J.
C. Kapteyn and _Ludwig Struve_ (born 1858), son of Otto Struve; but
these must be reserved for a later chapter.

Richard Anthony Proctor, born at Chelsea, in London, in 1837, graduated
at Cambridge in 1860. For the next twenty-eight years he earned his
living by publishing many volumes on astronomy, popular and technical,
fifty-seven having appeared at the time of his death, which took place
at New York on September 12, 1888. Notwithstanding the vast amount of
work bestowed on his books, his original investigations were permanent
contributions to astronomical science. In 1870 he undertook to chart the
directions and amounts of 1600 proper motions. While engaged on this
work, it occurred to him that it would be “desirable and useful to
search for subordinate laws of motion.” He found, from the laborious
process of charting, that five of the seven stars of the Plough had a
motion in common—that is to say, were moving in the same direction at
the same rate. This phenomenon was termed by Proctor “star-drift.” He
also recognised other instances of star-drift in other portions of the
heavens.

The subject was soon afterwards taken up by the French astronomer,
Camille Flammarion. Born in 1842 at Montigny-le-Roi, in Haute Marne,
Flammarion was appointed assistant to Le Verrier in 1858, but gave up
his post in 1862. Employed successively at the Bureau des Longitudes,
and as editor of scientific papers, he founded in 1882 his private
observatory at Juvisy-sur-Orge, where he has since continued his
investigations.

Following up Proctor’s discovery of star-drift, Flammarion drew charts
of proper motions. He demonstrated the “common proper motion” of Regulus
and an eighth-magnitude star, Lalande 19,749, from a comparison of his
measures in 1877 with those of Christian Mayer a century previously;
while he discovered many other instances. His reflections on these
motions, as given in his ‘Popular Astronomy,’ are worthy of
reproduction: “Such are the stupendous motions which carry every sun,
every system, every world, all life, and all destiny in all directions
of the infinite immensity, through the boundless, bottomless abyss; in a
void for ever open, ever yawning, ever black, and ever unfathomable;
during an eternity, without days, without years, without centuries, or
measures. Such is the aspect, grand, splendid, and sublime, of the
universe which flies through space before the dazzled and stupefied gaze
of the terrestrial astronomer, born to-day to die to-morrow, on a
globule lost in the infinite night.”

Measures of proper motion only enable us to determine the motion of
stars across the line of sight. They do not tell us whether the star is
advancing or receding. Here, however, the spectroscope comes to our aid
by means of Doppler’s principle, described in the chapter on the Sun. It
occurred to Huggins that, by observing the displacement of the lines in
the spectra of the stars, he could determine their motion in the line of
sight. His first results were announced in 1868. In the case of Sirius,
the displacement of the line marked F was believed to indicate a
velocity of recession of 29 miles a second. Some time later Huggins
announced that Betelgeux, Rigel, Castor, and Regulus were retreating,
while Arcturus, Pollux, Vega, and Deneb were approaching. Soon after
this successful work the subject was taken up by Maunder at Greenwich
and by Vogel at Bothkamp; but the delicacy of the measurements prevented
satisfactory results from being reached through visual observations, and
accordingly the measurements were very discordant.

In 1887 H. C. Vogel, working at Potsdam Astrophysical Observatory,
applied photography to the measurement of radial motion. Assisted by
_Julius Scheiner_ (born 1858), he determined the radial motions of
fifty-one bright stars by photographing the stellar spectra and
measuring the photographs. Vogel found 10 miles a second to be the
average velocity of stars in the line of sight, the tendency of the eye
being to exaggerate the displacements. The swiftest of the stars
measured by Vogel proved to be Aldebaran, with a velocity of recession
of 30 miles a second. Since 1892 the subject has been pursued by Vogel
himself with the new 30-inch refractor at Potsdam, by Campbell at the
Lick Observatory, Bélopolsky at Pulkowa, and other observers. Towards
the end of 1896 Campbell undertook, with the 36-inch Lick refractor, a
series of measures on radial motion, and many important discoveries were
made. These, however, must be reserved for the chapter dealing with
double stars.

Herschel’s great discovery, from the apparent motions of the stars, of
the movement of the Solar System was not accepted by the next generation
of astronomers. Bessel declared in 1818 that there was absolutely no
evidence to show that the Sun was moving towards Hercules. Even Sir John
Herschel rejected his father’s views, although some confirmatory results
had been reached by Gauss. At length, in 1837, Argelander, in a
memorable paper, based on his observations at Abo, in Finland, attacked
the problem, and demonstrated, from a discussion of the motions of 390
stars, quite independently of Herschel’s work, that the Solar System was
moving towards Hercules. This was confirmed in 1841 by Otto Struve, in
1847 by _Thomas Galloway_, and in 1859 and 1863 by Airy and _Edwin
Dunkin_ (1821-1898), assistant at Greenwich Observatory.

Meanwhile, in 1886, _Arthur Auwers_, permanent Secretary of the Berlin
Academy of Sciences, completed the re-reduction of Bradley’s
observations at Greenwich, and brought out 300 reliable proper motions,
which were utilised by Ludwig Struve, whose investigation removed the
solar apex from Hercules to the neighbouring constellation Lyra: this
slight change was confirmed by _Oscar Stumpe_, of Bonn, and _Lewis_
_Boss_ (born 1847), director of the Observatory at Albany, New York. An
investigation by Newcomb fully confirmed the previous results. In 1900,
1901, and 1902 Kapteyn made three distinct investigations on the solar
motion, and still further confirmed the previous investigations.

These investigations are fully confirmed by the application to the
question of Doppler’s principle of measuring radial motion. The
spectroscopic researches of Campbell at the Lick Observatory place the
solar apex very near the position assigned to it by Newcomb and Kapteyn.
Campbell finds the solar velocity to be about 12 miles a second, and
Kapteyn thinks a velocity of about 11 miles a second is “the most
probable value that can at present be adopted.”




                               CHAPTER X.
                        THE LIGHT OF THE STARS.


“That a science of stellar chemistry should not only have become
possible, but should already have made material advances, is assuredly
one of the most amazing features in the swift progress of knowledge our
age has witnessed.” So writes Miss Agnes Mary Clerke, the historian of
modern astronomy. As long ago as 1823 Fraunhofer observed the spectra of
the brighter stars, and gathered the first hint of the grouping of the
stars into three classes. Then, after Fraunhofer’s death, the subject
lay in abeyance for thirty-seven years. At length, in 1860, on
Kirchhoff’s explanation of the Fraunhofer lines, the study of stellar
spectra was inaugurated at Florence by Donati, who carefully fixed the
positions of the more important lines. His instrumental means, however,
were very limited, and his observations were not successful. In 1862
Rutherfurd, in New York, commenced the study of stellar spectra, but
shortly afterwards turned his attention to astronomical photography. The
actual founders of stellar spectroscopy were the eminent Italian
observer, Angelo Secchi, and the illustrious Englishman, William
Huggins.

Angelo Secchi was born in 1818 at Reggio, in the Emilia. Educated in the
Collegio Romano, he was ordained priest in 1847, but his love of
science, and particularly astronomy, dates from the beginning of his
career. In 1849 he succeeded Di Vico as director of the Observatory of
the Collegio Romano. This post he filled with conspicuous ability for a
period of twenty-nine years, until his death on February 26, 1878. To
Secchi is due the credit of the first spectroscopic survey of the
heavens. He reviewed the spectra of 4000 stars, and classified them into
four distinct groups, which are recognised to this day. The first type
embraces over half of those which Secchi examined. This type is
represented by Sirius, Vega, Altair, and other bluish-white stars, and
is characterised by the intensity of the hydrogen lines. The second type
embraces the yellow stars, such as Capella, Arcturus, Aldebaran, Pollux,
and the Sun itself, and is known as the Solar type. The spectra of these
stars closely resemble that of the Sun, and are distinguished by
innumerable lines. Secchi’s third type, or red stars, represented by
Betelgeux, Antares, and others, are characterised by strong absorption
bands, and the spectra have been described as “fluted.” The third-type
stars are comparatively scarce compared with the first and second, and
the fourth is even less numerous. The fourth-type stars are also red
with broad absorption lines. To Secchi’s four types a fifth was added in
1867 by Wolf and Rayet of Paris Observatory—namely, the gaseous stars.
Secchi aimed at a comprehensive survey of the stellar spectra, and he
accomplished much valuable work. He did not devote his time to analysing
individual stars. This branch of study—analysis of spectra and the
determination of the elements in the stars—was undertaken by his
contemporary, William Huggins, one of the greatest astronomers whom
England has ever produced.

Born in London in 1824, William Huggins commenced his astronomical
researches at the age of twenty-eight. In 1856 he erected, at Tulse
Hill, London, an observatory which he equipped at great expense. He
commenced observations on the usual astronomical lines, taking times of
transits and making drawings of the surfaces of the planets. But he soon
tired of the routine of ordinary astronomical work, and on the
publication of Kirchhoff’s explanation of the Fraunhofer lines in the
solar spectrum, he commenced to investigate the spectra of the stars.
Having constructed a suitable spectroscope, he commenced observations in
1862 in conjunction with his friend, William Allen Miller, Professor of
Chemistry in London. He exhaustively investigated the two red stars,
Betelgeux and Aldebaran, ascertaining the existence in the former star
of sodium, iron, calcium, magnesium, and bismuth; and in the latter star
the same elements, with the addition of tellurium, antimony, and
mercury.

In 1863 Huggins made an attempt to photograph the spectra of the stars,
and, indeed, obtained prints of Sirius and Capella, but no lines were
visible in them. In 1874 Draper of New York obtained a photograph of the
spectrum of Vega, showing four lines. Two years later Huggins again
attacked the problem, and secured a photograph of the spectrum of Vega,
showing seven strong lines. In 1879 he was enabled to communicate
satisfactory results of his work to the Royal Society, and since then he
has secured many admirable representations. In 1899 the monumental work,
‘An Atlas of Representative Stellar Spectra,’ the joint work of Sir
William and Lady Huggins, was published.

In 1874 the German Government established at Potsdam the Astrophysical
Observatory, for the spectroscopic study of the Sun and stars. A
position on the staff was given to Hermann Carl Vogel, whose researches
in astronomical spectroscopy rank with those of Secchi and Huggins. Born
in Leipzig in 1842, he was from 1865 to 1869 employed in the Leipzig
Observatory. Called to Bothkamp as director in 1870, he resigned his
post in 1874 to accept a position on the staff at Potsdam Observatory.
In 1882 he became director of that Institution, which position he still
retains.

In 1874 Vogel revised Secchi’s classification of stellar spectra, and in
1895 he further improved on it. His classification improves rather than
supersedes the previous work of Secchi; nevertheless, he approached the
question from a different standpoint. Vogel concluded in 1874 that a
rational scheme of stellar classification “can only be arrived at by
proceeding from the standpoint that the phrase of development of the
particular body is, in general, mirrored in its spectrum.” Vogel divides
Secchi’s first type into three classes. In the first type, designated
I_a_,—represented by Sirius and Vega,—the metallic lines are “very faint
and fine,” and the hydrogen lines conspicuous. In I_b_ no hydrogen lines
are visible, while in I_c_ the hydrogen lines are bright. This class
includes the gaseous stars. In 1895, after the recognition of helium in
the stars by his assistant, Scheiner, Vogel separated the stars of class
I_b_ from the first type altogether. These stars are sometimes
designated as “Type O,” and sometimes as helium stars and Orion stars,
as the majority of the stars in Orion are of that type. The solar type
is divided into two classes, II_a_ being represented by the Sun,
Capella, and other well-known stars, while II_b_ includes the Wolf-Rayet
stars. Secchi’s third and fourth types are both classified by Vogel as
of the third type. These red stars were specially studied from 1878 to
1884 by Dunér at Lund. His results were published in a descriptive
catalogue which appeared at Stockholm in 1884. His researches related to
the spectra of 352 stars, 297 of Secchi’s third type and 55 of his
fourth. Dunér is perhaps the greatest authority on stars with banded
spectra.

Vogel’s classification of spectra is generally adopted by astronomers,
although others have been proposed by Lockyer and by _Edward Charles
Pickering_ (born 1846), director of the Harvard Observatory. Lockyer’s
classification was designed to fit in with his “meteoritic hypothesis,”
discussed in the chapter on Celestial Evolution. The stars were divided
by Lockyer into seven groups, according to his views of their
temperature, rising through gaseous stars, red stars of Secchi’s third
type, and a division of solar stars to the Sirian type, and falling
through a second division of the solar type to red stars of Secchi’s
fourth type.

The first spectroscopic star-catalogue was published in 1883 by Vogel,
assisted by _Gustav Müller_ (born 1851), a son-in-law of Spörer. The
catalogue contained details of 4051 stars to the seventh magnitude, and
more than half of these proved to be of Secchi’s first type. Vogel’s
work was completed in different latitudes by Dunér at Upsala, and by
_Nicolaus Thege von Konkoly_ (born 1842) at O’Gyalla in Hungary.

The famous ‘Draper Catalogue’ ranks as the greatest catalogue of stellar
spectra. It was undertaken at Harvard Observatory by E. C. Pickering, in
the form of a memorial to Henry Draper, the successful spectroscopist.
Commenced in 1886, and published in 1890, it contains photographs of the
spectra of no fewer than 10,351 stars, down to the eighth magnitude.
Pickering subdivided Secchi’s types into various classes, the first or
Sirian into four classes, the second into eight, while the third and
fourth types each constitute a separate class. Pickering designated his
classes by the capital letters of the alphabet.

Much useful work has been done also in the analysis of the various
spectra. Julius Scheiner, now “chief observer” at Potsdam Astrophysical
Observatory, has, since 1890, done much valuable work in this direction.
Special attention was devoted to the spectrum of Capella, 490 lines in
the spectrum of which were measured by Scheiner. In his own words, “he
believes a complete proof of the absolute agreement between its spectrum
and that of the Sun to be thereby furnished.” Other stars of the Sirian
and solar classes were exhaustively studied by Scheiner.

The study of the exact brilliance of the stars was a branch of research
long neglected, yet it is of much importance in astronomy, for it is
only through exact measurement of stellar brilliance that stellar
variation can be detected. Herschel commenced the study, which was
continued by his son at the Cape, but it is only within the last twenty
years that stellar photometry has become a recognised branch of
astronomy; and the credit of this is due to the energy and zeal of the
great American observer, Edward Charles Pickering.

Born in Boston in 1846, Edward Charles Pickering was appointed in 1865
Instructor of mathematics in the Lawrence Scientific School at Harvard,
after a distinguished university career. In 1876 he succeeded Winlock as
director of the Harvard Observatory, and in the following year he
commenced his photometric studies. He invented an instrument named the
meridian photometer, with the aid of which he succeeded in determining,
in the years 1879 to 1882, the exact brilliance of 4260 stars to the
sixth magnitude between the north celestial pole and thirty degrees of
south declination. At a later date he devised a larger photometer, with
which he made over one million observations. Pickering next extended his
survey to the southern hemisphere, erecting the photometer on the slope
of the Andes, where the Harvard auxiliary station at Arequipa is now
located, and where 8000 determinations of stellar brilliance were made.
Meanwhile Pritchard, at Oxford, published in 1885 his ‘Uranometria Nova
Oxoniensis,’ with photometric determinations of the brilliance of 2784
stars from the pole to ten degrees of south declination. Both of these
catalogues were epoch-making works, and testify to the enthusiasm and
perseverance of the astronomers who designed them.

The study of stellar photometry glides into that of stellar variation.
At the beginning of the nineteenth century the number of known variable
stars was very small, as a glance at the list given in Brewster’s
edition of Ferguson’s Astronomy (1811) will show. Some remarkable
investigations were due to the English astronomer, _John Goodricke_
(1764-1786), who rediscovered the variability of the star Algol, and
accurately determined its period in 1782. Goodricke suggested that the
regular variations in the light of Algol were due to the partial eclipse
of its light by a dark satellite, a hypothesis now fully confirmed. Two
years later, in 1784, Goodricke discovered other two variables, δ Cephei
and β Lyræ. He died in 1786 at the age of twenty-one, and thus
variable-star astronomy was deprived of its founder.

The foundation of variable-star astronomy as an exact branch of the
science is due to Argelander. In the years 1837-1845, while residing at
Bonn during the erection of the observatory, of which he had been made
director, he erected a temporary observatory, and there he carefully
determined the magnitudes of all stars visible in Central Europe. From
this he was led to the discussion of stellar variation, to which subject
he continued to give much attention. He was the first to describe a
method of observing variable stars scientifically and accurately,—a
method consisting in estimating in “steps” or “grades” the difference in
brilliance between the variable, or suspected variable, and other stars
which are selected for comparison, and which are of various degrees of
brilliance, so that they may be available for comparison with the
variable throughout its fluctuations. Argelander’s “steps” are tenths of
a magnitude, and Gore describes the method of observation as follows:
“If we call _a_ and _b_ the comparison stars, and _v_ the variable, _a_
being brighter than _b_, and if _v_ is judged to be midway in brightness
between _a_ and _b_, we write _a_5_v_5_b_. If _v_ is slightly nearer to
_b_, we write _a_6_v_4_b_. We may also write _a_3_v_7_b_, or
_a_7_v_3_b_, the sum of the steps being always ten.”

This method, described in 1844, led to many discoveries at Bonn in the
following twenty years by Argelander and his assistants Schmidt and
Schönfeld. At this time Eduard Heis (1806-1877), at Münster, who also
ranks as one of the founders of meteoric astronomy, while engaged on the
construction of his great atlas, attentively determined the change of
magnitude of stars visible to the naked eye; and by means of the naked
eye, the opera-glass, and a small telescope, he amassed a large number
of observations. At the same time two English observers, Hind and
Pogson, were making remarkable discoveries which greatly increased the
number of known variables. Among Hind’s discoveries were S Cancri of the
Algol type; while Schmidt discovered another of the same class, δ Libræ,
and also the famous ζ Geminorum. While director of the Observatory of
Mannheim, an institution equipped with very antiquated instruments,
Schönfeld devoted himself to the study of variable stars, and increased
the number of known variables considerably. In the southern hemisphere
Gould, in South America, did for the observation of variable stars what
Argelander did in the northern.

In 1874 a very important, although not so obvious, service to
variable-star astronomy was rendered by the Danish observer, _Hans Carl
Fredrik Christian Schjellerup_ (1827-1887). He translated from Arabic
into French the works of the Persian astronomer of a thousand years ago,
Al-Sufi, and thus rendered his observations available to modern
astronomers. Al-Sufi was a most accurate observer, and, by comparing his
catalogue with those of modern observers, it can be found whether stars
have changed in brilliance during the past thousand years.

The study of variable stars has been pursued in recent years by many
astronomers, both by means of photography and by the visual method. The
most important names in the visual discovery of variables are Gustav
Müller and _Paul Friedrich Ferdinand Kempf_ (born 1856) of Potsdam;
_Alexander William Roberts_ of Lovedale, South Africa; Seth Carlo
Chandler of Boston; Nils Christopher Dunér at Upsala; and _John Ellard
Gore_ (born 1845) in Dublin.

The researches of J. E. Gore are a brilliant example of how much may be
done for astronomy by means of very moderate instruments. Born in 1845
at Athlone, in Connaught, he went to India in 1868 as engineer on the
Sirhind Canal in the Punjab. While in India he erected his small
telescopes on brick pillars, and took observations, many of them of
stellar brilliance. In 1879 he returned to Ireland, and since then has
devoted himself to astronomy with zeal and enthusiasm. His discoveries
and investigations of variables have been made by means of the
binocular. On December 13, 1885, he discovered a remarkable star in
Orion, which was at first considered to be temporary, and called “Nova
Orionis,” but was afterwards found to be a long-period variable star.

Recently photography has come much to the front in the discovery of
variable stars. Pickering at Harvard, and Wolf at Heidelberg, have
particularly distinguished themselves in this branch, and the number of
known variables is now very large, as every year brings fresh
discoveries, mostly by aid of photography. Many of these
newly-discovered variables are in star-clusters and nebulæ.

Pickering proposed in 1880 the following classification of variable
stars, which has been adopted all over the scientific world: Class I.,
temporary star; Class II., stars undergoing in several months large
variations, such as Mira Ceti and U Orionis; Class III., irregular
variables, such as Betelgeux and α Herculis; Class IV., short-period
variables, such as δ Cephei, ζ Geminorum, and β Lyræ; Class V., “Algol
variables,” which undergo variations lasting but a few hours. It is
doubtful whether new stars should be included in a classification of
variables, although in one case, at least, a new star was found to be a
long-period variable. To these a sixth class may now be added. This
class, the detection of which is mainly due to the profound
investigations of Gore, is composed of what have been termed “secular
variables,” which undergo slow fluctuations in periods of many years,
and sometimes of centuries. This Class includes δ Ursæ Majoris, Al-Fard,
λ Draconis, θ Serpentis, ε Pegasi, 83 Ursæ Majoris, ζ Piscis Australis,
β Leonis, α Ophiuchi, η Crateris, and others. The secular variations of
some of these stars have been detected by Gore himself during the past
thirty years, while in other cases he has detected them by comparison of
the most important star-catalogues, from Hipparchus and Al-Sufi down to
our own time. In some cases the star in question seems to be slowly
gaining in brilliance, in others slowly diminishing.

Thanks to the application of the spectroscope, much is now known of the
cause of the light changes in variable stars. Goodricke’s theory of the
variations of Algol was theoretically confirmed by the researches of E.
C. Pickering in 1880. In 1889 Vogel proved beyond a doubt that the
variation in the light of Algol is due to the partial eclipse of its
light by a dark satellite. It was obvious to Vogel that, as both Algol
and its companion are in revolution round their common centre of
gravity, the motion of Algol in the line of sight might be detected by
the spectroscopic method of observation. Vogel found that before each
eclipse Algol was retreating from our system, while on recovering it
gave signs of rapid approach, proving conclusively that both the star
and its dark satellite were in revolution round their centre of
gravity,—Algol suffering partial eclipse only because the plane of the
orbit lies in our line of sight. Algol, therefore, is not inherently a
variable star, but merely a binary. Following up his researches, Vogel,
assuming that the bright and dark stars are of equal density, arrived at
the conclusion that Algol is a globe about one and a half million miles
in diameter, the satellite equalling the size of the Sun, and the
centres of the stars being separated by about 3,230,000 miles. Thus,
variable stars of the Algol type are not variable in the true sense of
the word. Even the most irregular of the Algol variables have been
explained. Perhaps the most irregular was Y Cygni, discovered by
Chandler in 1886. It was soon found, however, that the variations
recurred with great irregularity: in less than two years the phases
differed by as much as seven hours from the predicted times. At length
the subject was taken up by Dunér at Upsala. A series of observations
made with the 14-inch refractor at Upsala in 1891 and 1892 convinced him
in the latter year that two eclipses take place in the course of one
revolution: one star occults the other. Dunér showed that the intervals
between minima were thus—1 day 9 hours; 1 day 15 hours; 1 day 9 hours,
and so on. Thus, the first, third, fifth, and seventh sets of minima
obeyed a different law from the second, fourth, sixth, and eighth. Dunér
proved that two stars revolve round their centre of gravity in less than
three days, alternately occulting each other, while the ellipticity of
the orbit explains the irregularity of the light changes. In April 1900
Dunér gave his final conclusions as follows: “The variable star Y Cygni
consists of two stars of equal size and equal brightness, which move
about their common centre of gravity in an elliptical orbit, whose major
axis is eight times the radius of the stars.” He also stated the exact
period of revolution and the eccentricity of the orbit.

In the case of the short-period variables, such as β Lyræ, δ Cephei, ζ
Geminorum, and η Aquilæ, the variations do not seem to be due to
eclipse. It was discovered by Professor Pickering that β Lyræ is a
spectroscopic binary, but Vogel and Keeler showed that the supposed
orbit is incompatible with the eclipse theory. Vogel says: “I am
convinced that β Lyræ represents a binary or multiple system, the
fundamental revolutions of which in 12 days 22 hours in some way control
the light change.” The eclipse theory, however, is still maintained by
Bélopolsky, who has framed a hypothesis according to which the chief
minimum of the star’s light corresponds with the obscuration of the
lesser star, the lesser minimum with that of the primary, implying that
the primary is much less luminous in proportion to its light than its
satellite,—a state of affairs which Miss Clerke concludes to be
improbable.

The variable stars, δ Cephei and η Aquilæ, were both found in 1894 by
Bélopolsky to be binaries; but as the times of minimum light do not
correspond with those of eclipses in the hypothetical orbits, he
concludes that the variations cannot be explained on the eclipsing
satellite theory. Miss Clerke is inclined to the theory that the
increase of luminosity in short-period variables is due to tidal action,
so that while the revolutions of the stars control their variability,
they are inherently unstable in light. A large number of these stars are
known, and it is a remarkable fact that the majority of these variables
lie on or near the Galaxy, so that their variations have probably
something to do with their vicinity.

We now come to the long-period variables of which Mira Ceti, χ Cygni,
and U Orionis are examples. Although varying in regular periods,
generally of about a year, they are subject to remarkable
irregularities, so that an exact period cannot be assigned even to Mira
Ceti, of which the maxima are at times retarded and at others
accelerated with no apparent law. The spectroscopic investigations of
Campbell in 1898 have shown that Mira Ceti is a solitary star, while
bright lines of hydrogen appear in its spectrum at maximum, showing that
the variations are due to periodical conflagrations in its atmospheres.
In many other long-period variables bright lines have been observed.

A remarkable fact regarding these stars is the amount of their light
change. Mira Ceti, for instance, varies from the first to the ninth
magnitude, and U Orionis from the sixth to the twelfth. As M. Flammarion
says, “the longer the period the greater the variation.” Another
remarkable fact is that their light curves show a curious resemblance to
the curves of the solar spots, only on a vastly greater scale, which
indicates that, relatively, these long-period variables are much older
than our Sun, the small variations in the light of which are
imperceptible. “Here, if anywhere,” says Miss Clerke, “will be found the
secret of stellar variability.”

To the irregular variables no period can be assigned. Betelgeux, in
Orion, the variation of which was noted by Sir John Herschel in 1840, is
a typically irregular variable. But the most extraordinary of all
variables is η Argus, in the southern hemisphere, which is probably a
connecting link between variable and temporary stars. The traveller
Burchell, from 1811 to 1815, observed the star as of the second
magnitude, but in 1827 he noted it to be of the first magnitude. In the
following year it fell to the second magnitude. In 1834 Sir John
Herschel noted the star to be between the first and second magnitude,
and in 1838 it rose to the first, being equal to α Centauri. After a
decline, it became in 1843 equal to Canopus, and not much inferior to
Sirius. Then it began to fade, and in 1868 it was only of the sixth
magnitude. In 1899 Innes estimated it as 7·71. Rudolf Wolf suggested a
period of 46 years, and Loomis 67 years; but astronomers generally agree
with Schönfeld that the star has no regular period.

The first temporary star of the nineteenth century was discovered by
Hind, in London, April 28, 1848. It was of the fifth magnitude at
maximum, and soon after began to fade, falling to the tenth magnitude.
In 1860 a new star appeared in the cluster Messier 80 in Scorpio, and
was discovered by Auwers at Königsberg. It reached only the seventh
magnitude.

On the night of May 12, 1866, a new star of the second magnitude blazed
out in the constellation Corona Borealis. It was first observed at Tuam,
in Ireland, by the Irish astronomer, _John Birmingham_. Four hours
earlier Schmidt had been observing that part of the heavens, and it was
not then visible. Birmingham at once communicated the discovery to
Huggins, at Tulse Hill, who had commenced his spectroscopic
observations. On May 16 Huggins observed its spectrum. In the words of
Miss Clerke, “The star showed what was described as a double spectrum.
To the dusky flutings of Secchi’s third type, four brilliant rays were
added. The chief of these agreed in position with lines of hydrogen; so
that the immediate cause of the outburst was plainly perceived to have
been the eruption, or ignition, of vast masses of that subtle kind of
matter.” Nine days after the appearance of the new star it was invisible
to the naked eye, and afterwards fell to the tenth magnitude. In 1856
Schönfeld had observed it at Bonn as a telescopic star, so that it was
not a “new star” in the true sense of the word.

The next temporary star observed was discovered by Schmidt, at Athens,
November 24, 1876. It was of the third magnitude, situated in the
constellation Cygnus. On December 2 its spectrum was examined at Paris
by _Alfred Cornu_ (1841-1902), and some days later at Potsdam by Vogel
and Lohse. It was closely similar to that of the new star of 1866,
bright lines of hydrogen and other elements standing out in front of an
“absorption” spectrum. By the end of 1876 the star was of the seventh
magnitude. On September 2, 1877, Nova Cygni was observed at Dunecht, and
its spectrum was found to have been transformed into that of a planetary
nebula. Three years later, however, the ordinary stellar spectrum
reappeared.

A new star appeared in the centre of the great nebula in Andromeda in
August 1885. The first announcement of the discovery was by _Karl Ernst
Albrecht Hartwig_ (born 1851), who observed the new star on August 31;
but it had been previously seen by several other observers. On September
1 it was of the seventh magnitude, and by March of the following year
had fallen to the sixteenth. Observed by Vogel, Young, and Hasselberg,
the new star gave a continuous spectrum, but Huggins and Copeland
succeeded in discerning bright lines. Hall, at Washington, undertook a
series of measures to detect the parallax of Nova Andromedæ, but his
efforts were unsuccessful.

The discovery of the next temporary star was announced February 1, 1892,
by the Rev. _Thomas_ _D. Anderson_, a Scottish amateur astronomer, in a
post-card to the Astronomer-Royal of Scotland. The star was situated in
the constellation Auriga. An examination of photographs, taken at
Harvard Observatory, showed that the new star had appeared December 10,
1891, and had risen to a maximum of the fourth magnitude ten days later.
On a photograph taken by Max Wolf on December 8 the new star was not
visible. After Anderson’s visual discovery, the spectrum of the new star
was studied by Copeland, Huggins, Lockyer, Vogel, Campbell, and others.
Bright hydrogen lines were visible in the spectrum, which appeared to be
actually double, giving support to the theory that the outburst was the
result of a collision between two dark bodies; and this was confirmed by
the measurements of radial motion by the Potsdam astronomers.

After March 9, 1892, the new star steadily faded, falling to the
sixteenth magnitude on April 26. But on August 17 _Edward Singelton
Holden_ (born 1846), director of the Lick Observatory, and his
assistants, Schaeberle and Campbell, found it of the tenth magnitude. On
August 19 Barnard found it transformed into a planetary nebula: while
various spectroscopic observations of the revived Nova revealed the
nebular lines. By the end of 1894 the new star had faded to the eleventh
magnitude, and early in 1901 was observed as a minute nebula.

After 1892 several new stars appeared, and were detected on photographic
plates by _Mrs Fleming_ (born 1857), of Harvard Observatory. The first
of these, in the southern constellation Norma, was discovered in 1893 by
its peculiar spectrum on a Draper spectrographic plate taken at Harvard.
But the new star rose only to the seventh magnitude. Other new stars
were discovered in Carina (Argo) in 1895, in Centaurus in 1895, in
Sagittarius in 1898, and in Aquila in 1900. Nova Sagittarii was, at its
brightest, fully equal to Nova Aurigæ, and was plainly visible to the
naked eye, but was never observed visually.

A temporary star, appropriately designated “the new star of the new
century,” blazed out in Perseus on the night of February 21, 1901. It
was discovered independently by several observers: on February 21, by
Borisiak, a student at Kiev, in Russia; on the following morning, by
Anderson in Edinburgh; and on the next evening, by Gore at Dublin,
Nordvig in Denmark, Grimmler at Erlangen, and other observers. When
first seen by Anderson, it was equal to Algol, of the second magnitude.
A photograph by Williams at Brighton showed that it must have been
fainter than the twelfth magnitude on February 20. On the evening of
February 23 the star was brighter than Capella, and was then the
brightest star in the northern hemisphere. On February 25 it fell to the
first magnitude; on March 1 to the second, and on March 6 to the third.
During the spring and summer the light fluctuated considerably, but in
September and October faded to the 6·7 magnitude. In March 1902 it was
of the eighth magnitude, and in July 1903 of the twelfth.

The spectrum of Nova Persei was found by Pickering to be of the Orion
type on February 22 and 23. On February 24 the spectrum had become one
of the bright and dark lines, and the hydrogen lines indicated a
velocity of 700 to 1000 miles a second. Measures of the sodium and
calcium lines, by Campbell and others, indicated a velocity of only
three miles a second, so that the displacements of the hydrogen lines
may have been due to an outburst of hydrogen in the star. The spectrum
was carefully studied during the spring and summer by Pickering,
Lockyer, Huggins, Vogel, and others. On June 25 Pickering reported that
the spectrum was slowly changing into that of a gaseous nebula. In
August and September 1901 the nebular spectrum became more apparent.

In August 1901 Wolf at Heidelberg discovered a faint trace of nebula
near the nova. On September 20 this nebula was photographed by _George
Ritchey_ at the Yerkes Observatory, and was seen to be of a spiral form.
This was confirmed by Perrine, who also found, from plates taken in
November, that the nebula was moving at the rate of eleven minutes of
arc a year. This extraordinary velocity was exceedingly puzzling to
astronomers, and at length Kapteyn suggested that the nebula shone only
by reflected light from the new star, and that the apparent motion was
an illusion caused by the flare of the explosion travelling out from the
nova.

On March 16, 1903, _Herbert Hall Turner_ (born 1861), Professor of
Astronomy at Oxford, discovered a new star of the seventh magnitude in
the constellation Gemini, from an examination of photographic plates.
Photographs taken at Harvard showed that on March 1 it must have been
fainter than the twelfth magnitude, while five days later it was of the
fifth. In August 1903 Pickering found its spectrum nebular. In August
1905 another small nova was found by Mrs Fleming on the Harvard
photographs, situated in Aquila.

Many theories have been advanced to account for temporary stars.
Flammarion has shown that a body surrounded by a hydrogen atmosphere, on
grazing a dark body enveloped in oxygen, would produce a tremendous
explosion. In 1892 Huggins suggested that the outburst of Nova Aurigæ
was due to the near approach of two bodies with large velocities,
disturbances of a tidal nature resulting and producing enormous
outbursts. Vogel suggested that the new star was due to the encounter of
a dark star with a worn-out system of planets; while Lockyer believes
all new stars to be due to the collision of swarms of meteors. Perhaps
the most probable theory is that of Seeliger, which attributes these
outbursts to the movement of a dark body through nebulous matter, which
is extensively diffused throughout space. This theory explains the
changes in the spectra as well as the revivals of brightness which
characterised Nova Aurigæ and the fluctuations of Nova Persei. In a
paper read to the Royal Society of Edinburgh in November 1904, the
German astronomer, _Jacobus Halm_, of the Royal Observatory, Edinburgh,
extended and developed Seeliger’s theory, showing also that the
necessary consequence of such an encounter as the theory assumes is the
formation of an atmosphere of incandescent gases, followed by that of a
revolving ring of nebulous matter. In the hands of Halm, therefore,
Seeliger’s theory leads to the nebular hypothesis as advanced by Laplace
and Herschel.




                              CHAPTER XI.
                      STELLAR SYSTEMS AND NEBULÆ.


The study of double stars, commenced by Herschel, was taken up after his
death by several of the foremost astronomers, and has since been pursued
by quite a number of observers and computers. Herschel’s immediate
successor in the study of double stars was his son, who ranks only
second to his father as a student of stellar systems. Born at Slough on
March 7, 1792, John Frederick William Herschel passed his childhood
“within the shadow of the great telescope.” Although his early life was
spent with his father and aunt, astronomy does not appear to have taken
up his attention as a boy. Chemistry, however, always interested him,
and, as his aunt recorded, even while a child he was fond of making
experiments. He was educated at Hitcham, and afterwards at Eton. He was
delicate, however, so his mother removed him from school, and he was
trained at Slough by Mr Rogers, a Scottish mathematician. At the age of
seventeen Herschel entered the University of Cambridge, and Caroline
Herschel, who was exceedingly proud of him, recorded in her memoirs that
he gained all the first prizes without exception. He left the University
in 1813.

John Herschel did not turn his attention to astronomy until he had
attained the age of twenty-four. In a letter to a friend, September 10,
1816, he said, “I am going, under my father’s directions, to take up
star-gazing.” It was only reverence for his father that made him turn to
astronomy, and he gave up the science he loved most—chemistry. But his
unselfishness received its reward. In 1820 John Herschel constructed his
first reflector under his father’s guidance. Four years previously he
had begun to observe double stars, which had been for long studied by
his father, who discovered their revolutions. These observations were
continued from 1821 to 1823 at the Observatory of Sir _James South_
(1786-1867). John Herschel and South measured 380 of the elder
Herschel’s double stars. These investigations gained for Herschel and
South the Lalande Prize of the French Academy and the Gold Medal of the
Royal Astronomical Society.

When his mother died Sir John Herschel decided to sail to the Cape of
Good Hope to make an investigation of the stars of the southern
hemisphere, which until then had been much neglected. He was offered a
free passage in a ship of war, but declined. In November 1833 he left
England, taking with him his great telescopes. In two months he arrived
at Cape Town, and erected his astronomical instruments at Feldhausen, a
short distance off. In October 1835 he informed his aunt that he had
almost completed his survey of the southern hemisphere. During his
“sweeps” of the heavens he discovered 1202 double stars, and 1708 nebulæ
and star-clusters. In 1838 he returned to England, and devoted the
remainder of his life to the publication of his results, as well as to
other branches of science. He died at Collingwood, in Kent, on May 11,
1871, at the age of seventy-nine.

John Herschel’s favourite objects of study were double stars, of which
he discovered 3347 in the northern hemisphere, and 1202 in the southern.
He also computed several stellar orbits; but the first calculation of a
stellar orbit was made by the French astronomer _Felix Savary_
(1797-1841), who computed the orbit of ξ Ursæ Majoris, and found the
period to be about sixty years. Contemporary with John Herschel was his
great rival in double-star astronomy, Friedrich Georg Wilhelm Struve.
Born at Altona in 1793, Struve took his degree in 1811 at the Russian
University of Dorpat. In 1813 he became director of the Dorpat
Observatory, and was in 1839 promoted to Pulkowa, as director of the
great Observatory there, remaining at its head until within three years
of his death, on November 23, 1864. Struve’s first recorded observation
was on the double star Castor. In 1819 he commenced to measure the
position-angles of double stars, of which he published a catalogue of
795. In 1825 he commenced a review of the heavens down to fifteen
degrees south, and thus discovered 2200 previously unknown objects. The
results were published in Struve’s ‘Mensuræ Merometricæ,’ which appeared
in 1836, giving the places, distances, colours, position-angles, and
relative brilliance of 3112 double and multiple stars.

Struve’s successor in this branch of astronomy was his son, Otto Wilhelm
von Struve, born in 1819 at Dorpat, who became in 1837 assistant to his
father, and in 1861 succeeded him as director of the Pulkowa
Observatory. In 1890 he retired from this post, settling in Germany, at
Carlsruhe, where, on April 14, 1905, he died in his eighty-sixth year.
Otto Struve detected 500 double stars, among them γ Andromedæ,
discovered in 1842, and δ Equulei, discovered in 1852, within a period
of between five and eleven years.

Various other astronomers have devoted themselves to the observation of
double stars, among them _Ercole Dembowski_ (1815-1881), of Milan; _Karl
Hermann Struve_ (born 1854), son of Otto Struve; _William Doberck_ (born
1845); _William J. Hussey_ (born 1864), now director of the Detroit
Observatory; Camille Flammarion; N. C. Dunér; G. V. Schiaparelli;
_Thomas Jefferson Jackson See_ (born 1866). But the greatest living
_discoverer_ is _Sherburne Wesley Burnham_ (born 1838), now employed at
the Yerkes Observatory, in Wisconsin. Born in 1838 at Thetford, Vermont,
he commenced his career as a shorthand reporter, studying astronomy in
his leisure hours. With a small 6-inch refractor, mounted in a home-made
observatory, Burnham commenced in 1871 his discoveries of double stars,
which soon attracted the attention of noted astronomers, who permitted
him to use larger telescopes, with which he continued his researches.
His first official appointment was in 1888, when he became chief
assistant at the Lick Observatory, which position he resigned in 1892.
Some years later he became astronomer in the Yerkes Observatory.
Altogether he has discovered 1308 double stars, with telescopes ranging
from a 6-inch refractor to the gigantic 40-inch of the Yerkes
Observatory.

The computation of double-star orbits has been undertaken by various
astronomers, among them Mädler, Klinkerfues, Dunér, Flammarion,
Seeliger, See, Gore, Burnham, _Robert Grant Aitken_ (born 1864) of the
Lick Observatory, and _Giovanni Celoria_ (born 1842), who was, from 1866
to 1900, assistant in the Brera Observatory of Milan, and since 1900
director of that institution. On June 9, 1890, Gore presented to the
Royal Irish Academy a catalogue of computed binaries containing
reference to fifty-nine stars.

In 1844 Bessel discovered a remarkable irregularity in the proper motion
of Sirius. He ascribed this to the gravitational influence of some
obscure body, probably a large satellite. In 1857 Peters calculated an
orbit for the supposed satellite with a period of 50 years. In 1861 an
orbit was computed by _Truman Henry Safford_ (1836-1901), which
indicated the position of the satellite. Close to this position it was
accidentally discovered by _Alvan Clark_ (1832-1897), the famous
American optician. The period of the star seems to be about 50 years. In
1844 Bessel noticed irregularities in the proper motion of Procyon, and
put forward the idea of a disturbing satellite, as in the case of
Sirius. This was confirmed by Mädler, and in 1874 an orbit was computed
by Auwers, who found a period of 40 years. In 1896 the satellite was
found by Schaeberle with the 36-inch refractor of the Lick Observatory.
A period of 40 years was found by See, in agreement with the
hypothetical orbit.

In putting forward these theories as to invisible stellar satellites,
Bessel remarked that “light is no real property of mass,” and that the
existence of countless visible stars is nothing against the existence of
countless invisible and dark ones. In this he laid the foundation of the
branch of science termed by Mädler the “Astronomy of the invisible.” In
recent years the astronomy of the invisible has become a recognised
branch of astronomical research, through the application and
interpretation of Doppler’s principle in spectroscopic observations. In
the course of photographing the stellar spectra for the Draper
Catalogue, E. C. Pickering photographed the spectrum of Mizar (ζ Ursæ
Majoris) in 1887 and again in 1889. On some of these photographs the
line K was seen double, while on others it was seen under its normal
aspect. This doubling of the lines indicated that the star which we see
as single is in reality composed of two bodies in revolution round their
centre of gravity, so close together that even the largest telescopes
cannot divide them. Pickering assigned a period of 104 days, but in 1901
Vogel diminished this to 20 days. In the same year the star β Aurigæ was
similarly found to be double; and in 1890 Vogel, from photographs taken
at Potsdam, independently inaugurated the discovery of spectroscopic
binaries. In the spectrum of Spica he discovered the spectral lines to
be, not doubled, but periodically displaced, indicating the existence of
a dark or nearly dark companion, both stars revolving round their centre
of gravity. Spica was seen to belong to the same class as Algol, only
that in the case of Algol the plane of the satellite’s orbit passes
through the Earth and eclipses the star, while in the case of Spica the
orbit is inclined, and the star is constant in light.

The line of research commenced by Vogel and Pickering was soon followed
up by these investigators, as well as by Bélopolsky at Pulkowa, Campbell
at the Lick Observatory, Slipher at the Lowell Observatory, and by
_Edwin Brant Frost_ (born 1866), now director of the Yerkes Observatory,
and his assistant, _Walter Adams_. In 1894 Bélopolsky discovered the
duplicity of several variable stars, and in 1896 that of Castor, in
Gemini. Late in 1896 Campbell undertook a systematic investigation of
radial motions, and has since discovered about sixty spectroscopic
binaries,—among them, in 1899, the Pole Star, and in 1900 Capella. The
latter discovery was made independently by _Hugh Frank Newall_ (born
1857) at Cambridge, in England. It was found by Campbell that the
revolution of the stars round their centre of gravity is performed in
104 days; and it soon became apparent that, owing to the large size of
the orbit, the duplicity of Capella might be observed telescopically. At
Greenwich the star was seen to be elongated, but at the Lick Observatory
it was seen persistently single.

Campbell finds that of 285 stars observed by him, more than one in nine
is a spectroscopic binary. He concludes that at least one star in five
or six will be found to be spectroscopically double, and considers that
“the proven existence of so large a number of stellar systems, differing
so widely in structure from the Solar System, gives rise to a suspicion
at least that our system is not of the prevailing type of stellar
systems.”

The study of triple and multiple stars is of deep interest, but the
orbits of these objects cannot be said to be fully investigated by any
means. The first application of the problem of three bodies to stellar
astronomy was made by Seeliger in 1889. His researches, relating to the
famous star, ζ Cancri, disclosed the existence of three stars revolving
round a dark body, apparently the most massive in the system. The system
of ζ Cancri, at least, seems to be modelled on the Ptolemaic design.

In the study of star-clusters and nebulæ, as in the investigation of
double stars, Herschel’s successor was his son. His observations, both
in England and at the Cape of Good Hope, resulted in a large number of
new discoveries, and the results of his studies in this direction were
published in 1864 in his catalogue of all known clusters and nebulæ,
amounting to 5079. This catalogue was enlarged and revised in 1888 by
_John Louis Emil Dreyer_ (born 1852), a Danish astronomer, but director
of the Observatory at Armagh, in Ireland; and the same observer
published from 1888 to 1894 a supplementary list, bringing the number of
known clusters and nebulæ to about 10,000.

In the early part of his career, John Herschel held firmly to the views
of his father of the difference between star-clusters and nebulæ,
considering the latter to be composed of “shining fluid.” But he fell
off from this view with the resolution into stars of many irresolvable
nebulæ. In 1845 _William Parsons_, third _Earl of Rosse_ (1800-1867),
erected at Birr Castle, in Ireland, his great 6-foot reflector, which
still surpasses all other telescopes in point of size. With this
instrument Lord Rosse believed himself to have resolved the Crab nebula
in Taurus and the Nebula in Orion, which was also said to have been
resolved by Bond with the 15-inch refractor at Harvard; and in 1854
Olmsted declared the “resolution” of these nebulæ to be the signal for
the renunciation of Herschel’s nebular theory. Most astronomers fell in
with the view that all the nebulæ were distant clusters, which would
eventually be resolved into stars, although it is only right to state
that the Scottish astronomer, _John Pringle Nichol_ (1804-1859), and
some other investigators, held to the theory of Herschel.

The solution of the great problem was in 1864, when on August 29 of that
year Huggins turned his spectroscope on a bright planetary nebula in
Draco. To his amazement the spectrum was one of bright lines, proving
conclusively that the nebula was not a star-cluster, but a mass of
glowing gas,—hydrogen, and some other unknown substance, now named
“nebulium.” By 1868 Huggins had observed the spectra of seventy nebulæ.
Of these one-third proved to be gaseous, among them the great Orion
nebula which Lord Rosse was believed to have resolved into stars. In the
spectrum of the latter, the “chief nebular line” was at first ascribed
by Huggins to nitrogen, but this was a mistake. Later, it was believed
by Lockyer to coincide with the fluting of magnesium, but this was
disproved by Huggins in 1889-90, and by Keeler in 1890-91. The great
nebula in Andromeda and the great spiral in Canes Venatici were found by
Huggins to display a continuous spectrum, and a similar discovery was
made in regard to the cluster M 13 in Hercules, and other star-clusters.
In the case of the nebulæ, it is not believed that the continuous
spectrum is due to the existence of sun-like bodies, as a gas under
pressure would give a continuous spectrum.

The Orion nebula has been more thoroughly studied than any other object
of its class. The application of photography to spectroscopy has done
much to further the study of the lines in the nebular spectrum. In 1886
Copeland detected in the spectrum of the Orion nebula the yellow ray of
helium. On February 13, 1890, Scheiner announced an important discovery,
namely, the possession by both the nebula and the stars in Orion—with
the exception of Betelgeux—of a line, which appeared bright in the
nebular spectra and dark in the stellar. This line was identified by
Vogel, Lockyer, and others with that of helium.

Nebular photography was inaugurated in 1880 by Draper, who obtained a
remarkably good representation of the Orion nebula in that year. His
work in this direction, cut short by his death in 1882, was taken up by
Janssen at Meudon, and by Common in England, who obtained, in 1883,
several excellent photographs. Later photographs have shown the Orion
nebula to be much more extended than visual observations would lead one
to expect. A photograph secured in 1890 by W. H. Pickering revealed the
nebulous matter in Orion in its true form, that of a gigantic spiral,
starting from near Bellatrix, sweeping past κ Orionis and Rigel to η,
and joining with the great nebula surrounding θ; the entire
constellation being thus shown to be enwrapped in nebulous haze.

In 1885 nebular photography was commenced by _Isaac Roberts_
(1829-1904), the English amateur astronomer, who secured admirable
representations of clusters and nebulæ. He published, in 1893 and 1900,
two volumes of collected photographs of clusters and nebulæ. This
monumental work was thus referred to by Dr _William James Lockyer_: “Dr
Roberts has not only nobly enriched astronomical science, but has raised
a monument to himself which will last as long as astronomy has any
interest for mankind.”

Perhaps the most remarkable revelation made by photography in this
branch of research has been the discovery of the nebulæ in the Pleiades.
In 1859 Tempel observed at Florence an elliptical nebula south of the
star Merope. On November 16, 1885, the brothers Henry obtained at Paris
a photograph of the Pleiades, revealing the existence of a small spiral
nebula. This was confirmed by visual observations, and particularly by
the photographs of Roberts, which also showed the entire cluster to be
nebulous, and that “the nebulosity extends in streamers and fleecy
masses, till it seems almost to fill the spaces between the stars, and
to extend far beyond them.” In 1888 a further advance was made by the
brothers Henry, who found seven stars to be strung on a nebulous streak.

Since 1890 nebular photography has been pursued by Max Wolf in Germany,
and by E. E. Barnard and J. E. Keeler in America. Wolf’s photographs of
the constellation Cygnus brought out the close connection between the
stars and the extensively diffused nebulosities discovered by him. In
1901 Wolf discovered a “nebelhaufen” or cluster of nebulæ, and in 1902
published a catalogue of 1528 nebulæ round the pole of the Galaxy,
showing them to be systematically distributed. Keeler made his memorable
observations with the great 36-inch reflecting telescope, which was
constructed in England many years ago by Common. It afterwards passed
into the hands of Mr Crossley of Halifax, who presented it to the Lick
Observatory. With this great instrument Keeler commenced to take
photographs of the heavens. On one occasion he photographed a well-known
nebula, and on developing the plate was surprised to find seven new
nebulæ besides that which he had photographed. On another occasion he
exposed a plate to a nebula in Pegasus. He was amazed to find altogether
twenty-one nebulæ included in the photograph. To give another instance,
a plate directed to the constellation Andromeda contained no fewer than
thirty-two nebulous objects. This has given an enormous extension to our
knowledge of the nebulæ. But even this is not all. Keeler found on his
plates numerous points of light which seem to be also nebulæ, either too
small or too remote to appear as such. Apparently, however, they are not
stars. Keeler’s work convinced him that, on a modest estimate, there
must be at least _one hundred and twenty thousand_ new nebulæ within
reach of the Crossley reflector. Half of these, he announced, were
probably spiral. An idea of the vast importance of Keeler’s work may be
gained if we reflect that the observations of all the earlier
astronomers resulted in the discovery of six thousand nebulæ. The
investigations of Keeler, in all probability, were the means of adding
120,000 more.

Many observations have been made on nebulæ, for the purpose of
ascertaining their proper motions—but without success. Measurements were
made by D’Arrest in 1857 and by Burnham in 1891, but none of these
revealed any motion of the nebulæ across the line of sight. Even the new
spectroscopic method of determining motions in the line of sight, in the
hands of Huggins, failed in the case of the nebulæ. With the great Lick
refractor at his disposal, Keeler attacked the subject in 1890, and
measured the radial velocities of ten nebulæ. He found that the
well-known planetary nebula in Draco was moving towards the Solar System
at the rate of 40 miles a second; for the Orion nebula he found a motion
of recession of 11 miles a second; but probably this belongs chiefly to
the movement of the Solar System in the opposite direction.

Unfortunately Keeler did not live to carry on his investigations in
nebular astronomy. His early death brought to an abrupt end these
fruitful investigations. Appointed director of the Lick Observatory in
1898, he died suddenly at San Francisco on August 12, 1900, at the early
age of forty-two.




                              CHAPTER XII.
        STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE.


After the death of Herschel there was little done in the direction of
furthering our knowledge of stellar distribution, or the construction of
the heavens. Here, as elsewhere, Herschel’s immediate successor was his
son, whose star-gauges, both in England and in South Africa, were a
worthy sequel to those of his father; but John Herschel, in his books on
astronomy, reproduced his father’s disc-theory, unaware that the elder
Herschel had himself abandoned it. The work of the younger Herschel was
entirely supplementary to that of his father.

To Wilhelm Struve belongs the credit of showing the disc-theory to be
untenable, and of demonstrating that Herschel had abandoned it. This he
was able to do after a perusal of Herschel’s papers, presented to him by
John Herschel. Having demonstrated this, he undertook a series of
investigations which resulted in his famous theory of the Universe. This
was published in his work ‘Études d’Astronomie Stellaire,’ which was
published in 1847. His researches were based on the star-catalogues of
Bessel, Piazzi, and others; and dealing with 52,199 stars, he discussed
the number of stars in each zone of Right Ascension. He found, in the
words of Mr Gore, “that the numbers increase from hour i to hour vi,
where they attain a maximum. They then diminish to a minimum at hour
xiii, and rise to another but smaller maximum at hour xviii, again
decreasing to a second minimum at hour xxii. As the hours vi and xviii
are those crossed by the Milky Way, the result is very significant.” He
concluded the Galaxy to be produced by a collection of
irregularly-condensed clusters, the stars condensed in parallel planes.
Next, he considered the Universe as perhaps infinitely extended in the
direction of the Galaxy, and accordingly he put forward the idea that
the light from the fainter and more distant stars was extinguished in
its passage through the ether of space, which he regarded as imperfectly
transparent. The theory, as Struve propounded it, was disposed of by Sir
John Herschel, who remarked that we were not permitted to believe that
at one part of the sky our view was limited by extinction, while at
another a clear view right through the Galaxy could be had; and by
_Robert Grant_ (1814-1892), director of the Glasgow Observatory, who
showed that, were the theory true, the Galaxy should present a uniform
appearance throughout its course. On the whole, Struve’s theory was no
improvement on Herschel’s; for, as Encke pointed out, Struve’s theory
was built on five assumptions, all of which were questionable.

At the time of Struve’s investigation Mädler, at Dorpat, was engaged in
an attempt to solve the question of the construction of the heavens by
quite another method, that of stellar proper motion. He determined to
investigate the subject of proper motion in order to discover the
central body of the Milky Way. If such a centre existed, however, the
motions near it would be somewhat different from those in the Solar
System. In our Solar System the planets nearest the Sun move swiftest,
owing to the strength of the force of gravitation. In the Sidereal
System, on the other hand, the movements at the centre, as Mädler
pointed out, would be slowest. As there would be no very large
preponderating body, the mutual attractions of the different stars would
cause the bodies at the boundaries of the Universe to move faster than
those at the centre, the central sun—the object of Mädler’s search—being
in a state of rest relative to the Sidereal System. Mädler accordingly
began to search the heavens for a region of sluggish proper motions.

In the constellation Taurus, Mädler noticed that the proper motions of
the stars were very slow. The idea occurred to him that the bright red
star Aldebaran might be the central sun, but its very large proper
motion was obviously against this inference. Star after star was now
subjected by Mädler to the most careful scrutiny. At length, after a
laborious investigation, he announced that the star which fulfilled the
conditions of a central body was Alcyone, the brightest of the Pleiades,
a group possessed of no proper motion except that due to the sun’s drift
in the opposite direction. In 1846 Mädler published his hypothesis in
his elaborate work, ‘The Central Sun.’ He announced that his
observations had led him to the conclusion that Alcyone occupied the
centre of gravity of the Sidereal System, and was the point round which
the stars of the Galaxy were all revolving. His profound imagination,
however, did not stop here. This speculation led him to the sublime
thought that the centre of the Universe was the Abode of the Creator. In
1847 Struve rejected Mädler’s theory as “much too hazardous,” and this
has been the general opinion of astronomers. Mädler’s theory is now
regarded as quite untenable.

Herschel’s earlier idea that the nebulæ were external galaxies was long
held by the majority of astronomers, in preference to his later and more
advanced ideas. The supposed resolution of the nebulæ by Lord Rosse’s
telescope gave support to this external galaxy theory. It was clearly
shown, however, by _William Whewell_ (1794-1866) in 1853, and by
_Herbert Spencer_ (1820-1903) in 1858, that the systematic distribution
of the nebulæ in regard to the stars precluded the possibility of their
being external galaxies. This was confirmed by the spectroscopic
discovery of the gaseous nature of some of the nebulæ, and by the later
researches of R. A. Proctor. Not only did Proctor make fresh
discoveries, but it fell to him to clear away the erroneous ideas
regarding the construction of the heavens, and to put the study on a new
basis. In 1870 Proctor plotted on a single chart all the stars, to the
number of 324,198, contained in Argelander’s ‘Durchmusterung’ charts.
This work gave the death-blow to the “disc-theory.” In his own words,
“In the very regions where the Herschelian gauges showed the minutest
telescopic stars to be most crowded, my chart of 324,198 stars shows the
stars of the higher orders (down to the eleventh magnitude) to be so
crowded, that by their mere aggregation within the mass they show the
Milky Way with all its streams and clusterings. It is utterly impossible
that excessively remote stars could seem to be clustered exactly where
relatively near stars were richly spread.”

Proctor showed also that in all probability the stars composing the
nebulous light of the Galaxy are much smaller than the brighter stars,
and not at such a great distance as their faintness would lead us to
suppose,—a conclusion confirmed by the work of Celoria. Proctor was not
so fortunate in theorising as in direct investigation. He thought that
the Magellanic clouds were probably external galaxies; and further, he
put forward the idea that the Milky Way is a spiral, the gaps and
coal-sacks being due to loops in the stream, but neither of these ideas
has found favour with astronomers. But the chief work accomplished by
Proctor was a revision of our knowledge of the Universe, which he thus
describes: “Within one and the same region coexist stars of many orders
of real magnitude, the greatest being thousands of times larger than the
least. All the nebulæ hitherto discovered, whether gaseous and stellar,
irregular, planetary, ring-formed, or elliptic, exist within the limits
of the Sidereal System.”

Proctor’s discovery of the excess of bright stars on the Galaxy was
confirmed by _Jean Charles Houzeau_ (1820-1888), director of the
Brussels Observatory. Some time later J. E. Gore carefully examined the
positions of all the brighter stars in the northern and southern
hemisphere. Following this, he made an enumeration of the stars in the
atlas of Heis and in the charts constructed by Harding; the outcome of
the investigation being to show that stars of each individual magnitude
taken separately tend to aggregate on the Galaxy, the aggregation being
noticed even in first-magnitude stars. Gore further pointed out many
cases of close connection between the lucid stars and the galactic
light. A similar investigation was undertaken by Schiaparelli in 1889.
Schiaparelli, basing his work on the catalogue of Gould and the
photometric measures of Pickering, constructed a series of planispheres
which demonstrated the crowding of the lucid stars towards the plane of
the Galaxy. These investigations were still further continued by Simon
Newcomb, who demonstrated that “the darker regions of the Galaxy are
only slightly richer in stars visible to the naked eye than other parts
of the heavens, while the bright areas are between 60 and 100 per cent
richer than the dark areas.” The Dutch astronomer, _Charles Easton_,
finds a connection between the distribution of ninth-magnitude stars and
the luminous and obscure spots in the Galaxy.

It was noticed by Gould, from observations made at Cordova, that “a belt
or stream of bright stars appears to girdle the heavens very nearly in a
great circle which intersects the Milky Way.” According to Gould, the
belt includes Orion, Canis Major, Argo, Crux, Centaurus, Lupus, and
Scorpio in the southern hemisphere, and Taurus, Perseus, Cassiopeia,
Cepheus, Cygnus, and Lyra in the northern. This was interpreted by
Celoria as indicating the existence of two galactic rings, but Gould
considered the zone of bright stars to form with the Sun a subordinate
cluster of about five hundred stars within the Galaxy.

Perhaps the most elaborate investigations on the structure of the
Universe have been those of Kapteyn, commenced in 1891. In that year he
demonstrated that stars are bluer and more easily photographed in the
Galaxy than elsewhere, a discovery independently made by Gill at the
Cape, and Pickering at Harvard. In 1893 Kapteyn announced his
conclusions, derived from a novel method of studying the distance of the
stars from their proper motions. In order to reach a definite idea of
the distances of the stars, he made use of the component of the proper
motion, measured at right angles to a great circle of the sphere which
passes through a given star and the apex of the solar motion. He found
that stars of the first spectral type have smaller proper motions than
those of the second, indicating that stars of the second type are on the
average nearer to the Solar System than those of the first, the near
vicinity containing almost exclusively second-type stars. Kapteyn
concluded that the group of second-type stars formed one system, named
the solar cluster, which he considered to be roughly spherical in shape.
In 1902 he abandoned this idea, retaining, however, his opinions as to
the relative distances of the different types. That the second-type
stars are nearer to the Sun than the first is, he remarked in a letter
to the writer, incontrovertible.

In the investigation of the motions in, and extent of, the Universe, the
name of Simon Newcomb stands out pre-eminently. Born in 1835 at Wallace,
in Nova Scotia, he went to the States in 1853. In 1862 he received an
appointment at Washington Observatory, and he retained an official
position until 1897. Throughout his scientific career he has been
specially attracted by the question of the construction of the heavens,
which he fully discussed in his book on ‘The Stars’ in 1901. Newcomb’s
investigations have shown that some of the stars are not permanent
members of the Sidereal System, among them the swiftly-moving 1830
Groombridge. He has shown that the Stellar Universe does not possess
that form of stability which is seen in the Solar System. Newcomb
considers the Universe to be limited in extent, as opposed to the
opinions of Struve and others, who believed it to be infinite. He has
brought clearly before his readers a calculation, based on the known law
that there are three times as many stars of any given magnitude as of
that immediately brighter, the increase of number compensating for the
decrease of brilliance. Were the Universe infinitely extended, the whole
heavens would shine with the brilliance of the Sun. Newcomb, therefore,
concludes that “that collection of stars which we call the Universe is
limited in extent.”

Positive evidence that this is the case was obtained by Giovanni
Celoria, now director of the Milan Observatory, in the course of a
series of star-gauges at the north galactic pole. Using a small
refractor, showing stars barely to the eleventh magnitude, he found he
could see exactly the same number of stars as Herschel’s large
reflector, indicating that increase of optical power will not increase
the number of stars visible in that direction. Celoria’s observation can
only be explained on the assumption that the Universe is limited in
extent, as otherwise Herschel’s telescope should have shown more stars
than Celoria’s, even granting an extinction of light,—a theory which
Newcomb, Schiaparelli, and others have shown to be quite untenable. That
the Universe is limited in extent is about all that is known for
certain, although even this has been called in question, notably by E.
W. Maunder and H. H. Turner. The problem of the construction of the
heavens is by no means solved, although several more or less probable
theories have been advanced.

A series of investigations on stellar distribution, from 1884 to 1898,
led Hugo Seeliger, director of the Munich Observatory, to some
remarkable deductions. He believes the Universe to be flattened at the
galactic poles. The Galaxy is the zone of stellar condensation, and he
concludes the distance of the Solar System from the inner border of the
zone to be 500 times the distance of Sirius, while the external border
is 1100 times that distance. The Universe is finite in extent, its
limits being about 9000 light years from the Solar System. In Seeliger’s
opinion the extinction of light may come into play beyond our Universe,
and prevent us seeing other collections of stars.

The question of external universes is purely a hypothetical one,
although there is undoubtedly much to be said in its favour. These
universes have never been seen, and we can only speculate as to their
existence. The last word on the subject is by Gore, in 1893, in his
elaborate work, ‘The Visible Universe.’ He regards the Solar System as a
system of the first order, and the Galaxy and its fellow-universes of
the second. He makes a calculation of the possible distance of an
external universe of his second order. He assumes the distance of the
nearest universe from our Galaxy as proportional to that separating the
Sun from α Centauri, and reaches the amazing conclusion that the
distance of the nearest Galaxy is no less than
520,149,600,000,000,000,000 miles,—a distance which light, with its
inconceivable velocity of 186,000 miles a second, would take almost
ninety millions of years to traverse.

These calculations absolutely overwhelm the mind, which is unable to
comprehend such vast distances. Our universe is indeed, as Flammarion
expresses it, a point in the infinite. The calculations of J. E. Gore
represent our highest scientific conception of the universe. He sums up
his investigations with the following words: “Although we must consider
the number of _visible_ stars as strictly finite, the numbers of stars
and systems really existing, but invisible to us, may be practically
infinite. Could we speed our flight through space on angel wings beyond
the confines of our limited universe to a distance so great that the
interval which separates us from the remotest fixed star might be
considered as merely a step on our celestial journey, what further
creations might not then be revealed to our wondering vision? Systems of
a higher order might there be unfolded to our view, compared with which
the whole of our visible heavens might appear like a grain of sand on
the ocean shore,—systems perhaps stretching out to infinity before us,
and reaching at last the glorious ‘mansions’ of the Almighty, the Throne
of the Eternal.”




                             CHAPTER XIII.
                          CELESTIAL EVOLUTION.


In the second chapter we outlined the nebular hypothesis as propounded
by Herschel. Some time earlier the French mathematician, Laplace, had
put forward his theory of the evolution of the Solar System. _Pierre
Simon Laplace_ was born at Beaumont-en-Auge, near Honfleur, in 1749, and
was educated in the Military School of his native town. In 1767 he
became Assistant Professor of Mathematics at Beaumont, and some years
later at the Military School in Paris, which position he retained for
many years. Member of the Institute and Minister of the Interior under
Napoleon, he was created a Marquis by Louis XVIII., and died at Arcuile
on March 5, 1827.

In the last chapter of his popular work, the ‘Système du Monde,’ Laplace
put forward his nebular theory “with that distrust which everything
ought to inspire that is not the result of observation or calculation.”
Laplace noticed that in the Solar System all the planets revolved round
the Sun in the same direction, from west to east, and that the
satellites of the planets obeyed the same law. He also observed that the
Sun, Moon, and planets rotated on their axes in the same direction as
they revolved round the Sun; also that the planets moved round the Sun,
and the satellites round their primaries, in almost the same plane as
the Earth’s orbit, the plane of the ecliptic. It was evident that these
remarkable congruities were not the result of chance, and accordingly
Laplace expressed his belief that the Solar System originated from a
great nebula, which in condensing detached various rings in the process
of rotation. These rings condensed into the various planets and their
satellites.

Laplace’s theory was powerfully supported by Herschel’s observations of
the various nebulæ in the heavens. But, with the supposed resolution of
the various nebulæ after the erection of the Rosse reflector in 1845,
the evidence in favour of the nebular theory seemed to be greatly
reduced. In 1864, however, the discovery of the gaseous nebulæ, by means
of the spectroscope, gave further support to the theory. Powerful aid
was lent to the nebular hypothesis by the famous German physicist,
_Hermann Ludwig Ferdinand von Helmholtz_ (1821-1894), in 1854, in his
theory of the maintenance of the Sun’s heat. Many theories had been
already advanced to account for this. After the discovery of the
conservation of energy, _Julius Robert Mayer_, one of the discoverers,
put forward the theory that the Solar heat was sustained by the inflow
of meteorites from space, and this idea was developed in 1854 by Sir
_William Thomson_, now _Lord Kelvin_ (born 1824), but it was soon
apparent that the supply of meteors required to sustain the Solar heat
was such as would have increased the mass of the Sun very considerably.
Accordingly the hypothesis was partially abandoned, and was succeeded by
that of Helmholtz, who pointed out that the radiation of the Sun’s heat
was the result of its contraction through cooling. The rate was then
estimated at 380 feet yearly, or a second of arc in 6000 years. This
theory was at once generally accepted. It assumes the Sun to be still
contracting, and therefore, on going backwards in imagination, we reach
a period when the Sun must have been much larger than now, and, in fact,
extended beyond the orbit of Neptune.

Several objections to Laplace’s nebular theory were urged by various
investigators. Among these was the retrograde motions of the satellites
of Uranus and Neptune, and the extremely rapid revolution of the inner
satellite of Mars. Other objections were urged by Babinet, Kirkwood, and
others, and at length a sweeping reform of the nebular theory was
proposed by Faye in 1884, in his work, ‘Sur l’Origine du Monde.’ Faye
put forward the idea that all the planets interior to the orbit of
Uranus were formed inside the solar nebula, while Uranus and Neptune
came into existence after the development of the Sun was far advanced.
But the objections to Faye’s theory are formidable, and the hypothesis
has not been accepted.

A popular exposition of the nebular theory was given in 1901 in Ball’s
work on ‘The Earth’s Beginning.’ He exhaustively discusses the whole
question, and explains the retrograde motion of the satellites of Uranus
and Neptune as due to the fact that the planes of the orbits of the
satellites will eventually be brought to coincide with the ecliptic.
These motions, says Ball, do not disprove the nebular theory. “They
rather illustrate the fact that the great evolution which has wrought
the Solar System into its present form has not finished its work: it is
still in progress.”

The theory that the Sun’s heat was maintained by meteors, was extended
by Proctor in 1870 to explain the growth of the planets through meteoric
aggregation as well as nebular condensation. Certainly the theory, as
developed by Proctor, accounted fairly well for the various features of
the Solar System; but the highest development of the meteoritic theory
is due to Lockyer, who published his views in 1890, in his work, ‘The
Meteoritic Hypothesis.’ Lockyer claims that his views are merely
extensions of Schiaparelli’s ideas regarding the concentration of
celestial matter. He considered the chief nebular line to be identical
with the remnant of the magnesium fluting, which is conspicuous in
cometic and meteoric spectra; but Huggins and Keeler, with more powerful
instruments, disproved the supposed coincidence. Lockyer considers that
“all self-luminous bodies in the celestial space are composed either of
swarms of meteorites or of masses of meteoric vapour produced by heat.
The heat is brought about by the condensation of meteor swarms, due to
gravity, the vapour being finally condensed into a solid globe.”

Lockyer divided the stars into seven groups, according to temperature,
the order of evolution being from red stars through a division of
second-type stars to Sirian stars, regarded as the hottest stars;
through a second division of solar stars to fourth-type stars. In fact,
the theory aspires to give a complete explanation of all celestial
phenomena, from meteors to nebulæ. Newcomb, however, considers that the
objections to the theory are insuperable, and his opinion is shared by
the majority of astronomers, many of whom, however, consider that there
are elements of truth in the theory; but Lockyer undoubtedly carried his
ideas to an extravagant extent.

Lockyer’s evolutionary order of the stars is not supported by Vogel.
Zöllner suggested in 1865 that yellow and red stars are simply white
stars in a further stage of cooling; but Angström showed that
atmospheric composition is a safer criterion of age than colour. Vogel’s
classification, first published in 1874, and further developed in 1895,
is from the standpoint of evolution. He considers Orion stars and Sirian
stars to be the youngest orbs. Solar stars are considered by Vogel to
have wasted much of their store of radiation, and red stars are viewed
as “effete suns, hastening rapidly down the road to final extinction.”
He considers stars of Secchi’s fourth type to be also dying suns, both
types representing alternative roads for stars of the Solar type in
their decline into dark stars. This view is supported by Dunér, and is
distinctly confirmed by Hale’s observations with the Yerkes telescope.
Vogel’s views, in fact, are generally accepted among astronomers. The
nebular theory, modified by subsequent research, seems destined to hold
its own against all attacks.

Distinctly supplementary to the nebular theory are the remarkable
researches, commenced in 1879, by Sir _George Howard Darwin_ (born
1845), son of Charles Darwin the great biologist. George Howard Darwin
was born in 1845, at Downe in Kent, was educated at Cambridge, and
studied for the law; but in 1873 he returned to Cambridge, where he
became Plumian Professor of Astronomy in 1883. In 1879 he communicated
to the Royal Society the first of his papers on tidal friction, which
were summed up in his book on ‘The Tides,’ published in 1898. He finds
that the tides act upon the Earth as a brake does upon a machine,—they
tend to retard its rotation. Consequently, the day is growing longer,
the Moon’s orbit is becoming enlarged, and its period of revolution is
being lengthened.

At present the day is about twenty-four hours long, and the month about
twenty-seven days. The day, however, will be lengthened at a more rapid
rate than the month, and in the remote future the day and month will
both last fifty-five of our present days. The Moon will revolve round
the Earth in the same period that the Earth rotates on its axis, and the
two bodies will perform their circuit round the Sun as if united by a
bar.

Not only can we foresee the future of the Earth-Moon System, but we can
also read the past. According to Darwin’s theory, the Earth, in the
remote past, was probably rotating on its axis in a very short period,
between three and five hours. The Moon must then have been much nearer
us than it is now, and was probably revolving round its primary in the
same period that the Earth took to rotate on its axis. The two globes,
then gaseous, must have been revolving almost in actual contact. Had the
month been even a second shorter than the day, the Moon must inevitably
have fallen back on the Earth. As it was, the condition of affairs could
not endure. The condition of the Moon resembled that of an egg balanced
on its point. The Moon must either recede from the Earth or fall back
upon it. The solar tide here interfered, and caused the Moon to recede
from its primary until it reached its present distance of 239,000 miles.

The fact that the Earth and Moon were almost in contact suggests that
they were probably in contact. In other words, the Moon originally
formed part of the Earth, which, in consequence of its short-rotation
period, and probably also owing to the interference of the solar tide,
split into two portions, and the smaller of these now forms the Moon. It
is likely that the matter now forming the Moon was detached from the
Earth in separate particles. Just as the tides raised by the Moon tend
to retard the motion of the Earth, so the Earth tides raised in the Moon
have already done their work. The Moon now rotates on its axis in the
same time as it revolves round the Earth. Part of the evolution of the
Earth-Moon system is completed. Schiaparelli’s discovery that the
rotation periods of both Venus and Mercury coincide with their times of
revolution is distinctly confirmatory of Darwin’s theory.

In his chapter on the “Evolution of Celestial Systems” in his book on
‘The Tides,’ Darwin discusses the distribution of the satellites of the
Solar System. He says of the evolution of a planet: “We have seen that
rings should be shed from the central nucleus when the contraction of
the nebula has induced a certain degree of augmentation of rotation.
Now, if the rotation were retarded by some external cause, the genesis
of a ring might be retarded or entirely prevented.” He then remarks that
probably the formation of the Moon was retarded, and in the case of
Mercury and Venus, solar tidal friction prevented satellite formation.
This explains why Mercury and Venus have no satellites, the Earth only
one, Mars two, while the exterior planets have each several satellites.

The theory of tidal friction was extended in 1892 to the explanation of
the double stars by the American astronomer, See. See showed by
mathematical calculation the effects of tidal friction in shaping the
eccentric orbits of the binary stars, the course of evolution being
traced from double stars, revolving almost in contact, which the
spectroscope reveals, to the telescopic doubles. See’s researches have
done much to supplement those of Darwin, who considers that there are
two types of cosmical evolution,—the Laplacian, and the “second” or
lunar type.

Lowell, in his work on ‘The Solar System’ (1903), adds six congruities
to those remarked by Laplace and his successors. These are, “All the
satellites turn the same face to their primaries (so far as we can
judge); Mercury, and probably Venus, do the same to the Sun; one law
governs position and size in the Solar System and in all the satellite
systems; orbital inclinations in the satellite systems increase with
distance from the primary; the outer planets show a greater tilt of axis
to orbit-plane with increased distance from the Sun (so far as
detectable); the inner planets show a similar relation.”

The fate of the average solar star is sketched out by Vogel’s
classification, and by any evolutionary hypothesis which we may adopt.
In the words of Lowell: “Though we cannot as yet review with the mind’s
eye our past, we can, to an extent, foresee our future. We can with
scientific confidence look forward to a time when each of the bodies
composing our Solar System shall turn an unchanging face in perpetuity
to the Sun. Each will then have reached the end of its evolution set in
the unchanging stare of death. Then the Sun itself will go out, becoming
a cold and lifeless mass; and the Solar System will circle unseen,
ghostlike, in space, awaiting only the resurrection of another cosmic
catastrophe.”

As to what this cosmic catastrophe will be, science gives no definite
idea; nor can astronomers say with certainty whether the Universe will
come to an end by the extinction of its luminaries, or whether the suns
and planets will be brought back to luminosity again; but the human mind
shrinks from the idea of a dead Universe. At this point science has said
its last word, and must give place to religion. In our day we may repeat
with deeper meaning the words of the Scottish astronomer, Thomas Dick:
“Here imagination must drop its wing, since it can penetrate no further
into the dominions of Him who sits on the Throne of Immensity.
Overwhelmed with a view of the magnificence of the Universe, and of the
perfections of its Almighty Author, we can only fall prostrate in deep
humility and exclaim, ‘Great and marvellous are Thy works, Lord God
Almighty.’”




                                 INDEX.


                                   A
  Absolute parallax, 158.
  Adams, J. C., 78, 116, 117, 118, 119, 120, 140.
  Adams, W., 205.
  Aerolites, 147, 148, 149.
  Airy, Sir G. B., 27, 104, 117, 120.
  Aitken, R. G., 202.
  Alcyone (η Tauri), 217.
  Aldebaran (α Tauri), 151, 166, 170, 172.
  Algol (β Persei), 178, 182, 183, 184, 193, 204.
  Al-Sufi, 180, 183.
  Altair (α Aquilæ), 170.
  Anderson, T. D., 191, 192.
  Andromeda nebula, 180, 208.
  Andromedæ (γ), 201.
  Andromedæ (Nova), 180.
  Andromedid meteors, 142, 149.
  Angström, A. J., 50, 51.
  Antares (α Scorpii), 171.
  Aquila, 195.
  Aquilæ (η), 185, 186.
  Arago, F. J. D., 6, 11, 31, 37, 40, 118, 120, 129.
  Arcturus (α Bootis), 165, 170.
  Arequipa Observatory, 75.
  Argelander, F. W. A., 27, 159, 167, 178, 179, 180, 218.
  Argo Navis, 221.
  Argus (η), 187, 188.
  Armagh Observatory, 206.
  Asteroids, 19, 62, 97-102.
  Astronomer-Royal of Scotland, 134, 155, 191;
      of England, 59, 17;
      of Ireland, 151, 156.
  Astronomy of the invisible, 203.
  Aurigæ (Nova), 191, 192, 195.
  Auwers, A., 167, 188, 203.


                                   B
  Babinet, 230.
  Baily, F., 159.
  Bakhuyzen, H. G., 91.
  Ball, Sir R. S., 23, 34, 108, 141, 149, 156, 158, 230.
  Barnard, E. E., 19, 95, 107, 108, 110, 111, 113, 136, 191, 211.
  Beer, W., 68, 69, 90.
  Bellatrix (γ Orionis), 209.
  Bélopolsky, A., 87, 110, 166, 185, 186, 204, 205.
  Berlin Observatory, 119, 120.
  Bessel, F. W., 24, 82, 116, 151, 152, 153, 154, 159, 167, 202,
          203.
  Betelgeux (α Orionis), 165, 171, 172, 182, 187.
  Biela, W., 128.
  Biela’s comet, 128, 129, 142, 143, 146, 149.
  Birmingham, J., 189.
  Bode, J. E., 97, 98, 152.
  Bode’s Law, 97.
  Boeddicker, O., 77.
  Bond, G. P., 103, 109, 130, 136, 207.
  Bond, W. C., 109, 112, 120.
  ‘Bonn Durchmusterung,’ 159, 160, 218.
  Bonn Observatory, 88, 97, 160.
  Böotis (ε), 30.
  Borisiak, 192.
  Boss, L., 168.
  Bouvard, A., 115, 116.
  Bradley, J., 159, 167.
  Brédikhine, T. A., 105, 131, 132, 133, 134, 135.
  Brewster, Sir D., 50, 101, 178.
  Brinkley, J., 151.
  Brünnow, F., 156.
  Bruno, G., 35.
  Buffon, 103.
  Bunsen, R. W., 51.
  Burchell, 188.
  Burnham, S. W., 201, 202, 212.


                                   C
  Callandreau, O., 136.
  Callandrelli, 151.
  Cambridge Observatory, 116.
  Campbell, T. (Poet), 2.
  Campbell, W. W., 24, 107, 110, 166, 168, 187, 191, 193, 204, 205.
  Canals of Mars, 91, 92, 93, 94, 95.
  Cancri (ζ), 206.
  Cancri (S), 180.
  Canis Major, 188.
  ‘Cape Durchmusterung,’ 161, 162.
  Cape Observatory, 155, 157.
  Capella (α Aurigæ), 170, 176, 193, 205.
  Carnera, L., 100.
  Carpenter, J., 73.
  Carrington, R. C., 45, 46, 59.
  Cassini, D., 21.
  Cassiopeia, 221.
  Castor (α Geminorum), 30, 200, 205.
  Celoria, G., 202, 218, 221, 223, 224.
  Centauri (α), 155, 188, 225.
  Centaurus, 221.
  Cephei (δ), 178, 182, 185, 186.
  Cepheus, 221.
  Ceres, 19, 98, 101.
  Cerulli, V., 86, 91, 94.
  Chacornac, 161.
  Challis, J., 116, 119, 120.
  Chambers, G. F., 31.
  Chandler, S. C., 88, 89, 181, 184.
  Chladni, E., 138.
  Chromosphere, solar, 55, 56.
  Clark, A., 202.
  Clerke, Miss A. M., 3, 5, 8, 12, 13, 15, 25, 26, 34, 42, 58, 75,
          86, 92, 105, 109, 124, 125, 131, 132, 133, 140, 142, 169,
          186, 187, 189.
  Clerk-Maxwell, J., 109, 110.
  Coggia’s comet, 131, 132, 133.
  Comet families, 135.
  Comets, 24, 123-137, 141, 142, 143, 144, 146, 149, 152.
  Common, A. A., 107, 209.
  Copeland, R., 134, 135, 190, 208.
  Cornu, A., 189.
  Corona Borealis, 188.
  Corona, solar, 55, 57, 64.
  Coronæ (Nova), 188, 189.
  Crossley, E., 211.
  Crux, 221.
  Cygni (61), 152, 158.
  Cygni (Y), 184, 185.
  Cygni (Nova), 189, 190.
  Cygnus, 152, 189, 221.


                                   D
  Damoiseau, 78.
  D’Arrest, H. L., 96, 119, 142, 212.
  Dartmouth Observatory, 56.
  Darwin, Sir G. H., 233, 234, 235, 236.
  Dawes, W. R., 90, 117.
  De la Rue, W., 52, 75.
  Delaunay, C. E., 78, 79.
  Dembowski, E., 201.
  Deneb (α Cygni), 165.
  Denning, W. F., 84, 85, 91, 95, 105, 111, 112, 144, 145, 146.
  Deslandres, H., 110.
  Dick, T., 85, 238.
  Disc-theory, 32, 36, 38, 39, 214, 218.
  Di Vico, F., 85, 86, 170.
  Doberck, W., 201.
  Donati, G. B., 130, 131, 169.
  Donati’s comet, 130, 133, 136.
  Doppler, C., 57, 58.
  Doppler’s Principle, 58, 59, 87, 110, 165, 168, 203.
  Douglass, A. E., 92, 107.
  Draconis (λ), 182.
  Draper, H., 136, 172, 175.
  Dreyer, J. L. E., 206.
  Dunecht Observatory, 157.
  Dunér, N. C., 58, 59, 174, 175, 181, 184, 185, 201, 202, 233.
  Dunkin, E., 27, 167.
  Dunsink Observatory, 156.


                                   E
  Earth, 76, 97, 103, 104, 147, 148, 149, 153, 154, 156, 236.
  Earth-Moon system, 234, 235.
  Easton, C., 221.
  Eclipses, lunar, 77.
  Eclipses, solar, 56, 57, 80, 81.
  Edinburgh (Royal) Observatory, 195.
  Electrical repulsion theory, 126.
  Elger, T. G., 74.
  Elkin, W. L., 157.
  Encke, J. F., 30, 61, 119, 127, 128, 216.
  Encke’s comet, 127, 128, 137.
  Erman, 140.
  Eros, 62, 101.
  Ertborn, 85.
  Euler, L., 88, 89.
  Evolution, planetary, 228, 229, 230, 231.
  Evolution, stellar, 33, 34, 231, 232.


                                   F
  Faye, H., 60, 129, 230.
  Faye’s comet, 129, 137.
  Ferguson, J., 9, 178.
  Flammarion, C., 87, 91, 95, 121, 147, 164, 187, 195, 201, 202,
          226.
  Flamsteed, J., 5.
  Fleming, Mrs, 192, 195.
  Forbes, G., 122.
  Fraunhofer, J. 47, 48, 49, 50, 3, 151, 153, 169.
  Fraunhofer lines, 48, 49, 50, 51, 169, 172.
  Frost, E. B., 205.


                                   G
  Galactic poles, 35, 224.
  Galaxies, external, 32, 218, 225, 226.
  Galaxy, or Milky Way, 32, 36-42, 186, 211, 215, 216, 217, 219,
          220, 221, 224, 225.
  Galileo, 44, 107.
  Galle, J. G., 62, 108, 109, 119, 142.
  Galloway, T., 167.
  Gambart, 128.
  Gauss, C. F., 27, 98, 167.
  Gautier, A., 45.
  Gemini, 11, 194.
  Geminorum (Nova), 194.
  Geminorum (ζ), 180, 182, 185.
  George III., 11, 23.
  Gill, Sir D., 62, 136, 155, 157, 160, 161, 221.
  Glasgow Observatory, 216.
  Goodricke, J., 178, 183.
  Gore, J. E., 24, 38, 179, 181, 182, 183, 192, 202, 215, 220, 225,
          226.
  Gould, B. A., 135, 160, 163, 180, 220, 221.
  Grant, R., 216.
  Gravitation, law of, 29.
  Greenwich Observatory, 59, 117.
  Grimmler, 192.
  Groombridge (1830), 156, 162, 223.
  Groombridge (1618), 156.
  Gruithuisen, 87.


                                   H
  Hale, G. E., 55, 57, 233.
  Hall, A., 96, 111, 112, 156, 190.
  Hall, Maxwell, 121.
  Halley, E., 138.
  Halley’s comet, 123, 130, 152.
  Halm, J., 195, 196.
  Hansen, P. A., 61, 78, 79.
  Hansky, A., 57.
  Harding, K. L., 99, 153, 220.
  Hartwig, E., 190.
  Harvard Observatory, 174, 175, 191.
  Hasselberg, B., 148, 190.
  Heis, E., 179, 220.
  Heliometer, 153, 157.
  Helium stars, 174.
  Helmholtz, H., 61, 229.
  Hencke, K. L., 99.
  Henderson, T., 154, 155.
  Henry, Paul and Prosper, 100, 114, 210.
  Hercules, 167.
  Herculis (α), 182.
  Herculis (λ), 26.
  Herschel, William, 1-42, 43, 60, 63, 65, 69, 74, 77, 85, 90, 99,
          103, 109, 111, 112, 115, 123, 150, 162, 167, 176, 196,
          197, 207, 214, 216, 218, 224, 227.
  Herschel, A., 144.
  Herschel, Caroline, 6, 8, 9, 12, 13, 14, 30, 35, 127, 198.
  Herschel, Sir J., 4, 17, 27, 30, 37, 50, 112, 113, 120, 130, 144,
          167, 187, 188, 197, 198, 199, 200, 214, 215.
  Hind, J. R., 99, 129, 135, 180, 188.
  Hoek, 135.
  Holden, E. S., 191.
  Hough, G., 105.
  Houzeau, J. C., 220.
  Huggins, Lady, 172.
  Huggins, Sir W., 54, 57, 74, 95, 106, 114, 131, 136, 165, 170,
          171, 172, 173, 189, 190, 191, 193, 195, 207, 208, 212,
          231.
  Humboldt, A., 44, 139.
  Hussey, W. J., 116, 201.


                                   I
  Innes, R., 188.
  Intra-Mercurial planet, 80, 81.
  Italian spectroscopists, 54, 55.


                                   J
  Janssen, P. J. C., 52, 53, 54, 57, 59, 112, 209.
  Juno, 19, 99, 101.
  Jupiter, 20, 75, 97, 101-108, 112, 114, 121, 122, 135, 144, 146.
  Juvisy Observatory, 164.


                                   K
  Kaestner, 65, 124.
  Kaiser, F., 90.
  Kant, I., 34, 35, 101, 103.
  Kapteyn, J. C., 27, 158, 161, 162, 168, 221, 222.
  Keeler, J. E., 95, 114, 185, 208, 211, 212, 213, 231.
  Kelvin, Lord, 229.
  Kempf, P., 181.
  Kepler, J., 5, 35, 137.
  Kirchoff, G. R., 61, 169, 172.
  Kirkwood, D., 140, 230.
  Klein, H. J., 73.
  Klinkerfues, E., 142, 202.
  Konkoly, N., 175.
  Küstner, F., 88.


                                   L
  Lalande, 23, 152.
  Lambert, J. H., 34.
  Lamont, J., 44.
  Langley, S. P., 77.
  Laplace, P. S., 20, 33, 34, 77, 109, 148, 152, 195, 227, 228, 229.
  Lassell, W., 112, 115, 117, 120.
  Latitude, variation of, 88, 89.
  Leipzig Observatory, 173.
  Leonid meteors, 139, 140, 142.
  Leonis (β), 183.
  Lescarbault, 80.
  Le Verrier, U. J. J., 61, 80, 81, 118, 119, 120, 142, 164.
  Leyden Observatory, 91.
  Libræ (δ), 180.
  Lick Observatory, 93, 107, 166, 168, 191, 213.
  Light, extinction of, 40, 215, 216, 224, 225.
  Lindsay, Lord, 157.
  Linné, 71, 72.
  Lockyer, Sir J. N., 52, 53, 54, 55, 58, 149, 174, 191, 193, 195,
          208, 209, 231, 232.
  Lockyer, W. J. S., 210.
  Loewy, M., 75.
  Lohrmann, W. G., 68, 71.
  Lohse, W. O., 88, 105.
  Loomis, 188.
  Lowell, P., 83, 84, 86, 87, 91, 92, 93, 94, 122, 236, 237.
  Lowell Observatory, 92, 94, 106, 114.
  Lund Observatory, 59.
  Lupus, 221.
  Luther, R., 100.
  Lyra, 221.
  Lyra (β), 178, 182, 185.
  Lyrid meteors, 122.


                                   M
  Maclaurin, C., 9.
  Maclear, Sir T., 155.
  Mädler, J. H., 27, 68, 69, 71, 96, 104, 202, 203, 216, 217, 218.
  Magellanic clouds, 219.
  Magnetism, 44, 60.
  Mars, 18, 19, 90-97, 101, 144, 236.
  Maunder, E. W., 59, 60, 94, 95, 134, 145, 166, 224.
  Mascari, A., 86.
  Mayer, C., 164.
  Mayer, J. R., 229.
  Mazapil meteorite, 149.
  Méchain, 127.
  Mee, A., 68.
  Melloni, 76.
  Mercury, 18, 80, 81-84, 97, 236.
  Messier, C., 30.
  Meteorites, 147, 148, 149, 229, 231.
  ‘Meteoritic Hypothesis,’ 231, 232.
  Meteors, 138-149.
  Meudon Observatory, 59.
  Milan Observatory, 82, 202, 224.
  Milky Way. See Galaxy.
  Miller, W. A., 50, 172.
  Mira Ceti, 11, 182, 186, 187.
  Mitchel, O. M., 31.
  Mizar (ζ Ursæ Majoris), 204.
  Möller, A., 129.
  Moon, the, 10, 24, 65-79, 90, 95, 148, 228.
  Moscow Observatory, 132.
  Mouchez, A., 161.
  Müller, G., 175, 181.
  Munich Observatory, 44, 224.


                                   N
  Napoleon, 67, 127.
  Nasmyth, J., 73, 103.
  Nebulæ, 30, 31, 207-213, 228.
  Nebular Hypothesis, 33, 195, 227, 228, 229, 230, 233.
  Neison (Nevill), E., 73.
  Neisten, 86, 105.
  Neptune, 120, 121, 135, 229, 230.
  Newall, H. F., 205.
  Newcomb, S., 27, 64, 78, 89, 94, 162, 168, 220, 222, 223, 224,
          232.
  Newton, H. A., 140, 141.
  Newton, Sir I., 2, 17, 29, 77.
  Nichol, J. P., 31, 207.
  Nordvig, L., 192.


                                   O
  Olbers, H. W. M., 19, 20, 69, 98, 99, 123, 124, 125, 126, 127,
          129, 130, 139, 148, 152, 153.
  Olbers’ comet, 125.
  Olmsted, D., 138, 207.
  Ophiuchi (α), 183.
  Orion, 221.
  Orion nebula, 10, 33, 207, 208, 209, 213.
  Orion stars, 174, 193, 209, 232.
  Orionis (κ), 209.
  Orionis (η), 209.
  Orionis (θ), 209.
  Orionis (U), 181, 182, 186, 187.


                                   P
  Palisa, J., 100.
  Pallas, 19, 99, 101.
  Parallax, solar, 61, 62, 63, 101.
  Parallax, stellar, 150-158, 190.
  Paris Congresses, 161.
  Paris Observatory, 78, 118, 171.
  Perrine, C. D., 81, 108, 194.
  Perrine’s comet, 136.
  Perrotin, H., 86, 91, 100, 114.
  Peck, W., 162.
  Persei (Nova), 192, 193, 194, 195.
  Perseid meteors, 122, 141.
  Perseus, 192, 221.
  Peters, C. H. F., 100.
  Peters, C. A. F., 142, 153, 155, 202.
  Photography, astronomical, 54, 56, 57, 59, 75, 81, 94, 108, 113,
          136, 158, 160, 161, 172, 175, 192, 193, 194, 203, 208,
          209, 210, 211, 212.
  Photometry, 176, 177.
  Piazzi, G., 19, 20, 98, 150.
  Pickering, E. C., 174, 175, 176, 177, 181, 182, 183, 185, 193,
          194, 203, 204, 220, 221.
  Pickering, W. H., 75, 76, 81, 91, 92, 93, 107, 113, 209.
  Plana, G., 78, 79.
  Pleiades, 124, 210, 217.
  Pogson, N. R., 142, 180.
  Pole Star, 205.
  Pollux, 165, 170.
  Pons, J. L., 127.
  Pontécoulant, 79.
  Potsdam Observatory, 46, 173, 176.
  Pritchard, C., 158, 177.
  Proctor, R. A., 4, 20, 38, 41, 90, 91, 104, 148, 163, 164, 218,
          219, 220, 231.
  Procyon (α Canis Minoris), 151, 203.
  Prominences, solar, 52, 53, 55, 64.
  Puiseux, P., 75.
  Pulkowa Observatory, 200.


                                   Q
  Quetelet, A., 139.


                                   R
  Radiant points, meteoric, 139, 144, 145, 146.
  Ranyard, A. C., 106, 146.
  Red spot on Jupiter, 105, 106.
  Regulus (α Leonis), 164, 165.
  Relative parallax, 157.
  Réseau, Photospherique, 59.
  Resisting medium, 128.
  Respighi, L., 55.
  Reversing layer, 56, 57.
  Ricco, A., 87, 105.
  Rigel (β Orionis), 165, 209.
  Ritchey, G., 194.
  Roberts, A. W., 181.
  Roberts, I., 209, 210.
  Roche, E., 109.
  Roman College Observatory, 85.
  Rosse, third Earl of, 141, 156, 207, 208, 218.
  Rosse, fourth Earl of, 77.
  Rotation of the Sun, 58, 59;
      of the planets, 82, 83, 84, 85, 86, 87, 104, 111, 112.
  Rowland, H. A., 52.
  Rutherfurd, L. M., 75, 169.


                                   S
  Sabine, Sir E., 44.
  Safford, T. H., 202.
  Santini, G., 159.
  Savary, F., 30, 199.
  Satellites, 96, 107, 108, 112, 113, 115, 120, 121, 236.
  Saturn, 20, 21, 22, 97, 103, 108-113, 121, 135.
  Schaeberle, J. M., 93, 107, 191, 203.
  Scheiner, C., 44.
  Scheiner, J., 166, 174, 176.
  Schiaparelli, G. V., 82, 83, 84, 85, 86, 87, 91, 92, 114, 141,
          143, 149, 201, 220, 224, 231, 235.
  Schjellerup, H., 180.
  Schmidt, J. F. J., 69, 70, 71, 72, 73, 104, 179, 180, 189.
  Schönfeld, E., 160, 179, 180, 188, 189.
  Schröter, J. H., 16, 65, 66, 67, 68, 69, 70, 74, 81, 82, 84, 85,
          86, 87, 97, 99, 153.
  Schwabe, S. H., 18, 43, 44, 46, 55.
  Schwassman, A., 100.
  Secchi, A., 52, 54, 55, 60, 72, 90, 114, 141, 170, 171, 173.
  Secchi’s types of stellar spectra, 170, 171, 173, 174, 175, 189,
          232.
  See, T. J. J., 201, 202, 236.
  Seeliger, H., 110, 195, 196, 202, 206, 224, 225.
  Serviss, G. P., 158.
  Sirius (α Canis Majoris), 151, 170, 173, 188, 202, 225.
  Slipher, V. M., 106, 114, 204.
  Sime, J., 27.
  Sola, J. C., 112.
  Solar cluster, 221, 222.
  Solar system, motion of, 26, 27, 167, 168.
  South, Sir J., 198.
  Spectroscopic binaries, 203, 204, 205.
  Spencer, H., 218.
  Spica (α Virginis), 204.
  Spörer, F. W. G., 45, 46, 54, 59.
  Star-catalogues, 159, 160, 161, 162.
  Star-clusters, 30, 31, 32, 206, 210.
  Star-drift, 164.
  Star-gauging, 36, 40, 41, 224.
  Stars, distance of, 150-158.
  Stars, distribution of, 35, 39, 40, 198-214.
  Stars, double, 28, 29, 30, 197-206.
  Stars, gaseous, 171, 174.
  Stars, proper motion of, 162, 163, 164, 165.
  Stars, radial motion of, 165, 166.
  Stars, temporary, 156, 182, 188-196.
  Stars, triple and multiple, 206.
  Stars, variable, 177-188.
  Stellar spectra, 169-176, 187, 189, 190, 191, 193, 194.
  Stellar universe, 35-42, 214, 215-226.
  Stereo-comparator, 100, 101.
  Stokes, Sir G., 50.
  Stone, E. J., 157, 160.
  Stroobant, P., 88.
  Struve, F. G. W., 3, 37, 38, 40, 42, 128, 151, 153, 200, 214, 215,
          216, 218.
  Struve, H., 201.
  Struve, L., 27, 163, 167.
  Struve, O. W., 27, 110, 115, 120, 153, 156, 163, 200, 201.
  Stumpe, O., 167.
  Sun, 15, 16, 17, 40, 43-64, 65, 80, 81, 105, 125, 128, 170, 222,
          228, 229, 230, 237.
  Swift, L., 81.
  Swift’s comet, 136.


                                   T
  Tacchini, P., 55, 86, 87.
  Taurus, 217, 221.
  Tempel, E., 210.
  Tennyson, 96.
  Tidal friction, 79, 87, 233, 234, 235, 236.
  Tisserand, F. F., 146.
  Todd, D. P., 122.
  Trans-Neptunian planet, 121, 122.
  Trouvelot, E., 86, 87.
  Tschermak, 148.
  Tulse Hill Observatory, 171.
  Turner, H. H., 194, 224.
  Twining, A. C., 139.


                                   U
  Upsala Observatory, 59.
  Uranometria Argentina, 160.
  Uranus, 11, 20, 22, 23, 97, 113, 114, 115, 118, 121, 135, 141,
          230.
  Ursa Major, 162, 164.
  Ursæ Majoris (δ), 182.
  Ursæ Majoris (ξ), 199.


                                   V
  Venus, 18, 84-88, 97, 235, 236.
  Venus, transits of, 61, 62, 87.
  Vega (α Lyræ), 151, 165, 170, 172, 173.
  Very, F. W., 77.
  Vesta, 19, 99, 101, 102.
  Vogel, H. C., 84, 88, 95, 102, 106, 114, 131, 148, 166, 173, 174,
          175, 183, 184, 185, 190, 191, 193, 195, 204, 209, 232,
          233, 237.
  Vulcan, 81.


                                   W
  Washington Observatory, 96, 223.
  Watson, J. C., 81, 100.
  Webb, T. W., 72, 73, 104.
  Weinek, L., 75.
  Weiss, E., 142.
  Well’s comet, 134.
  Whewell, W., 218.
  Williams, A. S., 110, 193.
  Wilson, A., 16.
  Winlock, J., 177.
  Winnecke, F. A. T., 131.
  Witt, K. G., 101.
  Wolf, Max, 100, 181, 191, 194, 211.
  Wolf, R., 44, 45, 188.
  Wolf and Rayet, 171.
  Wolf-Rayet stars, 171, 174.
  Wollaston, W. H., 48.
  Wright, T., 34, 110.


                                   Y
  Yale Observatory, 157.
  Yerkes Observatory, 55, 111, 202.
  Young, C. A., 54, 56, 57, 58, 60, 87, 114, 190.


                                   Z
  Zach, F. X., 97, 98, 152.
  Zantedeschi, 77.
  Zenger, 85, 88.
  Zöllner, J. C. F., 54, 58, 60, 84, 103, 132, 232.


CORRIGENDA.

  P.  30, l.  5, _for_ “objects” _read_ “orbits.”
  P.  36, l. 13, _for_ “unable” _read_ “able.”
  P.  61, l. 17, _for_ “8″.371” _read_ “8″.571.”
  P.  63, l. 21, _for_ “bases” _read_ “gases.”
  P. 100, l. 16, _for_ “Schwussmann” _read_ “Schwassmann.”
  P. 167, l. 28, _for_ “Strumpe” _read_ “Stumpe.”
  P. 184, l. 11, _for_ “star-variables” _read_ “variable stars.”
  P. 199, l. 23, _for_ “2102” _read_ “1202.”

                                THE END.

                 PRINTED BY WILLIAM BLACKWOOD AND SONS.




                          Transcriber’s Notes


—Silently corrected a few typos, and incorporated the corrigenda into
  the text.

—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by
  _underscores_.