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SHILLING SCIENTIFIC SERIES




[Illustration: DR. AITKEN'S DUST-COUNTER.

R is the test-receiver; P the air-pump; M the measuring apparatus; L the
illuminating arrangements; G the Gasometer; A the pipe through which the
tested air is drawn.]




  METEOROLOGY;
  OR,
  WEATHER EXPLAINED.


  BY
  J. G. M'PHERSON, Ph.D., F.R.S.E.,

  GRADUATE WITH FIRST-CLASS HONOURS, AND FOR NINE YEARS
  EXTENSION LECTURER ON METEOROLOGY AND MATHEMATICAL
  EXAMINER IN THE UNIVERSITY OF ST. ANDREWS;
  AUTHOR OF "TALES OF SCIENCE," ETC.


  LONDON: T. C. & E. C. JACK,
  34 HENRIETTA STREET, W.C.
  AND EDINBURGH.
  1905.




THE SHILLING SCIENTIFIC SERIES


_The following Vols. are now ready or in the Press_:--

BALLOONS, AIRSHIPS, AND FLYING MACHINES. By GERTRUDE BACON.

MOTORS AND MOTORING. By Professor HARRY SPOONER.

RADIUM. By Dr. HAMPSON.

TELEGRAPHY WITH AND WITHOUT WIRES. By W. J. WHITE.

ELECTRIC LIGHTING. By S. F. WALKER, R.N., M.I.E.E.

LOCAL GOVERNMENT. By PERCY ASHLEY, M.A.

_Others in Preparation_

  Printed by BALLANTYNE, HANSON & CO.
  At the Ballantyne Press




CONTENTS


  CHAP.                                             PAGE

       I. INTRODUCTION                                 9

      II. THE FORMATION OF DEW                        13

     III. TRUE AND FALSE DEW                          17

      IV. HOAR-FROST                                  20

       V. FOG                                         23

      VI. THE NUMBERING OF THE DUST                   26

     VII. DUST AND ATMOSPHERIC PHENOMENA              29

    VIII. A FOG-COUNTER                               31

      IX. FORMATION OF CLOUDS                         34

       X. DECAY OF CLOUDS                             37

      XI. IT ALWAYS RAINS                             40

     XII. HAZE                                        43

    XIII. HAZING EFFECTS OF ATMOSPHERIC DUST          47

     XIV. THUNDER CLEARS THE AIR                      49

      XV. DISEASE GERMS IN THE AIR                    53

     XVI. A CHANGE OF AIR                             55

    XVII. THE OLD MOON IN THE NEW MOON'S ARMS         58

   XVIII. AN AUTUMN AFTERGLOW                         62

     XIX. A WINTER FOREGLOW                           65

      XX. THE RAINBOW                                 68

     XXI. THE AURORA BOREALIS                         71

    XXII. THE BLUE SKY                                74

   XXIII. A SANITARY DETECTIVE                        78

    XXIV. FOG AND SMOKE                               80

     XXV. ELECTRICAL DEPOSITION OF SMOKE              83

    XXVI. RADIATION FROM SNOW                         86

   XXVII. MOUNTAIN GIANTS                             88

  XXVIII. THE WIND                                    92

    XXIX. CYCLONES AND ANTI-CYCLONES                  95

     XXX. RAIN PHENOMENA                              98

    XXXI. THE METEOROLOGY OF BEN NEVIS               102

   XXXII. THE WEATHER AND INFLUENZA                  107

  XXXIII. CLIMATE                                    110

   XXXIV. THE "CHALLENGER" WEATHER REPORTS           114

    XXXV. WEATHER-FORECASTING                        116

          INDEX                                      124




PREFATORY NOTE


I am very much indebted to Dr. John Aitken, F.R.S., for his great kindness
in carefully revising the proof sheets, and giving me most valuable
suggestions. This is a sufficient guarantee that accuracy has not been
sacrificed to popular explanation.

J. G. M'P.

  RUTHVEN MANSE,
  _June 10, 1905_.




METEOROLOGY




CHAPTER I

INTRODUCTION


Though by familiarity made commonplace, the "weather" is one of the most
important topics of conversation, and has constant bearings upon the work
and prospects of business-men and men of pleasure. The state of the
weather is the password when people meet on the country road: we could not
do without the humble talisman. "A fine day" comes spontaneously to the
lips, whatever be the state of the atmosphere, unless it is peculiarly and
strikingly repulsive; then "A bitter day" would take the place of the
expression. Yet I have heard "_Terrible_ guid wither" as often as
"_Terrible_ bad day" among country people.

Scarcely a friendly letter is penned without a reference to the weather,
as to what has been, is, or may be. It is a new stimulus to a lagging
conversation at any dinner-table. All are so dependent on the weather,
especially those getting up in years or of delicate health.

I remember, when at Strathpeffer, the great health-resort in the North of
Scotland, in 1885, an anxious invalid at "The Pump" asking a
weather-beaten, rheumatic old gamekeeper what sort of a day it was to be,
considering that it had been wet for some time. The keeper crippled to the
barometer outside the doorway, and returned with the matter-of-fact
answer: "She's faurer doon ta tay nur she wass up yestreen." The barometer
had evidently fallen during the night. "And what are we to expect?" sadly
inquired the invalid. "It'll pe aither ferry wat, or mohr rain"--a poor
consolation!

Most men who are bent on business or pleasure, and all dwellers in the
country who have the instruments, make a first call at the barometer in
the lobby, or the aneroid in the breakfast-parlour, to "see what she
says." A good rise of the black needle (that is, to the right) above the
yellow needle is a source of rejoicing, as it will likely be clear, dry,
and hard weather. A slight fall (that is, to the left) causes anxiety as
to coming rain, and a big depression forebodes much rain or a violent
storm of wind. In either case of "fall," the shutters come over the eyes
of the observer. Next, even before breakfast, a move is made to the
self-registering thermometer (set the night before) on a stone, a couple
of feet above the grass. A good reading, above the freezing-point in
winter and much above it in summer, indicates the absence of killing
rimes, that are generally followed by rain. A very low register accounts
for the feeling of cold during the night, though the fires were not out;
and predicts precarious weather. Ordinarily careful observers--as I, who
have been in one place for more than thirty years--can, with the morning
indications of these two instruments, come pretty sure of their
prognostics of the day's weather. Of course, the morning newspaper is
carefully scanned as to the weather-forecasts from the London
Meteorological Office--direction of wind; warm, mild, or cold; rain or
fair, and so on--and in general these indications are wonderfully accurate
for twenty-four hours; though the "three days'" prognostics seem to
stretch a point. We are hardly up to that yet.

The lower animals are very sensitive as to the state of approaching
extremes of weather. "Thae sea beass," referring to sea-gulls over the
inland leas during ploughing, are ordinary indicators of stormy weather.
Wind is sure to follow violent wheelings of crows. "Beware of rain" when
the sheep are restive, rubbing themselves on tree stumps. But all are
familiar with Jenner's prognostics of rain.

Science has come to the aid of ordinary weather-lore during the last
twenty years, by leaps and bounds. Time-honoured notions and revered
fictions, around which the hallowed associations of our early training
fondly and firmly cling, must now yield to the exact handling of modern
science; and with reluctance we have to part with them. Yet there is in
all a fascination to account for certain ordinary phenomena. "The man in
the street," as well as the strong reading man, wishes to know the "why"
and the "how" of weather-forecasting. They are anxious to have
weather-phenomena explained in a plain, interesting, but accurate way.

The freshness of the marvellous results has an irresistible charm for the
open mind, keen for useful information. The discoveries often seem so
simple that one wonders why they were not made before.

Until about twenty years ago, Meteorology was comparatively far back as a
science; and in one important branch of it, no one has done more to put
weather-lore on a scientific basis than Dr. John Aitken, F.R.S., who has
very kindly given me his full permission to popularise what I like of his
numerous and very valuable scientific papers in the _Transactions of the
Royal Society of Edinburgh_. This I have done my best to carry out in the
following pages. "The way of putting it" is my only claim.

Many scientific men are decoyed on in the search for truth with a spell
unknown to others: the anticipation of the results sometimes amounts to a
passion. Many wrong tracks do they take, yet they start afresh, just as
the detective has to take several courses before he hits upon the correct
scent. When they succeed, they experience a pleasure which is
indescribable; to them fame is more than a mere "fancied life in others'
breath."

Dr. Aitken's continued experiments, often of rare ingenuity and
brilliancy, show that no truth is altogether barren; and even that which
looks at first sight the very simplest and most trivial may turn out
fruitful in precious results. Small things must not be overlooked, for
great discoveries are sometimes at a man's very door. Dr. Aitken has shown
us this in many of his discoveries which have revolutionised a branch of
meteorology. Prudence, patience, observing power, and perseverance in
scientific research will do much to bring about unexpected results, and
not more so in any science than in accounting for weather-lore on a
rational basis, which it is in the power of all my readers to further.

"The old order changeth, giving place to new." With kaleidoscopic variety
Nature's face changes to the touch of the anxious and reverent observer.
And some of these curious weather-views will be disclosed in these pages,
so as, in a brief but readable way, to explain the weather, and lay a safe
basis for probable forecastings, which will be of great benefit to the man
of business as well as the man of pleasure.

  "Felix, qui potuit rerum cognoscere causas."
                                          --VIRGIL.




CHAPTER II

THE FORMATION OF DEW


The writer of the Book of Job gravely asked the important question, "Who
hath begotten the drops of dew?" We repeat the question in another form,
"Whence comes the real dew? Does it fall from the heavens above, or does
it rise from the earth beneath?"

Until about the beginning of the seventeenth century, scientific men held
the opinion of ordinary observers that dew fell from the atmosphere. But
there was then a reaction from this theory, for Nardius defined it as an
exhalation from the earth. Of course, it was well known that dew was
formed by the precipitation of the vapour of the air upon a colder body.
You can see that any day for yourself by bringing a glass of very cold
water into a warm room; the outer surface of the glass is dimmed at once
by the moisture from the air. M. Picket was puzzled when he saw that a
thermometer, suspended five feet above the ground, marked a lower
temperature on clear nights than one suspended at the height of
seventy-five feet; because it was always supposed that the cold of evening
descended from above. Again he was puzzled when he observed that a buried
thermometer read higher than one on the surface of the ground. Until
recently the greatest authority on dew was Dr. Wells, who carefully
converged all the rays of scientific light upon the subject. He came to
the conclusion that dew was condensed out of the air.

But the discovery of the true theory was left to Dr. John Aitken, F.R.S.,
a distinguished observer and a practical physicist, of whom Scotland has
reason to be proud. About twenty years ago he made the discovery, and it
is now accepted by all scientific men on the Continent as well as in Great
Britain. What first caused him to doubt Dr. Wells' theory, so universally
accepted, that dew is formed of vapour existing at the time in the air,
and to suppose that dew is mostly formed of vapour rising from the ground,
was the result of some observations made in summer on the temperature of
the soil at a small depth under the surface, and of the air over it, after
sunset and at night. He was struck with the unvarying fact that the
ground, a little below the surface, was warmer than the air over it. By
placing a thermometer among stems below the surface, he found that it
registered 18° Fahr. higher than one on the surface. So long, then, as
the surface of the ground is above the dew-point (_i.e._ the temperature
when dew begins to be formed), vapour must rise from the ground; this
moist air will mingle with the air which it enters, and its moisture will
be condensed and form dew, whenever it comes in contact with a surface
cooled below the dew-point.

You can verify this by simple experiments. Take a thin, shallow, metal
tray, painted black, and place it over the ground after sunset. On dewy
nights the _inside_ of the tray is dewed, and the grass inside is wetter
than that outside. On some nights there is no dew outside the tray, and on
all nights the deposit on the inner is heavier than that on the outside.
If wool is used in the experiments, we are reminded of one of the forms of
the dewing of Gideon's fleece--the fleece was bedewed when all outside was
dry.

You therefore naturally and rightly come to the conclusion that far more
vapour rises out of the ground during the night than condenses as dew on
the grass, and that this vapour from the ground is trapped by the tray.
Much of the rising vapour is generally carried away by the passing wind,
however gentle; hence we have it condensed as dew on the roofs of houses,
and other places, where you would think that it had fallen from above. The
vapour rising under the tray is not diluted by the mixture with the drier
air which is occasioned by the passing wind; therefore, though only cooled
to the same extent as the air outside, it yields a heavier deposit of
dew.

If you place the tray on bare ground, you will find on a dewy night that
the inside of the tray is quite wet. On a dewy night you will observe that
the under part of the gravel of the road is dripping wet when the top is
dry. You will find, too, that around pieces of iron and old implements in
the field, there is a very marked increase of grass, owing to the deposit
of moisture on these articles--moisture which has been condensed by the
cold metal from the vapour-charged air, which has risen from the ground on
dewy nights.

But all doubt upon this important matter is removed by a most successful
experiment with a fine balance, which weighs to a quarter of a grain. If
vapour rises from the ground for any length of time during dewy nights,
the soil which gives off the vapour must lose weight. To test this, cut
from the lawn a piece of turf six inches square and a quarter of an inch
thick. Place this in a shallow pan, and carefully note the weight of both
turf and pan with the sensitive balance. To prevent loss by evaporation,
the weighing should be done in an open shed. Then place the pan and turf
at sunset in the open cut. Five hours afterwards remove and weigh them,
and it will be found that the turf has lost a part of its weight. The
vapour which rose from the ground during the formation of the dew accounts
for the difference of weight. This weighing-test will also succeed on bare
ground.

When dealing with hoar-frost, which is just frozen dew, we shall find
visible evidence of the rising of dew from the ground.

You know the beautiful song, "Annie Laurie," which begins with--

  "Maxwelton's braes are bonnie,
  Where early fa's the dew"--

well, you can no longer say that the dew "falls," for it rises from the
ground. The song, however, will be sung as sweetly as ever; for the spirit
of true poetry defies the cold letter of science.




CHAPTER III

TRUE AND FALSE DEW


Ever since men could observe and think, they have admired the diamond
globules sparkling in the rising sun. These "dew-drops" were considered to
be shed from the bosom of the morn into the blooming flowers and rich
grass-leaves. Ballantine's beautiful song of Providential care tells us
that "Ilka blade o' grass keps it's ain drap o' dew."

But, alas! we have to bid "good-bye" to the appellation "dew-drop." What
was popularly and poetically called dew _is not dew at all_. Then what is
it?

On what we have been accustomed to call a "dewy" night, after the
brilliant summer sun has set, and the stars begin to peep out of the
almost cloudless sky, let us take a look at the produce of our vegetable
garden. On the broccoli are found glistening drops; but on the peas,
growing next them, we find nothing.

A closer examination shows us that the moisture on the plants is not
arranged as would be expected from the ordinary laws of radiation and
condensation. There is no generally filmy appearance over the leaves; the
moisture is collected in little drops placed at short distances apart,
along the edges of the leaves all round.

Now place a lighted lantern below one of the blades of the broccoli, and a
revelation will be made. The brilliant diamond-drops that fringe the edge
of the blade are all placed at the points where the nearly colourless
veins of the blade come to the outer edge. The drops are not dew at all,
but the exudation of the healthy plant, which has been conveyed up these
veins by strong root-pressure.

The fact is that the root acts as a kind of force-pump, and keeps up a
constant pressure inside the tissues of the plant. One of the simplest
experiments suggested by Dr. Aitken is to lift a single grass-plant, with
a clod of moist earth attached to it, and place it on a plate with an
inverted tumbler over it. In about an hour, drops will begin to exude, and
the tip of nearly every blade will be found to be studded with a
diamond-like drop.

Next substitute water-pressure. Remove a blade of broccoli and connect it
by means of an india-rubber tube with a head of water of about forty
inches. Place a glass receiver over it, so as to check evaporation, and
leave it for an hour. The plant will be found to have excreted water
freely, some parts of the leaves being quite wet, while drops are
collected at the places where they appeared at night.

If the water pressed into the leaf is coloured with aniline blue, the
drops when they first appear are colourless; but before they grow to any
size, the blue appears, showing that little water was held in the veins.
The whole leaf soon gets coloured of a fine deep blue-green, like that
seen when vegetation is rank; this shows that the injected liquid has
penetrated through the whole leaf.

Again, the surfaces of the leaves of these drop-exuding plants never seem
to be wetted by the water. It is because of the rejection of water by the
leaf-surface that the exuded moisture from the veins remains as a drop.

These observations and experiments establish the fact that the drops which
first make their appearance on grass on dewy nights are not dew-drops at
all, but the exuded watery juices of the plants.

If now we look at dead leaves we shall find a difference of formation of
the moisture on a dewy night: the moisture is spread equally over, where
equally exposed. The moisture exuded by the healthy grass is always found
at a _point_ situated near the tip of the blade, forming a drop of some
size; but the true dew collects later on _evenly_ all over the blade. The
false dew forms a large glistening diamond-drop, whereas the true dew
coats the blade with a fine pearly lustre. Brilliant globules are produced
by the vital action of the plant, especially beautiful when the deep-red
setting sun makes them glisten, all a-tremble, with gold light; while an
infinite number of minute but shining opal-like particles of moisture
bedecks the blade-surfaces, in the form of the gentle dew--

  "Like that which kept the heart of Eden green
  Before the useful trouble of the rain."




CHAPTER IV

HOAR-FROST


All in this country are familiar with the beauty of hoar-frost. The
children are delighted with the funny figures on the glass of the bedroom
window on a cold winter morning. Frost is a wonderful artist; during the
night he has been dipping his brush into something like diluted schist,
and laying it gracefully on the smooth panes.

And, as you walk over the meadows, you observe the thin white films of ice
on the green pasture; and the clear, slender blades seem like crystal
spears, or the "lashes of light that trim the stars."

You all know what hoar-frost is, though most in the country give it the
expressive name of "rime." But you are not all aware of how it is formed.
Hoar-frost is just frozen dew. In a learned paper, written in 1784,
Professor Wilson of Glasgow made this significant remark: "This is a
subject which, besides its entire novelty, seems, upon other accounts, to
have a claim to some attention." He observed, in that exceptionally cold
winter, that, when sheets of paper and plates of metal were laid out, all
began to attract hoar-frost as soon as they had time to cool down to the
temperature of the air. He was struck with the fact that, while the
thermometer indicated 36 degrees of frost a few feet above the ground and
44 degrees of frost at the surface of the snow, there were only 8 degrees
of frost at a point 3 inches below the surface of the snow. If he had
only thought of placing the thermometer on the grass, under the snow, he
would have found it to register the freezing-point only. And had he
inserted the instrument below the ground, he would have found it
registering a still higher temperature. That fact would have suggested to
him the formation of hoar-frost; that the water-vapour from the warm soil
was trapped by a cold stratum of air and frozen when in the form of dew.

One of the most interesting experiments, without apparatus, which you can
make is in connection with the formation of hoar-frost, when there is no
snow on the ground, in very cold weather. If it has been a bright, clear,
sunny day in January, the effect can be better observed. Look over the
garden, grass, and walks on the morning after the intense cold of the
night; big plane-tree leaves may be found scattered over the place. You
see little or no hoar-frost on the _upper_ surface of the leaves. But turn
up the surface next the earth, or the road, or the grass, and what do you
see? You have only to handle the leaf in this way to be brightly
astonished. A thick white coating of hoar-frost, as thick as a layer of
snow, is on the _under_ surface. If a number of leaves have been
overlapping each other, there will be no coating of hoar-frost under the
top leaves; but when you reach the lowest layer, next the bare ground, you
will find the hoar-frost on the under surface of the leaves. Now that is
positive proof that the hoar-frost has not fallen from the air, but has
risen from the earth.

The sun's heat on the previous day warmed the earth. This heat the earth
retained till evening. As the air chilled, the water-vapour from the
warmer earth rose from its surface, and was arrested by the cold surface
of the leaves. So cold was that surface that it froze the water-vapour
when rising from the earth, and formed hoar-frost in very large
quantities. When this happens later on in the season, one may be almost
sure of having rain in the forenoon.

As hoar-frost is just frozen dew, I can even more surely convince you of
the formation of hoar-frost as rising from the ground by observations made
by me at my manse in Strathmore, in June 1892. I mention this particularly
because then was the most favourable testing-time that has _ever_ occurred
during meteorological observations. June 9th was the warmest June day
(with one exception) for twenty years. The thermometer reached 83° Fahr.
in the shade. Next day was the coldest June day (with one exception) for
twenty years, when the thermometer was as low as 51° in the shade. But
during the night my thermometer on the grass registered 32°--the freezing
point. On the evening of the sultry day I examined the soil at 10 o'clock.
It was damp, and the grass round it was filmy moist. The leaves of the
trees were crackling dry, and all above was void of moisture. The air
became gradually chilly; and as gradually the moisture rose in height on
the shrubs and lower branches of small trees. The moon shone bright, and
the stars showed their clear, chilly eyes. The soil soon became quite wet,
the low grass was dripping with moisture, and the longer grass was
becoming dewed. This gave the best natural evidence of the rising of the
dew that I ever witnessed. But everything was favourable for the
observation--the cold air incumbent on the rising, warm, moist vapour from
the soil fixing the dew-point, when the projecting blades seized the
moisture greedily and formed dew. Had the temperature been a little below
the freezing-point, hoar-frost would have been beautifully formed.




CHAPTER V

FOG


To many nothing is more troublesome than a dense fog in a large town. It
paralyses traffic, it is dangerous to pedestrians, it encourages theft, it
chokes the asthmatic, and chills the weak-lunged.

In the country it is disagreeable enough; but never so intensely raw and
dense as in the city. On the sea, too, the fog is disagreeable and fraught
with danger. The fog-horn is heard, in its deep, sombre note, from the
lighthouse tower, when the strong artificial light is almost useless.

But a peculiar sense of stagnation possesses the dweller of the large
town, when enveloped in a dense fog. Sometimes during the day, through a
thinner portion, the sun will be dimly seen in copper hue, like the moon
under an eclipse. The smoke-impregnated mass assumes a peculiar "pea-soup"
colour.

Now, what is this fog? How is it formed? It has been ascertained that fogs
are dependent upon _dust_ for their formation. Without dust there could
be no fogs, there would be only dew on the grass and road. Instead of the
dust-impregnated air that irritates the housekeeper, there would be the
constant dripping of moisture on the walls, which would annoy her more.

Ocular demonstration can testify to this. If two closed glass receivers be
placed beside each other, the one containing ordinary air, and the other
filtered air (_i.e._ air deprived of its dust by being driven through
cotton wool), and if jets of steam be successively introduced into these,
a strange effect is noticed. In the vessel containing common air the steam
will be seen rising in a dense cloud; then a beautiful white foggy cloud
will be formed, so dense that it cannot be seen through. But in the vessel
containing the filtered air, the steam is not seen at all; there is not
the slightest appearance of cloudiness. In the one case, where there was
the ordinary atmospheric dust, fog at once appeared; in the other case,
where there was no dust in suspension, the air remained clear and
destitute of fog. Invisible dust, then, is necessary in the air for the
formation of fogs.

The reason of this is that a free-surface must exist for the condensation
of the vapour-particles. The fine particles of dust in the air act as
free-surfaces, on which the fog is formed. Where there is abundance of
dust in the air and little water-vapour present, there is an
over-proportion of dust-particles; and the fog-particles are, in
consequence, closely packed, but light in form and small in size, and take
the lighter appearance of fog. Accordingly, if the dust is increased in
the air, there is a proportionate increase of fog. Every fog-particle,
then, has embosomed in it an invisible dust-particle.

But whence comes the dust? From many sources. It is organic and inorganic.
So very fine is the inorganic dust in the atmosphere that, if the
two-thousandth part of a grain of fine iron be heated, and the dust be
driven off and carried into a glass receiver of filtered air, the
introduction of a jet of steam into that receiver would at once occasion
an appreciable cloudiness.

This is why fogs are so prevalent in large towns. Next the minute
brine-particles, driven into the air as fog forms above the ocean surface,
are the burnt sulphur-particles emanating from the chimneys in towns. The
brilliant flame, as well as the smoky flame, is a fog-producer. If gas is
burnt in filtered air, intense fog is produced when water-vapour is
introduced. Products of combustion from a clear fire and from a smoky one
produce equal fogging. The fogs that densely fill our large towns are
generally less bearable than those that veil the hills and overhang the
rivers.

It is the sulphur, however, from the consumed coals, which is the active
producer of the fogs of a large town. The burnt sulphur condenses in the
air to very fine particles, and the quantity of burnt sulphur is enormous.
No less than seven and a half millions of tons of coals are consumed in
London. Now, the average amount of sulphur in English coal is one and a
quarter per cent. That would give no less than 93,750 tons of sulphur
burned every year in London fires. Now, if we reckon that on an average
twice the quantity of coals is consumed there on a winter day that is
consumed on a summer day, no less than 347 tons of the products of
combustion (in extremely fine particles) are driven into the
superincumbent air of London every winter day. This is an enormous
quantity, quite sufficient to account for the density of the fogs in that
city.




CHAPTER VI

THE NUMBERING OF THE DUST


If the shutters be all but closed in a room, when the sun is shining in,
myriads of floating particles can be seen glistening in the stream of
light. Their number seems inexhaustible. According to Milton, the follies
of life are--

                    "Thick and numberless,
  As the gay motes that people the sunbeams."

Can these, then, be counted? Yes, Dr. Aitken has numbered the dust of the
air. I shall never forget my rapt astonishment the day I first numbered
the dust in the lecture-room of the Royal Society of Edinburgh, with his
instrument and under his direction.

This wonderfully ingenious instrument was devised on this principle, that
every fog-particle has entombed in it an invisible dust-particle. A
definite small quantity of common air is diluted with a fixed large
quantity of dustless air (_i.e._ air that has been filtered through
cotton-wool). The mixture is allowed to be saturated with water-vapour.
Then the few particles of dust seize the moisture, become visible in fine
drops, fall on a divided plate, and are there counted by means of a
magnifying glass. That is the secret!

I shall now give you a general idea of the apparatus. Into a common glass
flask of carafe shape, and flat-bottomed, of 30 cubic inches capacity, are
passed two small tubes, at the end of one of which is attached a small
square silver table, 1 inch in length. A little water having been
inserted, the flask is inverted, and the table is placed exactly 1 inch
from the inverted bottom, so that the contents of air right above the
table are 1 cubic inch. This observing table is divided into 100 equal
squares, and is highly polished, with the burnishing all in one direction,
so that during the observations it appears dark, when the fine
mist-particles glisten opal-like with the reflected light in order that
they may be more easily counted. The tube to which the silver table is
attached is connected with two stop-cocks, one of which can admit a small
measured portion of the air to be examined. The other tube in the flask is
connected with an air-pump of 10 cubic inches capacity. Over the flask is
placed a covering, coloured black in the inside. In the top of this cover
is inserted a powerful magnifying glass, through which the particles on
the silver table can be easily counted. A little to the side of this
magnifier is an opening in the cover, through which light is concentrated
on the table.

To perform the experiment, the air in the flask is exhausted by the
air-pump. The flask is then filled with filtered air. One-tenth of a cubic
inch of the air to be examined is then introduced into the flask, and
mixed with the 30 cubic inches of dustless air. After one stroke of the
air-pump, this mixed air is made to occupy an additional space of 10 cubic
inches; and this rarefying of the air so chills it that condensation of
the water-vapour takes place on the dust-particles. The observer, looking
through the magnifying-glass upon the silver table, sees the
mist-particles fall like an opal shower on the table. He counts the number
on a single square in two or three places, striking an average in his
mind. Suppose the average number upon a single square were five, then on
the whole table there would be 500; and these 500 particles of dust are
those which floated invisibly in the cubic inch of mixed air right above
the table. But, as there are 40 cubic inches of mixed air in the flask and
barrel, the number of dust-particles in the whole is 20,000. That is,
there are 20,000 dust-particles in the same quantity of common air
(one-tenth of a cubic inch) which was introduced for examination. In other
words, a cubic inch of the air contained 200,000 dust-particles--nearly a
quarter of a million.

The day I used the instrument we counted 4,000,000 of dust-particles in a
cubic inch of the air outside of the room, due to the quantity of smoke
from the passing trains. Dr. Aitken has counted in 1 cubic inch of air
immediately above a Bunsen flame the fabulous number of 489,000,000 of
dust-particles.

A small instrument has been constructed which can bring about results
sufficiently accurate for ordinary purposes. It is so constructed that,
when the different parts are unscrewed, they fit into a case 4-1/2 inches
by 2-1/2 by 1-1/4 deep--about the size of an ordinary cigar-case.

After knowing this, we are apt to wonder why our lungs do not get clogged
up with the enormous number of dust-particles. In ordinary breathing, 30
cubic inches of air pass in and out at every breath, and adults breathe
about fifteen times every minute. But the warm lung-surface repels the
colder dust-particles, and the continuous evaporation of moisture from the
surface of the air-tubes prevents the dust from alighting or clinging to
the surface at all.




CHAPTER VII

DUST AND ATMOSPHERIC PHENOMENA


Dr. Aitken has devoted a vast amount of attention to the enumeration of
dust-particles in the air, on the Continent as well as in Scotland, to
determine the effects of their variation in number.

On his first visit to Hyères, in 1890, he counted with the instrument
12,000 dust-particles in a cubic inch of the air: whereas in the following
year he counted 250,000. He observed, however, that where there was least
dust, the air was very clear; whereas with the maximum of dust, there was
a very thick haze.

At Mentone, the corresponding number was 13,000, when the wind was blowing
from the mountains; but increased to 430,000, when the wind was blowing
from the populous town.

On his first visit to the Rigi Kulm, in Switzerland, the air was
remarkably clear and brilliant, and the corresponding number never
exceeded 33,000; but, on his second visit, he counted no less than
166,000. This was accounted for by a thick haze, which rendered the lower
Alps scarcely visible. The upper limit of the haze was well defined; and
though the sky was cloudless, the sun looked like a harvest moon, and
required no eagle's eye to keep fixed on it.

Next day there was a violent thunder-storm. At 6 P.M. the storm commenced,
and 60,000 dust-particles to the cubic inch of air were registered; but in
the middle of the storm he counted only 13,000. There was a heavy fall of
hail at this time, and he accounts for the diminution of dust-particles by
the down-rush of purer upper air, which displaced the contaminated lower
air.

At the Lake of Lucerne there was an exceptional diminution of the number
in the course of an hour, viz. from 171,000 to 28,000 in a cubic inch. On
looking about, he found that the direction of the wind had changed,
bringing down the purer upper air to the place of observation. The bending
downwards of the trees by the strong wind showed that it was coming from
the upper air.

Returning to Scotland, he continued his observations at Ben Nevis and at
Kingairloch, opposite Appin, Mr. Rankin using the instrument at the top of
the mountain. These observations showed in general that on the mountain
southerly, south-easterly, and easterly winds were more impregnated with
dust-particles, sometimes containing 133,000 per cubic inch. Northerly
winds brought pure air. The observations at sea-level showed a certain
parallelism to those on the summit of the mountain. With a north-westerly
wind the dust-particles reached the low number of 300 per cubic inch, the
lowest recorded at any low-level station.

The general deductions which he made from his numerous observations during
these two years are that (1) air coming from inhabited districts is always
impure; (2) dust is carried by the wind to enormous distances; (3) dust
rises to the tops of mountains during the day; (4) with much dust there is
much haze; (5) high humidity causes great thickness of the atmosphere, if
accompanied by a great amount of dust, whereas there is no evidence that
humidity alone has any effect in producing thickness; (6) and there is
generally a high amount of dust with high temperature, and a low amount of
dust with low temperature.




CHAPTER VIII

A FOG-COUNTER


Next to the enumeration of the dust-particles in the atmosphere is the
marvellous accuracy of counting the number of particles in a fog. The same
ingenious inventor has constructed a fog-counter for the purpose; and the
number of fog-particles in a cubic inch can be ascertained. This
instrument consists of a glass micrometer divided into squares of a known
size, and a strong microscope for observing the drops on the stage. The
space between the micrometer and the microscope is open, so that the air
passes freely over the stage; and the drops that fall on its surface are
easily seen. These drops are very small; many of them when spread on the
glass are no more than the five-hundredth of an inch in diameter.

In observing these drops, the attention requires to be confined to a
limited area of the stage, as many of the drops rapidly evaporate, some
almost as soon as they touch the glass, whilst the large ones remain a few
seconds.

In one set of Dr. Aitken's observations, in February 1891, the fog was so
thick that objects beyond a hundred yards were quite invisible. The number
of drops falling per second varied greatly from time to time. The greatest
number was 323 drops per square inch in one second. The high number never
lasted for long, and in the intervals the number fell as low as 32, or to
one-tenth.

If we knew the size of these drops, we might be able to calculate the
velocity of their fall, and from that obtain the number in a cubic inch.

An ingenious addition is put to the instrument in order to ascertain this
directly. It is constructed so as to ascertain the number of particles
that fall from a known height. Under a low-power microscope, and
concentric with it, is mounted a tube 2 inches long and 1-1/2 inch in
diameter, with a bottom and a cover, which are fixed to an axis parallel
with the axis of the tube, so that, by turning a handle, these can be slid
sideways, closing or opening the tube at both ends when required. In the
top is a small opening, corresponding to the lens of the microscope, and
in the centre of the bottom is placed the observing-stage illumined by a
spot-mirror. The handle is turned, and the ends are open to admit the
foggy air. The handle is quickly reversed, and the ends are closed,
enabling the observer to count on the stage all the fog-particles in the
two inches of air over it.

The number of dust-particles in the air which become centres of
condensation depends on the rate at which the condensation is taking
place. The most recent observations show that quick condensation causes a
large number of particles to become active, whereas slow condensation
causes a small number. After the condensation has ceased, a process of
differentiation takes place, the larger particles robbing the smaller ones
of their moisture, owing to the vapour-pressure at the surface of the
drops of large curvature being less than at the surface of drops of
smaller curvature.

By this process the particles in a cloud are reduced in number; the
remaining ones, becoming larger, fall quicker. The cloud thus becomes
thinner for a time. A strong wind, suddenly arising, will cause the
cloud-particles to be rapidly formed: these will be very numerous, but
very small--so small that they are just visible with great care under a
strong magnifying lens used in the instrument. But in slowly formed clouds
the particles are larger, and therefore more easily visible to the naked
eye.

Though the particles in a fog are slightly finer, the number is about the
same as in a cloud--that is, generally. As clouds vary in density, the
number of particles varies. Sometimes in a cloud one cannot see farther
than 30 yards; whereas in a few minutes it clears up a little, so that we
can see 100 yards. Of course, the denser the cloud the greater the number
of water-particles falling on the calculating-stage of the instrument.

Very heavy falls of cloud-particles seldom last more than a few seconds,
the average being about 325 on the square inch per second, the maximum
reaching to 1290. This is about four times the number counted in a fog.
Yet the particles are so very small that they evaporate instantly when
they reach a slight increase of temperature.




CHAPTER IX

FORMATION OF CLOUDS


In our ordinary atmosphere there can be no clouds without dust. A
dust-particle is the nucleus that at a certain humidity becomes the centre
of condensation of the water-vapour so as to form a cloud-particle; and a
collection of these forms a cloud.

This condensation of vapour round a number of dust-particles in visible
form gives rise to a vast variety of cloud-shapes. There are two distinct
ways in which the formation of clouds generally takes place. Either a
layer of air is cooled in a body below the dew-point; or a mass of warm
and moist air rises into a mass which is cold and dry. The first forms a
cloud, called, from being a layer, _stratus_; the second forms a cloud,
called, from its heap appearance, _cumulus_. The first is widely extended
and horizontal, averaging 1800 feet in height; the second is convex or
conical, like the head of a sheaf, increasing upward from a level base,
averaging from 4500 feet to 6000 feet in height.

There are endless combinations of these two; but at the height of 27,000
feet, where the cloud-particles are frozen, the structure of the cloud is
finer, like "mares' tails," receiving the name _cirrus_. When the cirrus
and cumulus are combined, in well-defined roundish masses, what is
familiarly described as a "mackerel sky" is beautifully presented. The
dark mass of cloud, called _nimbus_, is the threatening rain-cloud, about
4500 feet in height.

At the International Meteorological Conference at Munich, in 1892, twelve
varieties of clouds were classified, but those named above are the
principal. In a beautiful sunset one can sometimes notice two or three
distances of clouds, the sun shedding its gold light on the full front of
one set, and only fringing with vivid light the nearer range.

Although no man has wrought so hard as Dr. Aitken to establish the
principle that clouds are mainly due to the existence of dust-particles
which attract moisture on certain conditions, yet even twenty years ago he
said that it was probable that sunshine might cause the formation of
nuclei and allow cloudy condensation to take place where there was no
dust.

Under certain conditions the sun gives rise to a great increase in the
number of nuclei. Accordingly, he has carefully tested a few of the
ordinary constituents and impurities in our atmosphere to see if sunshine
acted on them in such a way as to make them probable formers of
cloud-particles.

He tested various gases, with more or less success. He found that ordinary
air, after being deprived of its dust-particles and exposed to sunshine,
does not show any cloudy condensation on expansion; but, when certain
gases are in the dustless air, a very different result is obtained.

He first used ammonia, putting one drop into six cubic inches of water in
a flask, and sunning this for one minute; the result was a considerable
quantity of condensation, even with such a weak solution. When the flask
was exposed for five minutes, the condensation by the action of the
sunshine was made more dense.

Hydrogen peroxide was tested in the same way, and it was found to be a
powerful generator of nuclei. Curious is it that sulphurous acid is
puzzling to the experimentalist for cloud formation. It gives rise to
condensation in the dark; but sunshine very conclusively increases the
condensation.

Chlorine causes condensation to take place without supersaturation;
sulphuretted hydrogen (which one always associates with the smell of
rotten eggs) gives dense condensation after being exposed to sunshine.

Though the most of these nuclei, due to the action of sunshine in the
gases, remain active for cloudy condensation for a comparatively short
space of time--fifteen minutes to half-an-hour--yet the experiments show
that it is possible for the cloudy condensation to take place in certain
circumstances in the absence of dust. This seems paradoxical in the light
of the former beautiful experiments; but, in ordinary circumstances, dust
is needed for the formation of clouds. However, supposing there is any
part of the upper air free from dust, it is now found possible, when any
of these gases experimented on be present, for the sun to convert them
into nuclei of condensation, and permit of clouds being formed in dustless
air, miles above the surface of the earth.

In the lower atmosphere there are always plenty of dust-particles to form
cloudy condensation, whether the sun shines or not. These are produced by
the waste from the millions of meteors that daily fall into the air.

But in the higher atmosphere, clouds can be formed by the action of the
sun's rays on certain gases. This is a great boon to us on the earth; for
it assures us of clouds being ever existing to defend us from the sun's
extra-powerful rays, even when our atmosphere is fairly clear. This is
surely of some meteorological importance.




CHAPTER X

DECAY OF CLOUDS


From the earliest ages clouds have attracted the attention of observers.
Varied are their forms and colours, yet in our atmosphere there is one law
in their formation. Cloud-particles are formed by the condensation of
water-vapour on the dust-particles invisibly floating in the atmosphere,
up to thousands--and even millions--in the cubic inch of air.

But observers have not directed their attention so much to the decay of
clouds--in fact, the subject is quite new. And yet how suggestive is the
subject!

The process of decay in clouds takes place in various ways. A careful
observer may witness the gradual wasting away and dilution into thin air
of even great stretches of cloud, when circumstances are favourable. In
May 1896 my attention was particularly drawn to this at my manse in
Strathmore. In the middle of that exceptionally sultry month, I was
arrested by a remarkable transformation scene. It was the hottest May for
seventy-two years, and the driest for twenty-five years. The whole parched
earth was thirsting for rain. All the morning my eyes were turned to the
clouds in the hope that the much-desired shower should fall. Till ten
o'clock the sun was not seen, and there was no blue in the sky. Nor was
there any haze or fog.

But suddenly the sun shone through a thinner portion of the enveloping
clouds, and, to the north, the sky began to open. As if by some magic
spell there was, in a quarter of an hour, more blue to be seen than
clouds. At the same time, near the horizon, a haze was forming, gradually
becoming denser as time wore on. In an hour the whole clouds were gone,
and the glorious orb of day dispelled the moisture to its thin-air form.

This was a pointed and rapid illustration of the decay from cloud-form to
haze, and then to the pure vapoury sky. It was an instance of the
_reverse_ process. As the sun cleared through, the temperature in the
cloud-land rose and evaporation took place on the surface of the
cloud-particles, until by an untraceable, but still a gradual process
through fog, the haze was formed. Even then the heat was too great for a
definite haze, and the water-vapour returned to the air, leaving the
dust-particles in invisible suspension.

But clouds decay in another way. This I will illustrate in the next
chapter on "It always rains."

What strikes a close observer is the difference of structure in clouds
which are in the process of formation and those which are in the process
of decay. In the former the water-particles are much smaller and far more
numerous than in the latter. While the particles in clouds in decay are
large enough to be seen with the unaided eye, when they fall on a properly
lighted measuring table, they are so small in clouds in rapid formation
that the particles cannot be seen without the aid of a strong magnifying
glass.

Observers have assumed that the whole explanation of the fantastic shapes
taken by clouds is founded on the process of formation; but Dr. Aitken has
pointed out that ripple-marked clouds, for instance, have been clouds of
decay. When what is called a cirro-stratus cloud--mackerel-like against
the blue sky--is carefully observed in fine weather, it will be found that
it frequently changes the ripple-marked cirrus in the process of decay to
vanishing. Where the cloud is thin enough to be broken through by the
clear air that is drawn in between the eddies, the ripple markings get
nearer and nearer the centre, as the cloud decays. And, at last, when
nearly dissolved, these markings are extended quite across the cloud.

Whether, then, we consider the cases of clouds gradually melting away back
into their original state of blue water-vapour, or the constant fine
raining from clouds and re-formation by evaporation, or the transformation
of such clouds as the cirro-stratus into the ripple-marked cirrus, we are
forced to the conclusion that in clouds there is not always development,
but sometimes degeneration; not always formation, but sometimes decay.




CHAPTER XI

IT ALWAYS RAINS


All are familiar with the answer given by the native of Skye to the irate
tourist on that island, who, for the sixth day drenched, asked the
question: "Does it always rain here?" "Na!" answered the workman, without
at all understanding the joke; "feiles it snaas" (sometimes it snows).
Yet, strange to say, the tourist's question has been answered in the
affirmative in every place where a cloud is overhead, visible or
invisible.

Whenever a cloud is formed, it begins to rain; and the drops shower down
in immense numbers, though most minute in size--"the playful fancies of
the mighty sky."

No doubt it is only in certain circumstances that these drops are
attracted together so as to form large drops, which fall to the earth in
genial showers to refresh the thirsty soil, or in a terrible deluge to
cause great destruction. But when the temperature and pressure are not
suitable for the formation of what we commonly know as the rain, the fine
drops fall into the air under the cloud, where they immediately evaporate
from their dust free-surfaces, if the air is dry and warm. This is, in
other words, the decay of clouds.

It is a curious fact that objects in a fog may not be wetted, when the
number of water-particles is great. It seems that these water-particles
all evaporate so quickly that even one's hand or face is not sensible of
being wetted. The particles are minutely small; and they may evaporate
even before reaching the warm skin, by reason of the heated air over the
skin.

There is a peculiarly warm sensation in the centre of a cumulus cloud,
especially when it is not dense. A glow of heat seems to radiate from all
points. Yet the face and hands are quite dry, and exposed objects are not
wetted; but it is really _always raining_. That is a curious discovery.

It is radiant heat that is the cause of the remarkable result. The rays of
the sun, which strike the upper part of the cloud, not only heat that
surface but also penetrate the cloud and fall on the surface of bodies
within, generating heat there. These heated surfaces again radiate heat
into the air attached to them. This warm air receives the fine raindrops
in the cloud, and dissolves the moisture from the dust-particles before
the moisture can reach the surfaces exposed. That a vast amount of radiant
heat rushes through a cloud is clearly shown by exposing a thermometer
with black bulb _in vacuo_. On some occasions, a thermometer would
indicate from 40° to 50° above the temperature of the air, thus proving
the surface to be quite dry.

These observations have been corroborated on Mount Pilatus, near
Lucerne--1000 feet higher and more isolated than the Rigi. The summit was
quite enveloped in cloud, and, though one might naturally conclude that
the air was dense with moisture, yet the wooden seats, walls, and all
exposed surfaces were quite dry. Strange to say, however, the thermometers
hung up got wet rapidly, and the pins driven into the wooden post to
support them rapidly became moist. A thermometer lying on a wooden seat
stood at 60°, while one hung up read only 48°. This difference was caused
by radiant heat.

It is well known that, when bodies are exposed to radiant heat, they are
heated in proportion to their size; the smaller, then, may be moist, when
the larger are dry by radiation. The effect of the sun's penetrating heat
through the cloud is to heat exposed objects above the temperature of the
air; and if the objects are of any size they are considerably heated, and
retain their heat more, while at the same time around them is a layer of
warm air which is quite sufficient to force the water-vapour to leave the
dust-particles in the fine rain.

Hence seats, walls, posts, &c., are quite dry, though they are in the
middle of a cloud. They are large enough to throw off the moisture by the
retained heat, or by the large amount of surrounding heat; whereas, small
bodies, which are not heated to the same degree and cannot therefore
retain their heat so easily, have not heat-power sufficient to withstand
the moisture, and they become wetted. Hence, by the radiant heat, the
large exposed objects are dry in the cloud; whereas small objects are
damp, and, in some cases, dripping with wet.

The fact is, then, that whenever a cloud overhangs, _rain is falling_,
though it may not reach the earth on account of the dryness of the stratum
of air below the cloud, and the heat of the air over the earth. So that on
a summer day, with the gold-fringed, fleecy clouds sailing overhead, it is
really raining; but the drops, being very small, evaporate long before
reaching the earth. As Ariel sings at the end of "The Tempest" of
Shakespeare, "The rain, it raineth every day." It rains, but much of the
melting of the clouds is reproduced by a wonderful circularity--the
moisture evaporating, seizing other dust-particles, forming
cloud-particles, falling again, and so on _ad infinitum_, during the
existing circumstances.




CHAPTER XII

HAZE


What is haze? The dictionary says, "a fog." Well, haze is _not_ a fog. In
a fog, the dust-particles in the air have been fully clothed with
water-vapour; in a haze, the process of condensation has been arrested.

Cloudy condensation is changed to haze by the reduction of its humidity.
Dr. Aitken invented a simple apparatus for testing the condensing power of
dust, and observing if water-vapour condensed on the deposited dust in
unsaturated air.

The dust from the air has first to be collected. This is done by placing a
glass plate vertically, and in close contact with one of the panes of
glass in the window, by means of a little india-rubber solution. The plate
being thus rendered colder than the air in the room, the dust is deposited
on it.

Construct a rectangular box, with a square bottom, 1-1/2 inches a side and
3/4 inch deep, and open at the top. Cover the top edge of the box with a
thickness of india-rubber. Place the dusty plate--a square glass mirror, 4
inches a side--on the top of the india-rubber, and hold it down by spring
catches, so as to make the box water-tight. The box has been provided with
two pipes, one for taking in water and the other for taking away the
overflow, with the bulb of a thermometer in the centre. Clean the dust
carefully off one half of the mirror, so that one half of the glass
covering the box is clean and the other half dusty. Pour cold water
through the pipe into the box, so as to lower the temperature of the
mirror, and carefully observe when condensation begins on the clean part
and on the dusty part, taking a note of the difference of temperature. The
condensation of the water-vapour will appear on the dust-particles before
coming down to the natural dew-point temperature of the clean glass. And
the difference between the two temperatures indicates the temperature
above the dew-point at which the dust has condensed the water-vapour.

Magnesia dust has small affinity for water-vapour; accordingly, it
condenses at almost exactly the same temperature as the glass. But
gunpowder has great condensing power. All have noticed that the smoke
from exploded gunpowder is far more dense in damp than in dry weather. In
the experiment it will be found that the dust from gunpowder smoke begins
to show signs of condensing the vapour at a temperature of 9° Fahr. above
the dew-point. In the case of sodium dust, the vapour is condensed from
the air at a temperature of 30° above the dew-point.

Dust collected in a smoking-room shows a decidedly greater condensing
power than that from the outer air.

We can now understand why the glass in picture frames and other places
sometimes appears damp when the air is not saturated. When in winter the
windows are not often cleaned, a damp deposit may be frequently seen on
the glass. Any one can try the experiment. Clean one half of a dusty pane
of glass in cold weather, and the clean part will remain undewed and
clear, while the dusty part is damp to the eye and greasy to the touch.

These observations indicate that moisture is deposited on the
dust-particles from air, which is not saturated, and that the condensation
takes place while the air is comparatively dry, _before_ the temperature
is lowered to the dew-point. There is, then, no definite demarcation
between what seems to us clear air and thick haze. The clearest air has
some haze, and, as the humidity increases, the thickness of the air
increases.

In all haze the temperature is above the dew-point. The dust-particles
have only condensed a very small amount of the moisture so as to form
haze, before the fuller condensation takes place at the dew-point.

At the Italian lakes, on many occasions when the air is damp and still,
every stage of condensation may be observed in close proximity, not
separated by a hard and fast line, but when no one could determine where
the clear air ended and the cloud began. Sometimes in the sky overhead a
gradual change can be observed from perfect clearness to thick air, and
then the cloud.

A thick haze may be occasioned by an increased number of dust-particles
with little moisture, or of a diminished number of dust-particles with
much moisture, above the point of saturation. The haze is cleared by this
temperature rising, so as to allow the moisture to evaporate from the
dust-particles.

Whenever the air is dry and hazy, much dust is found in it; as the dust
decreases the haze also decreases. For example, Dr. Aitken, at
Kingairloch, in one of the clearest districts of Argyleshire, on a clear
July afternoon, counted 4000 dust-particles in a cubic inch of the air;
whereas, two days before, in thick haze, he counted no fewer than 64,000
in the cubic inch. At Dumfries the number counted on a very hazy day in
October increased twenty-fold over the number counted the day before, when
it was clear.

All know that thick haze is usual in very sultry weather. The wavy,
will-o'-the-wisp ripples near the horizon indicate its presence very
plainly. During the intense heat there is generally much dust in the
atmosphere; this dust, by the high temperature, attracts moisture from the
apparently dry air, though above the saturation point. In all
circumstances, then, the haze can be accounted for by the condensing power
of the dust-particles in the atmosphere, at a higher temperature than that
required for the formation of fogs, or mists, or rain.




CHAPTER XIII

HAZING EFFECTS OF ATMOSPHERIC DUST


The transparency of the atmosphere is very much destroyed by the
impurities communicated to it while passing over the inhabited areas of
the country. Dr. Aitken devoted eighteen months to compare the amount of
dusty impurities in different masses of air, or of different airs brought
in by winds from different directions.

He took Falkirk for his centre of observations. This town lies a little to
the north of a line drawn between Edinburgh and Glasgow, and is nearly
midway between them. If we draw a line due west from it, and another due
north, we find that, in the north-west quadrant so enclosed, the
population of that part of Scotland is extremely thin, the country over
that area being chiefly mountainous. In all other directions, the
conditions are quite different. In the north-east quadrant are the fairly
well-populated areas of Aberdeenshire, Forfarshire, and the thickly
populated county of Fife. In the south-east quadrant are situated
Edinburgh and the well-populated districts of the south-east of Scotland.
And in the south-west quadrant are Glasgow and the large manufacturing
towns which surround it. The winds from three of these quadrants bring air
polluted in its passage over populated areas, whereas the winds from the
north-west come comparatively pure.

The general plan of estimating the amount of haze is to note the most
distant hill that can be seen through the haze. The distance in miles of
the farthest away hill visible is then called "the limit of visibility" of
the air at the time. For the observations made at Falkirk, only three
hills are available, one about four miles distant, the Ochils about
fifteen miles distant, and Ben Ledi about twenty-five miles distant--all
in the north-west quadrant. When the air is thick, only the near hill can
be seen; then the Ochils become visible as the air clears; and at last Ben
Ledi is seen, when the haze becomes still less. After Ben Ledi is visible,
it then becomes necessary to estimate the amount of haze on it, in order
to get the limit of visibility of the air at the time. Thus, if Ben Ledi
be half-hazed, then the limit of visibility will be fifty miles. In this
way all the estimates of haze have been reduced to one scale for
comparison.

As the result of all the observations it was found that, as the dryness of
the air increases, the limit of visibility also increases. A very marked
difference in the transparency of the air was found with winds from the
different directions. In the north-west quadrant the winds made the air
very clear, whereas winds from all other directions made the air very much
hazed. The winds in the other three areas are nearly ten times more hazed
than those from the north-west quadrant. That is very striking.

The conclusion come to is that the air from densely inhabited districts is
so polluted that it is fully nine times more hazed than the air that comes
from the thinly inhabited districts; in other words, the atmosphere at
Falkirk is about ten times thicker when the wind is east or south than it
would be if there were no fires and no inhabitants.

It is interesting to notice that the limit varies considerably for the
same wind at the same humidity. This is what might have been expected,
because from the observations made by the dust-counter the number of
particles varied greatly in winds from the same directions, but at
different times. This depends upon the rise and fall of the wind, changes
in the state of trade, season of the year, and other causes. During a
strike, the dearth of coal will make a considerable diminution in the
number of dust-particles in the air of large towns. With a north wind, the
extreme limits of visibility are 120 to 200 miles; and with a north-west
wind, from 70 to 250 miles. An east wind has as limits 4 to 50 miles, and
a south-east wind 2 to 60 miles.

One interesting fact to be noticed, after wading through these tables, is
this--that, as a general result, the transparency of the air increases
about 3·7 times for any increase in dryness from 2° to 8° of wet-bulb
depression. That is, the clearness of the air is inversely proportional to
the relative humidity; or, put another way, if the air is four times drier
it is about four times clearer.




CHAPTER XIV

THUNDER CLEARS THE AIR


The phrase "thunder clears the air" is familiar to all. It contains a very
vital truth, yet even scientific men did not know its full meaning until
just the other day. It came by experience to people who had been for ages
observing the weather; and it is one of the most pointed of the
"weather-lore" expressions. Folks got to know, by a sort of rule-of-thumb,
truths which scientifically they were unable to learn. And this is one.

Perhaps, therefore, we should respect a little more what is called
"folk-lore," or ordinary people's sayings. Experience has taught men many
wonderful things. In olden times they were keener natural observers. They
had few books, but they had plenty of time. They studied the habits of
animals and moods of nature, and they came wonderfully near to reaching
the full truth, though they could not give a reason for it. The
awe-inspiring in nature has especially riveted the attention of man.

And no appearance in nature joins more powerfully the elements of grandeur
and awe than a heavy thunder-storm. When, suddenly, from the breast of a
dark thunder-cloud a brilliant flash of light darts zigzag to the earth,
followed by a loud crackling noise which softens in the distance into
weaker volumes of sound, terror seizes the birds of the air and the cattle
in the field. The man who is born to rule the storm rejoices in the
powerful display; but kings have trembled at the sight.

Byron thus pictures a storm in the Alps:--

                              "Far along
    From peak to peak, the rattling crags among
  Leaps the live thunder! Not from one lone cloud,
    But every mountain now hath found a tongue,
  And Jura answers, through her misty shroud,
  Back to the joyous Alps, who call to her aloud!"

Franklin found that lightning is just a kind of electricity. No one can
tell how it is produced; yet a flash has been photographed. When the flash
is from one cloud to another there is sheet-lightning, which is beautiful
but not dangerous; but, when the electricity passes from a cloud to the
earth in a forked form, it is very dangerous indeed. The flash is
instantaneous, but the sound of the thunder takes some time to travel.
Roughly speaking, the sound takes five seconds or six beats of the pulse
to the mile.

All are now taught at school that it is the oxygen in the air which is
necessary to keep us in life. If mice are put into a glass jar of pure
oxygen gas, they will at once dance with exhilarating joy. It occurred to
Sir Benjamin Richardson, some time ago, that it would be interesting to
continue some experiments with animals and oxygen. He put a number of mice
into a jar of pure oxygen for a time; they breathed in the gas, and
breathed out water-vapour and carbonic acid. After the mice had continued
this for some time, he removed them by an arrangement. By chemical means
he removed the water-vapour and carbonic acid from the mixed air in the
vessel. When a blown-out taper was inserted, it at once burst into flame,
showing that the remaining gas was oxygen.

Again, the mice were put into this vessel to breathe away. But, strange to
say, the animals soon became drowsy; the smartness of the oxygen was gone.
At last they died; there was nothing in the gas to keep them in life; yet,
by the ordinary chemical tests, it was still oxygen. It had repeatedly
passed through the lungs of the mice, and during this passage there had
been an action in the air-cells which absorbed the life-giving element of
the gas. It is oxygen, so far as chemistry is concerned, but it has no
life-giving power. It has been _devitalised_.

But the startling discovery still remains. Sir Benjamin had previously
fitted up the vessel with two short wires, opposite each other in the
sides--part outside and part inside. Two wires are fastened to the outside
knobs. These wires are attached to an electric machine, and a flash of
electricity is made to pass between the inner points of the vessel. The
wires are again removed; nothing strange is seen in the vessel. But, when
living mice are put into the vessel, they dance as joyfully as if pure
oxygen were in it. The oxygen in which the first mice died has now been
quite refreshed by the electricity. The bad air has been cleared and made
life-supporting by the electric discharge. It has been again vitalised.

Now, to apply this: before a thunder-storm, everything has been so still
for days that the oxygen in the air has been to some extent robbed of its
life-sustaining power. The air feels "close," a feeling of drowsiness
comes over all. But, after the air has been pierced by several flashes of
lightning, the life-force in the air is restored. Your spirits revive; you
feel restored; your breathing is far freer; your drowsiness is gone. Then
there is a burst of heavenly music from the exhilarated birds. Thus a
thunder-storm "clears the air."

After the passage of lightning through the air ozone is produced--the gas
that is produced after a flash of electricity. It is a kind of oxygen,
with fine exciting effects on the body. If, then, the life-sustaining
power of oxygen depends on a trace of ozone, and this is being made by
lightning's work, how pleased should we be at the occasional
thunder-storm!




CHAPTER XV

DISEASE-GERMS IN THE AIR


The gay motes that dance in the sunbeams are not all harmless. All are
annoying to the tidy housekeeper; but some are dangerous. There are living
particles that float in the air as the messengers of disease and death.
Some, falling on fresh wounds, find there a suitable feeding-place; and,
if not destroyed, generate the deadly influence. Others are drawn in with
the breath; and, unless the lungs can withstand them, they seize hold and
spread some sickness or disease. From stagnant pools, common sewers, and
filthy rooms, disease-germs are constantly contaminating the air. Yet
these can be counted.

The simplest method is that of Professor Frankland. It depends on this
principle: a certain quantity of air is drawn through some cotton-wool;
this wool seizes the organisms as the air passes through; these organisms
are afterwards allowed to feed upon a suitable nutritive medium until they
reach maturity; they are then counted easily.

About an inch from each end of a glass tube (5 inches long and 1 inch
bore), the glass is pressed in during the process of blowing. Some
cotton-wool is squeezed in to form a plug at the farther constricted part
of the glass. The important plug is now inserted at the same open end, but
is not allowed to go beyond the constricted part at its end. A piece of
long lead tubing is attached to the former end by an india-rubber tube.
The other end of the lead tubing is connected with an exhausting syringe.
Sixty strokes of the 18 cubic inches syringe will draw 1080 cubic inches
of the air to be examined through the plugs, the first retaining the
organisms.

The impregnated plug is then put into a flask containing in solution some
gelatine-peptone. The flask is made to revolve horizontally until an
almost perfectly even film of gelatine and the organisms from the
broken-up plug cover its inner surface.

The flask is allowed to remain for an hour in a cool place, and is then
placed under a bell-jar, at a temperature of 70° Fahr. There it remains,
to allow the germs to incubate, for four or five days. The surface of the
flask having been previously divided into equal parts by ink lines, the
counting is now commenced. If the average be taken for each segment, the
number of the whole is easily ascertained. A simple arithmetical
calculation then determines the number of organisms in a cubic foot, since
the number is known for the 1080 cubic inches. That is the process for
determining the number of living organisms in a fixed quantity of air.

No less than thirty colonies of organisms were counted in a cubic foot of
air taken from the Golden Gallery of St. Paul's Cathedral, London, and 140
from the air of the churchyard. An ordinary man would breathe there
thirty-six micro-organisms every minute.

Electricity has a powerful effect in destroying these organisms. Ozone is
generated in the air by lightning, and it is detrimental to them. In fine
ozoned Highland air scarcely a disease-germ can be detected. Open sea air
contains about one germ in two cubic feet. It has been found that in Paris
the average in summer is about 140 per cubic foot of air, but in bedrooms
the number is double. During the twenty-four hours of the day the number
of germs is highest about 6 A.M., and lowest about mid-day.

Raindrops carry the germs to the ground. Hence the advantage of a thunder
plout in a sanitary way. A cubic foot of rain has been found to contain
5500 organic dust-germs, besides 7,000,000,000 of inorganic
dust-particles. In a dirty town the rain will bring down in a year, upon a
square foot of surface, no less than 3,000,000 of bacteria, many of them
being disease-bearing and death-bearing. No wonder, then, that scientific
men are using every endeavour to protect the human frame, as well as the
frame of the lower animals, from the baneful inroads of these floating
nuclei of disease and death.




CHAPTER XVI

A CHANGE OF AIR


For weakness of body and fatigue of mind a very common and essentially
serviceable recommendation is "a change of air." Of course, the change of
scene from coast to country, or from town to hillside, may help much the
depressed in body or mind; and this is very commendable. But, strange to
say, there is a healing virtue in breathing different air.

At first one is apt to think that air is the same all over, as he thinks
water is--especially outside smoky towns; but both have varied qualities
in different parts. You have only to be assured that in a cubic inch of
bedroom air in the denser parts of a large town there are about 20,000,000
of dust-particles, and in the open air of a heathery mountain-side there
are only some hundreds, to see that there is something after all on the
face of it in the "old wives' saw."

Not that the dust-particles are all injurious; for most of them are
inorganic, and many of the organic particles are quite wholesome; yet
there is a change wrought, often very marked, in going from one place to
another for different air.

Even in the country, especially in summer-time, one distinctly notices the
great difference in the air of lowland and highland localities. The ten
miles change from Strathmore to Glenisla shows a marked difference in the
air. Below, it is close, weakening, enervating; above, it is exhilarating,
invigorating, and strong.

So people must have a change--at least those who can afford it--for health
must be seen to first of all, if one has means to do so. Oh! the blessing
of good health! How many who enjoy it never think of the misery of its
loss! In fact, health is the soul that animates all enjoyments of life;
for without it those would soon be tasteless. A man starves at the
best-spread table, and is poor in the midst of the greatest treasures
without health.

In these days half of our diseases come from the neglect of the body in
the overwork of the brain. The wear and tear of labour and intellect go on
without pause or self-pity. Men may live as long as their forefathers, but
they suffer more from a thousand artificial anxieties and cares. The men
of old fatigued only the muscles, we exhaust the finer strength of the
nerves. Even more so now, then, do we require a change of air to soothe
our overwrought nervous system.

And when that magic power, concealed from mortal view, works such wonders
on the health, surely it is one's duty to save up and have it, when it is
within one's means. For is not health the greatest of all possessions?
What a rich colour clothes the countenance of the young after a month's
outing in the hill country! How fine and pure has the blood become! All
stagnant humours, accumulated in winter town life, have been dispelled by
the ozone-brightening charm. The weary looking office or shop man is now
transfigured into a sprightly youth once more, ready with strongly
recuperated power for another winter's labours. The pale wife, who has
been stifled for months in close-aired rooms, has now a healthy flush on
her becoming countenance that speaks of gladly restored health. And all
this has been brought about by a "change of air"!

For a thorough change to a town man, he should make for the Highlands.
There he is never tired of walking, for the air which he breathes is full
of ozone. This revivifying element in the air is produced by the
lightning-bursts from hill to hill. There is in the Highlands a continual
rush of electricity, whether seen or not. Hence the air is very pure, free
from organic germs, intensely exhilarating and buoyant.

Sportsmen are livingly aware of the recuperative power of the Highland
air. Perhaps these city men do not benefit so much by the easy walking
exercise on the grouse moors as in breathing the splendidly
delight-inspiring air. What a change one feels there in a very few hours!

"A change of air" is an old wives' adage. But much of the weather-lore of
our forefathers was based on real scientific principles only now coming to
light. Nature is ever true, but it requires patience to unravel her
secrets. We therefore advocate an occasional "change of air" to improve
the health--

                        "The chiefest good,
  Bestow'd by Heaven, but seldom understood."




CHAPTER XVII

THE OLD MOON IN THE NEW MOON'S ARMS


After the sun's broad beams have tired the sight, the moon with more sober
light charms us to descry her beauty, as she shines sublimely in her
virgin modesty. There is always a most fascinating freshness in the first
sight of the new moon. The superstition of centuries adds to this charm.
Why boys and girls like to turn over a coin in their pocket at this sight
one cannot tell: yet it is done. No young lady likes to look at the new
moon through a pane of glass. And farmers are always confident of a change
of weather with a new moon: at least in bad weather they earnestly hope
for it.

But, banishing all superstition, we welcome the pale silver sickle in the
heavens, once more appearing from the bosom of the azure. And no language
can equal these beautiful words of the youthful Shelley:--

                    "Like the young moon,
  When on the sunlit limits of the night
    Her white shell trembles amid crimson air,
  And while the sleeping tempest gathers might,
    Doth, as the herald of his coming, bear
  The ghost of its dead mother, whose dim form
    Bends in dark ether from her infant's chair."

That is a more charming way of putting the ordinary expression, "the old
moon in the new moon's arms." We are regularly accustomed to the
moonshine, but only occasionally is the _earthshine_ on the moon so
regulated that the shadowed part is visible. This is not seen at the
appearance of every new moon. It depends upon the positions of the sun and
moon, the state of the atmosphere, and the absence of heavy clouds. I
never in my life saw the phenomenon so marvellously beautiful as on May
7th, 1894, at my manse in Strathmore. I took particular note of it, as
some exceedingly curious things were connected with it.

At nine o'clock in the evening, the new moon issued from some clouds in
the western heavens, the sun having set, about an hour before. The
crescent was thin and silvery, and the outline of the shadowed part was
just visible. The sky near the horizon was clear and greenish-hued. As
the night advanced the moon descended, and at ten o'clock she was
approaching a purple stratum of clouds that stretched over the hills,
while the position of the sun was only known a little to the east, by the
back-thrown light upon the dim sky. Through the moisture-laden air the
sun's rays, reflected by the moon, threw a golden stream from the crescent
moon, for the silvery shell became more golden-hued.

The horns of the moon now seemed to project, and the shadowed part became
more distinct, though the circle appeared smaller. By means of a
field-glass I noticed that this was extra lighted, with points here and
there quite golden-tinged. The darker spots showed the deep caverns; the
brighter points brought into relief the mountain peaks.

Why was the surface brighter than usual? I cannot go into detail about the
phases of the moon; but, in a word, I may say that, while the sun can
illuminate the side of the moon turned towards it, it is unable to throw
any light on the shadow, seeing that there is no atmosphere around the
moon to refract the light.

If we, in imagination, looked from the moon upon the earth, we should see
the same phases as are now noticed in the moon; and when it is just before
new moon on the earth, the earth will appear fully illuminated from the
moon. We would also observe (from the moon) that the brightness of the
illuminated part of the earth would vary from time to time, according to
the changes in the earth's atmosphere. More light would be reflected to
the moon from the clouds in our atmosphere than from the bare earth or
cloudless sea, since clouds reflect more light than either land or sea.
Accordingly, we arrive at this curious fact--that the extra brightness of
the _dark_ body of the moon is mainly determined by the amount of _cloud
in our atmosphere_.

Accordingly, I concluded that there must be clouds to the west, though I
could not see them, which reflected rays of light and faintly illuminated
the shadowed part of the moon. It had become much colder, and I concluded
that during the night the cloud-particles, if driven near by the wind,
would condense into rain. And, assuredly, next morning I was gratified to
find that rain had fallen in large quantities, substantiating the theory.

There is much pleasure in verifying such an interesting problem. The dark
body of the moon being more than usually visible is one of our well-known
and oldest indications of coming bad weather. And at once came to my
memory the lines of Sir Patrick Spens, as he foreboded rain for his
crossing the North Sea:--

  "I saw the new moon late yestreen
  Wi' the auld moon in her arm;
  And if we gang to sea, master,
  I fear we'll come to harm."

This lunar indication, then, has a sound physical basis, showing that near
the observer there are vast areas of clouds, which are reflecting light
upon the moon at the time, before they condense into rain by the chilling
of the air. According to the old Greek poet, Aratus: "If the new moon is
ruddy, and you can trace the shadow of the complete circle, a storm is
approaching."




CHAPTER XVIII

AN AUTUMN AFTERGLOW


A brilliant afterglow is welcomed for its surpassing beauty and a
precursor of fine fixed weather.

A glorious sunset has always had a charm for the lover of nature's
beauties. The zenith spreads its canopy of sapphire, and not a breath
creeps through the rosy air. A magnificent array of clouds of numberless
shapes come smartly into view. Some, far off, are voyaging their
sun-bright paths in silvery folds; others float in golden groups. Some
masses are embroidered with burning crimson; others are like "islands all
lovely in an emerald sea." Over the glowing sky are splendid colourings.
The flood of rosy light looks as if a great conflagration were below the
horizon.

I remember witnessing an especially brilliant sunset last autumn on the
high-road between Kirriemuir and Blairgowrie. The setting sun shone upon
the back of certain long trailing clouds which were much nearer me than a
range behind. The fringes of the front range were brilliantly golden,
while the face of those behind was sparklingly bright. Then the sun
disappeared over the western hills, and his place was full of spokes of
living light.

Looking eastward, I observed on the horizon the base of the northern line
of a beautiful rainbow--"the shepherd's delight" for fine weather.

Soon in the west the light faded; but there came out of the east a lovely
flush, and the general sky was presently flamboyant with afterglow. The
front set of clouds was darker except on the edges, the red being on the
clouds behind; and the horizon in the east was particularly rich with dark
red hues.

Gradually the eastern glow rose and reddened all the clouds, but the front
clouds were still grey. The effect was very fine in contrast. The fleecy
clouds overhead became transparently light red, as they stretched over to
reach the silver-streaked west. The new moon was just appearing upright
against a slightly less bright opening in the sky, betokening the firm
hardness of autumn.

Soon the colouring melted away, and the peaceful reign of the later
twilight possessed the land.

Now why that brilliancy of the east, when the west was colourless? Most of
all you note the immense variety and wealth of reds. These are due to dust
in the atmosphere. We are the more convinced of this by the very
remarkable and beautiful sunsets which occurred after the tremendous
eruption at Krakatoa, in the Straits of Sunda, thirty years ago. There was
then ejected an enormous quantity of fine dust, which spread over the
whole world's atmosphere. So long as that vast amount of dust remained in
the air did the sunsets and afterglows display an exceptional wealth of
colouring. All observers were struck with the vividly brilliant red
colours in all shades and tints.

The minute particles of dust in the atmosphere arrest the sun's rays and
scatter them in all directions; they are so small, however, that they
cannot reflect and scatter all; their power is limited to the scattering
of the rays at the blue end of the spectrum, while the red rays pass on
unarrested. The display of the colours of the blue end are found in
numberless shades, from the full deep blue in the zenith to the
greenish-blue near the horizon.

If there were no fine dust-particles in the upper strata, the sunset
effect would be whiter; if there were no large dust-particles, there would
be no colouring at all. If there were no dust-particles in the air at all,
the light would simply pass through into space without revealing itself,
and the moment the sun disappeared there would be total darkness. The very
existence of our twilight depends on the dust in the air; and its length
depends on the amount and extension upwards of the dust-particles.

But how have the particles been increased in size in the east? Because, as
the sun was sinking, but before its rays failed to illumine the heavens,
the temperature of the air began to fall. This cooling made the
dust-particles seize the water-vapour to form haze-particles of a larger
size. The particles in the east first lose the sun's heat, and first
become cool; and the rays of light are then best sifted, producing a more
distinct and darker red. As the sun dipped lower, the particles overhead
became a turn larger, and thereby better reflected the red rays.
Accordingly, the roseate bands in the east spread over to the zenith, and
passed over to the west, producing in a few minutes a universal
transformation glow.

To produce the full effect often witnessed, there must be, besides the
ordinary dust-particles, small crystals floating in the air, which
increase the reflection from their surfaces and enhance the glow effects.
In autumn, after sunset, the water-covered dust-particles become frozen
and the red light streams with rare brilliancy, causing all reddish and
coloured objects to glow with a rare brightness. Then the air glows with a
strange light as of the northern dawn. From all this it is clear that,
though the colouring of sunset is produced by the direct rays of the sun,
the afterglow is produced by reflection, or, rather, radiation from the
illuminated particles near the horizon.

The effect in autumn is a stream of red light, of varied tones, and rare
brilliancy in all quarters, unseen during the warmer summer. We have to
witness the sunsets at Ballachulish to be assured that Waller Paton really
imitated nature in the characteristic bronze tints of his richly painted
landscapes.




CHAPTER XIX

A WINTER FOREGLOW


Little attention has been paid to foreglows compared with afterglows,
either with regard to their natural beauty or their weather forecasting.
But either the ordinary red-cloud surroundings at sunrise, or the western
foreglow at rarer intervals, betokens to the weather-prophet wet and
gloomy weather. The farmer and the sailor do not like the sight, they
depend so much on favourable weather conditions.

Of course, sunrise to the æsthetic observer has always its charms. The
powerful king of day rejoices "as a bridegroom coming out of his chamber"
as he steps upon the earth over the dewy mountain tops, bathing all in
light, and spreading gladness and deep joy before him. The lessening
cloud, the kindling azure, and the mountain's brow illumined with golden
streaks, mark his approach; he is encompassed with bright beams, as he
throws his unutterable love upon the clouds, "the beauteous robes of
heaven." Aslant the dew-bright earth and coloured air he looks in
boundless majesty abroad, touching the green leaves all a-tremble with
gold light.

But glorious, and educating, and inspiring as is the sunrise in itself in
many cases, there is occasionally something very remarkable that is
connected with it. Rare is it, but how charming when witnessed, though
till very recently it was all but unexplained. This is the _foreglow_.

It is in no respect so splendid as the afterglow succeeding sunset; but,
because of its comparative rarity, its beauty is enhanced. I remember a
foreglow most vividly which was seen at my manse, in Strathmore, in
January 1893. My bedroom window looked due west; I slept with the blind
up. On that morning I was struck, just after the darkness was fading away,
with a slight colouring all along the western horizon. The skeleton
branches of the trees stood out strongly against it. The colouring
gradually increased, and the roseate hue stretched higher. The old
well-known faces that I used to conjure up out of the thin blended boughs
became more life-like, as the cheeks flushed. There was rare warmth on a
winter morning to cheer a half-despairing soul, tired out with the long
hours of oil reading, and pierced to the heart by the never-ceasing
rimes; yet I could not understand it.

I went to the room opposite to watch the sunrise, for I had observed in
the diary that the appearance of the sun would not be for a few minutes.
There were streaks of light in the east above the horizon, but no colour
was visible. That hectic flush--slight, yet well marked--which was
deepening in the western heavens, had no counterpart in the east, except
the colourless light which marked the wintry sun's near approach. As soon
as the sun's rays shot up into the eastern clouds, and his orb appeared
above the horizon, the western sky paled, the colour left it, as if
ashamed of its assumed glory. A foreglow like that I have very rarely
seen, and its existence was a puzzle to me till I studied Dr. Aitken's
explanation of the afterglows after sunset. I had never come across any
description of a foreglow; and, of course, across no explanation of the
curious phenomenon. The western heavens were coloured with fairly bright
roseate hues, while the eastern horizon was only silvery bright before the
sun rose; whereas, after the sun appeared and coloured the eastern hills
and clouds, the western sky resumed its leaden grey and colourless
appearance. Why was that? What is the explanation?

I have not space enough to repeat the explanation given already in the
last chapter of the glorious phenomenon of the afterglow. But the
explanation is similar. Before sunrise, the rays of the sun are reflected
by dust-particles in the zenith to the western clouds. The colouring is
intensified by the frozen water-vapour on these particles in the west.

One thing I carefully noted. Ere mid-day, snow began to fall, and for some
days a severe snow-storm kept us indoors. Then, at any rate, the foreglow
betokened a coming storm. It was, like a rainbow in a summer morning, a
decided warning of the approaching wet weather.




CHAPTER XX

THE RAINBOW


The poet Wordsworth rapturously exclaimed--

  "My heart leaps up when I behold
  A rainbow in the sky."

And old and young have always been enchanted with the beautiful
phenomenon. How glorious is the parti-coloured girdle which, on an April
morning or September evening, is cast o'er mountain, tower, and town, or
even mirrored in the ocean's depths! No colours are so vividly bright as
when this triumphal arch bespans a dark nimbus: then it unfolds them in
due prismatic proportion, "running from the red to where the violet fades
into the sky."

A plain description of the formation of the rainbow is not very easily
given, but a short sketch may be useful. Beautiful as is the ethereal bow,
"born of the shower and colour'd by the sun," yet the marvellous effect is
more exquisitely intensified in its gorgeous display when the hand of
science points out the path in which the sun's rays, from above the
western horizon, fall on the watery cloud, indicating fine weather--"the
shepherd's delight."

One law of reflection is that, when a ray of light falls on a plane or
spherical surface, it goes off at the same angle to the surface as it
fell. One law of refraction is that, when a ray of light passes through
one medium and enters a denser medium (as from air to water), it is bent
back a little. By refraction you see the sun's rays long after the sun has
set; when the sun is just below the horizon, an observer, on the surface
of the earth, will see it raised by an amount which is generally equal to
its apparent diameter.

The rays of different colours are bent back (when passing through the
water) at different rates, some slightly, others more, from the red to the
violet end. The rainbow, then, is produced by refraction and reflection of
the several coloured rays of sunlight in the drops of water which make up
falling rain.

The sun is behind the observer, and its rays fall in a parallel direction
upon the drops of rain before him. In each drop the light is dispersively
refracted, and then reflected from the farther face of the drop; it
travels back through the drop, and comes out with dispersing colours.

According to the height of the sun, or the slope of its rays, a higher or
lower rainbow will be formed. And, strange, no two people can see the very
same bow; in fact the rainbow, as seen by the one eye, is not formed by
the same water-drops as the rainbow seen by the other eye.

When the primary bow is seen in most vivid colours on a dark cloud, a
second arch, larger and fainter, is often seen. But the order of the
colours is quite reversed. At a greater elevation, the sun's ray enters
the lower side of a drop of rain-water, is refracted, reflected _twice_,
and then refracted again before being sent out to the observer's eye. That
is why the colours are reversed.

_A one-coloured rainbow_ is a curious and rare phenomenon. It is a strange
paradox, for the very idea of a rainbow brings up the seven colours--red,
orange, yellow, green, blue, indigo, and violet. Yet Dr. Aitken tells us
of a rainbow with one colour which he observed on Christmas Day, in 1888.

He was taking his walk on the high ground south of Falkirk. In the east he
observed a strange pillar-like cloud, lit up with the light of the setting
sun. Then the red pillar extended, curved over, and formed a perfect arch
across the north-eastern sky. When fully developed, this rainbow was the
most extraordinary one which he had ever seen. There was no colour in it
but red. It consisted simply of a red arch, and even the red had a
sameness about it.

Outside the rainbow there was part of a secondary bow. The Ochil Hills
were north of his point of observation. These hills were covered with
snow, and the setting sun was glowing with rosy light. Never had he seen
such a depth of colour as was on them on this occasion. It was a deep,
furnacy red. The sun's light was shorn of all the rays of short-wave
length on its passage through the atmosphere, and only the red rays
reached the earth. The reason why the Ochils glowed with so deep a red
was owing to their being overhung by a dense curtain of clouds, which
screened off the light of the sky. The illumination was thus principally
that of the direct softer light of the sun.




CHAPTER XXI

THE AURORA BOREALIS


He must be a very careless observer who has not been struck with the
appearance of the streamers which occasionally light up the northern
heavens, and which farmers consider to be indicators of strong wind or
broken weather.

The time was when the phenomenon was considered to be supernatural and
portentous, as the chroniclers of spectral battles, when "fierce, fiery
warriors fought upon the clouds, in ranks and squadrons, and right form of
war." And even in the rural districts of Britain, the blood-coloured
aurora, of October 24th, 1870, was considered to be the reflection of an
enormous Prussian bonfire, fed by the beleaguered French capital.

In joyful spirit, the Shetlanders call the beautiful natural phenomenon,
"Merry Dancers." Burns associated their evanescence with the
transitoriness of sensuous gratification:--"they flit ere you can point
their place." And Tennyson spoke of his cousin's face lit up with the
colour and light of love, "as I have seen the rosy red flushing in the
northern night."

Yet this phenomenon is to a great extent under the control of cosmical
laws. One of the most difficult problems of our day has been to
disentangle the irregular webwork of auroræ, and bring them under a law of
periodicity, which depends upon the fluctuations of the sun's photosphere
and the variations on the earth's magnetism, and which have such an
important influence upon the fluctuations of the weather.

The name "Aurora Borealis" was given to it by Gassendi in 1621.
Afterwards, the old almanacs described it as the "Great Amazing Light in
the North." In the Lowlands of Scotland, the name it long went by, of
"Lord Derwentwater's Lights," was given because it suddenly appeared on
the night before the execution of the rebel lord. In Ceylon auroræ were
called "Buddha Lights."

The first symptom of an aurora borealis is commonly a low arch of pale,
greenish-yellow light, placed at right angles to the magnetic meridian.
Sometimes rays cover the whole sky, frequently showing tremulous motion
from end to end; and sometimes they appear to hang from the sky like the
fringes of a mantle. They are among the most capricious of natural
phenomena, so full of individualities and vagaries. To the glitter of
rapid movement they add the charm of vivid colouring. It is strongly
asserted that auroræ are preceded by the same general phenomena as
thunder-storms. This was borne out by Piazzi Smith (late Astronomer-Royal
for Scotland), who observed that their monthly frequency varies inversely
with that of thunder-storms--both being safety-valves for the discharge of
surplus electricity.

Careful observers have, moreover, noticed a remarkable coincidence
between the display of auroræ and the maxima of the sun's spots and of the
earth's magnetic disturbances. Some have supposed that the light of the
aurora is caused by clouds of meteoric dust, composed of iron, which is
ignited by friction with the atmosphere. But there is this difficulty in
the way, shooting stars are more frequent in the morning, while the
reverse is the case with the aurora. The highest authorities have
concluded, pretty uniformly, that auroræ are electric discharges through
highly rarefied air, taking place in a magnetic field, and under the sway
of the earth's magnetic induction. They are not inappropriately called
"Polar lightnings," for when electricity misses the one channel it must
traverse the other.

The natives of the Arctic regions of North America pretend to foretell
wind by the rapidity of the motions of the streamers. When they spread
over the whole sky, in a uniform sheet of light, fine weather ensues.
Fitzroy believed that auroræ in northern latitudes indicated and
accompanied stormy weather at a distance. The same idea is still current
among many farmers and fishermen in Scotland.

Is there any audible accompaniment to the brilliant spectacle? The natives
of some parts, with subtle hearing-power, speak of the "whizzing" sound
which is often heard during auroral displays. Burns tells of their
"hissing, eerie din," as echoes of the far-off songs of the Valkyries.
Perhaps the most striking incident which corroborates this opinion
occurred during the Franco-Prussian War. Rolier, a practised aëronaut,
left Paris in a balloon, on his mission of city defence, and fourteen
hours afterwards landed in Norway. He had reached the height of two and a
half miles. When descending, he passed through a peculiar cloud of
sulphurous odour, which emitted flashed light and a slight scratching or
rustling noise. On landing, he witnessed a splendid aurora borealis. He
must, therefore, have passed through a cloud in which an electrical
discharge of an auroral nature was proceeding, accompanied with an audible
sound. There is, moreover, no improbability of such sounds being
occasionally heard, since a somewhat similar phenomenon accompanies the
brush discharge of the electric machinery, to which the aurora bears
considerable resemblance.

Though no fixed conclusions are yet established about the causes of the
brilliant auroral display, yet, as the results of laborious observations,
we are assured that the stabler centre of our solar system holds in its
powerful sway the several planets at their respective distances, supplying
them all with their seasonable light and heat, vibrating sympathetic
chords in all, and even controlling under certain--though to us still
unknown--laws the electric streamers that flit, apparently lawlessly, in
the distant earth's atmosphere.




CHAPTER XXII

THE BLUE SKY


If we look at the sky overhead, when cloudless in the sunshine, we wonder
what gives the air such a deep-blue colour. We are not looking, as
children seem to do, into vacancy, away into the far unknown. And even, if
that were the case, would not the space be quite colourless? What, then,
produces the blueness?

Some of the very fine dust-particles, even when clothed with an
exceedingly thin coating of water-vapour, are carried very high; and,
looking through a vast accumulation of these, we find the effect of a
deep-blue colour.

Why so? Because these particles are so small that they can only reflect
the rays of the blue end of the spectrum; and the higher we ascend, the
smaller are the particles and the deeper is the blue. But it is also
because water, even in its very finest and purest form, is blue in colour.
For long this was disputed. Even Sir Robert Christison concluded, after
years of experimenting on Highland streams, that water was colourless.

Of course, he admitted that the water in the Indian and Pacific Oceans has
frequent patches of red, brown, or white colour, from the myriads of
animalcules suspended in the water. Ehrenberg found that it was vegetable
matter which gave to the Red Sea its characteristic name. But these, and
similar waters, are not pure.

It is to Dr. Aitken that the final discovery of the real colour of water
is due. When on a visit to several towns on the shores of the
Mediterranean, he set about making some very interesting experiments,
which the reader will follow with pleasure.

It is a well-known fact that colour transmitted through different bodies
differs considerably from colour reflected by them. In his first
experiment he took a long empty metal tube, open at one end, and closed
at the other end by a clear-glass plate. This was let down vertically into
the water, near to a fixed object, which appeared of most beautiful deep
and delicate blue at a depth of 20 feet. Scientific men know that, if the
colour of water is due to the light reflected by extremely small particles
of matter suspended in the water, then the object looked at through it
would have been illuminated with yellow (the complementary colour of
blue). A blackened tube was then filled with water (which had a
clear-glass plate fixed to the bottom), and white, red, yellow, and purple
objects were sunk in the water, and these colours were found to change in
the same way as if they were looked at through a piece of pale-blue glass.
The white object appeared blue, the red darkened very rapidly as it sank,
and soon lost its colour; at the depth of seven feet a very brilliant red
was so darkened as to appear dark brick-red. The yellow object changed to
green, and the purple to dark blue.

But, still further to satisfy himself that water is really blue in itself,
even without any particles suspended in it, he tested the colour of
_distilled_ water. He filled a darkened tube with this water (clear-glass
plates being at the ends of the tube), and looked through it at a white
surface. The effect was the same as before, the colour was blue, almost
exactly of the same hue as a solution of Prussian blue.

This is corroborated by the fact that, the purer the water is in nature,
the bluer is the tint when a large quantity is looked through. Some
Highland lochs have crystal waters of the most extraordinary blue. Of
course, some cling to the old idea that this is accounted for by the
reflected blue of the clear heavens above. No doubt, if the sky be deep
blue, then this blue light, when reflected by the surface of the water,
will enrich and deepen the hue. But the water itself is _really_ blue.

At the same time, the dust-particles suspended in the water have a great
effect in making the water appear more beautiful, brilliant, and varied in
its colouring; because little or no light is reflected by the interior of
a mass of water itself. If a dark metal vessel be filled with a weak
solution of Prussian blue, the liquid will appear quite dark and void of
colour. But throw in some fine white powder, and the liquid will at once
become of a brilliant blue colour. This accounts for the change of depth
and brilliancy of colour in the several shores of the Mediterranean.

When, then, you look at the face of a deep-blue lake on a summer
evening--the heavens all aglow with the unrivalled display of colour from
the zenith, stretching in lighter hues of glory to the horizon--though to
you the calm water appears like a lake of molten metal glowing with
sky-reflected light, so powerful and brilliant as entirely to overpower
the light which is internally reflected, yet blue is the normal colour of
the water: _blueness is its inherent hue_.

Looking upwards, we observe three distinct kinds of blue in the sky from
the horizon to the zenith. All painters in water-colours know that. Newton
thought that the colour of the sky was produced in the same way as the
colours in thin plates, the order of succession of the colours gradually
increasing in intensity.




CHAPTER XXIII

A SANITARY DETECTIVE


The impure state of the air in the rooms of a house can now be determined
by means of colour alone. Dr. Aitken has invented a very simple instrument
for that purpose; and this ought to be of great service to sanitary
officers. It is called the koniscope--or dust-detective.

The instrument consists of an air-pump and a metal tube with glass ends.
Near one end of the test-tube is a passage by which it communicates with
the air-pump, and near the other end is attached a stop-cock for admitting
the air to be tested. It is not nearly so accurate as the dust-counter;
but it is cheaper, more easily wrought, and more handy for quick work. All
the grades of blue, from what is scarcely visible to deep, dark blue, may
be attached alongside the tube on pieces of coloured glass; and opposite
these colours are the numbers of dust-particles in the cubic inch of the
similar air, as determined by the dust-counter.

While the number of particles was counted by means of the dust-counter,
the depth of blue given by the koniscope was noted; and the piece of glass
of that exact depth of blue attached. A metal tube was fitted up
vertically in the room, in such a way that it could be raised to any
desired height into the impure air near the ceiling, so that supplies of
air of different degrees of impurity might be obtained. To produce the
impurity, the gas was lit and kept burning during the experiments. The air
was drawn down through the pipe by means of the air-pump of the koniscope,
and it passed through the measuring apparatus of the dust-counter on its
way to the koniscope. It may be remarked that, by a stroke of the
air-pump, the air within the test-tube is rarefied and the dust-particles
seize the moisture in the super-saturated air to form fog-particles;
through this fog the colour is observed, and the shade of colour
determines the number of dust-particles in the air. These colours are
named "just visible," "very pale blue," "pale blue," "fine blue," "deep
blue," and "very deep blue."

When making a sanitary inspection, the pure air should be examined first,
and the colour corresponding to that should be considered as the normal
health colour. Any increase from the depth would indicate that the air was
being gradually contaminated; and the amount of increase in the depth of
colour would indicate the amount of increase of pollution.

As an illustration of what this instrument can detect, a room of 24 by 17
by 13 feet was selected. The air was examined before the gas was lighted,
and the colour in the test-tube was very faint, indicating a clear
atmosphere. In all parts of the room this was found the same. A small tube
was attached to the test-tube, open at the other end, for taking air from
different parts of the room. Three jets of gas were then lit in the centre
of the room, and observations at once begun with the koniscope.

Within thirty-five seconds of striking the match to light the gas, the
products of combustion had extended near the ceiling to the end of the
room; this was indicated by the colour in the koniscope suddenly becoming
a deep blue. In four minutes the deep-blue-producing air was got at a
distance of two feet from the ceiling. In ten minutes there was strong
evidence of the pollution all through the room. In half-an-hour the
impurity at nine feet from the floor was very great, the colour being an
intensely deep blue.

The wide range of the indications of the instrument, from pure clearness
to nearly black blue, makes the estimate of the impurity very easily taken
with it; and, as there are few parts to get out of order, it is hoped it
may come into general use for sanitary work.




CHAPTER XXIV

FOG AND SMOKE


Just two hundred and forty years ago, Mr. John Evelyn, F.R.S., a
well-known writer on meteorology, sent a curious tract to King Charles
II., which was ordered to be printed by his Majesty. It was entitled
"Fumifugium," and dealt with the great smoke nuisance in London. I find
from the thesis that he had a very hazy idea of the connection between fog
and smoke; and no wonder, for it is only lately that the connection has
been fully explained.

We know that without dust-particles there can be no fog, and that smoke
supplies a vast amount of such particles. Therefore, in certain states of
the atmosphere, the more smoke the more fog. In Mr. Evelyn's day the fog,
which he called "presumptuous smoake," was at times so dense that men
could hardly discern each other for the "clowd." His Majesty's only sister
had complained of the damage done to her lungs by the contamination, and
Mr. Evelyn was disgusted at the apathy of the people to do anything to
remedy the nuisance. He deplored that that glorious and ancient city of
London should wrap her stately head in "clowds of smoake, so full of stink
and darknesse." He was of opinion that a method of charring coal so as to
divest it of its smoke, while leaving it serviceable for many purposes,
should be made the object of a very strict inquiry. And he was right. For
it is now known that fog in a town is intensified by much smoke.

In a city like London or Glasgow, where a great river, fed by warm streams
of water from gigantic works, passes through its centre, fogs can never be
entirely obliterated, for the dust-particles in the air (often four
millions and upwards in the cubic inch) will seize with terrible avidity
the warm vapour rising from the river. That is the main reason why fogs
cannot there be put down. Smoke is being consumed to a great extent; yet
we find particles of sulphur remaining, which seize the warm vapour and
form fogs dense enough to check all traffic. The worst form of city fogs
seems to be produced when the air, after first flowing slowly in one
direction, then turns on its tracks and flows back over the city,
bringing with it a black pall, the accumulated products of previous days,
to which gets added the smoke and other impurities produced at the time.

What irritated Mr. Evelyn was that, outside of London, the air was clear
when passengers could not walk in safety within the city. So vexed was he
about the contamination, that he made it the occasion of all the "cathars,
phthisicks, coughs, and consumption in the city." He appealed to common
sense to testify that those who repair to London soon take some serious
illness. "I know a man," he said, "who came up to London and took a great
cold, which he could never afterwards claw off again."

Mr. Evelyn proposed that, by an Act of Parliament, the nuisance be
removed; enjoining that all breweries, dye-works, soap and salt boilers,
lime-burners, and the like, be removed five or six miles distant from
London below the river Thames. That would have materially helped his
cause.

But there is more difficulty in the purification than he anticipated. Yet
there was pluck in the old man pointing out the killing contamination and
suggesting a possible remedy. He had the fond idea that thereby a certain
charm, "or innocent magick," would make a transformation scene like
Arabia, which is therefore "styl'd the Happy, attracting all with its gums
and precious spices." In purer air fogs would be less dense, breathing
would be easier, business would be livelier, life would be happier.

Few, I suppose, have laid their hands on this curious Latin thesis, or its
quaint translation, directing the King's attention to the fogs that were
ruining London. Since that time the city has increased, from little more
than a village, to be the dwelling-place of six millions of human beings,
yet too little improvement has been made in the removal of this fog
nuisance. King Edward's drive through London would be even more dangerous
on a muggy, frosty day than was Charles II.'s, when science was little
known.




CHAPTER XXV

ELECTRICAL DEPOSITION OF SMOKE


A good deal of scientific work is being done in the way of clearing away
fog and smoke; and this, through time, may have some practical results in
removing a great source of annoyance, illness, and danger in large towns.
Sir Oliver Lodge and Dr. Aitken have been throwing light upon the
deposition of smoke in the air by means of electricity.

If an electric discharge be passed through a jar containing the smoke from
burnt magnesium wire, tobacco, brown paper, and other substances, the dust
will be deposited so as to make the air clear. Brush discharge, or
anything that electrifies the air itself, is the most expeditious.

If water be forced upwards through a vertical tube (with a nozzle
one-twentieth of an inch in diameter), it will fall to the ground in a
fine rain; but, if a piece of rubbed (electrified) sealing-wax be held a
yard distant from the place where the jet breaks into drops, they at once
fall in large spots as in a thunder-shower. If paper be put on the ground
during the experiment, the sound of pattering will be observed to be
quite different. If a kite be flown into a cloud, and made to give off
electricity for some time, that cloud will begin to condense into rain.

Experiments with Lord Kelvin's recorder show that variations in the
electrical state of the atmosphere precede a change of weather. Then, with
a very large voltaic battery, a tremendous quantity of electricity could
be poured into the atmosphere, and its electrical condition could be
certainly disturbed. If this could be made practically available, how
useful it would be to farmers when the crops were suffering from excessive
drought! It might be more powerfully available than the imagined
condensation of a cloud into rain by the reverberation caused by the
firing of a range of cannon.

But what is the practical benefit of this information? If electricity
deposits smoke, it might be made available in many ways. The fumes from
chemical works might be condensed; and the air in large cities, otherwise
polluted, might be purified and rendered innocuous. The smoke of chimneys
in manufacturing works might be prevented from entering the atmosphere at
all. In flour-mills and coal-mines the fine dust is dangerously explosive.
In lead, copper, and arsenic works, it is both poisonous and valuable.

Lead smelters labour under this difficulty of condensing the fume which
escapes along with the smoke from red-lead smelting furnaces; and it was
considered that an electrical process of condensation might be made
serviceable for the purpose. At Bagillt, the method used for collecting or
condensing the lead fume is a large flue two miles long; much is retained
in this flue, but still a visible cloud of white-lead fume continually
escapes from the top of the chimney. There is a difficulty in the way of
depositing fumes in the flue by means of a sufficient discharge of
electricity, viz. the violent draught which is liable to exist there, and
which would mechanically blow away any deposited dust.

But Dr. Aitken suggests that regenerators might be used along with the
electricity. The warm fumes might be taken to a cold depositor, where (by
the ordinary law of cold surfaces attracting warm dust-particles) the
impurities would be removed, and, when purified, the air would again be
taken through a hot regenerator before being sent up the chimney. By a
succession of these chambers, with the assistance of electric currents,
the air, impregnated with the most deleterious particles, or valuable
dust, could be rendered innocuous.

The sewage of our towns must be cleaned of its deleterious parts before
being run into the streams which give drink to the lower animals, because
an Act of Parliament enforces the process. Why, then, ought we not to have
similar compulsion for making the smoke from chemical and other noxious
works quite harmless before being thrown into the air which contains the
oxygen necessary for the life of human beings?

There seems to be a good field before electricians to catch the smoke on
the wing and deposit its dust on a large scale. This seems to be a matter
beyond our reach at present, except in the scientist's laboratory; but
certainly it is a "consummation devoutly to be wished."




CHAPTER XXVI

RADIATION FROM SNOW


One night a most interesting paper by Dr. Aitken, on "Radiation from
Snow," was read by Professor Tait to the Fellows of the Royal Society of
Edinburgh. I remember that Dr. Alex. Buchan--the greatest meteorologist
living--spoke afterwards in the very highest terms of the subject-matter
of the paper. This was corroborated by Lord Kelvin, Lord MacLaren, and
Professor Chrystal.

Dr. Aitken had been testing the radiating powers of different substances.
Snow in the shade on a bright day at noon is 7° Fahr. colder than the air
that floats upon it, whereas a black surface at the same is only 4°
colder. This difference diminishes as the sun gets lower; and at night
both radiate almost equally well.

I select, among the careful and numerous observations, the notes on
January 19, 1886; for I took note of the cold of that day in my diary. It
was the coldest day of the whole of that winter. The barometer was 28·8
inches, and the thermometer 4°--that is, 28° of frost. According to Dr.
Buchan, that January had only two equal in average cold for fifty years.

On January 19, at 10 A.M., when the air was at 20° and the sky clear, a
black surface registered 16° and the upper layer of snow 12°, showing a
difference of 4° when both surfaces were colder than the superincumbent
air. It is curious to note that, on February 5 of the same year, at the
same hour, when the sky was overcast, the air was at 23°, the black
surface registered 29°, and the snow 25°, showing again the difference of
4°; but, in this case, both surfaces were warmer than the air. In both
cases the radiation at night was equal.

This small absorbing power of snow for heat reflected and radiated from
the sky during the day must have a most important effect on the
temperature of the air. The temperature of lands when covered with snow
must be much lower than when free from it. And, when once a country has
become covered with snow, there will be a tendency towards glacial
conditions.

But, besides being a bad absorber of heat from the sky, snow is also a
very poor conductor of heat. On that very cold night (January 18), when
there was a depth of 5-1/2 inches of snow on the ground, and the night
clear, with strong radiation, the temperature of the surface of the snow
was 3° Fahr., and a minimum thermometer on the snow showed that it had
been down to zero some time before. A thermometer, plunged into the snow
down to the grass, gave the most remarkable register of 32°. Through the
depth of 5-1/2 inches of snow there was a difference of temperature of
29°. This was confirmed by removing the snow, and finding that the grass
was unfrozen. As the ground was frozen when the snow fell, it would appear
that the earth's heat slowly thawed it under the protection of the snow.

The protection afforded by the bad-conducting power of snow is of great
importance in the economy of nature. How vegetation would suffer, were it
exposed to a low temperature, unprotected by the snow-mantle! So that,
though the continued snow cools the air for animals that can look after
their own heating, it keeps warm the soil; and vegetation prospers under
the genial covering. The fine rich look of the young wheat-blades, after a
continued snow has melted, must strike the most careless observer. Instead
of the half-blackened tips and semi-sickly blades, which we see in a field
of young wheat after a considerable course of dry frost without snow, we
have a rich, healthy green which shows the vital energy at work in the
plants. Or even in the town gardens, after a continued snow has been
melted away by a soft, western breeze, we are struck with the white,
peeping buds of the snowdrop and the finely springing grass in the sward.

Yet the snow-covering gives durability to cold weather. This has been
demonstrated by Dr. Woeikof, the distinguished Russian meteorologist. On
this account the spring months of Russia and Siberia are intensely cold.
The plants, then, which in winter are unable by locomotion to keep
themselves in health, are protected by the snow-mantle which chills the
air for animals that can keep themselves in heat by exercise. What a grand
compensating power is here!




CHAPTER XXVII

MOUNTAIN GIANTS


Some mysterious physical phenomena can be clearly explained by the aid of
science. The mountain giants that at times haunt the lonely valleys, and
strike with fear the superstitious dwellers there, are only the enlarged
shadows of living human beings cast upon a dense mist.

The two most startling of these "eerie" phenomena are the spectres of
Adam's Peak and the Brocken.

The phenomena sometimes to be observed at Adam's Peak, in Ceylon, are very
remarkable. Many travellers have given vivid accounts of these. On one
occasion the Hon. Ralph Abercromby, in his praiseworthy enthusiasm for
meteorological research, went there with two scientific friends to witness
the strange appearance. The conical peak, a mile and a half high,
overlooks a gorge west of it. When, then, the north-east monsoon blows the
morning mist up the valley, light wreaths of condensed vapour pass to the
right of the Peak, and catch the shadows at sunrise.

This party reached the summit early one morning in February. The foreglow
began to brighten the under-surface of the stratus-cloud with orange, and
patches of white mist filled the hollows. Soon the sun peeped through a
chink in the clouds, and they saw the pointed shadow of the Peak lying on
the misty land. Then a prismatic circle, with the red inside, formed round
the shadow. The meteorologist waved his arms about, and immediately he
found giant shadowy arms moving in the centre of the rainbow.

Soon they saw a brighter and sharper shadow of the Peak, encircled by a
double bow, and their own spectral arms more clearly visible. The shadow,
the double bow, and the giant forms, combined to make this phenomenon the
most marked in the whole world.

The question has been frequently asked: Why are such aërial effects not
more widely observed? There are not many mountains of this height and of a
conical shape; and still fewer can there be where a steady wind, for
months together, blows up a valley so as to project the rising morning
mist at a suitable height and distance on the western side, to catch the
shadow of the peak at sunrise.

The most famous place in Europe for witnessing the awe-inspiring
phenomenon is the Brocken, in Germany--3740 feet in height. The only great
disappointment there is that the conditions rarely combine at sunrise or
sunset to have "the spectre" successful.

In July 1892, my daughter and I were spending some weeks at Harzburg, and,
of course, we had to visit the Brocken and take stock of the world-known
phenomenon. At mid-day, the air at the flat summit was cold, clear, and
hard. The boulders are of enormous size; and near the "Noah's Ark" Hotel
and Observatory many are piled up in a mass, on which the observers stand
at the appointed time for having their shadows projected on the misty air
in the valleys.

At five o'clock in the afternoon the sky was brilliantly clear on the
summit of the Brocken; but the wind was rising from the sun's direction,
and the mist was filling up the wide-spread eastern valley. We stood on
the "spectre" boulders, and our shadows were thrown on the grass, just as
at home. However, they fell upon large patches of white heather, which
there is very plentiful.

At six o'clock the sun was still shining beautifully, and we anxiously
waited for the time when it would be low enough to raise our shadows to
the misty wall. An hour afterwards, a hundred visitors were out, and many
of us were on the "spectre" stones. There was great excitement in
anticipation of the weird appearances, which had attracted us from such a
distance.

But, almost at the moment of success, the sun descended behind a belt of
purple cloud, and all we saw was part of a rainbow on the misty hollow.
For the sun never appeared again. This was intensely saddening, seeing
that, but for that stratum of cloud above the horizon, the phenomenon
would have been graphically displayed.

The cold became suddenly intense, and we had to sleep with a freezing mist
enveloping the hotel. In vain did we wait for the wakening call, to tell
us of sunrise; for the sun could not pierce the mist, and we had to return
home disappointed.

Sometimes the rainbow colours assume the shapes of crosses instead of
circles. Occasionally a bright halo will be seen above the shadow-head of
the observer, concentric rainbows enclosing all. In some recorded cases
the grand effect must have been simply glorious.

Scientific observation has done much to dispel the superstition which has
clung so tenaciously to the Highland mind. The lonely grandeur of the
weird mountain giants has been clearly explained as perfectly natural, yet
the awe-striking feeling cannot be entirely driven off.




CHAPTER XXVIII

THE WIND


Once was the remark pointedly made: "The wind bloweth where it listeth."
And that is nearly true still. The leading winds are under the calculation
of the meteorologist, but the others will not be bound by laws.

Yet there are instruments for measuring the velocity and force of the
wind, after it is on; but "whence it comes" is a different matter. A
gentle air moves at the rate of 7 miles an hour; a hurricane from 80 to
150 miles, pressing with 50 lbs. on the square foot exposed to its fury.
Some of the gusts of the Tay Bridge storm, in 1879, had a velocity of 150
miles an hour, with a pressure of 80 to 90 lbs. to the square foot.

Before steamers supplanted so many sailing vessels, seamen required to be
always on the alert as to the direction and strength of the wind, and the
likelihood of any sudden change; and they chronicled twelve different
strengths from "faint air" to a "storm."

In general, the wind may be considered to be the result of a change of
pressure and temperature in the atmosphere at the same level. The air of a
warmer region, being lighter, ascends, and gives place to a current of
wind from a colder region. These two currents--the higher and the
lower--will continue to blow until there is equilibrium.

The trade winds are regular and constant. These were much followed in the
days of old. A vast amount of air in the tropics gets heated and ascends,
being lighter, and travels to the colder north. A strong current rushes in
from the north to take its place. But the earth rotates round its axis
from west to east, and the combined motions make two slant wind
directions, which are called the "trade winds," because they were so
important in trade navigation.

Among the periodical winds are the "land and sea breezes." During the day,
the land on the sea coast is warmer than the sea; accordingly, the air
over the land becomes heated and ascends, the fine cool breeze from the
sea taking its place. Towards evening there is the equilibrium of
temperature which produces a temporary calm. Soon the earth chills, and
the sea is counterbalancingly warm--as sea-water is steadier as to
temperature than is land--the air over the sea becomes warmer, and
ascends, the current from the land rushing in to take its place. Hence
during the night the wind is reversed, until in the morning again the
equilibrium is restored and there is a calm, so far as these are
concerned. These are therefore called the "land and sea breezes." Of
course, it is within the tropics that these breezes are most marked. By
the assistance of other winds, a hurricane will there occasionally destroy
towns and bring about much damage and loss of life; but better that
hundreds should perish by a hurricane than thousands by the pestilence
which, but for the storm, would have done its dire work.

In countries where the differences of pressure are more marked than the
differences of temperature, in the surrounding regions the strength of
the wind thereby occasioned is far stronger than the land and sea breezes.

The variable winds are more conflicting. These depend on purely local
causes for a time, such as "the nature of the ground, covered with
vegetation or bare; the physical configuration of the surface, level or
mountainous; the vicinity of the sea or lakes, and the passage of storms."
Among these winds are the simoom and sirocco.

The _east_ winds, which one does not care about in the British Islands
during the spring time, are occasioned by the powerful northern current
which rushes south from the northern regions in Europe. Dr. Buchan points
out a very common mistake among even intelligent observers who shudder at
the hard east winds. It is generally held that these winds are damp. They
are unhealthy, but they are dry. It is quite true that many easterly winds
are peculiarly moist; all that precede storms are so far damp and rainy;
and it is owing to this circumstance that, on the east coast of Scotland,
the east winds are searching and carry most of the annual rainfall there.
But all of these moist easterly winds, however, soon turn to some westerly
point. The real east wind, so much feared by invalids, does not turn to
the west; it is exceeding dry. Curious is it that brain diseases, as well
as consumption, reach their height in Britain while east winds prevail.
Once in Edinburgh, during the early spring, I had rheumatic fever, and
during my convalescence my medical adviser, Dr. Menzies, would not let me
have a short drive until the wind changed to the west. The first thing I
anxiously watched in the morning was the flag on the Castle; and for
nearly two months it always waved from the east. How heart-depressing!

Creatures are we in the hands of nature's messengers. We so much depend
upon the weather for our happiness. Joyful are we when the honey-laden
zephyr waves the long grass in June, or when

  "The gentle wind, a sweet and passionate wooer,
  Kisses the blushing leaf."

Compared with this, how terrible is Shakespeare's allusion to the
appalling aspects of the storm:--

  "I have seen tempests, when the scolding winds
  Have rived the knotty oaks; and I have seen
  The ambitious ocean swell, and rage and foam,
  To be exalted with the threat'ning clouds;
  But never till to-night, never till now,
  Did I go through a tempest dropping fire."




CHAPTER XXIX

CYCLONES AND ANTI-CYCLONES


The criticism of the weather in the meteorological column of our daily
newspapers invariably speaks of "cyclones." It is, therefore, advisable to
give as plain an explanation of these as possible. Cyclones are
"storm-winds." Their nature has to be carefully studied by meteorologists,
who are industriously at work to ascertain some scientific basis for the
atmospheric movements.

What is the cause of the spiral movement in storm-winds? In their centre
the depression of the barometer is lowest, because the atmosphere there is
lightest. As the walls of the spiral are approached, the barometer rises.

Dr. Aitken has ingeniously hit upon an experiment to illustrate a spiral
in air. All that is necessary is a good fire, a free-going chimney, and a
wet cloth. The cloth is hung up in front of the fire, and pretty near it,
so that steam rises readily from its surface; and, when there are no
air-currents in the room, the steam will rise vertically, keeping close to
the cloth. But if the room has a window in the wall, at right angles to
the fireplace, so as to cause the air coming from it to make a
cross-current past the fire, then a cyclone will be formed, and the vapour
from the cloth will be seen circling round. When the cyclone is well
formed, all the vapour is collected into the centre of the cyclone, and
forms a white pillar extending from the cloth to the chimney. This
experiment shows that no cyclone can form without some tangential motion
in the air entering the area of low-pressure.

Now to illustrate the spiral approach. Fill with water a cylindrical glass
vessel, say 15 inches in diameter and 6 inches deep. Have an orifice with
a plug a little from the centre of the bottom. Remove the plug, the water
runs out, passing round the vessel in a vortex form. But, as the passage
between the orifice (or centre of the cyclone) and the temporary division
is narrower than in any other place, the water has to pass this part much
more quickly than at any other place. And this curious result is observed:
the top of the cyclone no longer remains over the orifice, but _travels_
in the direction of the water which is moving most speedily. Similar to
this is the cyclone in the atmosphere; its centre also moves in the
direction of the quickest flowing wind that enters it.

Dr. Aitken is of opinion that, in forecasting storms, too little attention
has been paid to the _anti-cyclones_. They do more than simply follow and
fill up the depression made by the cyclones. They initiate and keep up
their own circulation, and collect the materials with which the cyclones
produce their effect. Neither could work efficiently without the other.

Suppose a large area on the earth over which the air is still in bright
sunshine. After a time, when the air gets heated and charged with vapour,
columns of air would begin to ascend in a disorderly fashion. But suppose
an anti-cyclone is blowing at one side of this area. When the upper air
descends to the earth, it spreads outwards in all directions; but the
earth's rotation interferes and changes the radial into a spiral motion.
The anti-cyclonic winds will prevent the formation of local cyclones, and
drive all the moist, hot air to its circumference, just above the earth.
The anti-cyclone forces its air tangentially into the cyclone, and gives
it its direction and velocity of rotation, also the direction and rate of
travel of the centre of depression. The earth's rotation is the original
source of the rotatory movements, but both intensify the initial motion.

Accordingly, the cyclone must travel in the direction of the strongest
winds blowing into it, just as the vortex in the vessel with the eccentric
orifice travelled in the direction of the quickest moving water. This is
verified by a study of the synoptic charts of the Meteorological Office.

The sun's heat has always been looked upon as the main source of the
energy of our winds, but some account must also be taken of the effects of
cold. It is well known that the mean pressure over Continental areas is
high during winter and low during summer. As the sun's rays during summer
give rise to the cyclonic conditions, so the cooling of the earth during
winter gives rise to anti-cyclonic conditions. It is found during the
winter months in several parts of the Continent that as the temperature
falls the pressure rises, producing anti-cyclones over the cold area;
whereas, when the temperature begins to rise, the pressure falls, and
cyclones are attracted to the warming area.

Small natural cyclones are often seen on dusty roads, the whirling column
having a core of dusty air, and the centre of the vortex travelling along
the road, tossing up the dust in a very disagreeable way to pedestrians.
Sometimes such a cyclone will toss up dry leaves to a height of four or
five feet. They are very common; but it is only when dust, leaves, or
other light material is present that they are visible to the eye.




CHAPTER XXX

RAIN PHENOMENA


The soft rain on a genial evening, or the heavy thunder-showers on a
broiling day, are too well known to be written about. Sometimes rain is
earnestly wished for, at other times it is dreaded, according to the
season, seed-time or harvest. Some years, like 1826, are very deficient in
rainfall, when the corn is stunted and everything is being burnt up; other
years, like 1903, there is an over-supply, causing great damage to
agriculture. The year 1903 will long be remembered for its continuous
rainfall; it is the record year; no year comes near it for the total
rainfall all over the kingdom.

Rain is caused by anything that lowers the temperature of the air below
the dew-point, but especially by winds. When a wind has blown over a
considerable area of ocean on to the land, there is a likelihood of rain.
When this wind is carried on to higher latitudes, or colder parts, there
is a certainty of rain. Of course, in the latter case the rain will fall
heavier on the wind side than on the lee side.

For short periods, the heaviest falls or "plouts" of rain are during
thunder-storms. When the raindrops fall through a broad, cold stratum of
air, they are frozen into hail, the particles of which sometimes reach a
large size, like stones. Of course, water-spouts now and again are of
terrible violence.

One of the heaviest rainfalls yet recorded in Great Britain was about
2-1/4 inches in forty minutes at Lednathie, Forfarshire, in 1887. Now 1
inch deep of rain means 100 tons on an imperial acre; so the amount of
water falling on a field during that short time is simply startling. The
heaviest fall for one day was at Ben Nevis Observatory, being fully 7-1/4
inches in 1890. In other parts of the world this is far exceeded. In one
day at Brownsville, Texas, nearly 13 inches fell in 1886. On the Khasi
hills, India, 30 inches on each of five successive days were registered.
At Gibraltar, 33 inches were recorded in twenty-six hours.

The heaviest rainfalls of the globe are occasioned by the winds that have
swept over the most extensive ocean-areas in the tropics. On the summer
winds the rainfall of India mainly depends; when this fails, there is most
distressing drought. Reservoirs are being erected to meet emergencies.

From Dr. Buchan's statistics it is found that the annual rainfall at
Mahabaleshwar is 263 inches; at Sandoway 214; and at Cherra-pungi 472
inches, the largest known rainfall anywhere on the globe. Over a large
part of the Highlands of Scotland more than 80 inches fall annually, while
in some of the best agricultural districts there it does not exceed 30
inches.

Of all meteorological phenomena, rainfall is the most variable and
uncertain. Symons gives as tentative results from twenty years'
observations in London--(1) In winter, the nights are wetter than the
days; (2) in spring and autumn, there is not much difference; (3) in
summer, nearly half as much again by day as by night.

The wearisomeness of statistics may be here relieved by a short
consideration of the _splash_ of a drop of rain. Watching the
drop-splashes on a rainy day in the outskirts of the city, when unable to
get out, I brought to my recollection the marvellous series of experiments
made by Professor A. M. Worthington in connection with these phenomena. Of
course, I could not see to proper advantage the formation of the
splashes, as the heavy raindrops fell into these tiny lakes on the quiet
road. There is not the effect of the huge thunder-drops in a stream or
pool. The building up of the bubbles is not here perfect, for the domes
fail to close, nor are the emergent columns visible to the naked eye. It
is a pity; for R. L. Stevenson once wrote of them in his delightful
"Inland Voyage," when he canoed in the Belgian canals, as thrown up by the
rain into "an infinity of little crystal fountains."

Beautiful is this effect if one is under shelter, every dome seeming quite
different in contour and individuality from all the rest. But terrible is
it when out fishing on Loch Earn, even with the good-humoured old Admiral,
when the heavy thunder-drops splash up the crystal water, and one gets
soaked to the skin, sportsman-like despising an umbrella.

There is, however, a scientific interest about the splash of a drop. The
phenomenon can be best seen indoors by letting a drop of ink fall upon the
surface of pure water in a tumbler, which stands on white paper. It is an
exquisitely regulated phenomenon, which very ideally illustrates some of
the fundamental properties of fluids.

When a drop of milk is let fall upon water coloured with aniline dye, the
centre column of the splash is nearly cylindrical, and breaks up into
drops before or during its subsequent descent into the liquid. As it
disappears below the surface, the outward and downward flow causes a
hollow to be again formed, up the sides of which a ring of milk is
carried; while the remainder descends to be torn a second time into a
beautiful vortex ring. This shell or dome is a characteristic of all
splashes made by large drops falling from a considerable height, and is
extremely pretty. Sometimes the dome closes permanently over the
imprisoned air, and forms a large bubble floating upon the water. The most
successful experiments, however, have been carried through by means of
instantaneous photography, with the aid of a Leyden-jar spark, whose
duration was less than the ten-millionth of a second. But the simple
experiments, without the use of the apparatus, will while away a few hours
on a rainy afternoon, when condemned to the penance of keeping within
doors.




CHAPTER XXXI

THE METEOROLOGY OF BEN NEVIS


Several large and very important volumes of the Royal Society of Edinburgh
are devoted to statistics connected with the meteorology of Ben Nevis.
Most of the abstracts have been arranged by Dr. Buchan; while Messrs.
Buchanan, Omond, and Rankine have taken a fair share of the work.

This Observatory, as Mr. Buchanan remarks, is unique, for it is
established in the clouds; and the observations made in it furnish a
record of the meteorology of the clouds. It is 4406 feet above the level
of the sea; and as there is a corresponding Observatory at Fort William,
at the base of the mountain, it is peculiarly well fitted for important
observations and weather forecasting. The mountain, too, is on the west
sea-coast of Scotland, exposed immediately to the winds from the Atlantic,
catching them at first hand. It is lamentable to think that, when the
importance of the observations made at the two Observatories was becoming
world known, funds could not be got to carry them on. Ben Nevis is the
highest mountain in the British Islands, best fitted for meteorological
observations; yet these have been stopped for want of money.

Dr. Buchan's valuable papers were published before any one dreamed of the
stoppage of the work, which had such an important bearing on men engaged
in business or taken up with open-air sport. From these I shall sift out a
few facts that even "mute, inglorious" meteorologists may be interested in
knowing.

For a considerable time the importance of the study of the changes of the
weather has come gradually to be recognised, and an additional impetus was
given to the prosecution of this branch of meteorology when it was seen
that the subject had intimate relations to the practical question of
weather forecasts, including storm warnings. Weather maps, showing the
state of the weather over an extensive part of the surface of the globe,
began to be constructed; but these were only indicators from places at the
level of the sea.

The singular advantages of a high-level observatory occurred to Mr. Milne
Home in 1877; and Ben Nevis was considered to be in every respect the most
suitable in this country. The Meteorological Council of the Royal Society
of London offered in 1880, unsolicited, £100 annually to the Scottish
Meteorological Society, to aid in the support of an Observatory, the only
stipulation being that the Council be supplied with copies of the
observations.

From June to October, in 1881, Mr. Wragge made daily observations at the
top of the Ben; and simultaneous observations were made, by Mrs. Wragge,
at Fort William. A second series, on a much more extended scale, was made
in the following summer.

Funds were secured to build an Observatory; and, in November 1883, the
regular work commenced, consisting of hourly observations by night as well
as by day. Until a short time ago, these were carried on uninterruptedly.
Telegraphic communications of each day's observations were sent to the
morning newspapers; and now we are disappointed at not seeing them for
comparison.

The whole of the observations of temperature and humidity were of
necessity eye-observations. For self-registering thermometers were
comparatively useless when the wind was sometimes blowing at the rate of
100 miles an hour. Saturation was so complete in the atmosphere that
everything exposed to it was dripping wet. Every object exposed to the
outside frosts of winter soon became thickly incrusted with ice.
Snowdrifts blocked up exposed instruments. Accordingly, the observers had
to use their own eyes, often at great risks.

The instruments in the Ben Nevis Observatory, and in the Observing Station
at Fort William, were of the best description. Both stations were in
positions where the effects of solar and terrestrial radiation were
minimised. No other pair of meteorological stations anywhere in the world
are so favourably situated as these two stations, for supplying the
necessary observations for investigating the vertical changes of the
atmosphere. It is to be earnestly hoped, therefore, that funds will be
secured to resume the valuable work.

The rate of the decrease of temperature with height there is 1° Fahr. for
every 275 feet of ascent, on the mean of the year. The rate is most rapid
in April and May, when it is 1° for each 247 feet; and least rapid in
November and December, when it is 1° for 307 feet. This rate agrees
closely with the results of the most carefully conducted balloon ascents.
The departures from the normal differences of temperature, but more
especially the inversions of temperature, and the extraordinarily rapid
rates of diminution with height, are intimately connected with the
cyclones and anti-cyclones of North-Western Europe; and form data, as
valuable as they are unique, in forecasting storms.

The most striking feature of the climate of Ben Nevis is the repeated
occurrence of excessive droughts. For instance, in the summer and early
autumn of 1885, low humidities and dew-points frequently occurred.
Corresponding notes were observed at sea-level. During nights when
temperature falls through the effects of terrestrial radiation, those
parts of the country suffer most from frosts over which very dry states of
the air pass or rest; whereas, those districts, over which a more humid
atmosphere hangs, will escape. On the night of August 31 of that year, the
potato crop on Speyside was totally destroyed by the frost; whereas at
Dalnaspidal, in the district immediately adjoining, potatoes were
scarcely--if at all--blackened.

The mean annual pressure at Ben Nevis was 25·3 inches, and at Fort William
29·8, the difference being 4-1/2 inches for the 4400 feet.

For the whole year, the difference between the mean coldest hour, 5 A.M.,
and the warmest hour, 2 P.M., is 2°. For the five months, from October to
February, the mean daily range of temperature varied only from O·6 to 1·5.
This is the time of the year when storms are most frequent; and this small
range in the diurnal march of the temperature is an important feature in
the climatology of Ben Nevis; for it presents, in nearly their simple
form, the great changes of temperature accompanying storms and other
weather changes, which it is so essential to know in forecasting weather.

The daily maximum velocity of the wind occurs during the night, the daily
differences being greatest in summer and least in winter. A blazing sun in
the summer daily pours its rays on the atmosphere, and a thick envelope of
cloud has apparently but little influence on the effect of the sun's rays.
Thunder-storms are essentially autumn and winter phenomena, being rare in
summer.

According to Mr. Buchanan, the weather on Ben Nevis is characterised by
great prevalence of fog or mist. In continuously clear weather it
practically never rains on the mountain at all. In continuously foggy
weather, on the other hand, the average daily rainfall is 1 inch. There is
a large and continuous excess of pressure in clear weather over that of
foggy weather. The mean temperature of the year is 3-1/2 degrees higher
in clear than in foggy weather. In June the excess is 10 degrees. The
nocturnal heating in the winter is very clearly observed. This has been
noticed before in balloons as well as on mountains. The fog and mist in
winter are much denser than in summer. Whether wet or dry, the fog which
characterises the climate of the mountain is nothing but _cloud_ under
another name.




CHAPTER XXXII

THE WEATHER AND INFLUENZA


Some remarkable facts have been deduced by the late Dr. L. Gillespie,
Medical Registrar, from the records of the Royal Infirmary of Edinburgh.
He considered that it might lead to interesting results if the admissions
into the medical wards were contrasted with the varying states of the
atmosphere. The repeated attacks of influenza made him pay particular
attention to the influence of the weather on that disease.

The meteorological facts taken comprise the weekly type of weather, _i.e._
cyclonic or anti-cyclonic, the extremes of temperature for the district
for each week, and the mean weekly rainfall for the same district. More
use is made of the extremes than of the mean, for rapid changes of
temperature have a greater influence on disease than the actual mean.

The period which he took up comprises the seven years 1888-1895. There was
a yearly average of admissions of 3938; so that he had a good field for
observation. Six distinct epidemics of influenza, varying in intensity,
occurred during that period; yet there had been only twenty-three attacks
between 1510 and 1890. Accordingly, these six epidemics must have had a
great influence on the incidence of disease in the same period, knowing
the vigorous action of the poison on the respiratory, the circulatory, and
the nervous systems. The epidemics of influenza recorded in this country
have usually occurred during the winter months.

The first epidemic, which began on the 15th of December 1889 and continued
for nine weeks, was preceded by six weeks of cyclonic weather, which was
not, however, accompanied by a heavy rainfall. Throughout the course of
the disease, the type continued to be almost exclusively cyclonic, with a
heavy rainfall, a high temperature, and a great deficiency of sunshine.
The four weeks immediately following were also chiefly cyclonic, but with
a smaller rainfall.

The summer epidemic of 1891 followed a fine winter and spring, during
which anti-cyclonic conditions were largely prevalent. But the epidemic
was immediately preceded by wet weather and a low barometer. It took place
in dry weather, and was followed by wet, cyclonic weather in turn.

The great winter epidemic of 1891 followed an extremely wet and broken
autumn. Simultaneously with the establishment of an anti-cyclone, with
east wind, practically no rain, and a lowering temperature, the influenza
commenced. Great extremes in the temperature followed, the advent of
warmer weather and more equable days witnessing the disappearance of the
disease.

The fourth epidemic was preceded by a wet period, ushered in by dry
weather, accompanied by great heat; and its close occurred in slightly
wetter weather, but under anti-cyclonic conditions. The fifth outbreak
began after a short anti-cyclone had become established over our islands,
continued during a long spell of cyclonic weather with a considerable
rainfall, but was drowned out by heavy rains. The last appearance of the
modern plague, of which Dr. Gillespie's paper treats, commenced after cold
and wet weather, continued in very cold but drier weather, and subsided in
warmth with a moderate rainfall.

The conditions of these six epidemics were very variable in some respects,
and regular in others. The most constant condition was the decreased
rainfall at the time, when the disease was becoming epidemic.
Anti-cyclonic weather prevailed at the time.

According to Dr. Gillespie, the tables seem to suggest that a type of
weather, which is liable to cause catarrhs and other affections of the
respiratory tract, precedes the attacks of influenza; but that the
occurrence of influenza in _epidemic form_ does not appear to take place
until another and drier type has been established. As the weather changes,
the affected patients increase with a rush.

He is of opinion that the supposed rapid spread of influenza on the
establishment of anti-cyclonic conditions may be explained in this way.
The air in the cyclonic vortex, drawn chiefly from the atmosphere over the
ocean, is moist, and contains none of the contagion; the air of the
anti-cyclone, derived from the higher strata, and thus from distant
cyclones, descending, blows gently over the land to the nearest cyclone,
and, being drier, is more able to carry suspended particles with it. He
considers that temperature has nothing to do with the problem, except in
so far as the different types of weather may modify it. The Infirmary
records point to the occurrence of similar phenomena, recorded on previous
occasions. Accordingly, if such meteorological conditions are not
indispensable to the spread of influenza in epidemic form, they at least
afford favourable facilities for it.




CHAPTER XXXIII

CLIMATE


One is not far up in years, in Scotland at any rate, without practically
realising what climate means. He may not be able to put it in words, but
easterly haars, chilling rimes, drizzling mists, dagging fogs, and
soddening rains speak eloquently to him of the meaning of climate.

Climate is an expression for the conditions of a district with regard to
temperature, and its influence on the health of animals and plants. The
sun is the great source of heat, and when its rays are nearly
perpendicular--as at the Tropics--the heat is greater on the earth than
when the slanted rays are gradually cooled in their passage. As one passes
to a higher level, he feels the air colder, until he reaches the
fluctuating snow-line that marks perpetual snow.

The temperature of the atmosphere also depends upon the radiation from the
earth. Heat is quite differently radiated from a long stretch of sand, a
dense forest, and a wide breadth of water. Strange is it that a newly
ploughed field absorbs and radiates more heat than an open lea. The
equable temperature of the sea-water has an influence on coast towns. The
Gulf Stream, from the Gulf of Mexico, heats the ocean on to the west coast
of Britain, and mellows the climate there.

The rainfall of a district has a telling effect on the climate. Boggy land
produces a deleterious climate, if not malaria. Over the world, generally,
the prevailing winds are grand regulators of the climate in the
distinctive districts. A wooded valley--like the greatest in Britain,
Strathmore--has a health-invigorating power: what a calamity it is, then,
that so many extensive woods, destroyed by the awful hurricane twelve
years ago, are not replanted!

Some people can stand with impunity any climate; their "leather lungs"
cannot be touched by extremes of temperature; but ordinary mortals are
mere puppets in the hands of the goddess climate. Hence health-resorts are
munificently got up, and splendidly patronised by people of means. The
poor, fortunately, have been successful in the struggle for existence, by
innate hardiness, otherwise they would have had a bad chance without ready
cash for purchasing health.

It may look ludicrous at first sight, but it seems none the less true,
that the variation of the spots on the sun have something to do with
climate, even to the produce of the fields. On close examination, with a
proper instrument, the disc of the sun is found to be here and there
studded with dark spots. These vary in size and position day after day.
They always make their first appearance on the same side of the sun, they
travel across it in about fourteen days, and then they disappear on the
other side. There is a great difference in the number of spots visible
from time to time; indeed, there is what is called the minimum period,
when none are seen for weeks together, and a maximum period, when more are
seen than at any other time. The interval between two maximum periods of
sun-spots is about eleven years. This is a very important fact, which has
wonderful coincidences in the varied economy of nature.

Kirchhoff has shown, by means of the spectroscope, that the temperature of
a sun-spot must be lower than that of the remainder of the solar surface.
As we must get less heat from the sun when it is covered with spots than
when there are none, it may be considered a variable star, with a period
of eleven years. Balfour Stewart and Lockyer have shown that this period
is in some way connected with the action of the planets on the
photosphere. As we have already mentioned, the variations of the magnetic
needle have a period of the same length, its greatest variations occurring
when there are most sun-spots. Auroræ, and the currents of electricity
which traverse the earth's surface, follow the same law. This remarkable
coincidence set men a-thinking. Can the varying condition of the sun exert
any influences upon terrestrial affairs? Is it connected with the
variation of rainfall, the temperature and pressure of the atmosphere,
and the frequency of storms? Has the regular periodicity of eleven years
in the sun-spots no effect upon climate and agricultural produce?

Mr. F. Chambers, of Bombay, has taken great trouble to strike, as far as
possible, a connection between the recurring eleven years of sun-spots and
the variation of grain prices. He arranged the years from 1783 to 1882 in
nine groups of eleven years; and, from an examination of his tables, we
find that there is a decided tendency for high prices to recur at more or
less regular intervals of about eleven years, and a similar tendency for
low prices. An occasional slight difference can be accounted for by some
abnormal cause, as war or famine.

Amid all the apparently irregular fluctuations of the yearly prices, there
is in every one of the ten provinces of India a periodical rise and fall
of prices once every eleven years, corresponding to the regular variation
which takes place in the number of sun-spots during the same period. If it
were possible to obtain statistics to show the actual out-turn of the
crops each year, the eleven yearly variations calculated therefrom might
reasonably correspond with the sun-spot variations even more closely than
do the price variations.

This is a remarkable coincidence, if nothing more. What if it were yet
possible to predict the variations of prices in the coming sun-spot cycle?
Such a power would be of immense service. By its aid it could be predicted
that, as the present period of low prices has followed the last maximum of
sun-spots, which was in the year 1904, it will not last much longer, but
that prices must gradually keep rising for the next five years. Could
science really predict this, it would be studied by many and blessed by
more. Yet the strange coincidence of a century's observations renders the
conclusions not only possible, but to some extent probable.




CHAPTER XXXIV

THE "CHALLENGER" WEATHER REPORTS


The _Challenger_ Expedition, commenced by Sir Wyville Thomson, and after
his death continued by Sir John Murray, with an able staff of assistants
for the several departments, was one of the splendid exceptions to the
ordinary British Government stinginess in the furtherance of science. The
results of the Expedition were printed in a great number of very handsome
volumes at the expense of the Government.

And the valuable deductions from the _Challenger's_ Weather Reports by Dr.
Alex. Buchan, in his "Atmospheric Circulation," have thrown considerable
light upon oceanic weather phenomena. For some of his matured opinions on
these, I am here much indebted to him.

Humboldt, in 1817, published a treatise on "Isothermal Lines," which
initiated a fresh line for the study of atmospheric phenomena. An isotherm
is an imaginary line on the earth's surface, passing through places having
a corresponding temperature either throughout the year or at any
particular period. An isobar is an imaginary line on the earth's surface,
connecting places at which the mean height of the barometer at sea-level
is the same. To isobars, as well as to isotherms, Dr. Buchan has devoted
considerable attention. In 1868, he published an important series of
charts containing these, with arrows for prevailing winds over the earth
for the months of the year. In this way what are called synoptic charts
were established.

In the _Challenger_ Report are shown the various movements of the
atmosphere, with their corresponding causes. It is thus observed that the
prevailing winds are produced by the inequality of the mass of air at
different places. The air flows from a region of higher to a region of
lower pressure, _i.e._ from where there is an excessive mass of air to
fill up some deficiency. And this is the great principle on which the
science of meteorology rests, not only as to winds, but as to weather
changes.

Of the sun's rays which reach the earth, those that fall on the land are
absorbed by the surface layer of about 4 feet in thickness. But those that
fall on the surface of the ocean penetrate, as shown by the observations
of the _Challenger_ Expedition, to a depth of about 500 feet. Hence, in
deep waters the temperature of the surface is only partially heated by the
direct rays of the sun. In mid-ocean the temperature of the surface
scarcely differs 1° Fahr. during the whole day, while the daily variation
of the surface layer of land is sometimes 50°. The temperature of the air
over the ocean is about three times greater than that of the surface of
the open sea over which it lies; but, near land, this increases to five
times.

The elastic force of vapour is seen in its simplest form on the open sea,
as disclosed by these Reports. It is lowest at 4 A.M. and highest at 2
P.M. The relative humidity is just the reverse. When the temperature is
highest, the saturation of the air is lowest, and _vice versâ_. So on land
when the air, by radiation of heat from the earth, is cooled below the
dew-point, dew is produced, and, at the freezing-point, hoar-frost.

The _Challenger_ Reports, too, show that the force of the winds on the
open sea is subject to no distinct and uniform daily variation, but that
on nearing land the force of the wind gives a curve as distinctly marked
as the ordinary curve of temperature. That force is lowest from 2 to 4
A.M., and highest from 2 to 4 P.M. Each of the five great oceans gives the
same result. At Ben Nevis, on the other hand, these forces are just
reversed in strength.

It is also shown by the _Challenger_ observations that on the open sea the
greatest number of thunder-storms occur from 10 P.M. to 8 A.M. And, from
this, Dr. Buchan concludes that over the ocean terrestrial radiation is
more powerful than solar radiation in causing those vertical disturbances
in the equilibrium of the atmosphere which give rise to the thunder-storm.




CHAPTER XXXV

WEATHER-FORECASTING


To foretell with any degree of certainty the state of the weather for
twenty-four hours is of immense advantage to business men, tourists,
fishermen, and many others. The weather is everybody's business. And the
probabilities of accurate forecasts are so improving that all are more or
less giving attention to the morning meteorological reports.

Weather-forecasting depends on the principle from vast experience that, if
one event happens, a second is likely to follow. According to the extent
and accuracy of the data, will be the strength of the probability of
correct forecasts. And the great end of popular meteorology is to
demonstrate this.

We have given some explanations of the weather in some respects unique;
and a careful consideration of these explanations will the more convince
the reader of the importance of the subject. No doubt the changes of the
weather are extremely complex, at times baffling; and the wonder is that
forecasts come so near the truth.

For instance, the year 1903 almost defied the ordinary rules of weather,
for it broke the record for rainfall. And, last year, so repulsive and
unseasonable was the spring, that there seemed to be a virtual
"withdrawal" of the season. I wrote on it as "The Recession of Spring."
Speak about Borrowing Days! We had the equinoctial gales of March about
the middle of April. On very few days had we "clear shining to cheer us
after rain," for the bitter cold dried up any genial moisture. An old
farmer remarked that "We're gaun ower faur North." No one could account
for the backwardness of the season. Unless for the cheering songs of the
grove-charmers, one would have forgotten the time of the year.

In March of this year, at Strathmore, the barometer fell from 30·5 inches
(the highest for years) to 28·65 in five days without unfavourable weather
following. It again rose to 30·05, then fell to 28·45, followed by a rise
to 28·7 without any peculiar change. But in two days it fell to 28·4 (the
lowest for years), followed by a deluge of rain and a perfect hurricane
for several hours, while the temperature was fortunately mild. It was only
evident at the end that this universal storm had been "brewing" some days
before.

All are familiar with the ordinary prognostics of good and bad weather. A
"broch" round the moon, in her troubled heaven, indicates a storm of rain
or wind. When the dark crimson sun in the evening throws a brilliant
bronzed light on the gables and dead leaves, we are sure that there is an
intense radiation from the earth to form dew, or even hoar-frost.

According to the meteorological folk-lore, the weather of the summer
season is indicated by the foliation of the oak and ash trees. If the oak
comes first into leaf, the summer will be hot and dry, if the ash has the
precedence it will be wet and cold. Looking over the observations of the
budding of these two trees for half a century, I find that the
weather-lore adage has been pretty correct. The ash was out before the oak
a full month in the years 1816, '17, '21, '23, '28, '29, '30, '38, '40,
'45, '50, and '59; and the summer and autumn in these years were
unfavourable. Again, the oak was out before the ash several weeks in the
years 1818, '19, '20, '22, '24, '25, '26, '27, '33, '34, '35, '36, '37,
'42, '46, '54, '68, and '69; the summers during these years were dry and
warm, and the harvests were abundant. One can never think of this weather
prognostic from nature without recalling the Swallow Song of Tennyson's
"Princess":--

  "Why lingereth she to clothe her heart with love,
  Delaying, as the tender ash delays
  To clothe herself, when all the woods are green?"

On a muggy morning a sudden clearness in the south "drowns the ploughman."
And yet enough blue in the sky "tae mak' a pair o' breeks" cheers one with
the assurance of coming dry and sunny weather. The low flying of the
swallows betokens rain, as well as any unseasonable dancing of midges in
the evening. Sore corns on the feet, and rheumatism in the joints, are
direful precursors. The leaves are all a-tremble before the approach of
thunder. But throughout this volume I have given many illustrations.

But one of the largest and most important practical problems of
meteorology is to ascertain the course which storms follow, and the causes
by which that course is determined, so that a forecast may thereby be
made, not only of the certain approach of a storm, but the particular
direction and force of the storm. The method of conducting this large
inquiry most effectively was devised by the French astronomer, Le
Verrier--the great aspirant, with our own Couch Adams, for the discovery
of the planet Neptune. He began to carry this out in 1858 by the daily
publication of weather data, followed by a synchronous weather map, which
showed graphically for the morning of the day of publication the
atmospheric pressure and the direction and force of the wind, together
with tables of temperature, rainfall, cloud, and sea disturbances from a
large number of places in all parts of Europe. It is from similar maps
that forecasts of storms are still framed, and suitable warnings issued;
and a mass of information is being collected by telegraph from sixty
stations in the British Islands, &c., of the state of the weather at eight
o'clock every morning, and analysed and arranged at the Meteorological
Office in London for the evening's forecasts over the different districts
of the country. A juster knowledge is being now acquired of those great
atmospheric movements, and other changes, which form the groundwork of
weather-forecasting.

The Meteorological Office, Westminster (entirely distinct from the Royal
Meteorological Society), is administered by a Council (Chairman, Sir R.
Strachey; Scottish member, Dr. Buchan), selected by the Royal Society. It
employs a staff of over forty. The chief departments relate to: (1) Ocean
Meteorology, including the collection, tabulation, and discussion of
meteorological data from British ships, the preparation of ocean weather
charts, and the issue of meteorological instruments to the Royal Navy and
Mercantile Marine; (2) Weather Telegraphy, including the reception of
telegrams thrice a day from selected stations for the preparation of the
daily reports and weather forecasts. Representatives of newspapers, &c.,
receive copies of the 11 A.M. forecast based on the 8 A.M. observations;
and also of the 8.30 P.M. forecasts based on the observations received
earlier in the day. In summer and autumn harvest forecasts are issued by
telegraph to individuals who will defray the cost. The Office also
collects climatological data from a number of voluntary and some
subsidised stations. The "first order" stations include Valentia,
Falmouth, Kew, and Aberdeen. These have self-recording instruments of high
precision, giving a continuous record of the meteorological elements.

A Government Commission which sat last year, under the Rt. Hon. Sir
Herbert Maxwell, Bart., have issued a Report, recommending a number of
changes in the management and constitution of the Meteorological Office;
and considerable modifications are not unlikely to take place in the near
future. In his evidence before that Commission, the Chairman of the
Council acknowledged that the great function of meteorologists is the
collection of facts; but the interpretation of those collected facts, in a
scientific manner, is still in a very immature condition. Dr. Buchan, in
his evidence, confessed that forecasting by the Council is purely "by rule
of thumb." It is not possible to lay down hard and fast rules for
forecasting.

With regard to the storm-warning telegrams, as a rule, the earliest
trustworthy indication of the approach of a dangerous storm to the coasts
of the British Isles precedes the storm by only a few hours. Delays are
therefore very serious.

It is admitted by the best British meteorologists that the observations of
the United States are better conducted, although the best instruments in
the world are set and registered at Kew, in England. The work of weather
forecasts and storm warnings is carried on with the highest degree of
promptitude and efficiency at the Washington Central Office. This is
because the work of predictions has been hitherto the chief work of the
Office: the entire time of the observers, on whose telegraphic reports the
forecasts are based, is controlled by the United States Weather Bureau;
and the right of precedence in the use of wires is maintained.

Professor Brückner, of Berne, has devoted a lifetime to the comparatively
new treatment of climatic oscillations, based upon observations made at
321 points on the earth's surface, distributed as follows: Europe, 198;
Asia, 39; N. America, 50; Cen. and S. America, 16; Australia, 12; Africa,
6. One of his conclusions is that an average time of about thirty-five
years is found to intervene between one period of excess or deficiency of
warmth and the next, accompanied by the opposite relative condition of
moisture.

All are familiar with the hoisting of cone-warning as indication of a
coming storm. This work is exceedingly important, especially for those
connected with the sea by business or pleasure. On the known approach of a
cyclone of dangerous intensity, special messages are sent from the London
Meteorological Office, warning the coasts likely to be affected. When the
cone is hoisted with its apex downwards, it means that strong south or
south-west winds are to be looked for. When the cone is hoisted with its
apex upwards, it indicates that strong winds from the north or north-east
are expected. Of course they are merely useful precautions; but they are
universally attended to by people on the sea-coast.

Though one may have reasonable doubts about the use that can be made of
weather forecasts for three days, such as are now regularly issued, on
account of the finical, coy, spasmodic interludes on short notice, yet
there is a wonderful certainty in the daily prognostics of the direction
and strength of the wind, the temperature of the air, and the likelihood
of rainy or fair weather, dependent on the broad uniformity of nature.
This is very serviceable for people who have now to live at high pressure
in business, in the enthralling days of keen competition. And it is a
great boon to those who are in search of health by travelling, or who, in
innocent pleasure, desire to live as much as possible in the open air.
Very little credit is given to the "gas" of the isolated "weather
prophet"; but those who have confidence in the usual weather forecasts
from the Meteorological Office are satisfied in their belief; and those
who, in self-confidence, ignore all weather prognostics, are still weak
enough to read them and act up to them.

       *       *       *       *       *

In practical meteorology, in the scientific explanation of popular
weather-lore, and in the study of atmospheric phenomena, which so
powerfully influence us, for gladness or discomfort, we may, as with other
branches of science, even all our days, cheerfully go on in "the noiseless
tenor of our way,"

                        "Nourishing a youth sublime,
  With the fairy tales of science and the long results of time."




INDEX


  Abercromby, spectre on Adam's Peak, 89

  Adam's Peak, spectre, 89

  Afterglow described, 62;
    dust-particles to form, 64

  Air, change of, 55;
    clearness and dryness, 49;
    devitalised, 52;
    disease-germs in, 53;
    thunder-clouds, 49

  Aitken, Dr., afterglows, 67;
    anti-cyclones, 97;
    colour of water, 75;
    condensing power of dust, 2;
    decay of clouds, 39;
    dew-formation, 14;
    dust and atmospheric phenomena, 29;
    electrical deposition of smoke, 83;
    false dew, 18;
    fog-counter, 82;
    foreglows, 67;
    formation of clouds, 35;
    haze, 44;
    hazing effects of atmospheric dust, 47;
    Kingairloch experiments, 30;
    one-coloured rainbow, 70;
    radiation from snow, 86;
    regenerators, 85;
    sanitary detective, 78

  Ammonia and cloud formation, 36

  Annie Laurie, 17

  Anti-cyclones, forecasting by, 97;
    formation, 97;
    cause of influenza, 109

  Aratus, forecasting by moon, 61

  Ariel's song, 42

  Aurora Borealis, 71;
    forebodings, 71-73;
    name by Gassendi, 72;
    other names, 72;
    safety valve of electricity, 72;
    sun's spots, 72;
    sun control, 74;
    symptoms, 72


  Bagillt, condensing lead fumes, 84

  Ballachulish, sunsets, 64

  Ballantine's song, 17

  Barometer, indications, 10

  Ben Nevis, dust-particles, 30;
    instruments, 104;
    meteorology, 102;
    observations, 105;
    rainfall, 103;
    regret at stoppage of Observatory, 103

  Blairgowrie, personal description of afterglow, 62

  Blue sky, 74;
    cause of, 75, 77

  Borrowing days, 117

  Brocken, spectre, 89;
    personal description, 90;
    Noah's Ark, 90

  Brückner, climatic oscillations, 122

  Buchan, Dr., Aitken's radiation from snow, 86;
    Ben Nevis, papers on, 103;
    _Challenger_ Reports, 114;
    cold of 1886, 86;
    east winds, 94;
    isobars, 115;
    rainfall statistics, 100;
    on forecasting, 121

  Buchanan, Ben Nevis Observatory, 102;
    great prevalence of fog, 106

  Buddha's Lights, of Ceylon, 72

  Burns, allusions to aurora, 71, 73

  Byron, storm in Alps, 50


  _Challenger_ Expedition, 114;
    temperature, 115;
    thunder-storms, 116;
    winds, 116

  Chambers on sun-spots and grain prices, 113

  Change of air, 55;
    Strathmore to Glenisla, 56

  Charles II., fog and smoke, 80

  Chlorine and cloud formation, 36

  Christison and colour of water, 75

  Chrystal on Aitken's radiation from snow, 86

  Cirro-stratus cloud, mackerel-like, 39

  Climate, _Challenger_ notes, 115;
    cone-warnings, 120;
    Gulf Stream, 111;
    oscillations, 120;
    rainfall, 111;
    sun-spots on, 112;
    wooded country on, 111

  Clouds, decay of, 37;
    distances of, 35;
    dry, 42;
    even without dust, 36;
    formation of, 34;
    height of, 34;
    numbering of cloud-particles, 34;
    sunshine on cloud formation, 35;
    varieties of, 35

  Cone-warnings, 121

  Continental winds, 98

  Cyclones, 95;
    formation of, 96, 98;
    small natural, 98


  Decay of clouds, 37;
    in thin rain, 41;
    process, 38;
    ripple markings, 39

  Dew, evidence of rising, 22;
    experiments, 15, 16;
    false dew, 17;
    formation of, 13

  Disease-germs in air, 53;
    causes, 53;
    deposited by rain, 55

  Diseases, and east wind, 94;
    personal notes, 95

  Dumfries, dust in air at, 46

  Dust, condensing power, 43;
    from meteors, 37;
    generally necessary for cloud formation, 26;
    hazing effects, 47;
    numbering, 26;
    instruments for numbering, 27;
    produces afterglows, 64;
    produces foreglows, 67;
    quantity in Bunsen flame, 28;
    at Ben Nevis, 30;
    Hyères, Mentone, Rigi Kulm, 29;
    Lucerne, Kingairloch, 30;
    when not necessary, 36

  Dust enumeration, deductions on, 31


  Earn, Loch, splash of drop at, 101

  Earthshine, 59

  Ehrenberg, on colour of water, 75

  Evelyn, fumifugium, 80;
    remedy for smoke, 82


  Falkirk, Dr. Aitken's experiments on haze, 47

  False dew, 19

  Fitzroy on aurora as a foreboder, 73

  Fog, counter, 31;
    dry, 41;
    formation, 24;
    more in towns, 25;
    and smoke, 80

  Folk-lore, 50

  Foreglow, described, 66;
    how produced, 67

  Fort William Observatory, 102

  Frankland, disease-germs, 53

  Franklin, lightning, 51


  Gassendi, named aurora, 72

  Gillespie, Dr., on weather and influenza, 107

  Glasgow, fog, 81

  Glass, appearing damp, 44

  Glenisla, ozoned air, 56

  Grain crops and sun-spots, 112;
    Chambers' tables, 113

  Great amazing light in the north, 72

  Gulf Stream, effects on climate, 111

  Gunpowder, great condensing power, 44


  Haze, what is, 43;
    how produced, 44;
    in clearest air, 45;
    stages of condensation, 46;
    in sultry weather, 46;
    dryness of air and visibility, 48

  Health improved by change of air, 56

  Highland air, few disease-germs, 55

  Hoar-frost, frozen dew, 20;
    on under surfaces, 21

  Humboldt, isotherms, 114

  Hydrogen peroxide and cloud formation, 36

  Hyères, dust-particles, 29


  Indian Ocean, colour, 75

  Influenza, weather and, 107;
    six distinct epidemics, 108;
    spread of anti-cyclonic conditions, 109

  Isobars by Buchan, 115

  Isotherms by Humboldt, 114

  Italian lakes, stages of condensation, 45


  Job, on dew formation, 13


  Kelvin recorder, 84;
    Aitken's radiation from snow, 86

  Kew, instruments set, 121

  Kingairloch, dust-particles, 30, 46

  Kirchhoff, lower temperature of sun-spot, 112

  Krakatoa, eruption of, dust-particles, 63


  Le Verrier and weathercharts, 119

  Lockyer, and sun-spots, 112

  Lightning, electricity, 51;
    photographed, 51;
    sheet and forked, 51;
    ozone, 52

  Lodge, electrical deposition of smoke, 83

  London, coals consumed, 25;
    sulphur and fog, 25;
    fog in reign of Charles II., 81;
    Meteorological Office, 11, 120

  Lord Derwentwater's Lights, 72

  Lower animals, sensitiveness, 11

  Lucerne, dust-particles, 30


  MacLaren, Aitken's radiation from snow, 86

  Magnesia, small affinity for water-vapour, 44

  Man in the street, 11

  Mediterranean, brilliant colour, 77

  Mentone, dust-particles, 29

  Merry Dancers of Shetland, 71

  Meteors, producing dust, 37

  Meteorological Council, London, 103;
    Office, 120;
    cone-warnings, 121;
    regular forecasts, 123

  Milne Home on Ben Nevis, 103

  Milton, dust numberless, 26

  Moon, old, in new moon's arms, 58;
    weather indications, 59, 61

  Mountain giants, 88;
    Adam's Peak, 89;
    Brocken, 89

  Munich, International Meteorological Conference, 35

  Murray, _Challenger_ Expedition, 114


  Nardius, dew exhalation, 13

  Newton, colour of sky, 77

  Nimbus, cloud, 35


  Oak and ash, on climate, 118

  Ochils, one-coloured rainbow, 70


  Pacific, colour, 75

  Paris, aurora, 71;
    disease-germs, 55

  Paton, Waller, bronze tints in sunsets, 64

  Piazzi Smith, aurora, 72

  Picket, dew-formation, 14

  Pilatus, fine rain, 42

  Polar lightnings, 72


  Radiant heat, producing fine rain, 41

  Radiation from snow, 86

  Rain, 98;
    heavy rainfalls, 99

  Rainbow, 68;
    forecasts, 62, 69;
    formation, 69;
    one-coloured, 70

  Rains, it always, 40;
    radiant heat in process, 41;
    Ariel's song, 43

  Rankin, dust-particles, Ben Nevis, 30

  Richardson, devitalised air, 51

  Rigi Kulm, dust-particles, 29

  Rolier, aurora, 73


  St. Paul's, London, disease-germs in air, 54

  Sanitary detective, 78

  Shakespeare, tempest, 95

  Shelley, old moon in new moon's arms, 59

  Simoom and sirocco, 94

  Skye, rainy, 40

  Smoke, electrical deposition of, 83;
    regenerators, 85

  Smoking-room, condensing power, 44

  Snow, bad conducting, 87;
    radiation from, 86

  Sodium dust, condensing power, 45

  Spens, forebodings of moon, 61

  Splash of a drop, experiments, 101

  Stevenson, R. L., splash of drop, 101

  Stewart, sun-spots, 112

  Strachey on forecasts, 121

  Strathmore, observations on hoar-frost, 22;
    on decay of clouds, 38;
    to Glenisla, change of air, 56;
    observations on old moon in new moon's arms, 59;
    afterglow described, 62;
    foreglow, 66;
    cold of 1886, 86;
    healthy by woods, 111;
    observations on barometer, 118

  Strathpeffer, 9

  Sulphur as a fog-former, 25

  Sulphuretted hydrogen and cloud-formation, 36

  Sunshine on cloud-formation, 35

  Sun's spots, and aurora, 72, 112;
    and grain crops, 112

  Symons, rainfall, 100

  Synoptic charts, 98


  Tait, on Aitken's radiation from snow, 86

  Tay Bridge, fall of, 92

  Tennyson, aurora, 71;
    dew, 19;
    oak and ash, 119

  Thermometer, indications, 10

  Thomson, Wyville, _Challenger_ Expedition, 114

  Thunder-storm described, 50


  Valkyries, aurora, 73

  Visibility, limit of, 48


  Washington, Meteorological Office, 121

  Water, pressure to show plant exudation, 18;
    colour of, 75;
    experiments on distilled, 76;
    dust-particles vary colour, 77

  Weather and influenza, 107

  Weather-forecasting, 116;
    advantages, 117;
    principle, 117;
    examples, 118;
    old moon in new moon's arms, 59;
    by moon, 61;
    oak and ash, 118;
    cone-warnings, 122;
    three days', 123

  Weather-lore, 50, 118

  Weather talisman, 9;
    call on barometer and thermometer, 10;
    exceptional years, 117

  Wells, Dr., on dew, 14

  Wilson, Prof., on hoar-frost, 20

  Wind, 92;
    rates, 92;
    trade, 93;
    land and sea, 93

  Woeikof, durability of cold, 88

  Wordsworth, rainbow, 68

  Worthington, splash of drop, 100

  Wragge, observations at Ben Nevis, 104


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  Edinburgh & London