The story of the earth's atmosphere

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^ THE STORY OF THE

EARTH'S ATMOSPHERE

DOUGLAS ARCHIBALD, M.A.

Fellow and sotnetine Viee-PresidaU
of the Royal Meieorological Society ^ Loudon

WITH FORTY-FOUR ILLUSTRATIONS

LONDON
GEORGE NEWNES, LIMITED

SOUTHAMPTON STREET, STRAND
1897

J C~iujUxj .4-U-/ruC0 '

PKEFACE.

I HAVE desired in the present little work to put
forward the main features of our knowledge of
the conditions which prevail in our atmosphere
as they are interpreted through the science of
to-day. The Atmosphere, unlike its solid part-
ner, contains no gold or coal mines with which
to stimulate scientific research. Its study has
consequently been somewhat neglected until of
late years, and is even now only just emerging
from the stage of myth and speculation into
that of fact and certainty.

This desirable result has been chiefly attained
by the disuse of vague hypothesis and the ap-
plication of the known laws of physics.

I have therefore written, not for the minority,
who vaguely wonder at the relation of extra-
ordinary facts and pass on, but for what I
believe to be that much more numerous section
who are not content with a mere collection of
facts, but want to know the reason why.

I have levied largely upon ihe original works
of the more modem school of meteorologists
which is so ably represented in America, India,
and Germany — and am under especial obliga-
tions to those of Prof. Davis of Harvard, Prof.

6 PREFACK

Loomis of Yale, Mr Ferrel of Washington, Prof.
Sprung, and Prof. Waldo.

1 have purposely omitted the subject of
weather and descriptions of instruments, and
only briefly touched upon climate, and have
rather endeavoured to shew, especially in the
chapter on Flight, that the Atmosphere pos-
sesses growing uses and interests quite apart
from, and in addition to, its consideration as
a vehicle of weather.

DOUGLAS ARCHIBALD.
1897.

CONTENTS.

CHAP. PAOB

I. THE ORIGIN AND HKIOHT OF THE ATMOSPHERE 9

II. THE NATURE AND COMPOSITION OF THE ATMO-
SPHERE 17

III. THE PRESSURE AND WEIGHT OF THE ATMO-

SPHERE 26

IV. THE TEMPERATURE OP THE ATMOSPHERE . 88
V. THE GENERAL CIRCULATION OF THE ATMO-
SPHERE 68

VI. THE LAWS WHICH RULE THE ATMOSPHERE . 100
VII. THE DEW, FOG, AND CLOUDS OF THE ATMO-
SPHERE . . . . . . . 118

VIII. THE RAIN, SNOV/^, AND HAIL OF THE ATMO-
SPHERE 127

IX. THE CYCLONES OF THE ATMOSPHERE . . 133

X. THE SOUNDS OF THE ATMOSPHERE . . 146
XI. THE COLOURS AlO) OPTICAL PHENOMENA OF THE

ATMOSPHERE 150

XII. WHIRLWINDS, WATERSPOUTS, TORNADOES, AND

THUNDERSTORMS OF THE ATMOSPHERE . 169

XIII. SUSPENSION AND FLIGHT IN THE ATMOSPHERE 173

XIV. LIFE IN THE ATMOSPHERE .... 195

LIST OF ILLUSTKATIONS.

Frontispiece.

Fig. 1— Strato
flow) .

- Cumulus

.

. 11

Fig. 2— Strato
Fig. 8— Ciito-(

- Cumulus

Dumulus

15
18

Fig. 4 .

35

Fig. 5 .

37

Fig. 6 .

45

Fig. 7 .

48

Fig. 8— Distribution of

Atmospheric

Tempera

ture in Latitude for

January, July, and the

year .

50

Fig. 9 .

56

Fig. 10 .

59

Fig. 11 .

60

Fig. 12 .

61

Fig. 18 .

71

Fig. 14 .

73

Fig. 15 .

76

Fig. 16 .

77

Fig. 17 .

78

Fig. 18 .

80

Fig. 19 .

82

Fig. 20 .

86

Fig. 21 .

88

Fig. 22 .

91

103

. 107

120
122
124

PAGB

Fig. 23— " After tho

Storm "
Fig. 24 — Diffusive Limits

of the Component Gases

of the Atmosphere
Fig. 25--Cirrus Cloud {var

Tracto Cirrus, 1889)
Fig. 26 . . .
Fig. 27 . . .
Fig. 28— Festooned Cimiu-

lus .... 126
Fig. 29 . .129

Fig. 30 . . 131

Fig. 31 . .132

Fig. 32 . . 137

Fig. 33 . . 140

Fig. 34 . .141

Fig. 35 . . 141

Fig. 36— Tornado Fimnel

Cloud .
Fig. 37— Thunderstorm in

Section
Fig. 38— Kestrel Hawk

Hovering
Fig. 39 .
Fig. 40 .

Fig. 41 • . . .
Fig. 42 ... .
Fig. 43— YachtinginSyd

ney Harbour

165

167

179
182
186
187
189

198

THE STORY OF THE EARTH'S
ATMOSPHERE.

CHAPTER I.

THE ORIGIN AND HEIGHT OF THE ATMOSPHERE.

The atmosphere of air in which we live and
breathe is really a part of the solid globe on
which we stand.

Until we think of it, we might be inclined to
imagine we were surrounded by mere space, but
when we place our heads under water we find we
cannot live more than a few seconds without in-
haling this same air, and we have only to look at
our ships sailing, our windmills rotating, and our
slates blowing off our roofs in a storm, to be cer-
tain that it is just as material as the solid earth
to which it clings.

Its past history, unlike that of its more solid
partner, is not written in the unmistakable lan-
guage of successive rock strata, or fossil remains,
and we can only infer something of its ancient
changes from analogy with what is now occurring
in the sun, and a knowledge of the physical his-
tory of the universe.

If we are to believe the "nebular theory,"

9

K

10 THE STORY OF THE EARTH'S ATMOSPHERE.

propounded years ago by the great French
astronomer, La Place, and which, far from being
upset, has rather been confirmed by recent dis-
covery, all existing suns and planets have been
simply condensed from clouds or nebulae of
matter originally scattered through space.

By the mutual attraction of their matter
(which force we now term gravitation), these
separate aggregations became highly heated
globular masses, every element of which was
at first in a state of fiery gaseous incandescence.
As they gradually cooled and threw off planetary
excrescences, these masses became condensed at
first into liquid spheres or suns, surrounded by
atmospheres of the lighter and less condensible
gases, still hot enough to be luminous. Of such
a type is our own sun.

A further stage of cooling took place, particu-
larly amongst the planetary offspring, during
which the liquid cooled enough on its external
surface to form a thin solid crust, beneath which
it still remained more or less liquid, and above
which enough gases still remained uncondensed
to form a thin atmosphere, through which light
and heat could penetrate, and yet substantial
enough to support animal life. This is the
present condition of our own planet.

We must not, however, suppose that this state
of things holds on every other planet. The rate
at which such changes progress is different for
each planet.

The planet Jupiter is still so hot that it is
believed to be partly self-luminous, and its at-
mosphere probably contains clouds and vapours

ORIGIN AND HEIGHT OF THE ATMOSPHERE. 11

of substances which on our cooler earth have long
since condensed into liquids or solids. Through
the telescope it is seen to be covered with dense
clouds, and most of its water probably still exists
in the form of vapour (or water gas), and not in
liquid seas as on our own globe. The planet

Photo by O. W. Wilson, Aberdeen.

Fig. 1.— Stkato-Cumulus (i^ow).

Mars, on the other hand, has so little water left
in its atmosphere or on its surface that, while
enough remains to supply its polar caps with
snow during the winter, its parched equatorial
deserts are believed by Mr Lowell, of the Arizona
Observatory, and others who have made it a
special study, to be irrigated thence by the

12 THE STOBY OF THE EARTH'S ATMOSPHERE.

system of so-called canals which intersect its
surface.

Finally, our moon presents a picture of the
condition eventually reached by a small globe —
viz., all solid, no liquid, and no gas left. There-
fore, according to our ideas, no life would be
possible on the moon. The liquid, which would
be chiefly water, has been absorbed into the solid
substance of the moon, while the last relics of
the gaseous atmosphere, which it once must
undoubtedly have possessed, have been either
absorbed into its mass or else diffused into space
beyond the power of recall by gravitation.

The condition of each globe at present de-
pends chiefly on the rate at which these changes
from all gas, to gas and liquid, and thence to gas,
liquid, and solid, occur — i.e., on their rate of cool-
ing. The larger the globe the longer it takes to
cool.

The final condition, however — viz., whether a
globe ultimately ceases to possess a liquid or
gaseous covering, and becomes like our moon, or
still retains an atmosphere and oceans like our
earth, depends on the attraction (gravity, as we
term it) by which it holds its gaseous portions to
it. This, again, directly depends on the amount
of matter it contains, and therefore again upon
its size. Thus, our earth will probably never lose
its atmosphere altogether, though considerable
quantities of the lighter gases, such as hydrogen,
have no doubt already escaped into space.

The fact, therefore, that we possess at the
present time a gaseous atmosphere of exactly
that particular degree of tenuity that suits our

ORIGIN AND HEIGHT OF THE ATMOSPHERE. 13

breathing apparatus, remarkable though it may
seem, is a direct consequence of the particular
size of the globe on which we stand.

Back through the corridors of time, before the
earth had sufficiently cooled to acquire a solid
crust, wo were a little sun, with an atmosphere
of hot, turbid, metallic vapours which poured
down metallic rain, only to be boiled off again
on approaching the heated surface. After a
time, however, such metallic rain would cease
to rise asain, and remain a part of the solidify-
ing earth, and by the time that geologic history
commenced and the surface was cool enough to
admit of animal and vegetable growth, the at-
mosphere must have been practicaMy as clear as
it is to-day.

In proof of this we find that those remarkable
trilobites or sea-lice of the Silurian period, which
is nearly the oldest of which we have any know-
ledge, were endowed with organs of vision, which
shew that as much light penetrated the seas then
as now. The atmosphere, therefore, must have
been equally transparent. Doubtless, more
vapour and carbonic acid were present. Indeed,
some of the latter has since been locked up in a
solid form in the coal measures and limestone
rocks of subsequent epochs.

Continuing our globe history, there came a
time when the atmosphere, after being heated
mostly from the still warm earth, began to find
its soHd partner no longer the warm friend of its
youth, and found itself compelled to depend on
the solar beams, albeit after they had travelled
through ninety-three million miles of space, to

14 THE STORY OF THE EARTH'S ATMOSPHERE.

protect it from the terrible cold of space. By
receiving and entrapping such rays, it is even now
enabled to keep some 500" Fahr. warmer than
outside space, while the heat which at present
reaches it from the earth is estimated as being
barely enough to raise it ydtf^^^ ^^ ^ degree in
temperature.

The atmosphere of our planet, therefore, is our
own individual property, and in no sense part of
a universal atmosphere spread all over space. In
fact, if such a general atmosphere existed at all,
it has been calculated by Dr Thiesen of Berlin
that our sun would, by virtue of its enormous
size — a million times that of our earth — and
gravity, which is twenty-seven times greater,
attach to itself a gaseous covering or atmosphere,
which would be as dense as our own, far beyond
the orbit of Venus. This, however, is known to
be contrary to fact.

The sun's atmosphere is not more than about
500,000 miles deep, while that of the earth is
certainly not more than 100 miles.

The height of our atmosphere has never been
measured as we measure distances on the earth's
surface, for the very simple reason that we can
never hope to reach the top. Indeed, we should
find it very difficult to know where the top was,
even if we were able to approach it, since the
air would shade off so gradually into where it
suddenly changed into the vacuum of space that
we should with difficulty discover the place
where we could say " thus far and no farther."

We can, however, arrive at some knowledge of
the probable height to which the air exists in

ORIGIN AND HEIGHT OF THE ATMOSPHERE, 15

such quantity as to possess weight and resistance
by calculation of the rate at which the pressure
of the atmosphere diminishes as we ascend, and
also by observation of the duration of twilight
and the heights at which meteorites (or, as they

Fhotoby O. W. Wi'ton^ Aberdeen.

Fig. 2.--STBATo-CimnLus (high).

are still popularly termed, falling stars) arc
visible.

Living as we do at the base of our ocean of
air, like the flat-fish live at the bottom of the
ocean of water, we are absurdly ignorant of the

16 THE STOEY OF THE EAKTH'S ATMOSPHERE.

condition of the atmosphere a few miles over-
head.

The highest ascent made by man up moun-
tains is believed to be that of Zurbriggen on
Aconcaqua, when he reached about 24,000 feet,
or a little over 4 miles, while the highest in a
balloon was that made jjy Dr Berson of Berlin,
who in 1894 ascended to a height of 30,000 feet.

Some years ago, in 1862, Qlaisher and Coxwell
made a memorable ascent over Wolverhampton,
when they became unconscious at 29,000 feet,
after whici they were supposed to have ascended
for a short time, to nearly 36,000 feet, but in Dr
Berson's case, by inhaling oxygen he was able to
observe his instruments and carefully note the
conditions around him.

His thermometer went down to 54 degrees
below zero Fahr., while the mercury in his
barometer sank from 30 to 9 inches. Six miles
is probably the limit to which man will ever
care to ascend into the atmosphere, since above
this height he can only survive by the aid of arti-
ficial assistance. For permanent habitation it is
found to be prejudicial to live at greater heights
than 15,000 feet, so that it is only within a thin
slice of our atmospheric blanket that human life
is lived. Actually, the marvellous complexity of
human thought and action, and the development
of modern civilisation on this earth, has taken
place, and will probably always remain confined
within the vertical distance of a London shilling
cab fare above the surface.

Apart from direct measurement, the pressure
of the atmosphere gives us some clue to its

NATURE AND COMPOSITION OF ATMOSPHERE. 17

height as well as to its weight. From the
pressure observations alone, it ought to dis-
appear somewhere about 38 miles, since at that
height the mercury column of the barometer,
which measures the weight of air above, would
tend to disappear. Observations of meteorites,
however, whose appearance depends upon their
heating to incandescence by friction against a
resisting medium, shew that some air exists at
100 miles, though at such great altitudes it is
probably in a condition of extreme rarity. Ob-
servations of the duration of twilight^ which is
due to reflection from particles of dust and air,
gave about 50 miles as the limit. Practically,
therefore, we may take 50 .miles to be about
the limit up to which the atmosphere exists
in a coherent form as we know it near the earth's
surface.

CHAPTER II.

THE NATURE AND COMPOSITION OF THE
ATMOSPHERE.

To one of those superior beings who, we believe,
inhabit the celestial regions, it must be infinitely
pathetic to see the poor little human mites on
this planet struggling for centuries through the
mist of error aM superstition, until they finally
discovered one day the composition of the atmo-
sphere in which they lived. By the Greeks the
air was considered to be one of the four elements,

B

18 THE STORY OF THE EARTH'S ATMOSPHERE.

and it was not until the middle of the last century
that Priestley discovered that air was a mixture
of oxygen and nitrogen, and that its neutral
character was due to the blending of a most
active- element, oxygen, with a most inactive ele-
ment, nitrogen.

A slight difference in the proportion of either

^ Fig. 8.— CibbO'Gumulus.

element would be fatal to life as we kuow it.
With more oxygen in the air our lives, short
enough as they are, would be still more brief,
and though we might be more witty and brilliant,
we should live in a state of such mental and
physical intoxication that we should never be
able to sit down quietly to do any solid work.

NATURE AND COMPOSITION OF ATMOSPHERE. 19

In fact, the human race would be converted into
a number of thoughtless, reckless, frivolous beings,
who would probably end by destroying each
other in a frenzy of over-excitement. On the
other hand, too much nitrogen would reduce us
to such a degree of dulness and inertia that our
supposed national characteristics would be inten-
sify and we should become like a row of statues
or mummies, without action or passion, lifeless —
in fact, matter without motion. The existing
proportion therefore is decidedly adapted to our
present requirements. The average proportion
in which the two principal components of the
atmosphere are found to occur is 21 of oxygen
to 79 of nitrogen by volume, and 23 of oxygen to
77 of nitrogen by weight.

The proportion in which the remaining con-
stituents enter is so small that it may be practi-
cally neglected when we consider the physical
properties of the atmosphere, though it cannot
be neglected when we regard its vital and
chemical functions. The other constituents are
carbonic acid, which occupies -nr^^nr^^ ^7
volume, traces of ammonia, ozone, and the
recently discovered argon.

Oxygen, which forms one-fifth of the atmos-
phere, represents the active vitalising principle, a
large proportion of which, by its former chemical
union with certain terrestniEd elements, such as
silicon and aluminium, has solidified into large
rock masses, by union with hydrogen, has pro-
duced the liquid ocean, and the gaseous vapour
of the atmosphere, and which, by its chemical
union with carbon through the tissues of plants

20 THE STORY OF THE EARTH'S ATMOSPHERE.

and animals, developes the energy which is
manifested in their life and movements.

Owing to the fact that the density of oxygen
is very nearly the same as that of nitrogen, and
to the constant mixture which takes place, the
proportions in which they are found at high
elevations differ but little from those at sea-
level.

Thus in a balloon ascent at Kew, the percent-
age of oxygen present at a height of 18,630
feet was found to be 20*88, while it was 20*92
at the surface. Here it varies chiefly according
to the lack of ventilation and the number of
people who inhabit confined spaces. In the pit
of a theatre the percentage is 20*7, in a law
court 20*6, and in the gallery of a theatre about
20-5.

So far as its chemical properties are concerned,
therefore, the atmosphere at great heights is just
as suitable for man as it is at sea-leveL The
only practical drawbacks arise from its greater
rarity and cold, as we ascend from the surface.

The Nitrogen, which forms three-fifths of the
atmosphere, represents the inert, negative ele-
ment which, though not actively hostile to life,
by diluting the oxygen, lessens the activity and
rapidity of the energy developed by the latter's
combustion, and thus tends to prolong life,
which would be used up too rapidly in pure
oxygen. It would not be easy, in fact, to find
any other diluent of oxygen which could take the
place of nitrogen without producing poisonous
effects like those of carbonic acid.

Regarded from a physical point of view.

NATURE AND COMPOSITION OF ATMOSPHKRK -21

nitrogen, being slightly denser than oxygen in
the proportion of 110 to 97, renders the air a
better vehicle for sound, support, and power
than it would be otherwise.

Nitrogen is also absorbed from the atmos-
phere by plants, through the agency of those
marvellous little bacilli parasites, the Nitragin,
which have recently been shewn by Prof. Dobb^
to nourish certain plants by abstracting the
nitrogen from the air and passing it into the
substance of the plants. Each plant, moreover,
appears to be fed by its own special bacillus,
but starved by that of any other plant.

The carbonic acid only forms a very small
percentage of the air, but nevertheless plays an
important part in the operations of nature.

Animals consume oxygen and exhale carbonic
acid as a product of their respiration. Plants, on
the other hand, under the action of light on their
green cells decompose the carbonic acid, absorb
the carbon, and liberate the oxygen. By these
means the balance between supply and consump-
tion is about maintained.

In former periods of the earth's history the
amount of carbonic acid in the atmosphere was
probably much greater than at present. Especially
during the carboniferous epoch of geology, when
owing to special climatic conditions enormous
quantities of trees and ferns grew which abstracted
the carbon from the then existing atmosphere,
and by burying it for centuries in the solid
form of coal all over the world materially re-
duced the subsequent proportion of carbonic acid
from what had previously existed. Though -03

22 THE STORY OF THE EARTH'S ATMOSPHERE.

per cent., the amount existing at present seems
a small quantity, it is yet as we know, enough to
supply all the vegetable world with its solid
carbon.

Huxley once calculated the amount of this gas
which is contained in a section of the atmosphere
resting on a square mile to be as much as 13,800
tons, while the amount of solid carbon which
could be extracted from such a quantity of the
gas would be about 3700 tons, enough to supply
a small forest of trees weighing 7400 tons.

Ozone, of which traces exist in the atmosphere,
is a peculiar form of oxygen, a molecule of which
is. composed of two atoms linked together, and a
third which, on the principle of two is company
and three is none, is inclined to walk oflF whenever
it meets with a suitable companion. Fortunately
for man the tastes of this third atom are distinctly
low, since it has a partiality for sewers and places
where matter is decomposing and which by its
active oxidising power it renders neutral and
harmless. Since towns usually contain more of
such deleterious conditions tnan the country,
more ozone is found on their windward than on
their leeward sides.

Ozone prevails most in the spring months and
least in the autumn, and while it probably acts
beneficially as a rule, by its active oxidation of
poisonous gases, its excess is associated with the
prevalence of certain forms of catarrhal disease.

Traces of ammonia occur which help to supply
nitrogen to the soil and plants when washed
down by rain. Every year about 30 lbs. of
ammonia are carried down to each acre of ground.

NATURE AND COMPOSITION OF ATMOSPHERE. 23

The above constituents are blended together like
different brands of spirit, but are free to enter
into combination with other substances. This
freedom of contract is implied in the term
mechanical union, which is employed to dis-
tinguish the mixture of oxygen and nitrogen
forming atmospheric air from that of the chemical
union between oxygen and hydrogen in the com-
pound water.

The vapour of water which as an invisible gas
is generally more or less associated with dry air
may be looked upon as a separate atmosphere of
gaseous water. The fact, however, that it is
impossible to distinguish it from dry air by sight
or smell, and that until it condenses out of the
latter as rain or cloud it virtually forms one of
its components, makes it desiraole for us to
regard it in this light, if we are careful to remem-
ber that its quantity (generally about 1 per cent,
by weight) is ever varying, and that the volume
of dry air it displaces and occupies itself, depends
on the temperature as well as the mass of it
present. When it occurs as an invisible gas it is
fths as dense as dry air at the same tempera-
ture and pressure. The peculiarity of the posi-
tion of aqueous vapour is, that if it existed sJone
on the earth, there would be only one temperature
at which it would change from a gas into a liquid,
and therefore only one level at which cloud would
form and whence rain would descend altering
with the time of day and season.

Since, however, it exists in combination with
air, it spreads upwards until it arrives at the
particular temperature at which the air fails to

24 THE STORY OF THE EARTH'S ATMOSPHERE.

support it in solution, when a layer of cloud
forms and perhaps rain falls. After this an
interval occurs in which the vapour is at first
in defect, but as we ascend, its relative amount
to that which is capable of being sustained
increases until another level and temperature is
reached at which condensation takes place, and
a second stratum of cloud is formed and so on.
Ultimately a point is reached at which the
vapour-sphere nearly vanishes, but this must be
very high, for although it is found that at a
height of 23,000 feet in the Himalaya the amount
of vapour in the air is only one-tenth of that
which exists at sea-level, while at 46,000 feet it
would only be one hundredth, cirrus clouds have
occasionally been seen above the latter level.

Dust is another constituent which plays an
important rdle. Mr John Aitken of Glasgow
has made this question the subject of special
investigation, and has found that the atmosphere,
especially in its lower parts over land, contains
thousands of particles of the finest dust. Over
the sea and in its loftier regions these particles
are much less numerous. He has also found
that the presence of this dust is necessary to the
formation of rain.

A recent series of observations by Mr E. D.
Fridlander, taken with Aitken's pocket dust
counter in various parts of the world, embracing
the Atlantic and Pacific Oceans, New Zealand,
California, the Indian Ocean, and Switzerland,
shewed that these tiny dust particles are found
in the lower atmospheric strata right out in the
middle of the Pacific Ocean as well as on land.

NATURE AND COMPOSITION OF ATMOSPHERE. 25

and especially in towns. They arc, however,
less numerous at sea, especially in the Pacific
and Indian Oceans. Thus comparing all three
oceans we have at sea-level.

Number of
dost particles per
cubic centimetre.*

Atlantic Ocean . . . 2053

' Pacific „ . . . 613

Indian „ - . . 512

As low a value as 210 was found in the Indian
Ocean after rain. On the other hand, over land
areas the number frequently rises to 3000 or
4000 per cc. In large cities such as Edinburgh,
Paris, and London, where the products of animal
and fuel combustion enter the atmosphere in
large quantities, the lower atmosphere is so
polluted that in some cases as much as 150,000
dust particles in a single cubic centimetre have
been counted.

As we rise above the surface the number of
dust particles is found to diminish pretty
regularly with the ascent. From observations
on the Bieshom, Frid lander found the number
gradually diminish in the following ratio.

Number of
particles per cc.

950

480

Height above
sea-level.

6,700 feet

8,200 „

8,400 „

10,665 „

11,000 „

13,200 „

13,600 „

513
406
257
219
157

* About 15 cubic centimetres are equal to 1 cubic inch.

26 THE STORY OF THE EARTH'S ATMOSPHERE.

The general rule for the diminution in the
number of dust particles may be simply expressed
thus : For every rise of 3000 feet the amount
is fths of what it was at the lower level. The
bearing of this fact on the question of the
beneficial influence of high mountain resorts on
pulmonary and other diseases is obvious.

These same minute dust particles, by their
scattering action on the small waves of light
at the violet end of the spectrum, have been
shewn by Lord Eayleigh to be the cause of
blue sky, while its gradual deepening into black
as we ascend is readily seen to be the result of
their gradual diminution in number.

CHAPTER III.

THE PRESSURE AND WEIGHT OF THE
ATMOSPHERE.

One of the first facts which is brought to our
notice in these days when those physical laws,
which the ancient philosophers discovered to-
wards the end of their lives, are taught us from
childhood, is that the air has weight and exerts
pressure. The story of the discovery of the
barometer or weight measurer is a romantic
chapter in the history of science.

About 1643, some Florentine gardeners found
that they were unable to pump up water higher
than thirty-three feet. Up to that time it was
aii accepted dogma that "Nature abhorred a

PRESSURE AND WEIGHT OF ATMOSPHERE. 27

vacuiim," and this apparent lapse on the part of
Nature was looked upon as inexplicable. When
Galileo was informed of it, soured as he was with
a world which had rejected some of his greatest
discoveries, he cynically remarked that Nature
evidently abhorred a vacuum up to thirty-three
feet. His pupil, Torricelli, however, was not
content with this perfunctory explanation, and
applying his genius to the question, conjectured
that the column of thirty-three feet of water
exactly balanced a similar column of air stretch-
ing to the limits of the atmosphere. Semem-
bering tfant merciurv was about thirteen times as
heavy as water, he inferred that if this were true,
a mercury pump would only raise mercury to a
height of about 30 inches. He thereupon filled
a long glass tube with mercury, and having
stopped up one end, placed his thumb over the
open end and inverted it over a basin of the
liquid metal. The result proved his anticipations
to have been well founded, since the mercury fell
in the tube until it exactly reached this height of
30 inches, leaving what is known as the Torri-
cellian vacuum in the upper part of the tube.

This is substantially tne mercorial barometer
by which to-day we measure what we term
atmospheric {uressure.

The reason the term pressme is employed and
not tveight is because air, in common with all
fluids, not merely presses downwards, but equally
in all other directions.

This is readily shewn by the familiar experi-
ment of placing a bit of paper over the mouth of
a bottle full of water, and inverting it, when the

28 THE STORY OF THE EARTH'S ATMOSPHERE.

water will be retained by the upward pressure of
the air on the surface of the paper.

When we want to measure the weight of air,
we must remember that, since air is elastic, it is
more compressed, and therefore weighs heavier
near the surface than up above.

At sea-level, where the barometer frequently
registers a height of 30 inches, we shall find that
at 32° Fahr. the column of mercury 30 inches
high resting on one square inch weighs 14*7
lbs. It is easy from this, knowing that mer-
cury is 13 6 times as dense as water, and air
^"ly l oVo ^o o ^^s as dense, to measure the weight
of a cubic foot of pure dry air, which under these
conditions will be about 565 grains (troy). On
the top of a mountain 18,000 feet high it would
only weigh half as much. The weight of a cubic
foot of water vapour under the same conditions
would be only 352 grains. From this it will be
understood that, when vapour is mixed with dry
air, the resulting compound is lighter — that is,
damp air is lighter than dry air.

The weight of the atmosphere on the earth
cannot be ignored.

A flood of water 33 feet high over the globe
would represent the same weight, and would
evidently exercise a very considerable pressure
on the surface. Westminster Hall alone con-
tains 75 tons of air, while the entire weight
of air resting on the earth has been estimated
by Sir John Herschel to amount to llf
trillions of pounds. Sudden alterations of
this pressure, which are indicated by the rise
and fall of the barometer, undoubtedly aflfect

PRESSURE AND WEIGHT OF ATMOSPHERE. 29

some persons of a sensitive temperament, while
the steady fall of pressure which occurs when we
ascend a mountain or rise in a balloon occasions
what is termed mat de marUagne in both men
and animals.

On the other hand, the excessive pressure ex-
perienced in diving-bells or in the Waterloo
tunnel, where the men are now working imder a
pressure of two or more atmospheres, is found to
bring on a species of paralysis.

To give a general idea of the decrease of
pressure with the height when the barometer
marks 30 inches at sea-level, we find the
following relative scale for air of an average
temperature and dampness.

Presanre.

AlUtnde.

30 inches . . . .

29 „

910

28 „

1,850

27 „

2,820

26 „

3,820

25 „

4,850

24 „

5,910

23 „

7,010

22 „

8,150

21 „

. 9,330

20 „

10,550

18 .,

13,170

16 ;,

16,000

At 18,000 feet the pressure is about half that
at sea-level.

It will be observed that at the lower elevations
the height in feet corresponding to one inch in

30 THE STORT OF THE EARTH'S ATMOSPHERE.

the barometer is less than at the higher. The
atmosphere is in fact more tightly packed near the
earth, so that while 1 inch of mercury represents
the weight of the first 900 feet of ascent, 1 inch
at 16,000 feet represents the weight of about
1500 feet, and the proportion increases at greater
heights.

Were the scale 1 inch of mercury to 910 feet
of atmospheric air preserved all the way up, we
should reach the limit of the atmosphere at about
26,220 feet, or 5 miles, which is the height of
what is termed a homogeneous atmosphere.

Comparing the atmosphere with the ocean, we
find that the volume of the former, assuming it
to reach to a height of 100 miles, is as 65 to 1,
while its mass bears to that of the latter the
ratio of only 1 to 300.

The pressure at the average depth of the ocean
— viz., two miles, is as much as 320 atmospheres.

The barometric pressure undergoes changes,
some of which are irregular, and due to the
passage of what are termed cyclones and anti-
cyclones, in which the air is moving round mov-
ing centres, while others, such as those which
complete their period in a year, are connected
with seasonal transfers Qf air between sea and land
and from hemisphere to hemisphere. Others,
again, which run through their course in a day,
are connected with the daily heating and cooling
of the air by the sun, while certain short and
nearly regular instantaneous changes over large
areas, such as the five-day pressure oscillations
recently noticed by Eliot in India, are still mys-
teries that require exphmation. The seasonal

PRESSURE AND WEIGHT OF ATMOSPHERE. 31

changes and the general distribution of pressure
will be alluded to in future chapters, where they
are considered with reference to dependent
phenomena.

The diurnal variation of barometric pressure
which is dependent on the daily rise and fall of
sun-heat is largest, as we should expect, in the
tropics, amounting to a range of as much as
twelve hundredths of an inch at Calcutta, and
diminishing thence as we travel polewards, until
at Greenwich it is only about "02 inch, or one-
sixth of its tropical value. Nearer the poles it
vanishes altogether. Between the tropics, the
irregular changes of pressure introduced by the
passage of storms are so small and infrequent
that the diurnal variation is noticeable above
all other changes, and is so regular that the late
Mr Broun, of Trevandrum Observatory in India,
used to declare he could tell the time of day by
simply noting the height of the barometer. The
rise and fall' of the mercury column is a double
one, reaching its greatest height at 10 A.M. and
10 P.M., and its least height at 4 A.M. and 4 P.M.
The causes are not yet thoroughly worked out,
since, although it undoubtedly depends on the
action of the sun, the total effect is made up of
a combination of direct and indirect motions of
the air. In temperate regions the diurnal change
of pressure is so small .that it is almost lost sight
of in those much larger pressure changes intro-
duced by the passage of cyclones, which frequently
amount to 1 or 2 inches' rise and fall of the mercury.

Barometric charts in which isobars^ or lines of
equal barometric pressure, are drawn over the

32 THE STORY OF THE EARTH'S ATMOSPHERE.

representation of different parts of the earth,
will be referred to in chap. V. These charts
are similar to those employed in weather bureaux
in order to forecast the probable weather for
the ensuing twenty-four hours.

One practical use of the barometer is to deter-
mine the altitude of a place above sea-level. The
science of measuring heights by this means is
termed hypsometry (from the Greek, hypsos,
height, meiron, measui*e). We have abeady
seen that the pressure descends in a certain pro-
portion as we ascend in the atmosphere, and
formulae have been determined by which the
height may be calculated under certain conditions
of temperature, humidity, etc. For rough and
ready purposes, however, the following rule gives
a very fair approximation : —

" The difference of level in feet between two alti-
tudes is equal to the difference of the barometric
pressures observed at each in inches divided by their
sum and multiplied by the number 55,761, when the
average of the temperatures at the two places is

When the average temperature of the two
stations is above 60* the multiplier must be
increased by 117 for every degree the average
is 'above this temperature, and decreased in like
manner for every degree it is below 60°. Thus,
if the values at the lower station are 30*15
inches pressure and 65° temperature, and
those at the upper station are 28*67 inches and
59°, a little household arithmetic will shew that
the difference of their heights is 1409 feet.

CHAPTER IV.

THE TEfi£PSRATURE OF THE ATMOSPHERE.

The temperature of the atmosphere, whether we
are aware of it or not, is a condition in which we
are more directly interested than any other.
The most common form of salutation in the
street involves a dictum or a query as to " how
cold it is to-day," "much warmer than yesterday,"
"I do hope we are going to have some really
warm weather now," or " some skating," as the
case may be. In all this the temperature of
the air is concerned, since it is the medium in
contact with us, and from which, chiefly by con-
duction, we derive our sensation of heat or cold.
When we talk of temperature we must take care
to know what we mean by the term. Heat, as
we know, is a "mode of motion," as Tvndall
used to call it, a vibration of the small molecules
of a body, and directly this mode of motion
is communicated to it, by what is termed
radiation, it tends to return the compliment to
other bodies in its neighbourhood, and set
all their molecules in a similar state of oscilla-
tion. The process, however, is an exchange all
round, and the temperature of any body measures
the rate at which it loses heat to or gains heat from
surrownding bodies. This rate depends upon its
capacity for heat, and its power of absorbing
and radiating heat rays, all of which vary in
different bodies.

In the case of the atmosphere, the radiating
power exceeds the absorbing power for rays
c ^

34 THE STORY OF THE EARTH'S ATMOSPHERE.

coming from the sun, but is considerably less for
the heat radiated back again from the earth.
So that, on the whole, the absorption power of
the lower air for all kinds of rays is about 2^
as great as its radiation power.

It is this property of the atmosphere which
allows us to keep decently warm. Otherwise,
were we bereft of this valuable covering or
envelope we should shiver in a temperature of
138 degrees below zero Fahrenheit, which is
probably the mean temperature of the moon's
surface. The only advantage that could be
claimed for such a temperature is, that it would
be 332 degrees higher than what would probably
ensue in the event of the sun becoming cold.

The temperature of the atmosphere is derived
chiefly from the solar radiation which is arrested
by the earth, and partly reflected, partly radiated
back through the atmosphere towards space.
Temperature is a result of radiation.

Consequently before we speak of the tem-
perature it is necessary to see how radiation
aflbcts the atmosphere, since the conditions which
regulate radiation, aflbct the temperature of the
atmosphere in a somewhat similar manner.

When the sun's radiations have reached the
earth's surface from which the lowest stratum of the
atmosphere chiefly derives its temperature, their
heating effect on a given area is modified by two
circumstances, (1) their angle of incidence or the
angle the direction of the sun makes with the
horizon, and (2) the thickness of atmosphere
they have traversed.

When a certain width of the sun's rays is con-

THE TEMPERATURE OF THE ATMOSPHERE. 35

sidered it will be found to cover a smaller area
in proportion as they fall more vertically or less
inclined. Thus in the accompanying diagram

Son vertical (as at noon
Eqolnoz on Equator).

the same width of rays is concentrated upon A B
in the one case, and spread over A C in the other,
consequently the heat received by the earth is
greatest when the sun is highest above the
horizon, and shines most directly upon the
ground. During a single day the heat received
on the ground is greater at noon than at any
other hour (about four times as great as at 10 A.M.
or 2 P.M.). It is also greater in the summer
when the sun is permanently at a higher angle
all through the day after it has risen, than it is in
the winter. These both operate together at any
place on the earth. When we change our lati-
tude we can, by travelling towards or from the
equator at the rate of about 18 miles per day,
obviate the seasonal change in the angle of the
sun above the horizon and secure the same
general amount of sun radiation. We should

36 THE STORY OF THE EARTH'S ATMOSPHERE.

not, however, be able to secure the same
average temperature since the direct effects of
the radiation on temperature are modified by
what goes on over entire hemispheres. More-
over the effect of changing our latitude intro-
duces another consideration which has a potent
influence upon the amount of heat falling in
the 24 hours — viz., the time during which
the sun remains above the horizon. This time
increases as we travel polewards in the hemi-
sphere which is enjoying summer. There are
thus two influences which work in opposite
directions, one, the general angle of the sun
above the horizon, which diminishes as we leave
the equator, and the other, the length of the
day, which increases under the same conditions.
The conjoint effect must therefore generally
reach its maximum value at some intermediate
latitude.

As a matter of fact, this important problem
has been worked out by several physicists, in-
cluding Lambert, Poisson, and Meech. The
last-named finds that on the average of the year,
as we should expect, more heat falls on the
equator than elsewhere. If we take the six
months of the northern summer, more heat falls
on latitude 25 degrees north (the latitude of
northern India) than on the equator. If again
we take the three months nearest midsummer,
i,e. from May 7th to August 7th, the zone of
greatest heat reception lies in 4r N., while from
May 31st to July 16th, more heat falls on the
North Pole than on any other part of the earth.
The temperature of the Pole does not of course

THE TBBIPBRATURB OF THE ATMOSPHERE. 37

at once respond to this heating, since the average
temperature effect lags about one month behind
the solar radiation, and near the Pole the heat
is mainly employed in melting the Arctic ice
floes, and in raising the temperature of the
water. At the same time this beneficial arrange-
ment obviously prevents the temperature there
from becoming as low as it otherwise would.

In addition to this, the amount of heat which
is transmitted through the atmosphere so as to
reach the surface at all, varies with the angle of

S

jAimt

Fio. 5.

the sun for a different cause — viz., the different
thickness of the atmosphere traversed in each case.

This is plain from the adjoining figure in
which as the sun's rays fall vertically or inclined,
we have the thicknesses A.P., B.P., and C.P.

This last circumstance exaggerates the differ-
ence caused by the hourly and seasonal changes in
the angle of the sun, especially as it approaches
the horizon.

38 THE STORY OF THE EARTH'S ATMOSPHERK

Direct observations of the sun-heat by means
of an instrument termed an actinometer, which
has been employed with great success by Prof.
S. P. Langley at Washington, have shown that
of the heat which falls vertically on the upper
surface of the atmosphere, 25 per cent is absorbed
^Langley says 40 per cent, but this seems doubt-
ful from other considerations) before it penetrates
to the earth. When the rays are inclined, instead
of 75 per cent being transmitted, only 64 per
cent arrives at an angle of 50 degrees, and only
16 per cent at an angle of 10 degrees. The light
varies in the same way. At simrise and sunset
the sun has only yA^th part of the brilliancy it
Dossesses when vertical overhead.

When we come to consider the actual quantity
of heat that is received from the sun, we shall
see how utterly it transcends all our means for
deriving warmth from (so-called) artificial sources.
The intensity of solar heat may be measured
by the temperature to which it would raise a
certain quantity of water. If we suppose the rays
which fall vertically on an area one square foot
at the outside of the atmosphere, before any
absorption has taken place to be applied to
warming up 10 lbs. of water, they would raise it
1 degree on a Fahrenheit thermometer in 1 minute.

By the time these rays have reached the earth,
as we have seen, about ^th of the original radia-
tion has been absorbed or scattered by the at-
mosphere, and therefore only about 7 lbs. of water
could be raised 1 degree per minute. This how-
ever gives us some faint idea of the enormous
quantity of heat which is continually falling on

THE TBUPBBATURS OF THE ATMOSPHERE. 39

either the earth or the clouds. If we take the
heat which falls on a square mile of the earth's
surface per minute, we shall find that it would
be enough to raise 560 tons of water from the
freezing to the boiling point.

In a year, assuming that the sun's heat (con-
tinually penetrated to the ground, this heat
would suffice to melt a layer of ice about 178
feet thick over the whole earth, or not much
below the monument in London.

The general effect has been popuiarly put by
one writer in the following graphic manner,
in which the different amount of neat received
when the sun is inclined at different angles is
properly considered.

''Suppose the earth one vast stable covered
with horses, and suppose that as the sun's ande
varied according to season and latitude, the
horses arranged themselves so that no horse's
shadow fell upon or underneath his neighbour;
then the solar heat falling upon the earth con-
verted into horse power, would be always repre-
sented by all these horses working continuously
at their utmost strength."

Some of this heat energy is, no doubt, con-
verted into mechanical energy in the winds, rivers,
and rainfall, but a vast proportion of it is wasted
so far as man is concerned, and it is plain, as
both Lord Kelvin and Edison have recently
pointed out, that we have still an immense source
of power comparatively untouched, which can
be drawn upon when our coal supply shows
symptoms of giving out.

One effect has not been alluded to — viz., the

40 THB STORY OF THE EARTH'S ATMOSPHERE.

change in the distance between the earth and the
sun, which are nearer to one another in December
than in July. Theoretically the effect would in
any case be small. Practically it is counteracted
by the large mass of water in the southern
hemisphere, which responds more slowly to an
increase of heat than the northern land, so that
on latitude 20 degrees S., where it reaches its
greatest effect, it only adds i^th to what would
occur if the distance were invariable.

Since the temperature of the atmosphere
results from the accumulation of altered solar
rays, in surrounding objects which radiate them
to one another, instead of passing them back at
once into space, the temperature epochs will
always follow those of direct radiation. Thus
the highest temperature of the day does not
occur at noon, but an hour or two afterwards.
Similarly the highest temperature of the year
occurs on an average a month after midsummer
day, and a like retardation occurs for the lowest
temperatures. At the Pole, where one long day
and night occurs in the year, the coldest month
is delayed to February or March, in the northern
hemisphere. When the sun's rays fall upon water,
or where the locality is naturally moist, the heat
is conducted through the top layer, and in any
case takes longer to raise its temperature.
Where, as always occurs, part of the water is
evaporated, nearly 1000 times as much heat is
needed to convert it into vapour as will raise its
temperature 1 degree Fahr. Consequently, not
only does the temperature of the air over oceans
rise and fall less daily and seasonally than that

THE TEMPEKATURE OF THE ATMOSPHERE. 41

over t^e continents, but the highest temperature
of the year in maritime regions lags about 42
days behind midsummer day, while in the centre
of the large continents, the lag is reduced to 25
days.

This slowness to rise and fall in temperature
on the part of large masses of water, accounts
for the equable temperature of the atmosphere
of islands and coasts, compared with interiors of
continents, and exerts an important influence in
determining the changes in the general wind
and weather system over oceans and continents
in summer and winter.

The measurement of atmospheric temperature
dates back like that of the telescope to Galileo,
who in 1597 devised the first liquid thermo-
meter.

This consisted of a glass bulb, containing air,
terminating below in a long glass tube, which
dipped into a vessel containing a coloured fluid.
The variation of the volume of the enclosed air,
caused the fluid to rise and fall in the tube to
which an arbitrary scale was attached. Galileo
further invented the alcohol thermometer in
1611, which was adopted generally by the
Florentines of that time.

The determination of the zero and some fixed
point above it, by which to graduate the scale,
appears to have taken years to evolve. Newton
suggested a scale in which the freezing point of
water was degrees, and the blood of a healthy
man 12 degrees, and subsequently Fahrenheit,
to whose scale with characteristic conservatism
we still adhere in this country, in spite of the

42 THE STORY OF THE EARTH'S ATMOSPHERE.

universal use of that of Celsius on the continent
and in physical investigation, in 1714 took
blood heat and that of a freezing mixture of ice
and salt as his fixed points. Since then the
freezing and boiling points of water have been
taken as the fixed points on the thermometric
scale.

Unlike the early Florentine thermometers, the
modern alcohol and mercury thermometers con-
sist of a bulb and tube, partially filled with the
liquid, above which is a tolerably complete
vacuum, allowing the liquid to move with
perfect freedom up and down the tube.

For measuring rapid variations in the tempera-
ture of the atmosphere, it is necessary to have
the bulb small, since where the bulb is large, the
effect of an exposure to heat is considerably
delayed. Consequently, for determining the
true shade temperature of the atmosphere at
any moment where it is difficult to obtain
proper shade conditions, a small sling thermo-
meter is by far the most accurate. By tying
any thermometer to a string, and whirling it
round until the reading does not alter, a very
fair notion of the true temperature of the air
can be obtained.

For standard purposes where momentary
changes are not so important, thermometers with
large bulbs are preferred, since by this plan the
variations due to the expansibilitv of the glass
bear a small ratio to the volume of mercury.

We have now-a-days advanced so rapidly in
our methods of investigation, that instead of
being content with two or three readings a day.

THE TEMPERATURE OF THE ATMOSPHERE. 43

we require to know the continuous changes in
the temperature of the atmosphere in places
where it is impossible to make eye observations.

For this purpose the self-recording thermo-
graph is employed, and by the use of such an
instrument the temperature of the atmosphere
can be registered on the top of mountain peaks
only occasionally accessible, and in the free
atmosphere by the elevating power secured by
kites and captive or free balloons.

When we examine the observed facts as they
present themselves, we find in the first place a
constant diminution of the temperature of the
atmosphere as we ascend from the earth's surface.
This decrease of temperature with ascent varies
somewhat in different latitudes, and is not the
same in the free atmosphere as on mountain sides.

From Glaisher's balloon ascents the rate begins
quite near the surface at about 7 degrees in every
140 feet, and finally diminishes to 1 decree in
every 400 feet at 10,000 feet, the entire diminu-
tion of temperature from sea-level up to 10,000
feet being 34 degrees, or 1 degree in 300 feet. If
therefore we take the temperature of London to
be about 50 degrees on the mean of the year, a
temperature of freezing point or in other words
the snow line would be reached at an elevation
of about 4500 feet or a little above Ben Nevis in
the free atmosphere (the mean temperature at
the top of Ben Nevis is about 30^ degrees F.).
We have no mountains above this level, conse-
quently we have no perpetual snows in these
islands.

In India the initial rate of decrease of tempera-

44 THE STORY OF THE EARTH'S ATMOSPHERE.

ture is very much more rapid, amounting to 1
degree in the first 33 feet, but it slackens down
to about 1 degree in 330 feet at 15,000 feet. On
the average it is about 1 degree in 270 feet in
stations away from the Himalaya where the
mountain range appears to reduce the rate.

If we take the region of the North West
Himalaya we shall find that the mean tempera-
ture of London would be reached at a height of
9600 feet, and the range of temperature through-
out the year would not diifer very much from
that of England.

Most of the Himalayan sanitaria lie between
6000 and 7000 feet where the temperature is
about 60 degrees, and possess climates similar
to those of the Eiviera and the coast irom Mar-
seilles to Genoa.

When therefore we wish to vary our climate
as far as temperature is concerned, we can do so
without changing our latitude by remembering
that the temperature cools on an average about 1
degree for every 300 feet we ascend, or warms at
the same rate as we descend the same distance.
Since the mean temperature at the north pole is
about degree F. and at the equator between 80
and 90 degrees F., we can similarly alter our
temperature 1 degree F. by travelling north
or south about 70 to 80 English miles. As an
illustration of a combination of these facts we
can imagine a series of planes rising upwards
from different points of the earth towards the
equator along which the temperature would
range on either side of a certain average through-
out the year. These would rise to their highest

THE TEMPERATURE OF THE ATMOSPHERE. 45

level over the equator, and their height in any
latitude would show us at what elevation we
shoidd experience some particular temperature
all the year round.

The vertical scale above sea-level is of course
immensely exaggerated.

fEET

Seaifs^i

Eartfi

MPole.

Fig. e.

It will be seen that at an elevation of 27,000
feet over the equator, the temperature is about
de^ee F., and that the snowline or line of
freezing point cuts the surface at sea-level about
latitude 69° North.

In the Himalaya this line throughout the year
is about 15,400 feet above the sea, or about
17,850 feet in the summer months. Even this
great altitude would still leave about 11,000 feet

46 THE STORY OF THE EARTH'S ATMOSPHERE.

of the higher summits mantled with perpetual
snow during the summer.

There is perhaps no point about which so much
perplexity is generally felt and expressed as the
reason for this decrease of temperature as we
ascend. It is often popularly expressed as being
due to the greater rarity of the air above, but
this simply leaves the matter as obscure as before.
Like most other facts regarding the atmosphere,
it results from the operation of a definite physical
law.

It is well known that the rapidity of the cool-
ing of a body depends on the perfection of its
enclosure, whether solid or ^seous. At the
earth's surface the enclosure is nearly perfect,
but as we ascend, the upper side of the enclosure
weakens owing to the thinning of the air, until
at the top of the atmosphere the enclosure is only
half what it was at the earth's surface. The heat
radiated from the earth is moreover intercepted
to a large extent at the higher levels by the
intervening lower air. Consequently on both
accounts the temperature of the air remains
cooler in proportion to the altitude.

The distribution of the temperature of the
atmosphere in a horizontal direction as ordin-
arily measured has reference merely to the
temperature of the lowest stratum. Unlike
the barometer which gives us the sum total of
the pressures of the superincumbent layers, a
thermometer near sea-level simply gives us the
temperature of the particular stratum in which
it lies. The magnitude of the daily and seasonal
changes vary according as the locality is con-

THE TEMPKRATURE OF THE ATMOSPHERE. 47

tinental or maritime, and its soil is dry and
rocky or damp and alluvial, and the average
itself depends largely on these and other con-
ditions besides mere latitude.

The general distribution however shews de-
cidedly that latitude is one of the principal
causes which affect the mean annual tempera-
ture. The map (fig. 7) shews the distribution
of the heat in the lowest atmospheric stratum
over the earth's surface on the average of the
year, by lines of equal average temperature
(isothermals). The principal points to notice
are the widening out of the area between the
isothermals of SO** over the land areas and
the contraction that takes place over the
seas, particularly the Atlantic. Also that wher-
ever a marked dip of the line, particularly that
of 70*, occurs toward the pole over the land,
an equally marked dip of the line occurs in the
opposite direction close alongside.

This is specially visible m California, Peru,
and in S.W. Africa, and is plainly due to the
known existence of cold marine currents flowing
along these several coasts towards the Equator.
So far as the British Islands are concerned it is
equally plain that the northward flow of the
gulf-stream of the Atlantic raises the isotherm
of 50** F. which normally belongs to latitude
40° and passes through Nippon (Japan) ten
degrees further north so as to make it pass
through London. We thus get in these islands
an atmosphere artificially heated irp about 10
degrees (the isothermal of 40** F. properly
belongs to our latitude) more than we are

THE TEMPERATURE OF THE ATMOSPHERE. 49

entitled to by our latitude. In like manner
Peru no doubt enjoys several degrees less heat
than it would otherwise have owing to the cool
Antarctic stream (Humboldt's current) which
flows up its coast and cools the lower atmos-
phere.

The area of greatest heat is where the largest
land masses lie near the Equator, and of greatest
cold where the largest land masses, such as N.
Asia and N. America, lie nearest the Pole.

The reason for this is too important to be
omitted.

If the same anumnt of sun lieat falls ujmi an
equal area of land and water it raises the temperature
.of Ihe former four or fire times as much as that of
the latter.

Less heat energy is spent in agitating the mole-
cules of dry earth than those of water. Conse-
quently its eflTects are more patent. In tne case
of water the heat energy is not lost, no energy. ever
is in this Universe — but it is more latent and the
expressed temperature is less. The atmosphere
is more readily heated by the radiation from the
hotter (as we say) earth than the cooler water.
Consequently the lowest stratum over the land
areas under the more direct sun near the Equator
exhibits a generally higher temperature than
that which lies over oceans in the same latitude.
Since heating and cooling are reciprocal opera-
tions it is easy to see that the reverse applies to
the temperature over polar seas and continents.

The migration of the sun north and south of
the equator by reason of the inclination of the
earth's axis to the plane in which the centres of
D

50 THE STORY OF THE EARTH'S ATMOSPHERE.

the sun and planets lie, causes a similar migra-
tion in the area of greatest heat north and south
of the geographical equator. While the sun shifts
from 23J" N. to 23J" S., however, the central
line pf greatest heat (or heat-equator) migrates
to a less amount, particularly over the oceans.
On the Pacific it moves onlj 15° to 20° in
latitude. On the Atlantic still less. On th^

FlO. 8.— DiSTRIBITTIOK OF ATMOSPHERIC

Tbmpjbraturb in latitude, roB January,
July, and the tear.

land it shifts as much as 43° in Africa and
50° in America, while in India it runs up
to the deserts of Persia in latitude 33° N. in the
summer, and down to only 10* S. in the winter,
because there are no southern lands to attract
it further.

The general distribution in latitude and migra-
tion of the temperature may be best seen in the

THE TEMPERATURE OF THE ATMOSPHERE. 51

accompanying diagram (fig. 8) plotted out from
the means given by the late Mr Ferrel of the
American Weather Department.

The position of the thermal equator to the
north of the line and the greater annual range
of temperature in the northern hemisphere are
plainly visible.

If we were to undertake a balloon voyage
round the world at an average altitude of about
5000 feet we should find very few signs of this
peculiar distribution which prevails near the
surface.

The temperature over the equator would be
about 64** F., an agreeable summer tempera-
ture in England, and though if we preserved the
same elevation, the temperature would descend
to about 34° over London, the seasonal and
daily changes would be very much less con-
spicuous than near the surface.

By means of thermometers and thermographs
the temperature of the atmosphere near the
surface is read at certain hours or recorded con-
tinuously, and for various purposes particular
attention is paid to its maximum, minimum,
and average, either for a day of twenty-four
hours, a month of 30 days, a year, or a series
of years.

Where it is difl&cult to have continuous hourly
readings taken, three hours are chosen, which,
when combined in a simple manner, give a
value which is found by experience to closely
approximate to the average of the day.

Thus, the average of a single reading at 9 A.M.
gives a very close approximation to the mean of

*52 THE STORY OF THE EARTH'S ATMOSPHERE.

the twenty-four hours. Or we may add the
readings at six, fourteen, and twenty-two hours
and divide by three, or take the lowest and
highest readings and divide by two. Where
the self-recording thermograph is employed, the
mean can be found by measuring the area traced
out by the recording pencil and bisecting it.

The maximum and minimum in this case
correspond to the highest and lowest points of
the curve traced out, but usually they are
measured by separate maximum and minimum
thermometers.

Apart from the general distribution of its mean
annual values shown in Buchan's isothermal
chart, the temperature of the lowest air stratum
and proportionally of those lying above it, is
subject to regularly recurring daily and seasonal
oscillations. These two series of changes are
so important in their relation to our general
comfort and welfare that it is of the highest
interest for us to know whether they eAibit
any signs of progi'essive change in obedience
to law as we vary our position on the earth.
As a general rule we find the greatest ranges
of the temperature of the lowest atmospheric
stratum between day and night occur in the
driest parts of the earth, in the interior of
continents, such as the Sahara, Arabia, Gobi
desert, Eajputana, Colorado, etc., where it often
amounts to 40** F., and the smallest ranges
in small oceanic islands, such as Honolulu,
Kerguelen, Madeira, Bermuda, where it is as
small as 50** F.

In India, which presents the greatest contrasts

THE TEMPERATURE OF THE ATMOSPHERE. 53

of dry interior and moist coast in the world, we
have daily ranges of SO** to 40" in the Punjab,
20' to 30° in the Central Provinces, 16** at
Calcutta, 8' at Bombay, and 6' at Galle in
Ceylon. The daily range also decreases gener-
ally from the Equator to the poles when we
take places at the same distance from the sea.
Thus, while it is 11 degrees at Colombo, it sinks
down to an average of only 3 degrees at Suchta
Bay, in latitude 73" N. Even at St Petersburg,
surrounded by large continents, it is only 7".

The reason for this is simple. The changes
in the solar altitude between sunrise and sunset
are manifestly more marked where the sun rises
higher in the sky than where its path is at a
small inclination to the horizon all day, while
at the Poles, where it takes a jrear to rise and
set once, the daily variation entirely vanishes.

The diurnal range of the temperature of the
air also diminishes with elevation above the sea-
level.

Thus in the N.W. Himalaya, while the mean
daily ranges at Mussourie and Ranikhet at 6000
feet above the sea are only 13" and 15° F., the
ranges at Bareilly and Roorkee on the adjacent
plains are 23" and 24" F. The reason is obvious
if we remember that the heat received during
the day is more absorbed by the denser air
near sea-level than the rarer air on the moun-
tains. Consequently, since the heat which falls
on the mountain-top is more freely radiated
back into space, the air over the mountains is
less expanded than that over the adjacent
plains.

54 THE STORY OF THE EARTH'S ATMOSPHERE.

During the day since the air over the latter
expands upwards about 13 feet for every degree
F. the temperature of the entire mass up to 6000
rises. Meanwhile since the mountain cannot
expand the air over it remains sensibly stationary.
In consequence a downflow takes place towards
the mountain somewhat like the sea-breeze
towards a coast which brings with it the cooler
temperature in the free air at the same level
and so cools that on the mountain.

At night when the mountain which is a good
radiator cools down rapidly and chills the air
which lies on it, this air by reason of its in-
creased density slides down the mountain side
and its place being taken by the adjacent less
cooled air, the temperature is again prevented
from descending too low.

In valleys on the other hand even at high
altitudes, the contrary conditions take place.

By day, owing to the greater perfection of the
atmospheric enclosure, the sun's heat is more
effective in warming up the lower stratum of
air, while at night the chilled air from the
surrounding mountain tops descends into the
valley and increases the cold. Hence, at Leh
in Thibet, which lies in a valley at 11,500 feet
elevation the daily range is as high as 29 degrees.
On a smaller scale it is practically recognised
that frosts prevail more in valleys than on hill
tops.

The atmosphere and the ocean thus exert a
similar tendency in reducing temperature ranges,
and the man who builds his house on a hill and
so rises into the atmosphere, enjoys similar ad-

THK TEMPERATURE OF THE ATMOSPHERE. 55

vantages to the one who takes up his residence
on the sea-coast or an island. In both cases
extremes of temperature are avoided.

The temperature is lowest as a rule on land
shortly before sunrise. In tropical countries,
such as India, where it occurs only just a few
minutes before sunrise, it is often the only toler-
able moment-of the 24 hours.

The highest temperature occurs nearly every-
where on laud between 2 and 3 P.M., but alters
according to seasQih

The greatest changes in the times at which the
daily temperature variation reaches its highest
and lowest points are related to the position of
the place as regards the ocean.

Put briefly, the drier and more inland or
continentally the place is situated, the later
will be the epochs, while out in the open ocean
theonid-day maximum occurs soon after noon
and the morning minimum as early as 4 a.m.

The annual range of temperature, or in other
words the diflTerence between the average tempera-
tures of the hottest and coldest months, in con-
trast to the diurnal range, increases from the
equator, where it is least, to the poles. It also
increases with the distance from the coast. Thus
while it is only 3 J** at Colombo, it is IT at
Bombay, 21° at Calcutta, and from W to 40'' in
N.W. India.

JThe accompanying map, in which the lines of
equal annual range of 5°, 10% 20*, 30% etc., are
drawn, shews at a glance its general distribution
over the earth, from which it is plain that while
it is least over a broad belt surrounding the

56 THE STORY OF THE EARTH'S ATMOSPHERE.

equator, it reaches its highest values in the
poleward centres of the continents.

In England the range of temperature between
summer and winter is about 20 degrees. In
Honolulu it is only 5 degrees, as near the

Fig. 9.

equator, while at Werkojansk in North Eastern
Siberia it amounts to no less than 120 degi-ees.
The man who boasts he can wear the same coat
summer and winter through would have to
change his habits in that district. There are
several remarkable features exhibited on this
map. One is that in all the northern continents
the position of the greatest annual temperature
lange is to the east of their geographical centres.
This is chiefly owing to the influence of the
warm currents which bathe their western shores
and the accompanying wind currents which
carry the moderating effect of the ocean over

THE TEMPERATURE OF THE ATMOSPHERE. 57

a large part of their western interiors. West-
ern Eurbpe is peculiarly favoured in this
respect.

Also the generally small range over the larger
oceans which is due to the slow response of
masses of water to the seasonal changes in the
amount of solar heat falling on it, a point which
has already been attended to. As a result, the
ranges over the southern hemisphere which is
mostly water are imiformly small. In New
Zealand, for example, December and June differ
by only 10 degrees.

Besides the diurnal and annual ranges of
temperature it is found that there are slow
periodic changes of a small amount in the mean
temperatures of the year, in correspondence with
the changes in the niunber and area of sunspots
which recur about every eleven years. What-
ever may be the exact cause, whether an increase
or decrease of solar radiation corresponding to a
great spot manifestation, the effects have been
proved through the labours of Prof. Piazzi
Smyth, Dr Stone, Dr Koppen of Hamburg,
and Prof. Fritz of Zurich, to be visible in the
temperature of the earth's atmosphere.

In years grouped round those of fewest spots,
such as 1811, -23, -34, -43, -56, -67, -77, -88, the
temperature is highest, and in those similarly
grouped round those of most spots such as 1860,
-71, -83, 93, it is lower than the average. The
effect is most noticeable in the tropics. For
example, in India, the difference between the
temperature at the two epochs varies from 1 to
2 degrees on the mean of the year.

58 THE STORY OF THE EARTH'S ATMOSPHERE.

A similar periodic change is found to prevail
in conditions which depend upon air tempera-
ture, such as fruit-harvests, vintages, rainfall,
glacier extension, storms, cloud proportion, etc.,
while the late Professor Jevons endeavoured to
shew that even commercial panics were brought
about periodically through the medium of such
indirect consequences.

Though there is much scepticism as to the
quantity of the temperature effect being of such
importance as to bring about panics through bad
harvests, there is no doubt that the condition of
the sun affects our atmosphere in some peculiar
way not only in regard to temperature, but
also magnetically, since the appearance ^f the
aurora is admitted by those who dispute the
heating effect to be closely connected with the
presence of sunspots and other forms of solar
disturbance.

This periodic oscillation of annual tempera-
ture does not of course involve any steady pro-
gressive change in the temperature of the
atmosphere. In fact, when some years ago the
people of Paris were temporarily afraid that
their climate was changing, the astronomer
Arago proved to their satisfaction, by a recourse
to statistics, that the temperature of Paris had
not sensibly changed for 100 years, and within
historical periods there does not seem to be any
evidence that the temperature anywhere is
sensibly changing permanently one way or
another.

We will now pass on to consider the circum-
stances that attend a local accession of heat over

THE TEMPERATURE OF THE ATMOSPHERK 59

land and water and the primary effects which it
produces.

Beginning with any area on a small scale. Let
fig. (10) represent a vertical section of the atmos-
phere and let the dotted times represent lines of
equal barometric pressure beginning with 30
inches at the earth's surface, and let us suppose
that the temperature of the central region is
raised by a certain amount.

2.8"

Fio. 10.

All the air thus warmed will expand. The
column H.A. will expand to height H.B., and as
each layer will expand all the way up, the sur-
face of the top layer will be most raised. Con-
sequently there will be a flow outwards of the
raised up air down the slopes marked by the
thick lines toward the neighbouring air of the
same pressure, which, not being expanded, lies at
a lower level.

The outflow will be greatest in the highest
layer since it is the most raised (the increase is

60 THE STORY OF THE EAIITH'S ATMOSPHERE.

denoted by the varying size of the arrows).
Meanwhile the loss of air above will lessen the
pressure on the earth's surface near the centre
of the area. Consequently the surrounding air
will flow in towards this centre chiefly in the
lowest layer, and the action having once started
will continue so long as the central area is more
heated than the neighbourhood.

r

^ ;^v^-S:rT:^^:^^^^JM^

Fig. 11.

We have already noticed that where sun heat
falls upon land it heats it up more readily than
water. Therefore particularly in the case of an
island lying in a tropical sea where the sun is
powerful the above action takes place as in fig.
(11) and we have the phenomenon known as the
local sea breeze. When the sun disappears at

THE TEMPERATURE OF THE ATMOSPHERE. 61

night the action is precisely reversed, and the air
near the surface flows outwards as the land breeze,
while above a certain height, which in local cases
is often as low as 1000 feet, the air streams in
over the rapidly cooling land.

After the action has once started things arrange
themselves as in fig. (12) where the lower curved

Fio. 12.

lines represent the depression caused by the loss
of air which has flowed outwards above, and
where NN'^ represents where the tendency to
flow in and out neutralise each other and there
is a plane of no motion called sometimes the
neutral plane.

The above action involves a certain amount of
upward motion of the air over the central part
of the heated area, and a corresponding downward
motion over the surrounding cooler area, but these
movements are evidently much smaller than the
horizontal outflow and inflow. The same action
also explains the origin ot the manifest monsoons
of Asia and Australia, where in the summer season
the air blows more or less towards a heated land.

62 THE STORY OF THE EARTH*S AtMOSPHERK.

area, and in the winter from it towards the
surrounding sea.

It also accounts in part for the annual changes
in the barometric pressure over large areas,
especially the low pressures in the middle of the
larger continents like Asia and America during
the summer, and the corresponding very high
pressures at the opposite season.

Unless some such system of rise and overflow
over the hotter areas and sinking and underflow
over the cooler areas took place, the barometer
would record a steady pressure over both areas,
and if we ascended over the mt)re heated area
we should find the pressure greater than at the
same level over the cooled area, because the air
being more expanded vertically, there would be
more top cover so to speak ovei* our heads.

As a matter of fact, notwithstanding the over-
flow which relieves this state of things, the
pressure at highly elevated stations like Leh
(11,800 feet) north of Kashmir, rises until the
beginning of May, and only falls very slightly in
June and July. Consequently the lowering of
pressure which appears so distinctly over Southern
Asia in the summer is confined to the lower half
(by mass) of the atpaosphere, that is to say below
18,000 feet, at which level the pressure is 15
inches. Above this level there is more or less an
outflow in the summer and an inflow in the winter.

A similar system of land and sea monsoonal
circulation exists everywhere, only in high lati-
tudes it is ordinarily masked by other motions
of the air, introduced by the frequent passage of
cyclones, and large travelling systems or waves

THE TEMPERATURE OF THE ATMOSPHERE 63

of high and low pressure. Even along Western
Europe the winds blow more towards the land
in summer and from it in the winter.

Where a small area on the land or sea is
heated up above its neighbourhood we have the
initial conditions for the formation of a dis-
turbance of equilibrium. In hot countries where
such a condition is more prevalent, there may
arise a cyclone, tornado, whirlwind, or thunder-
storm, under different conditions, which will be
alluded to later on, but in order that there may
be intense local action and a real * courani asce^vd-
anty^ the air must not be merely gently lifted
up and overflow, which is the only possible
condition when it is dry, but it must be nearly
saturated with vapour, in which case it will
flow upwards so long as the lowest stratum
continues to supply damp air. The part taken
by temperature in causing these phenomena will
be alluded to in a later chapter.

The present account of the temperature of the
atmosphere would be incomplete if it omitted to
notice the transfer of heat from one part of the
earth to another.

So far we have merely examined the heat
which falls locally or generally by means of the
direct solar radiation.

The temperature over any region is however
largely dependent on the heat brought to it by
winds. When they come from the sea their
temperature is modified by the influence of the
ocean currents, warm or cold, over which they
have travelled. When they come from the
interior of a continent, they are usually hotter in

64 THE STORY OF THE EARTH'S ATMOSPHERE.

the summer and colder in the winter than the
maritime regions towards which they advance.

Thus in summer our hottest wind in England
is the south-east, and the same wind often accom-
panies our most severe frosts in the winter. The
thermal effects of land winds thweforo change
with the season.

Sea winds, especially where they are connected
with ocean currents, and blow with some degree
of constancy, exercise a permanent influence upon
the temperature of countries over which they pre-
vail. The most marked warm sea winds are felt
on Western Europe, the Pacific slope north of
lat. 40**, and the eastern coast of South America.

These winds are not merely warm because they
have accompanied streams of warm water, such
as the Gulf stream of the Atlantic and the Japan
stream of the Pacific, but because their cooling
is retarded by the latent heat set free in* the
condensation of the vapour they bring from the
humid tropics.

Several attempts have been made to measure
the heat conveyed by both these streams. Dr
Haughton of Dublin some years ago estimated
that these two streams together carried one-third
of the total heat received by the northern
tropical zone towards the middle latitudes.
Ferrel, however, has more recently shown that it
is more probably one-sixth. As we have already
seen, the efi*ect on England is to raise the mean
temperature nearly 10 degrees above what it
would otherwise be. Norway is raised as much
as 16 de^ees, and Spitzbergen 19 degrees. On
the other hand compensating cold currents and

. THE TEMPERATURE OF THE ATMOSPHfiUE. 65

the winds which blow off them depress the
temperature of Eastern Canada, northern China,
wfistern South America, and western South
Africa. Newfoundland is thus about 10 degrees
colder than the normal for the latitude. The
North China coast about 7 degrees colder, and
even Honolulu, in the mid Pacific, has its
temperature reduced 5 degrees by the return
Japan stream cooled after losing its heat up north.

The general influence of the ocean currents in
reducing the difference which would exist
between the temperature at the equator and the
poles, may be inferred from the fact, that accord-
ing to Ferrel, if the surface of the earth were
entirely dry land, and there were consequently
no transfer of heat by oceanic or atmospheric
currents, theoretical considerations shew that
the temperature at the equator would stand at
about ISr F., while that at the Pole would
be 108' below Zero.

Observations, however, shew that the mean
temperature day and night at the Equator is
about 80° F., while that at the Pole is only
0° F. or Zero. Consequently the effiect of the
cii'culation of the ocean and the atmosphere, to-
gether is to depress the temperature at the
Equator about 50 degrees and raise that at the
Pole no less than 100 degrees, and in this
manner render the earth generally fit for human
habitation, since, if such extremes as those
mentioned prevailed, man would have been
forced to inhabit a very constricted zone in
middle latitudes. In like manner were the
earth deprived of its atmosphere the mean

66 THE STORY OF THE EARTH'S ATMOSPHERB.

temperature at the Equator would be 94 degrees
below zero F., while that at the poles would
be 328 degrees below zero F., and the mean
temperature of the whole globe 138 degrees
below zero F., — a terrible frost. In fact, even if
it were possible to do ^vithout air the human
species as at present constituted would in such
an event be quite unable to exist. With the
protection of an atmosphere the average
temperature of the earth, or more correctly of
the lowest stratum of the atmosphere is about
60° F. which is regarded as the most delight-
ful that can be enjoyed. So mvch do we owe
to the invisible envelope of atmospheric air,
which otherwise appears to constitute such a
flimsy blanket between us and the terrible cold
of stellar space.

Extreme local temperatures are due to the
concurrence of accidental causes tending to raise
or lower the temperature, such as the passage of
storms, prevalence of winds from north or south,
long continued clear weather, combined with those
of more regular incidence. Extremely high tem-
peratures will generally occur in these latitudes
soon after noon in July and August, and ex-
tremely low ones early in the morning in January
or February. Occasionally, however, the epochs
are considerably displaced.

The highest temperatures on the earth usually
occur in India, the Eed Sea, the Persian Gulf,
and Australia. Thus in the centre of the Sahara,
130 defflrees has been recorded. At Jacobabad
in the Kind desert, the temperature frequently
rises over 120** F. and even in New South Wales,

THE TEMPERATURE OF THE ATMOSPHERE, 67

120° and 121° have been recorded at Bourke
and Deniliquin. In February 1896, a tempera-
ture of 108 degrees was recorded at Sydney,
due to a remarkable prevalence of dry N. W.
winds blowing over it from the interior.

Paris has only once reached 106 degrees and
London has seldom recorded anything over 96°.

The coldest temperatures are founa not at the
poles themselves, where the water circulation
tends to bring heat from the equator but in the
north-east of Siberia and north-east America.

Werkojansk is the coldest place in the world.
In January the mean temperature there is 55° F.
below zero while all through the year the tem-
perature is only 5 degrees above zero.

During arctic expeditions, the Alert and
Discovery experienced 73 below zero, while
Capt Nares once saw the thermometer descend
to 84° F. below zero.

Of recent years, a great extension of our
knowledge of the phenomena of the atmosphere
has been made by the application of what is
known as thermo-aynamics.

Prof. Bezold of Berlin, the late Mr Ferrel of
Washington, Dr Hann of Vienna, and others
have cleared away much of the loose and misty
reasonings which characterize the work of their
predecessors, but the subject is too difficult and
technical to be alluded to here, A few of the
leading ideas However will be briefly touched
upon when some of the particular atmospheric
phenomena are being described further on.

CHAPTER V.

THE GENERAL CIRCULATION OF THE
ATMOSPHERE.

The * Story of the Winds ' is interesting and
important enough to form the subject of a
separate volume and within the compass of one
which endeavours to cover the varied phenomena
of the atmosphere generally, only the more
salient points in connection with atmospheric
motic>n can be reviewed. In these latter days,
in spite of the old saying that " the wind bloweth
where it listeth '' and the manifest and apparently
capricious changes which characterize its be-
haviour in these midway latitudes, we know that
there exists an independent dominating scheme
of general circulation between the poles and the
equator. This scheme results from the action of
nearly permanent differences of temperature
between these points in combination with certain
mechanical laws resulting from the shape of the
earth and its rotation on its axis.

In former days many guesses were made more
or less at variance with both facts and theory.
Even Maury's fascinating attempt in 1855 to
weave observation into a connected system, failed
owing to the imperfect knowledge existing at
that time of the winds of the entire globe as
well as of the true laws which operated.

The earliest attempt at any rational scheme of
accounting for the more obvious features of the
general circulation appears to have been made in
1735 by Hadley.

GENERAL CIRCULATION OF THE ATMOSPHERE. 69

The regularity of the * trade winds ' was then
attracting the attention of scientists, and in a
short paper in the Philosophical Transactions,
Hadley advanced a theory to account for this
which sounded so plausible, that for over a
century it remained unquestioned.

Hadley's theory in brief was, that owing to the
general difference of temperatures between the
polar and equatorial regions, a motion of the air
took place similar to that just described in the
last chapter, in the lower strata towards the
zone of greatest heat, while the easterly*
direction of the trades was attributed to the fact
that as the air continually arrived at parallels
where the earth's surface moved faster eastwards
than the part it had just left, it tended con-
tinually to lag behind in a westward direction,
and so appear to blow partly from the east.
Hence it became the north-east trade on the
northern and the south-east trade on the southern
side of the equator. Carried to its logical con-
clusions Hadley's theory would require the trades
to blow all the way from the poles to the
equator, the return current being confined almost
entirely to the upper air.

Moreover the highest pressure as measured
by the height of the mercury column in the
barometer air balance, should be found at or
near the poles.

As a matter of fact, however, it was found
that neither of these circumstances took place.

The trades extended no further than latitudes
30 degrees N. and S. of the equator, the pressure
* From the East.

70 THE STORY OF THE EARTH'S ATMOSPHERE.

at the poles, especially the south pole was
permanently lower than at the equator (about ^ths
of an inch of mercury) while the highest pres-
sure was found to occupy two belts between 30**
and 40° N. and S. of the equator.

Obviously therefore there was something
radically wrong with Hadley's theory.

In 1856, Mr Ferrel afterwards Professor in
the United States Weather bureau, tackled
the subject and found out that Hadley had
entirely overlooked the fact that the earth is a
sphere.

In consequence his theory contained two
serious errors, one of which was that only air
moving north and south was deflected by the
earth's rotation, while that moving in any other
direction remained unchanged.

The only circumstances to which Hadley's
theory could possibly apply would involve the sup-
position that the earth was' a perfectly flat plane
composed of separate planks parallel to a straight
line equator. Also that these planks moved
along with difierent speeds beginning with
1000 miles at the equator and gradually de-
creasing to about 850 miles at latitude 30°,
manifestly a very difl^erent affair from a spherical
surface like that of the earth.

Some few years before Ferrel approached the
question, the eminent French mathematician
Poisson in 1837 read a paper before the Paris
Academy, in which he demonstrated that when
a freely moving body passes over the earth's

surface in any direction^ the eff*ect of the earth's
rotation is to cause it to deviate (not lag) to

GENERAL CIRCULATION OF THE ATMOSPHERE. 71

the right of its path in the northern hemisphere,
and to the left in the southern.

Employing the same reasoning as Poisson, but
appilying it to masses of air instead of solid
bodies Ferrel gradually built up a satisfactory
explanation of the general circulation, and with
the help of suitable modifications, applied the
same principle to explain the leading features
of cyclones and tornados.

In general, if a mass of air initially tends to
move on a rotating sphere towards a certain point,
impelled in the first instance by a difference of
density or pressure, it tends to move continually to
the right when looked at from a point above the
N. pole of rotation and
unless prevented from
doing so by any extra
force resisting such
motion, would continue
to deviate until it had
turned through a com-
plete circle thus &g.
(13).

Suppose a particle
of air at A starts to
move towards B. In-
stead of moving in the straight line AB, it
will tend to move in the curve AC, and if
it is very near the pole it will eventually com-
plete a circle* as above in 12 hours, the size de-

* In other latitudes the inertia curve as it is termed is
more like the series of loops cut by a skater restricted
between certain limits of latitude. At the equator it
vanishes.

72 THE STORY OOF THE EARTH'S ATMOSPHERE.

pending on its velocity. Thus for a speed of —

Radios of inertia circle
in miles.

20 miles an hout . . 77

10 „ „ . . 38

5 „„ ... 19

We have the corresponding values of what is
termed the curve of inertia.

Prof. Davis of Harvard has suggested a very
pretty experiment which can be performed by
any one who wishes to have visual evidence of
the existence of this inertia curve.

Take a small circular table, and lay a sheet of
white paper over it. Then take a marble and
dip it in ink and lay it near the centre of the
table. Next tilt the table slightly so as to give
the marble a slight motion towards the edge, and
at the same time rotate the table about its centre.
Then the marble will be found to trace out in
ink a curved line on the paper which will fairly
represent the inertia curve or the curve of
successive deviations from a straight line by
which the particle through its inertia (or laziness)
is unable to accommodate itself to the varying
motion of the parts over which it rolls.

In whatever direction the table is tilted the
curve will still be traced out, the curvature
sharpening with increased rotation of the table
(analogous to increased latitude) and lessening
with increased tilt by which the velocity of
the particle is augmented.

This tendency of air to move to the right of
its original direction of motion, is what really
accounts for the development of those permanent

GENERAL CIRCULATION OF THE ATMOSPHERE. .73

or rapidly changing differences of barometric
pressure which accompany the large general or
small particular air motions. A difference of
temperature alone, for example, between the poles
and equator, or between two neighboming parts
of the earth would cause a very slight alteration
in the barometric pressures, but when the air
begins to move in the direction of the lower
pressure its tendency to push to the right, causes
a squeezing and heaping up. of air to the right of
its path, and a corresponding stretching apart or
lowering of density and pressure to its left, until
the difference of pressures becomes great enough
to prevent its further movement to the right and
it moves in a path regulated by these joint
tendencies, thus —

JfSom

Direction,

in which •

air starts to

move

Fia. 14

Jpth (curred

' tn which ear
ultimatelif
doe^ more.

Cold area

fcan
into which
car tendi to
be d9 flee tod
Fotce
exerted by
pressure gradient

A mass of air at the cold area will tend
initially to move towards the less dense warm
air, but once it starts it tends to move along the
inertia curve. Eventually the high pressure

74 THE STORY OF THE EARTH'S ATMOSPHERE.

(denoted by the shading) of the heaped up air
on this side exerts a force indicated by the
arrow directed towards the increased low pres-
sure to the left, and finally the air making a
compromise moves along a line between the two,
indicated by the direction, labelled ' Path ' etc.,
so that instead of moving directly from high to
low pressure it only partly moves towards the
latter, keeping the high pressure to the right
and the low pressure to the left of its path. In
the southern hemisphere owing to the reversed
point of view right becomes left and the high
pressure would be to the left of the path and the*
low pressure to the right.

This diagram will be found to supply the
explanation of the general relations between
pressure and wind, especially if it is remembered
that where, as on land and near the surface, the
air is prevented by friction from moving with
freedom, the back thrust in the opposite direc-
tion tends to make the ultimate path point
more towards the low pressure, while at
sea and at great altitudes, where friction is
small, it moves almost at right angles to the
line joining the central areas of high and low
pressures, or in the technical language borrowed
from engineering, at right angles to the direction
of the barometric gradient.

Before alluding to Ferrel's explanation of the
general circulation of air over the globe on these
principles, let us see what this circulation
really is like from observation.

In the two plates adjoining, figs. (15) and (16), in
which the actually observed barometric pressures

GENERAL CIRCULATION OF THE ATMOSPHERE. 75

and winds at two opposite seasons of the year are
represented, it will be noticed that, overlooking
minor features, there is a broad belt over the
equator, over which the barometric pressure is
about 29*80 inches, gradually rising on either side
to two belts of high pressure, in latitude 30"
in places reaching 30*2 inches, and generally
about 0*2 inches higher than over the equator.
Within this area, the trade winds blow throughout
the year on each side of the equator, except over
the North Indian Ocean, where in July they
blow in towards an area of excessively low
pressure and high temperature as the south-west
monsoon of the Indian seas, which brings the
rain, that has made India such a much more
fertile and populous country than the neigh-
bouring peninsula of Arabia. In the map
fig. (20), p. 86, the monsoon Mdnds are repre-
sented blowing over India during July. In
January, the south-west winds disappear, and
in. the general chart it will be seen that their
place is taken by Northerly or North-easterly
winds, blowing down towards the equator, from
the large area of high pressure which at this season
spreads over the whole of north-eastern Asia.

On the polar sides of these bands or nuclei of
high pressure, it will be observed that the winds
blow more or less towards the poles, especially
in the southern hemisphere.

The lines (isobars) on these maps, by which
the changes in the distribution of the mean
monthly barometric pressure is indicated, are
similar to the contour lines or lines of equal
elevation employed to represent the contour of

78 THE STORY OF THE EARTH'S ATMOSPHERE.

a hilly country. They do not necessarily repre-
sent real elevations or depressions of the atmos-
phere, because increased or decreased pressure is
more due to a greater or less squeezing or
density, than to a piling up of the atmosphere
into absolute heaps and hollows, but since the
effective results would be very much the same in
either case, they may practically be considered
as atmospheric contours. More correctly, they
are the lines along which atmospheric contours
intersect the earth's surface, the pressure over
which at sea-level, (about 30 inches), lies half way
up the atmospheric slope. The accompanying
figiu-e will render this clearer.

Fig. 17.

The sloping lines marked 30*2, 30, 29-8 etc.,
represent sections of the actual atmospheric
isobars or pressure contours. ByCyD, points where
these lines cut the earth's surface. The dotted
continuations represent how the lines would run
if the atmosphere took the place of the solid
earth. The curved lines starting from B and C
to E and F, denote the lines as they occur on the
earth's surface or appear on a plane chart, when
the contours curve round a central area A, where

GENERAL CIRCULATION OF THE ATMOSPHERE. 79

the pressure in this case is about 29*7 inches. In
considering the general circulation, A may be
taken to represent the North or South Pole, in
which case the diagram represents something like
what actually takes place. When we are deal-
ing with particular motions, A would correspond
with the centre of a cyclone or travelling
disturbance.

Returning to our story, it is plain from these
maps, that the circulation of the atmosphere
comprises a great deal more than a mere system
of trade winds, blowing towards the equator.

Any theory that pretended to explain the
entire system, would have to account for the
prevailing high pressures about latitude 30**
N. and S., and the poleward trend of the wind
on the polar sides of these atmospheric sierras.

Ferrel, in his first paper in 1856, not only
shewed that Hadley's theory was mathematic-
ally incorrect, but that Maury's fascinating
scheme, put forward in his Physical Geography
of the Sea, erred both in fact and the laws of
physics.

Reversing the usual procediu:e by which laws
are induced from observations and starting with
a few fundamental principles, such as the law of
deflection already noticed, he shewed that the
high pressure belt about 30° and the sy^m
of poleward winds on the polar sides of it were
necessary consequences of these principles.

Ferrel's final ideal chart of atmospheric circu-
lation on the earth is represented by fig. (18)
where the average motions near the surface are
represented in plan in the shaded circle, and

80 THE STORY OF THE EARTH'S ATMOSPHERE.

in vertical section at the border, and though
his explanation is a complicated piece of mathe-
matical reasoning, the following represents the
pith of it in simple language.

Assuming that the air over the equatorial zone
is heated above that near the pole, it expands

Fig. 18.

near the equator and contracts over the pole.
Consequently the uplifted air over the equator
tends to flow downhill as it were towards the
poles and a corresponding flow near the surface
takes place towards :the equator. If the earth

GENERAL CIRCULATION OF THE ATMOSPHERE. 81

did not rotate on its axis the upper air would
flow direct to the pole then descend and return
towards the equator along the surface. Since
however the earth does rotate, the upper air is
deflected in the northern hemisphere to the right
(increasingly as it travels polewards) so much
that were it not for the downward slope towards
the pole, it would eventually deviate towards the
equator. CJonsequently the pressure to the left
of its path, Le, towards the pole, is decreased and
the pressure to the right is increased. This
increases what would otherwise be an insignificant
slope to what is actually observed. By the time
this upper air has reached latitude 30° which
divides the hemisphere into two equal areas*
(though it is only a third of the actual distance
on a meridian) it has descended to the surface
and overpowers any tendency towards contrary
motion in the air there, and the entire atmos-
phere tends to move bodily eastwards from thence
to the pole.

Meanwhile the air near the surface between
latitude 30° and the equator, moving towards
the latter, deviates towards the west and heaps
up pressure to its right and lowers the pressure
to its left in the same way. Consequently on all
accounts there is a tendency on the part of the
air to heap up and increase in pressiu-e about
latitude 30° and to be reduced in density or
pressure near the poles and the equator. Also
since the air reaches a terminus at the poles and

* This most important fact is one of those thin^ which
is no< as a rule taught at school, though it is of immense
significance.

F

82 THE STORY OF THE EARTH'S ATMOSPHERE.

equator there will be calms at the surface at
both these points. Moreover since on either side
of latitude 30° or more correctly 35** the air moves
along the surface in contrary directions, there will
be an absence of prevailing winds over this region.
These calms are known to exist, and owing to
their proximity to the tropics used to be called
the calms of Cancer and Capricorn.

The preceding explanation may be better

VfAK^

realised if we take a vertical section of the atmo-
sphere along a meridian as it actually exists, and
draw sections of the general planes of equal baro-
metrical pressure as they exist by observation on
the average at different levels between the equator
and the poles as in ^g, (19). The poles are at N.
and S. and the equator is at Q.

The line ABODE F represents a section of
the general isobar of 30 inches at sea-level and
shews two cols or hills at latituude 30°. Then as

GENERAL CIRCULATION OF THE ATMOSPHERE. 83

we ascend these cols gradually disappear owing
to the absence of the surface trades and therefore
of the side pressure they create which keeps the
opposite pressures e3^erted by the winds on their
polar sides from pressing the air into one high
pressure belt over the equator. As we ascend,
therefore, these cols gradually coalesce into one
central hill from which the air descends as a
westerly upper current (blowing partly from the
south in the northern hemisphere and from the
north in the southern), into the two polar valleys
on either side.

The depth of the atmospheric valleys of pressure
below the top level of pressure at the surface of
the earth converted into feet of air is found to
be about 262 feet at the equator, about 320 feet
at the north pole, and 640 feet at the south pole.

At a height of 30,000 feet above the surface
the north polar valley is 2,800 feet and the south
polar valley 3,10.0 feet below the mean level.
The height at which the equatorial valley dis-
appears varies from 8,000 to 12,000 feet. Above
this level there is a downward slope all the
way to the arctic and antarctic circles and
possibly to the poles themselves.

Viewed as a whole, the general circulation of
the air according to Ferrel, may be considered as
consisting of two huge atmospheric whirls, or, as
they are technically termed cyclones, with the
poles as centres, in which the air rotates in each
hemisphere in the direction of axial rotation.
On the equatorial side of each of these whirls, a
belt occurs in which the motion of the air is con-
trary to that of axial rotation. These are the

84 THE STORY OF THE EARTH'S ATMOSPHERE.

trade wind belts. Between these two areas the
air is heaped up into two zones of high pressure,
reaching their highest values in the northern
hemisphere about latitude 35° and in the
southern about latitude 30°.

Since this system was established by Ferrel,
Dr Werner Siemens, von Helmholtz, Herr Moller,
Professor Oberbeck, Dr Sprung and others have
developed the theory by the aid of more modern
refined methods and closer reasoning, but their
conclusions are substantially the same as those
deduced by Ferrel, and the above sketch repre-
sents as far as can be attempted in a work like
the present the modern theory of the general
circulation of the atmosphere.

The most noticeable permanent modification
from the ideal condition of things is afforded by
the exceedingly low pressure round the south
pole and the strong north-west winds which pre-
vail south of the Atlantic, Indian, and Pacific
Oceans, and which enabled Australian clippers
in the old days to make passages of fabulous
rapidity. This is due to the fact that the south-
ern hemisphere is chiefly covered by water which,
by exerting less friction on the air than land,
allows its motions to occur with greater freedom.
In consequence of this the Antarctic barometric
depression is more developed and more sym-
metrical than the northern. For example the
pressure on the latitude corresponding to our
.Antipodes is permanently -^ths of an inch below
what we experience, while the wind velocity is
three or four times as great.

The seasonal changes and migrations as the

GENERAL CIRCULATION OF THE ATMOSPHERE. 85

sun moves north and south are scarcely notice-
able in the southern hemisphere for the same
reason. In July the pressure over the tropical *
belt as we may term it, is slightly increased, and
the belt lies a little further south than in January.
In the northern hemisphere on the other hand,
the seasonal changes are far more conspicuous.

The high pressure nuclei which in July lie on
the eastern sides of the Pacific and Atlantic
oceans have by January shifted on to the con-
tinents of America and Asia, while the low
pressures, which in July occupied the middle
parts of North America and the centre of Africo-
Asiatic continent, (the centre lying almost exactly
over the Persian Gulf which is the geographical
land centre) in January lie over the North Pacific
and North Atlantic. Meanwhile the equatorial
low pressure belt, or barometric equator as it may
be termed, which in January is confined between
its ideal equatorial limits, in July runs up into
the northern continents and in Africo-Asia in
particular, may be said to lie entirely over the
land surface, where it causes the Monsoon as
in figure (20). The relation of these extensive
migrations to the effect of seasonal changes in
solar heat on the air lying over land and water
surfaces is obvious.

The result of these large transfers of air and
air pressure north and south, and between the
oceans and the continents, is to cause what is
termed the annual variation of the barometric

* The term tropical is here used to signify on or near the
tropics or turning points and not to the entire space
between them as is nsnally the case.

86 THE STORY OF THE EARTH'S ATMOSPHERE.

pressure at any single place on the earth. This
annual variation reaching its extremes generally
in January and July, will be found to be largest
in the centres of continents such as Asia where
the barometric shift is greatest and least on the

Fio. 20.

coasts which are the axes of the annual pressure
see-saw between the oceans and the continents.

In England the change is small, amounting to
about 0.12 inches between January and July.

In India, it increases from 0*26 inches in

GENERAL CIRCULATION OF THD ATMOSPHERE. 87.

Bengal to 0*62 inches in the Punjab, while over
Siberia and central Asia it reaches about 1 inch.

The mean pressure over the whole earth is
29*89 inches. In the northern hemisphere the
mean pressure for January is 29*99, and for July
29 -87. For the southern hemisphere the pressures
in the same months are 29*79 and 29*91. From
this it is evident that there is a much greater
difference between the quantity of air over
the two hemispheres in the northern winter
in January than in the southern winter in
July.

This difference in favour of the northern
hemisphere really means that owing to the
greater cooling and contraction over the northern
land area in the winter 32,000,000 tons of air
have shifted over to supply the defect. There is
no protective tariff placed upon this valuable
import from the southern hemisphere.

A seasonal shift of the general wind system
of the lower strata occurs like that in the
barometric pressures, as the sun shifts no^th and
south.

The shift of the sun in latitude is 47°, but
the wind systems only shift from 5* to 8**
on the northern, and from 3° to 4° on the
southern side of the equator. The accom-
panying diagram fig. (21) ^ves an idea of
the effect of the shift. The central belt
(sub-equatorial) represents the district which is
alternately subject to the doldrums or equatorial
calms, formerly the bane of sailors, and the
attendant bordering trades as they oscillate north
and south.

88 THE STORY OF THE EARTH'S ATMOSPHERE.

The width of this belt varies from 350 miles
in the Atlantic to 200 in the Pacific and lies on
the north side of the equator all through the
year, owing to the fact that the system of
circulation in the southern hemisphere and round
the South pole, owing to smaller friction, is so
much more powerful than that in the northern

^ ^M-^i^i^!!>^^

%, ^ \ ^^ ^ jy'*»v<oOx. >i/i^j*« f H ^ '^'•7 \ * * S ^

Fio. 21.

that the pressure belts on that side are all pushed
northwards.

The sub-tropical belts, as the calms of Cancer
and Capricorn are now termed, are similarly the
alternate arena of westerlies, trades, and inter-
vening calms.

GENERAL CIRCULATION OF THE ATMOSPHERE. 89

So far, we have mostly considered the general
circulation with respect to the motion of the
atmosphere near the surface. The Upper
current which blows all the way from the
equator to the poles, is usually termed the
Anti-trade in the tropics, because it overlies
and blows in the contrary direction to the
latter from the south-west on the northern,
and from the north-west on the southern
side of the wind equator. In the equatorial
zone its lowest limit is about 10,000 feet,
and as we proceed polewards, this limit de-
scends so that along the ran^e of the Hima-
laya in the winter season, this upper current
descends to within 2000 or 3000 feet above
the sea.

On the Peak of TeneriflFe, Prof. Piazzi Smyth,
when he was conducting astronomical observa-
tions there in 1860, was able to walk up through
the north-east trade wind and find the south-
west upper current blowing continuously at his
station at Alta Vista, 10,000 feet up the
mountain side.

In like manner the smoke of lofty volcanoes
such as Cotopaxi 18,000 feet, and Coseguina
lying in the trada wind zone have been observed
to blow from the west, contrary to the surface
wind.

Nearer the poles about latitude 35' to 40**
the lower edge of this upper current touches
the earth, and its lower hklf breaks up into
what Prot Helmholtz terms vortices. In plain
language it separates into irregular currents
which form the cyclonic storms which are so

90 THE STORY OF THE EARTH'S ATMOSPHERE.

prevalent in high ktitudes on either side of the
equator. Meanwhile the upper part of the cur-
rent, except where it is affected by local disturb-
ances or whirls, continues to move generally
from the south or north-west. Its motion is
is motion determined by observation of the high
clouds which float at an average elevation,
according to the most recent measurements, of
about 27,000 feet.

The general circulation of the atmosphere and
its seasonal shifts is intimately bound up with
the general distribution of the rainfall of the
world, and the permanent occurrence and seasonal
shifting of zones of drought and rain. Locally,
rain is due, especially in high latitudes, to the
passage of cyclonic storms, but in the equatorial
and trade wind zones, the rainy season is
almost entirely determined by the shift of the
doldrums.

Without at present going into the question
of how rain is produced in all cases, it is easy
to see why the central belt of equatorial calms
is an area of constant cloud and rain. For the
air there, supplied with vapour bv the inflowing
trades on either side, is constantly rising up to
higher and colder levels, where it cannot contain
so much vapour as at sea-level. The surplus
therefore condenses first into cloud, and then
into raindrops which fall back to the sea-
level. On the equator, as at Singapore, it
rains everyday. As the doldrum belt oscil-
lates north or south, it may give rise to one
rainy and dry season, or in some cases to tyro,
thus,

GENERAL CIRCULATION OF THE ATMOSPHERE. 91

Extreme Northerrv Extreme Southern
position ofrainbelt position qfrairibelt

essjji

kVisf.

Fio. 23.

If a place is situated at a or b, just within the
edge of the rainbelt at its extreme positions, it is
within the belt during half of the year about,
and without it the other half. Consequently, it
has a rainy season for six months, and a dry
season for about six months. This is the case at
Panama, where it rains from May to November,
and is comparatively dry during the remaining
months. Similar equal periods occur in Bengal,
the Nile Basin, and Northern Australia. When
a place is situated at e or f, nearer the outer
edge of the rainbelt in its extreme position, the
rainy season is shorter, and the dry season
longer. The Punjab, Upper Burmah, northern
Mexico, and northern Centml Australia are
regions which underly the rain-belt for only three
months of the year, and if, as in the year 1896

92 THE STORY OF THE EARTH'S ATMOSPHERE.

some irregularity occurs in the arrival of the belt,
the rainy season may be so short as to cause
drought and famine.

Places such as g and h, lying outside the influ-
ence of the equatorial rainbelt altogether, would
be rainless except for the extension equatorwards
of the system of polar winds, which sometimes
descends as far south as latitude 35*" in the
winter. Between latitude 35" and latitude 20°
N. and S., except where as in India, monsoons
blow from the equator, or along coast lines,
no regular rainfall belt arrives, and the dry
desert zones of the earth tend to form.

The dry regions of California, Arizona, and
Colorado in North America, the great Sahara
and Nubian deserts of North Africa, and the
Arabian and Persian dry areas all occur
between these limits in the northern hemi-
sphere.

In the Southern hemisphere within the same
parallels are the dry regions of the Argentine and
Eastern Patagonia, a &rge dry region in South
Africa and one comprising the whole interior of
Australia. These areas coincide with the belts
of high atmospheric pressure and may be said to
mffer from permanent fine weather.

Places at c between the two positions of
the equatorial rainbelt, experience two short
dry seasons alternated by two short seasons of
rainfall. Such are Ceylon and southern India,
Colombia in South America, parts of the Nile
basin and Java.

In the latitudes between 35 degrees and the
poles, the seasonal rains are entirely regulated

GENERAL CIRCULATION OF THE ATMOSPHERE. 93

by the seasonal shifts in the polar system of
general winds.

Here again, owing to the shift in latitude of
a principal single rain belt corresponding to the
seasonal shift of the sun, fiome places in the middle
of the area between its extreme limits undergo
two rainy and two dry periods. In South
Europe, for example, the rain falls mostly in
the winter, because the system of winds circling
round the pole reaches its furthest extension
towards the equator at that season. In middle
Europe, the rains fall chiefly in spring and
autumn as the belt moves north and returns,
and in Northern Europe they fall chiefly in the
summer.

The general circulation of the atmosphere not
only determines the prevalent direction of the
winds on the surface and in the upper regions,
but also exercises a very large influence upon
their average velocity.

The practical importance of possessing a know-
ledge of the general velocity of motion of the
air above the earth's surface is evident when
we touch upon the subject of ballooning or the
coming flight of man.

When man is able to circumnavigate the ocean
of air with the same ease that he sails across the
ocean of water he will require to possess as
accurate a knowledge of atmography (to invent
a title) as he does at present of hydrography.
We have at present a hydrographer to the
admiralty, and we shall then require the services
of one who will tell us all about the movements
and conditions of the air, not only at sea-level,

94 THE STORY OF THE EARTH'S ATMOSPHERE.

but in the upper regions whither it will be
necessary to ascend in order to cross mountain
chains.

From the theory of circulation as developed
by Ferrel and Oberbeck, it appears that the
surface wind ought to reach its greatest average
velocity about latitude 50', and diminish thence
both towards the poles on the one side and
towards the equator on the other. As a
matter of observation this appears to be the
case.

Taking an average of the winds throughout the
year, the late Prof. Loomis found the following
average values for the wind in typical lati-
tudes: —

Mean velocity of wind
In miles per hour.

United States .
Europe

Southern Asia .
West Indies

9-5

10-3

6-5

6-2

In England the average surface wind is nearer
12 miles an hour. Like other elements the
surface wind varies with distance from the sea,
time of year, and time of day.

The movements of the air are much affected
by the nature of the surface over which it

It moves faster over water than over land, and
faster over flat bare land than where it is hilly
or covered with forest. In the interior of
continents it is much more sluggish than near
the coasts or out at sea.

GENERAL CIRCULATION OF THE ATMOSPHERE. 95

Thus in India, the wind velocity diminishes
as we leave the coast in the following
manner :—

rr«««» Velocity of wind In

^®^*- miles per diem.

^ . ^Bombay . 408

On coast jj^^^J^^^ ^^^

I'/^^^^i* [Calcutta . 123

100 miles ^j)^^^^

from sea j

600 miles W^^

from sea J

SOOmilesjjj^^^,^^^

from sea J

Again, while the velocity over Eiurope is 10
miles an hour, it is as much as 29 miles an hour
over the North Atlantic.

Everyone is aware of the great amount of
wind experienced even in summer when crossing
the Channel as compared with that felt on shore.

A similar difference of velocity is observed as
we ascend from the earth's surface.

This is partly due to the decrease of friction
and partly to the increased slope down which
the upper air raised by the equatorial heat tends
to flow towards the poles.

Near the surface and for the first 50 or 100
feet the increased velocity with height is entirely
due to the diminished friction encountered by
the air against the roughness of the surface,
trees, houses, and other obstacles. After that
the retardation of the lower layers is com-
municated to the upper ones in a gradually
decreasing scale by means of the friction of

96 THE STORY OF THE EARTH'S ATMOSPHERE.

the air molecules against each other, somewhat
as a spoon passed through treacle or honey drags
some of the surrounding mass along besides what
it pushes directly in front of it.

This property of the particles of a gas is termed
viscosity, owing to its similarity to the visible
behaviour of what is termed a viscous liquid like
melted glass.

Such resistance which neighbouring portions of
gas offer to one another's motion is due, on the
Kinetic theory of gases, to the collisions which
are constantly taking place between the rapidly
oscillating molecules.

A good parallel is offered by a crowd of per-
sons all moving along a road in the same general
direction towards some common point of interest.
If everyone moved at the same pace in parallel
lines, the speed of the crowd would be the same
as that of any individual person, but owing to
the fact that some persons cannot walk so
quickly as others, some stop to look at the shop
windows, others walk crookedly and jostle their
neighbours, while some walk back against the
crowd because they have left something behind
them, the average speed of the crowd is sensibly
reduced below that of the quickest walkers,
though it still remains above that of the
slowest.

In like manner if two streams of persons, one
moving faster than the other, join together and
personal interchanges take place between them,
the persons who walk across from the slower
stream into the quicker one tend to retard its
average pace. Those on the contrary who move

GENERAL CIRCULATION OF THE ATMOSPHERE. 97

across from the quicker stream into the slower
one tend to accelerate its average pace.

In a similar manner, adjacent strata of the
atmosphere tend to equalise each other's speed
on a small scale by interchange of molecules,
and where large masses intermingle, as in the
general circulation, by interchange of masses,
moving with different average speeds.

It is by this internal friction between inter-
mingling air masses in addition to that experi-
enced by the friction of the lowest layer against
the earth, which is gradually communicated to
those above, that the atmosphere does not assume
unheard-of velocities and storms are not more
violent than they are. The latter alone would
not be enough.

Helmholtz, for example, has calculated that if
the atmosphere generally started moving over the
earth with a certain average velocity, say of 20
miles an hour, it would take no less than 42,747
years to reduce this to 10 miles an hour by the
action of friction with the ground.

The first experiments to find the increase of
velocity of the air with the height above the
ground were undertaken by Mr T. Stevenson
of Edinburgh, who attached anemometers of the
Eobinson pattern at different heights on a 50-foot
pole. Here the retarding effect of the ground
was found to rapidly diminish in the first 15 feet.
Above this the rate decreased. For building and
efigineering purposes it is best to make experi-
ments in the locality and trust to no formula,
since the rule alters rapidly up to the first 100 feet.

Beyond this and up to 1800 feet experiments

G

98 THB STORY OF THE EARTH'S ATMOSPHERE.

conducted bv the author iu 1883-5, under a
grant from the Royal Society, resulted in estab-
lishing the fact that the average velocity at 1600
feet is just double the velocity at 100 feet.
Above the former height the rate appears to
increase, if we are to judge from the observations
of the clouds made at Blue Hill Observatory,
near Boston, U.S.A. Mr Clayton's recent obser-
vations there of the movements of the different
cloud strata reduced to English measure give
the following results throughout the year : —

m««^ i^-oi Height Average speed in

Cloud level ^^ ^J^j mumper hour.

Stratus . . . 1,676 19

Cumulus

Alto-cumulus

Cirro-cumulus

Cirrus

5,326 24

12,724 34

21,888 71

29,317 78

The rule in this case may be simply remem-
bered thus. For every 1000 feet of ascent add
on about 2 miles an hour to the velocitv of motion.

In winter the speeds are twice as large at the
upper levels as in summer. For the winter half
year the speed of the cirrus is as much as 96
miles an hour, considerably faster than our
express trains travel at present.

In Europe the velocities appear to increase
less rapidly, but are still large when compared
with those at the surface.

An average of closely concordant results,
obtained hyVr Vettin of Berlin and Hagstrom
and Dr Ekholm of Upsala in Sweden, make the
velocities at 4300 and 22,000 feet about 19 and
38 miles per hour respectively. Here the rule

GENERAL CIRCULATION OF THE ATMOSPHERE. 99

gives about 1 mile per hour for each 1000 feet of
elevation, which is probably nearer the mark for
Europe generally.

This great increase of Velocity of the average
motion of the aerial ocean as we rise above the
surface is scarcely realised by us tiny mortals
who dwell mostly at its base.

The loftiest building is scarce 1000 feet above
the ground, while the loftiest inhabited place is
but half-way through the mass and probably a
twentieth of the actual height of the atmosphere.
The great velocity often attained by balloons
is thus readily explained.

At the same time it is equally plain that no
navigable balloon will ever be able to stem the
currents above 5000 feet, while flying machines
would do well to travel with the wind above this
elevation. In fact they may eventually utilise
these rapid currents much in the same way as
the Australian clippers formerly utilised the
brave north-west wind which blows so powerfully
round the watery expanse of the southern ocean.

One very curious result of this great motion
at high altitudes has been recently pointed* out
by Herr MoUer, a German engineer. The
energy of the motion of the air or the power
it possesses of performing work is propor-
tional to its speed and mass. Holier has thus
calculated that the energy of the upper half of
the atmosphere — i.e., the half above 16,000 feet,
is no less than six times the energy possessed by
the lower half. Of this inconceivably enormous
store of energy we at present utilise a minute
proportion in sailing ships and driving windmills.
The rest is completely wasted.

CHAPTER VI.

THE LAWS WHICH RULE THE ATMOSPHERE.

The story of the earth is for the most part a
chapter of ancient history. The story of the
atmosphere is a tale of to-day, and even of
to-morrow. When we have opened up the
earth's crust to view we can trace the operation
of past changes in the physical and chemical
nature and position of the diflPerent rocks. All
the atmospheric motions and changes, on the
other hand, are going on before our eyes. Every
action, moreover, is subject to the reign of law.
The chaos which at first sight appears to sur-
round the infinite complexity of atmospheric
phenomena is reduced to harmony and order in
proportion as we learn the true laws which
operate in the grand laboratory of Nature. We
have been a long time learning our lesson, and we
are only now beginning to rise from superstitions
and guesses to those intellectual "Delectable
mountains " whence we may, even though it be
"through a glass darkly," snatch a glimpse of
the true character of the mysteries of our
wonderful atmosphere.

The earth is a symbol of rest, stability, and
permanence. The atmosphere, on the other
hand, is in ceaseless motion and constant ac-
tivity, under the influence of the heat of the sun,
the cold of space, the rotation of the earth, and
the changes of the seasons, as it moves in its
orbit round the sun. Physicists working in their
laboratories have discovered that certain laws

100

THE LAWS WHICH RULE THE ATMOSPHERE. 101

are obeyed by air in common with other gases,
and it is only when we know these laws that we
can interpret the phenomena which are daily and
hourly observed m the sky and air around us.
One of the first laws relating to the atmosphere
was discovered by Dr Boyle, the celebrated
Glasgow chemist, and Marriotte of France, and
is usually called Boyle and Marriotte's law.
This law states that if a volume of gas (which is
elastic and compressible), confined within certain
limits, such as an elastic bag, is subjected to
compression, its pressure increases in the same
proportion as its volume decreases. Thus, if 6
cubic feet of air at the ordinary atmospheric
pressure were squeezed together until they occu-
pied only 3 feet, the pressure or resistance of the
air would rise to that of two atmospheres ; and
if a mercury barometer, at first marking 30 inches,
were placed within the containing vessel, the
column of mercury would at the end of the ex-
periment rise to a height of 60 inches.

Human beings, when subjected to moral
pressure, frequently exhibit similar character-
istics, though their resistance cannot be measured
on a moral barometer. In the free atmosphere
such an ideal case seldom occurs, since the air
generally finds some escape from complete com-
pression by expansion or motion in various
directions.

One immediate result of this law is the great
density of the lower strata of the atmosphere,
due to the compression to which they are sub-
jected by the weight of the overlying layers. In
like manner the rarity of the upper air is due to

102 THE STORY OF THE EARTH'S ATMOSPHERE.

its smaller compression. Also, when any mass
of air h forced upwards, it comes under gradually
decreasing pressure, and consequently by Boyle's
law it expands. Conversely, if it is forced down-
wards, it contracts owing to the increasing pressure.

It is important to notice the distinction be-
tween cases where the air is forced upwards and
where it ascends by reason of an expansion al-
ready effected before it starts, by the action of
heat. In the former case it stops when the
forcing agency stops. In the latter it rises to
the level where the air all round is equally
expanded, and therefore of the same density.

The former case occurs in Nature, where a
wind blows athwart an abrupt chain of moun-
tains, and forces the air up their sides. The
latter occurs wherever air is locally heated above
or cooled below that surrounding it.

When, instead of being compressed, the air is
heated within a confined vessel, its pressure in-
creases in direct proportion to its rise in tempera-
ture (when reckoned from an absolute zero 461°
below zero Fahr.). If when heated it is
allowed to expand freely (that is to say, still
confined by ordinary atmospheric pressure) it
expands or increases in volume in like propor-
tion. This is called the law of Charles or Gay-
Lussac.

A third law which is really the converse of
this may, for convenience, be termed Poisson's
law, and asserts that, if air is sudderdp compressed
it rises proportionately in temperature, and if
suddenly allowed to expand, it falls in tempera-
ture. The suddenness is only necessary in order

THE LAWS WHICH RULE THE ATMOSPHERE. 103

that the heat engendered may not have time to
escape before it can be detected. These three
laws, in combination with a few special character-
istics displayed by water vapour, explain all the
varieties of atmospheric phenomena primarily

\

m

Photo by NegrtUi and Zanibra

Fio. 33.—" Aftrr thb Stobm. '

From the Grolx de fer Switzerland, 7000 feet above sea-level.

due to the action of heat and cold, such as wind,
storms, clouds, rain.

These laws are really only variations of one
grand principle which applies to everything in
the Universe — viz., what is termed the " conser-
vation of energy." Thus, to take a single example,
when a bullet hits a target it gets quite hot, and

104 THE STORY OF THE EARTH'S ATMOSPHERE.

the heat that is thus generated is the exact
equivalent of the motion that is lost.

Heat, as we have learnt, is "a mode of
motion." It is, in fact, a motion of the small
atoms or molecules that make up a body instead
of a motion of the body itself.

According to the modern theory of gases,
which applies equally to a mixture of gases such
as air, the tiny atoms or molecules which com-
pose them are in a state of constant motion
backwards and forwards, kicking, as it were,
against each other, and against anything that
obstructs their freedom to move. The pressiure
exerted by a gas on its neighbourhood, where
this is solid liquid or gas, is measured by the
number of kicks or impacts which its atoms per-
form in a given time.

If the gas contained within a bag (say) is
compressed, the paths of the atoms are shortened,
and, in consequence, the number of impacts with
each other and against the sides of the bag is
increased — i.e., the pressure increases. That is
Boyle's law.

In like manner, if, without compression, the
temperature of the gas is raised, the speed of the
movements is increased. (Molecular speed is
heat.) Therefore, the effect in this case is just the
same as when the gas was compressed and the
paths were shortened — ^viz., an increased number
of impacts or kicks, and therefore increased pres-
sure. That is Charles's law.

When the gas is suddenly compressed, the
atoms have been pushed towards each other, and
their speed has therefore been increased by this

THE LAWS WHICH RULE THE ATMOSPHERE. 105

push. Consequently, a rise of temperature takes
place. This is Poisson's law.

In all these cases, no energy is lost or created.
It is simply a transformation of motion into
different "modes," from which it can be retrans-
formed without sensible loss, though it is easier
to transform motion into heat than heat into
motion. Dr Joule, of Manchester, was the first
to determine the exact equivalence between what
we term motion and heat. His corrected law
maybe stated thus :

fFhen a pound weight falling through 783 feet is
arrested^ as much heat will he developed as will raise
one pound of water one degree above absolute zero}

The converse is equally true. Here we see
the true cause of the marvellous manifestations of
energy in the movements of the atmosphere, the de*
vastating hurricanes which overturn buildings and
destroy ships, and the terrible tornados of America
which have been known to carry solid objects
like wooden church spires a distance of 15 miles
and kill hundreds of people in the space of a few
minutes. They are all due to the heat of the
sun, which is converted into motion on the above
scale.

Charles's law, by which air expands by heat,
and Poisson's, bv which it cools by expansion
under diminished pressure, have a very import-
ant bearing on the formation of clouds, rain,
thunderstorms, tornados and tropical cyclones.

When a mass of dry air, or air only contain-
ing a small proportion of vapour, rises, it cools
at the rate of V F. for every 183 feet it
^ (i.e., 461° below zero Fahr.)

106 THE STORY OF THE EARTH'S ATMOSPHERE.

ascends. This would be about l^e** in 300 feet,
or about 5-2" in 1000 feet.

The rate, however, at which the air is found
by observation to remain cooler as we ascend in
the atmosphere is much slower than this. Con-
sequently, if dry air is warmed up until it
expands and gete lighter than the surrounding
air, it cannot rise very far before it is cooled by
ascent, down to the temperature and density of
the air around it at the same level, when it is
bound to stop.

When, however, air contains as much vapour as
it can hold invisibly, or is said to be saturated,
the case is different. As it ascends and cools, the
vapour condenses into cloud and finally falls out
as rain and at the same time gives out the heat
which it absorbed in the (wt of conversion from
water into vapour. This heat, which is latent so
long as it is vapour, becomes patent when it con-
denses and retards the cooling of such a mass of
air so much that under ordinary conditions of
temperature and pressure in these latitudes
(barometer 30 inches, thermometer 62°), it
only cools fths of a degree Fahr. in ascending
300 feet. An ascending current of air saturated
with vapour once started, therefore, could go on
ascending until nearly all the vapour had fallen
out of it, or until it had risen into very lofty
and cold regions of the atmosphere.

Air saturated with vapour is thus essential to
the formation of clouds, especially cumulus
clouds, like that in our frontispiece, and of up-
waixi currents generally, which are the chief
cause of local storms. Diffusion by which one

THE LAWS WHICH RULE THE ATMOSPHERE. 107

gas tends to work its way through another plays
an important rdle in atmospheric economy.
Were it not for dififusion the heavier gases would
all lie near the surface and the lighter near the
top, and we should all be poisoned off by even
the small amount of carbonic acid there is.

MILES

80|

70
60
50

40
30
20
10

62

32
31

HM/droytit,

-Water Vapour.
N'itrogen^,

9 - Carbonic acid

Fio. 34.— DiFFUsivB Limits or thb CoMPOKSirT Oasss or thb
Atmosphsre.

Some
founded

years ago the great chemist Dalton
the law of gaseous pressure, and
deduced that of diffusion from it, but it has
since been found that though his conclusions were
fairly correct, they are not due to the causes he
alleged. The rule is that the lighter the gas the

108 THE STORY OF THE EARTH'S ATMOSPHERE.

more rapidly it tends to wander through its
neighbours, and the tendency is for each gas
to behave as though its neighbours were not in
existence. Thus, the tendency is for the compo-
nent gases of the atmosphere to take up positions
in which they would exist as separate atmospheres
up to the limits in miles indicated opposite each
in the adjoining diagram.

This ideal final arrangement never takes place
owing to the constant motion, but since all the
gases diffuse upwards as well as downwards it is
quite possible that some of the hydrogen and
lighter gases have diffused upwards until they
have got beyond the power of recall by gravitation.

Long before the water vapour has reached 37
miles, a great deal is lost by being condensed
into rain. At a height of 9 miles above the sur-
face, for example, the actual amount of vapour
present is only ^th of what would exist if it
were incondensible.

The laws which determine the passage of the
sun's heat and light through the atmosphere
present problems which are even yet only par-
tially solved, partly because light and heat are
made up of a variety of wave movements in that
wonderful medium which pervades all space
termed aether, and partly because the conditions
in the atmosphere can never be exactly imitated
in the laboratory.

However, this much is known. Ordinary
white light from the sun when passed through a
prism is found to be made up of a variety of
coloured rays ranging from violet to red. The
visible violet rays are made up of the smallest

THE LAWS WHICH RULE THE ATMOSPHERE. 109

waves, about -00001 in. in length, and the visible
red rays of waves about 'OOOOS in. Beyond
these visible rays are invisible ones which affect
the atmosphere and earth, and whose existence
can be proved by photography. The rays to-
wards the red end produce more heat than light,
and those towards the violet end more light than
heat. The general action of the atmosphere on
these rays from the sun is twofold. In passing
through it they are partly absorbed and partly
scattered. The blue and violet rays are most
affected, and the red least. In fact, as Prof.
Langley has pointed out, so much of the blue
rays are filtered out by the atmosphere that if
we could see the sun as he appears at the outside
of our atmosphere he would be bhie instead of
white.

The absorption particularly of the heat rays
has been shown by the late Prof. Tyndall to
be mainly effected by the water-vapour present
in the atmosphere, while the scattering is effected
by the fine particles of dust and frozen vapour.

The red colours at sunset are due to the fact
that the sun's rays, before they reach our eyes
in this position, pass through a thickness of
atmosphere about 900 miles instead of 50 miles
when overhead. The blue, rays, in consequence,
are nearly all filtered out, and nothing is left but
the longer waves at the red end of the prismatic
spectrum. The red colour of the sun when seen
through a fog is due to a like excess of absorption
and scattering of blue rays by the particles of

\

Similarly, the blue colour of the sky when the

110 THE STORY OF THE EARTH'S ATMOSPHERE.

sun is high up in the heavens is explained by
Lord Rayieigh to be due to these missing blue
rays. Scattered mostly at right angles to the
direction of the sun's rays, these spumed rays
reach us from all parts of the sky, and shew up
blue against what would, in their absence, be the
black background of space.

When the luminous rays from the sun pass
through the air, they heat the dry part of it very
little. The absorption is mainly eflfected by the
small but valuable vapour constituent, which
Prof. Hill estimates as being 764 times as
effective as dry air. The remaining rays on
reaching the earth are changed into invisible
heat rays, which possess the peculiar property
of being unable to repenetrate the atmosphere.
Trapped thus like lobsters in a basket, they
expend their energy, first upon the earth's sur-
face and then upon the adjacent air. The
atmosphere, in fact, acts like the glass in a
greenhouse, which lets the luminous rays in and
prevents their escape when they have become
converted into dark heat.

The upper parts of the atmosphere would thus
remain colder than they are, were it not for the
conveyance (convection is the technical word) of
the heat from the lower strata to the upper
regions — in other words, from the ground floor
to the attics. This convection plays such an
important part in the atmospheric economy, and
in the formation of clouds, rain, and storms of
every kind, that it demands a brief consideration.

We have seen that in general, ascending
air cools at the rate of T F. for every 183

THE LAWS WHICH RULE THE ATMOSPHERE. HI

feet. Consequently, so long as the rate of de-
crease of temperature with the height is equal to
or dower than this, the atmosphere tends to
remain in vertical equilibrium-^that is to say,
vertical motions will not arise spontaneously.
The only interchange under such circumstances
would be due to expansion and overflow, such as
that described in Chap. IV., and which gives rise
to the general circulation between the equator
and the poles described in Chap. V. When,
however, the atmosphere near the surface is not
only heated by radiation of the changed solar rays
by the earth, but is surcharged with vapour,
it cools even at 62** F., our summer tem-
perature in £ngland, only T F. in every 400
feet of ascent, while at 92° F., as often
occurs within the tropics, it cools only V
F. in 600 feet. An upward movement once
started, therefore, is able to continue, since the
air is always warmer, and therefore lighter than
that which it reaches above. Similar downward
motions of the cool air above take place until a
large proportion of the heat received near the
surface is carried up aloft. The process is pre-
cisely analogous to that by which the hot water
from the kitchen boiler is conveyed through the
pipes to the cisterns at the top of a house.

These upward convection currents carry the
life-giving heat to the cold regions above us, just
as the arterial blood conveys warmth to our
extremities, and is quite as necessary to the life
of the atmosphere as the circulation of blood
heat is to that of the animal. Were it not for
this safe conveyance, moreover, of the surplus

112 THE STORY OF THE EARTH'S ATMOSPHERE.

heat away from our midst, there would often be
a dangerous accumulation which would render
life more insupportable than it is, especially in the
tropics. When the existing rate of decrease be-
comes anything like 1° F. in 100 feet, or greater,
rapid convection sets in, even if the air is com-
paratively dry, and clouds are formed with rain,
and often lightning. If the air over a large district
is affected, as sometimes occurs in the tropics, a
cyclone is formed by the inrush of surrounding
air, or, if the action is very intense and quite
local, a tornado or whirlwind may result. This
may be termed convection run riot.

!rrof. Abb6, of the U.S. Weather Bureau, con-
siders the limit of convection currents to be
about 30,000 feet, or about the height of Mount
Everest. Above this the temperature diminishes
very rapidly, as indeed we find from the observa-
tions recorded on the free balloons, nAerophile
in France and the Cirrus in Germany, which on
March 21st, 1892, and July 7th, 1894, reached
the same height of 10 miles. In the case of the
French balloon, the temperature descended to
104* F. below zero, and at the same rate the
cold of space — viz., minus 461*, would be reached
at a height of about 30 miles.

The ordinary rate of decrease is in general
about 1** in 320 feet after we rise above the
first 100 feet.

From what has been said about the slower
rate at which air saturated with vapour cools by
ascent when the temperature is Ugh near the
surface than when it is low, we can readily un-
derstand why cumulus clouds and rain showers

DEW, FOG, AND CLOUDS OF THB ATMOSPHERE. 113

occur in the daytime and in warm latitudes
more readily than at night and near the poles.
In fact, since at freezing-point and at sea-level
even saturated air would cool as rapidly as 1"* F.
in every 277 feet; if it were not for imported
convection systems and clouds there would be
very little ascent and precipitation of condensed
vapour at all in the arctic zones.

CHAPTER VII.

THE DEW, FOG, AND CLOUDS OF THE ATMOSPHERE.

When we gaze skywards and see the filmv wisps
of high-cirrus cloud, touching as it were, the very
vault of heaven, or when we notice the ragged
scud of the approaching storm, half covering the
low hills, we are witnessing one of the first stages
by which the water of our atmosphere becomes
visibly separated from its gaseous companions.

Another stage is manifested when the pearly
drops of dew gather on the blushing petals of our
roses, or the rain drops from the frowning storm
clouds. Still another transformation scene, and
the beautiful six-rayed flakes of snow fall like
flowers, scattered by an angel hand, and cover
up the gloomy earth with a mantle of dazzling
white.

Yet one more strange scene, and from the fiery
thunder-cloud white balls of ice rattle down as
though from some aerial glacier.

The chameleon character of this same water
element is indeed a most fortunate circumstance.

114 THE STORY OF THE EARTH'S ATMOSPHERE.

Imagine what a dull world it would be without
our gaudy sunset cloud tints. What a desert
if it never rained.

It happens, however, that, unlike the other
gases, water-vapoxu* can undergo all its changes
within the gamut of the temperatures we ex-
perience on this planet.

Solid at 32' F., liquid thence to 212" F., after
which it becomes gas.

Moreover, the air is thirsty as it were, and so,
from the liquid water, at all temperatiures, and
even the solid ice vapour, it is ever ascending by
evaporation and rendered invisible as it passes
through the other gases.

There is a limit, however, to the capacity of
air for such a temperance beverage, which, like
the thirst of men, depends on the tempera-
ture. Thus, while a cubic foot of air at zero
P. can hold but J a grain of vapour, at 60° F.
it can soak up 5^ grains. At 80° F. as much as
11 grains can remain invisible in the same space.

To give a larger example. Suppose a room,
20 feet square by 10 feet high at 60^ F., to be
supplied with vapour until it could hold no more,
then the air in such a room would weigh 304
lbs., while the vapoxu*, if it were condensed to
water, would weigh but 3 lbs., and fill three pint
measures.

When air can hold no more vapour it is said to
be saturated, and since, when it is cooled, it is able
to hold less and less water, it can, even when un-
saturated, be made satiu^ted by being cooled
down to a point of temperature some few degrees
below. This point is called its dew point, and

DBW, FOG, AND CLOUDS OF THE ATMOSPHERE. 115

depends partly on how damp, partly on how
warm, it was at first.

When very warm and moist, a very slight
lowering of temperature produces condensation
into cloud and finally rain. Hence clouds and
rain will form easier in warm countries, though
other conditions may make them more constant in
cold countries.

What we ordinarily term the dampness of the
air, is not simply a question of how much vapour
is present, since warm air may hold more than
cold air and yet feel drier.

It is determined by the nearness of the dew-
point to the existing temperature, and this de-
pends on both the amount of vapour present
and the temperature of the air in which it is
dissolved.

Ordinarily the dampness in England is about
60 per cent of what could be, but in very wet
weather it rises to 90 per cent.

Over the ocean it is generally high both in
warm and cold latitudes, while in we interior
of continents and deserts it is occasionally as low
as 15 per cent. As we rise above the earth
towards the level of the lower clouds the damp-
ness increases, until, at the cloud level, we rea^
dew- or cloud-point, where the air is saturated.

Dew itself is the moisture deposited on the
surface of bodies near the earth's surface, which
have cooled down by radiation below the dew-
point of the surrounding air.

Dr Wells, in 1783, was the first to offer this
explanation, and thought the moisture came
entirely from the air around. Of late, however,

116 THE STORY OF THE EARTH'S ATMOSPHERE.

Mr John Aitken, of Edinburgh, and others, have
shown that a large part of the moisture comes
from the ground and the plants on which it is
deposited. They are, in fact, constantly perspir-
ing like human beings. In the day-time this
perspiration is evaporated by the warmth and
earned off by the winds. Only in the cool and
calm of the night and early morning does it
become deposited in drops of water.

Hoar Frost is simply dew formed when the
dew-point happens to be below the freezing point
of water.

Fog, or mist, may be termed a cloud of vapour,
formed near the ground or water.

Sometimes, like dew, it is occasioned by the
earth parting with its heat so rapidly as to cool
down a stratum of air, just above it, below its
dew-point> as occurs in the quiet anti-cyclonic
weather, as it is termed, occasionally experienced
in these latitudes in winter. Such fogs are
usually fairly dry. Sometimes it occurs with a
wind, when it is wetter and warmer. In such
cases it usually comes off the sea, and is due to
a warm moist air from the sea passing over a
cool ground surface.

Locally, mists usually form in low river valleys,
where the air is nearly saturated with vapour,
rising from the water or moist land.

It seems strange, but it is nevertheless true,
that the pretty white valley mist of the country
is a near relative of the ugly nauseous fogs of our
larce cities. London fog is simply Thames river
valley mist, mixed with the smoke poured forth
by innumerable chimneys, which is unable to be

DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 117

carried off, owing to an absence of wind above.
When we can consume our own smoke, London
fog, as we know it, will disappear.

Fogs at sea occur most frequently in summer,
and especially near cold currents, as off New-
foundland, wnere the warm moist air off the Gulf
Stream passes over the cold Labrador current;
in the Behring Sea, where the Japan current
meets the Arctic ice ; off Cape Horn, &c.

At San Francisco, the Pacific Ocean breeds a
chilly fog, which rolls up the streets, and obliter-
ates the sun, even in May, but it is fortunately
white.

The clouds of heaven have ever been an object
of wonder and admiration to the sons of men.
Long before Aristophanes wrote his immortal
comedy, "The Clouds," we have numerous re-
ferences to the clouds in the ancient Scriptures.
Job says: '*He bindeth up the waters in his
thick, clouds." " For he maketh small the drops
of water ; they pour down rain according to the
vapour thereof." " Also can any understand the
spreadings of- the clouds"; while in Ecclesiastes
we have a wonderful insight into the whole
scheme of water circulation in the verse which
says, "All the rivers run into the sea^ yet the
sea is not full Unto the place from whence the
rivers come thither they return again."

The first person who seriously observed and
described the clouds was Luke Howard, the
Quaker, who was first attracted to the subject
in 1783.

Howard roughly divided clouds into three
primary forms — stratus, cumulus, and cirrus —

118 THE STORY OF THE EARTH'S ATMOSPHERE*

and the same division also roughly applies to
their height — ^low, intermediate, high.

Of late years, as our knowledge of their various
forms and mode of origin has increased, this
simple division has been found inadequate. The
late Rev. Clement Ley, Mr Ralph Abercromby,
Dr Hildebrandsson, Dr Vettin, and Messrs
Ekholm and Hagstrom have observed their forms,
behaviour, and heights, and developed quite a
science of clouds.

The outcome of their researches is briefly sum-
marised in the International Cloud Atlas, which
has just been published, as a result of the Inter-
national Committee held at Upsala, Sweden, in
1894.

Without going into details, the following gives
a general idea of the varieties and corresponding
heights, beginning with those near the siirface ;
the character of the cloud being indicated as tod
or dry according to the weather by which it is
usually accompanied : —

Varieties of Clouds.

Height in feet.

Name.

Char,
acter.

1. Sea-level
up to 3000

2. 4600 to ^

6000 1

3. 4600 to '

24,000 j

4. 6400

Stratus

Cumulus
Cumulo- -
nimbus

Strato-
cumulus

Elevated fog, so-
called

Rounded hean

Tower-like clouds
with round tops
and flat bases

Bolls of dark cloud

Dry&
wet
Dry

Wet

Dry

DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 119

Height in feet

Name.

Description.

Char-
acter.

6. 6400

Nimbus

Masses of dark
formless cloud

Wet

6. 10,000 to

Cirro-

Fleecy cloud, mac-

Dry

21,000

cumulus

kerel sky

Average height.

7. 27,000

Cirro-stratus

Fine whitish veil
giving halosround
sun and moon

Wet

8. 27,000

Cirrus

Isolated feathery
Avhite clouds

Dry

I have left out one or two purposely for
simplicity's sake.

Pictures of 1, 2, 4, 6, 8 are given in figs. (23) ;
frontispiece, (1) and (2), (3) and (25^ respectively.

It used to be thought that clouds were simply
produced by the mixture of cold and warm air.
In 1788 Dr Button of Edinburgh propounded
his celebrated law of mixtures, which briefly
asserted that, when two masses of air at different
temperatures mingled, the colder air caused the
vapour in the warmer air to condense, owing to
the lower temperature of the mixture not being
able to support the same amount of vapour in
solution.

Dr Von Bezold has recently (1890) investigated
the question, and has shewn that when saturated
warm air mixes with unsaturated cold air, more
cloud will be found than when saturated cold air
penetrates unsaturated warm air. These two

120 THE STORY OF THE EARTH'S ATMOSPHERE.

cases are successively illustrated by opening the
door of a laundry on a cold day and the door to
an ice-house on a hot day. In the former case
fog is at once formed, but not in the latter.

On the whole, however, he finds the effect of
mixture to be very small and ineffective. In
the case of Nature it usually occurs when a layer
of warm air overlies a layer of colder air

Fig. 25.— <:;ibbu8 Cloud {var Tracto Cirrus^ 1889).
p. Garnler, Boulogne Observatory.

The shallow stratus, cirro-cumulus, and cirro-
stratus clouds are partly due to this action.

Where one current crosses another at a different
speed it raises waves or billows in it, just as when
wind passes over the sea. In such waves there
will be cloud at the crests and clear spaces in the
troughs.

Mackerel sky, or cirro-cumulus, and the long

DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 121

rolls of strato cumulus which follow one another
at the rear of a storm, with showers and clear
intervals of several hundred feet, are examples
on a small and large scale of such aerial billows.

The most frequent cause of clouds, however,
is the cooling, due to expansion of air, which
ascends either freely, or by being forced up a
mountain-slope, or drawn up an aerial eddy.

When the existing rate of decrease of tempera-
ture with the height is greater than If degrees
per 300 feet, air locally warmed will ascend, and
cool at this rate until the dew-point is reached,
when vapour will be condensed and cloud formed.
After this the air, since it is now saturated, will
cool at a much slower rate — viz., about ^° F. in
300 feet.

Ascent after cloud level is reached is easy, there-
fore, so long as sufficient moist air is supplied.

Clouds are often found hugging mountains
when the surrounding plains are clear.

In popular language mountains are said to
"attract clouds." This, of course, is literally
incorrect. The clouds are due in this case chiefly
to the lower air being forced up the mountain
side, and cooled by expansion down to dew-point.

Clouds also occur in connection with cyclones
or large storms, both in the tropics and high
latitudes.

This is because the air in the centres of such
movements is damp to start with, and is con-
tinually rising and flowing out over the sur-
rounding drier air.

The approach of such a storm when miles away
is frequently heralded by the appearance of

122 THE STORY OF THE EARTH'S ATMOSPHERE,

tangled masses of the lofty cirrus cloud, which
appear to converge to a point below the horizon,
as in fig. 26, and which represent the overflow of
the ascending damp air from the centre of the
storm. These clouds are called the '^warning
cirrus," because their appearance and motion
(from the storm-centre, unlike the lower clouds
which move towards it) indicate the position and

Fio. 26.

character of the approaching disturbance. Soon
after this storm signal has been hung out by
the Celestial weather-bureau, sheets of cirro-
stratus appear below these cirrus wisps, p. 118,
No. 7, which hide the sun and often cause
a large halo, by refracting its rays through
the prisms of ice of which they are composed.
The final act in the aerial drama is ushered in by
the appearance of high stratus and nimbus. No.

DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 123

5, with ragged scud of stratus, No. 1, quite low
down. These lower clouds move round and in
towards the storm-centre, and thus make a con-
siderable angle with that of the cirrus flowing
out from it.

If a storm-centre, for example, bears S.W., the
eirrus will also bear S.W., the cirro-stratus S.,
the low clouds S.K, and the surface wind
KS.K

The storm culminates with the arrival of the
nimbus, from which rain or snow falls, after
which the clouds disappear in the reverse order
of their arrival, except that, owing to the more
rapid motion above, in the direction in which the
storm is moving, the upper clouds are blown
towards the front, and are less prevalent in the
clearing up showery weather in the rear. Fig.
(23) represents the upper sky as seen after a
storm, from a height of 7000 feet, in the Alps.
A sheet of lower cloud or fog is below the spec-
tator.

Fig. (27), overleaf, shows the position of
the clouds round a European cyclone. The con-
tinuous lines are the isobars, and the dotted lines
isotherms.

Almost the entire cloud mass, it will be noticed,
lies to the front of the central ring of low pres-
sure.

Amongst special forms of clouds may be men-
tioned the table-cloth of Table Mountain at the
Cape, and similar table-cloths on Mount Pilatus,
the Eock of Gibraltar, Atlas, &c. These are all
fcnmed by the passage of a warm moist current
over a cold mountain which condenses its moisture

124 THE STORY OF THE EARTH'S ATMOSPHERE.

while it is moving across the summit. When,
however, it has passed beyond the mountain, the
vapour cloud mixes with the warmer air around,
and, the vapour becoming reabsorbed, the cloud
gradually tails off into invisibility.

In New Zealand, when a wet north-wester is
blowing against the Southern Alps, and heavy

--*4

Fio. 27.

rain is falling on their western sides, a long bar
of cloud, due to similar causes, may be observed
stretching along their summit ridge, extending
some little dis^nce from it above the eastern
plains.

The eastern edge of this cloud roll remains

DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 125

fixed, though the wind may be blowing through
it, as it often is in these cases, with hurricane
violence.

Cumulus clouds indicate upward convection
movements, and are more frequent in summer
and warm countries.

They are frequently as tall and thick as they
are broad, and often pierce up right into the
cirrus.

Clouds of the stratus form, on the other hand,
are usually very shallow compared with their
area, which may extend for hundreds of miles.
They indicate horizontal motions, and prevail
mostly in winter and cold latitudes.

Clouds generally increase during the day-time,
and reach their greatest height in the afternoon
and evening.

The average rate at which clouds move in-
creases with their altitude. This accords with
the theory of the general circulation in Chap-
ter Y. ,

Cirrus clouds have occasionally been observed
to niove as fast as 250 miles per hour ; but their
average pace is more nearly 80 miles in America
and 40 in Europe.

The average velocities for the diflferent cloud
levels observed at Blue Hill, near Boston, have
already been given on p. 98.

The average heights of the clouds are greatest
in summer and least in winter. Their average
speed is least in the former and greatest in the
latter season.

In fig. (25) a cirrus cloud is shewn, in
which the delicate fibrous nature of this cloud

126 THE STORY OF THE EARTH'S ATMOSPHERE.

is plainly seen. The contrast between this ice
crystal cloud, like a puflf of white tobacco smoke,
and the cumulus in the frontispiece, composed of
heavy water drops, is very striking.

The Festooned cumulus in fig. (28) is a
special form of cloud associated with thunder
and hail-storms. It is a kind of inverted cumulus,
in which a cold, very moist air is moving over a

Fio. 28.— Festooned Gomulus.
Sydney N.S. Wales. Jan. 18, 1893.

hot and very dry air such as frequently occurs in
Colorado, Australia, and India. Under these
circumstances all the condensation occurs in the
cold layer, and none in the lower, hot one.
Portions of the upper layer drop down in large
bulbous masses like water balloons, which burst
like soap bubbles and drop their moisture like

RAIN, SNOW, AND HAIL OF THE ATMOSPHERE. 127

water running out of a cask. Sometimes this
water never reaches the ground, being re-
evaporated while passing through the hot
air.

CHAPTER Vni.

THE RAIN, SNOW, AND HAIL OF THE
ATMOSPHERE.

Rain is the final stage of condensation of vapour
back into water, of which cloud is a half-wajr
stage. The mist which composes a cloud is
formed of tiny drops of water about s^fcir-inch in
diameter. It used to be a puzzle to explain how
these water particles were sustained, and it was
at one time supposed that cloud particles were
hollow. We know now that this is neither
necessary nor true, since very small particles
even of gold will remain suspended for a long
time in air; the finer the particles the longer
they take to fall. A slight upward motion of
the air is therefore enough to keep them balanced.
As condensation proceeds these particles grow
larger by fresh coatings of water, and the larger
ones fall down against the smaller and mingle
with them until large drops from iJS to i\j i^c^
thick form, which are no longer capable of being
suspended and fall to the earth. Snow forms
when the temperature at which this further stage
of condensation occurs is below freezing point.
Every snow crystal is a variety of a six-rayed

128 THE STORY OF THE EARTH'S ATMOSPHERE.

cluster, and is similar to the crystals of salts
which are precipitated from a chemical solution.
No one has watched the formation of snow, but
.it must be very similar to that of crystallisation
out of a solution which is saturated with a
chemical salt.

Hail, unlike the delicate snow crystals, is
frozen water-drops. Its frequent association with
thunder-storms led to the belief that it was
caused in some way by electricity. This is,
however, found to be untenable in the search-
light of modem science, which shews that
electricity is mostly an effect^ not a cavm of such
mechanical disturbances. It is believed, that in
such storms the rain-drops found in one part of a
storm are carried upwaros by powerful ascending
currents (twenty-five miles an hour is enough
to sustain large drops) into higher regions of the
atmosphere where they are solidified by the
excessive cold, and being carried over with the
overflow which takes place near the top, fall
down until they are reorawn into the interior of
the storm and again whirled up aloft. Receiving
alternate meltings and freezings, and ^wing
larger with each circuit they make in the
atmospheric chum, they are finally thrown out
on either side of the storm centre. This explains
the fact that in a travelling hailstorm there are
two bands where hail falls on either side, while,
under the centre, it is often found that only rain
has fallen.

Hailstones have often fallen of enormous sizes.
In 1697, Robert Taylor found hailstones in Hert-
fordshire 14 inches in circumference.

RAIN, SNOW, AND HAIL OF THE ATMOSPHERE. 129

In India, the writer remembers a liailstorm on
the great Brahmaputra river when the hailstones
cut holes through the tarpaulin cover of the
steamer, which were so large that each one had
to be mended with a separate patch. Hailstorms

r

-T'

Fig. 29.

are intimately connected with tornadoes, and, like
these phenomena, are more frequent over flat plains
and in very hot and moist summers and countries.
The destruction dealt by hail on standing
crops, vineyards, and orchards has led to means
I

130 THE STORY OF THE EARTH'S ATMOSPHERE.

being proposed for its prevention. Under the
old idea of its connection with electricity, light-
ning rods were erected, but without avail. With
ironical waywardness it fell in several instances
on the rod-protected lands and avoided the others.

Planting of trees would be more effective,
since this would tend to check the rapid heating
up of the lowest stratum of air which is one of
the chief causes of tornado and hailstorm action.

The general distribution of rain in belts over
different areas of the earth's surface has already
been alluded to.

Rainfall, like clouds, is more prevalent in
mountainous than over fiat countries, and for
similar reasons, especially cooling by forced
ascent of air.

In the accompanying mean annual rainfall
maps of England and India, this will be readily
seen. In England the heaviest falls will be
observed to occur in the mountains of Cumber-
land and Wales, and generally along the hilly
country of the West and North. In Scotland
and Ireland it is the same. The lowest rainfalls
under 20 inches all occur on the eastern sides of
the country. This difference is partly due to the
fact that tne prevailing and most rainy winds are
south-west and drop a good deal of their moisture
before reaching the eastern parts, but even were
these barriers absent, the rainfall over the flatter
country on the eastern sides would not be very
much increased. In India, in like manner the
dark shading along the Western Ghats down the
Bombay coast and along the Himalaya shews the
influence of the mountains, the heaviest fall oc-

RAIN, SNOW, AND HAIL OF THE ATMOSPHERE. 131

curring near the north-east comer of the Bay of
Bengal in the Khasi^ hills, which offer an abrupt
wall 4000 feet high up which the southerly
monsoon winds, see fig (20), are forced.

Chirapunji, at the edge of these hills, has the
l;Lr;^i^t rainfall in the world (about 500 iiicbea),

Fio. 80.

half of which falls in June and July. On the
western side of the Ghats rain falls heavily
up to 250 inches at Mahalleshwar on their
summits, while the tableland of the Deccan on
their eastern lee side has a scanty supply and is
one of the areas liable to drought.
The greatest amount of rain in a vertical

132 THE STORY OF THE EARTH'S ATMOSPHERE.

direction occurs at altitudes where the lowest
cloud is thickest, that is, at about 3000 feet in
Europe and 4000 feet in India above sea-level.
In the interior of large continents where moun-
tain ranges are absent, especially when they lie
like Australia in the zone of perpetual high
pressure dividing the tropical from the polar

FiQ. 31.

wind systems, the rainfall tails off to a very few
inches as we go inland.

The accompanying rain map of Australia
shews this very plainly.

Over the whole of the lightly shaded area of
central and west Australia there are less than
10 inches per annum.

This district can never support a large popula-
tion.

CHAPTER IX.

THE CYCLONES OF THE ATMOSPHERE.

Every large storm of the atmosphere is now
called a cyclone, because it is found that the air
moves round and in towards a central area.

Formerly the word cyclone was only applied
to the rare but violent storms of the tropics,
while the words hurricane and tornado are still
popularly used to signify small and large storms,
indifferently.

The term tornado is now applied by meteor-
ologists entirely to certain storms of quite a
special class, differing from cyclones both in size,
mode of origin, and effects, and it is to be hoped
that the newspapers will learn eventually to give
up a habit which only leads to confusion.

A cyclone is a large disc of nearly horizontally
moving air circulating spirally round a central
area over which the barometric pressure varies
from one-fifth to as much as three inches below
that at its border.

The direction in which the wind circulates is
the same as that in which the earth's surface
would appear to rotate in each hemisphere, if we
stood several miles directly above the pole and
looked downwards.

Cyclonic storms range in diameter from 20 to
as much as 3000 miles.

A tornado, on the other hand, consists of a
narrow column of air varying in width from 20
feet to 1400 feet which is rotating with immense
velocity (up to 500 miles an hour) round a

183

134 THE STORY OF THE EARTH'S ATMOSPHERE.

central shaft up which it is also ascending with
a speed in some cases amounting to 100 miles
an hour.

A cyclone is an elephant, while a tornado is a
mouse, and they differ just as much in other
respects as these two animals.

Tornadoes will therefore be specially referred
to in the next chapter, together with whirlwinds,
waterspouts, and thunderstorms, which belong to
the same family.

Many poetic and graphic descriptions of the
awful grandeur of a real tropical cyclone have
been given. All descriptions, however, pale
before the real thing.

The writer once experienced in Eastern Bengal
the full violence of perhaps the most disastrous
cyclone in regard to destruction of human life
on record — viz., what is known as the Backer-
gunge cyclone of November 1st, 1876.*

After several days of unusually quiet, muggy
warmth and murky skies, lurid sunsets, and a
general sense of impending doom, the rain began
to fall in torrents and the wind to rise as the
night came on, until at last I had to pile up tbe
furniture against the windows to prevent their
being burst inwards. The lightning flashed un-
ceasingly, the thunder crashed, the wind tore
past like a raging fiend. All the elements
seemed to have broken loose, and one could
almost fancy that the sober laws of physics were
having "a night out." The very rarity of such

* June 12th of this same voar witnessed the heaviest
fall of rain ever measured in the world — yiz,, 40 inches in
24 hours, at Chirapunji, Assam.

THE CYCLONES OF THE ATMOSPHERE. 135

hurricane violence made it all the more alarming.
After a night of Tartarean gloom, mingled wiSi
truly horrible noise, morning broke sadly through
gaps in the rampart of furniture, and I awoke to
a knowledge that the plaster coating of my house
lay strewn all over the compound. Later on I
learnt to be thankful nothing worse had occurred,
when I heard the awful news that 100,000
natives in the adjoining province had been
drowned by the storm wave which was forced
up the Bay on to the low-lying islands of Dakhin
Shabaspore and Noakolly at the mouth of the
giant Brahmaputra.

While cyclones are comparatively rare in the
tropics, they are very prevalent, though fortunately
as a rule in a milder form in hi^er latitudes.
North and south of latitude 35"* continual streams
of small cyclones travel along the borders of the
large permanent areas of low pressure or polar
cyclones which surround either pole.

Sometimes one of these streams passes over
us, in which case we experience wet and stormy
weather. Sometimes they take a more northerly
or southerly track.

Their place is frequently occupied by large
areas of high pressure from which the air flows
quietly outwards to feed the cyclones. These
areas are termed anti-cyclones. Modem observa-
tions shew that the air which flows in towards
and up the centres of the cyclone hollows flows
out above and pours down the centres of these
anti-cyclone heaps. While the weather in the
cyclones, owing to the ascending damp air, is
cloudy and rainy, the weather in the anti-cyclones.

136 THE STORY OF THE EARTH'S ATMOSPHERE.

where it is descending, is dry and clear. Years
ago, until about 1830, there was little known
about the course of the winds in cyclones,
and ships which mostly experienced their full
fury were at their mercy or the individual
caprice of their commanders.

About the beginning of the century. Capper,
of the East India Company's Service, announced
that the storms of the Bay of Bengal were vast
whirlwinds.

In 1828 Professor Dov6 of Berlin, and soon
after Bedfield of America, Beid of England, and
Piddington of Bengal, developed, though with
much diversity of opinion, the memorable "Law
of Storms."

The chief point of this law was the fact that
the wind always circulated round the area of
lowest barometer in a nearly circular spiral (there
was much unnecessary dispute on this point)
against watch hands in the northern hemisphere.

They also ascertained that tropical cyclones
originated in a belt about 10** on either side of
the equator and travelled thence polewards along
parabolic paths, occasionally crossing the tropic^
belts of high pressure, where they were broken
on their western sides and into the north and
south temperate zones.

The accompanying fig. (32) shows their general
course under these circumstances.

An immediate consequence of these rough
rules was to enable the mariner to avoid what
was termed the " dangerous semicircle " (i,e,y the
front half in the direction of travel), and to tell
the direction of the storm-centre by noting the

THE CYCLONES OP THE ATMOSPHERE. 137

direction of the wind, A rule by which this is
simply remembered wajs afterwards enunciated
by Dr Buys Ballot, the eminent Dutch meteor-
ologist, thus :

"Stand with your hands stretched out on
either side and your back to the wind, then in
the northern hemisphere the centre of the cyclone

Fio. 82.

will be to your left hand." In the Southern
hemisphere substitute "right" for "left."

Stated in this form the incurvature of the wind
or its inclination to the isobars which contour
round the central area is completely overlooked.

138 THE STORY OF THE EARTH'S ATMOSPHERE.

This inclination is found to increase with the
distance from the centre, and the distance of the
storm from the equator, quite apart from the
fact that on land it is always greater than at sea.

Thus in the Philippine Isknds (latitude 14**)
it was found to be 62% in the Bay of Bengal
(latitude 20°^ to be 57\ in the United States iO%
over the Atlantic 30**, and in England the late
Eev. Clement Ley found it to be about 20*,
while Captain Toynbee found it to be as much
as 30^* for the Atlantic in latitude 50* N.

Near the equator, therefore, it would be mani-
festly unsafe for a mariner to trust to the famous
old "circular theory," which made the winds
blow directly along the isobars, since there
the centre of the storm, instead of being directly
to his left if he stood until the wind blew
directly upon his back, would actually be nearly
in front of him.

Dr Meldrum of Mauritius, who was one of the
most indefatigable reformers of the old circular
law, mentions a case where as recently as Jan-
uary 24, 1883, the captain of the ship Caledonien
deliberately ran his ship straight into the centre
of a storm by following the old rule. The
modem rules now advise the mariner (1) to
avoid runnine before the wind, (2) to lie to on
the starboard tack in the northern hemisphere
or the port tack in the southern. By this means
the vessel may be safely guided out of the
dangerous vortex.

Tropical cyclones occur most frequently on the
western sides of the N. Atlantic, the N. and S.
Pacific oceans, and the S. Indian Ocean, also in the

THE CYCLONES OF THE ATMOSPHERE. 139

Bay of Bengal and the China Sea, where they are
termed Taif uns. The months in which they occur,
September and October in the N. hemisphere,
and February and March in the Southern, are soon
after the periods when the equatorial calms or
doldrums which lag behind the sun have reached
their extreme northerly and southerly positions.

The air is calm and full of moisture, and
this, combined with the fact that they are pre-
ceded and accompanied by torrential rain, has
led to the conclusion that they are due to the
upward convection of damp air which causes an
indraft towards some central area. The heavy
clouds and thunder and lightning which accom-
pany them, fully bear out the same view.

Moreover, the energy supplied by the con-
densation of the vapour which allows the air to
recoup itself for the loss due to expansion has
been calculated to be sufficient to account for the
immense wind energies they exhibit. Professor
Reye of America calculated that the Cuban cyclone
of October 5, 1844, used up in three days 473
million horse-power. Indeed, when we consider
that the air in a cyclone 100 miles in diameter
and a mile high weighs as much as half a million
ocean steamers of 6000 tons a-piece, we can hardly
wonder at the enormous amount of energy required
to keep this in motion, at, say, 40 miles an hour.

On reaching land, tropical cyclones frequently
break up. They are nearly unknown on the
equator itself.

Ferrel again proved to be the Newton, who
was able to weave all the disconnected facts relat-
ing to cyclones into a reasonable theory of cause

140 THE STORY OF THE EARTH'S ATMOSPHERE.

and effect. Assumiag an inflow towards, and an
upflow over, a given area, he was able to shew
that at some little distance from the equator the
spiral rotation of the winds and all the other
phenomena of a cyclone would follow from the
law of inertia on a rotating sphere explained in
Chap. V. In an ideal case, where friction was
unconsidered, the air would tend to rotate round
a central area, as in fig. (33). At the centre the

pressure would
be very low,
gradually ris-
ing to a maxi-
mum on the
line separating
the interior
from the ex-
terior gyra-
tions. Outside
this line the gy-
rations would
be reversed.
The interior
region would
be the true
cyclone and the exterior a kind of anti-cyclone,
usually termed a peri-cyclone. Where, as in
nature, the air experiences friction, the pressure
near the centre would be moderately low, the
interior arrows would point inward towards
the centre, and the exterior arrows outwards.
The vertical circulation of the interior zone is
simply shewn in fig. (34).

All the results from theory agree with those

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1

|w

\

imr

Fia.88.

THE CYCLONES OP THE ATMOSPHERE. 141

Fia.84.

observed. For example, an increase of the
violence of the wind until it suddeidy dropB
near the centre,
where in tropi-
cal storms the
clouds also dis-
appear and the
airbecomesclear
and calm for a space of occasionally 20 miles.
This is called the eye of the storm, and the
course of the air is believed to be that in the
accompanying diagram, fig. (36), where the
violent rotation produces such a centrifugal
force as to cause some of the upper air to
descend to fill the vacuum. Though the air is
calm, the sea is here of that confused character
most apt to make a vessel founder.

The weather of the tropical regions is con-
trolled almost entirely by the regukr daily and
seasonal changes produced by the path of the
sun in the sky between sunrise and sunset,
together with that in its average altitude be-

tween summer and winter, and is scarcely
afi*ected except temporarily by the rare passage
of a cyclone.

In the extra tropics, that is to say from about
latitude 35° to the Pole, the weather on the
contrary is almost entirely made up of a succes-
sion of cyclones and anti-cyclones. The changes

14:2 THE STORY OF THE EARTH'S ATMOSPHERE.

introduced by these, completely dominate those
brought about by the daily and seasonal causes.

Extra-tropical cyclones are moreover believed
to be due to different causes to those of the
tropics. Unlike the latter, they are most
frequent in winter when the lower air is in a
stable condition, and they sometimes occur
without rain. They are supposed by modern
specialists to be for the most part eddies in the
large upper return currents, as they flow over
from the Equator, and crowd into the narrowing
space towards the poles. The effects at the
earth's surface are the same as though the air
rose spontaneously, since an eddy in mid-air
causes the lower air to ascend just as a whirlpool
sucks down water that is drawn into its vortex.
The air which is thus forced to rise, forms clouds
and usually rain, but the storm generally in such
cases belongs more to the upper regions of the
atmosphere, and is less violent near the earth.

The movement of cyclones is quite different
from that of the winds that blow round their
centres. The latter may vary from a gentle
breeze to a hurricane of 100 miles, and the centre
may remain stationary, but usually the cyclone
itself moves over the earth at a speed which
varies in different localities and for each disturb-
ance.

We have already noticed the general move-
ment of cyclones in fig. (32). In all cases when
once they are formed they appear to be guided
chiefly by the upper currents. In the tropics
the west and poleward motion results from a com-
bination of the lower westward moving trades and

THE CYCLONES OF THE ATMOSPHERE. U3

the upper poleward moving return (or anti-trade)
currents from the equator, but they often move
here as elsewhere in very different paths, though
generally north or south-westward.

Beyond the tropics they are driven eastward
by the prevalent west to east winds, both above
and below, and like eddies on a river are carried
along by the stream.

They also exhibit a tendency to move round
the anti-cyclone heaps which are here chiefly
forced down-flows (just as the cyclones are here
forced up-flows) so as to keep the anti-cyclones
on their right in the northern hemisphere. This
principle is made use of in forecasting their
probable motions. There appears to be little
known about the movements of detached anti-
cyclones or areas of fine weather, but they fre-
quently shew a tendency to move from E. to W.
as well as from W. to E.

The average speed of cyclones in different
parts of the world has been determined by the
late Professor Loomis from a very large number
of cases, thus: —

Average Speed of Cyclones over the Earth.

United States, .

28 miles per hour.

North Atlantic,

18

Europe, .

16

West Indies, .

U

Bay of Bengal and

China Sea, .

8

The greatest amount of cloud and rain and
the highest temperature occurs in their front
halves, and their vitality, especially in the

14:4: THE STORY OF THE EARTH'S ATMOSPHERE.

tropics, appears to depend on the continuance
of condensation and rainfall, which allows the
air to flow readily up their centres.

The vitality and longevity of some of these
storms is wonderful. Thus a few years ago Mr
C. Harding, of the English weather bureau, traced
a storm which started in the Japan seas right
across the Pacific, America, the Atlantic, and
Europe, until it was lost sight of in Siberia.

The forecasting of daily weather in high lati-
tudes requires a knowledge of the birth and
subsequent movements of particular storms. In
the tropics, where the daily weather changes are
small, the most important problem is the pre-
vision of average seasonal weather. This re-
quires something more than a mere knowledge
of travelling cyclones, and observes large waves
of pressure which take months to travelfrom
equator to pole, or from west to east. By this
means, seasonal forecasting six months ahead
is carried on 'in India, and a similar method
could be applied to the average weather of other
countries. The waves of pressure caused by the
passage of cyclonic storms compared with these
large waves are like the ripples on an ocean billow.

The passage of cyclones and anti-cyclones
introduce special winds which possess peculiar
characteristics, due to origin and direction.
The sirocco, of Italy and Greece, the leveche
and solano of Spain, the leste of Madeira, the
Khamsin of Egypt, the Kona of Hawaii, and the
brickfielder oi Southern Australia are all ex*
amples of the wind in the front half of a cyclone
which, coming from regions nearer the equator.

THE CYCLONES OF THE ATMOSPHERE. 146

is invariably warm, and dry and exhilarating, or
damp and muggy, according as they have tra-
velled over sea or land.

The lassitude and irritability produced by the
solano has given rise to the Spanish proverb,
"Ask no favour during the Solano." The
Italian sirocco induces similar weariness.

At the rear of the cyclones of the temperate
zone which travel from W. to E. in both hemi-
spheres, the wind blows from some polar direction.

Locally are thus produced the cold " Nortes **
of Mexico, the "blizzard" of the States, which
is accompanied by blinding snow, the " Mistral "
of the Khone Valley and the Gulf of Lyons,
the "pampero" of Mexico, and the "southerly
bursters " of Australia.

The "bora" of the Adriatic is of the same
species, but possesses a peculiar, penetrating cold
by being drawn down towards the cyclonic
depression from a lofty plateau where it has
acquired great cold by radiation.

A peculiar wind arises in connection with the
motion of cyclones over mountain ranges, caUed in
Switzerland the ^^foehn " and in America the
" Chinook/* In New Zealand it is locally known
as a " hot north-wester," and it is found to occur
everywhere on the lee side of mountain ranges
running athwart the paths in which the cyclones
travel. This wind is uncommonly hot and dry,
and melts the snow on the Alps in one night
more than the sun shining for several weeks.

In America the chino<^ blows on the eastern
side of the Eockies and raises the temperature
of a long belt of that part of the country per-

K

146 THE STORY OF THE EARTH'S ATMOSPHERE.

manently above what it would otherwise experi-
ence.

The way in which this heat is derived has
been revealed by a knowledge of atmospheric
physics, and is really quite simple when, as the
professor of legerdemain is fond of saying, " you
know how it's done.'*

On the windward side, the air after it has
reached cloud level, loses only about J° every 300
feet it ascends. When it has reached the top of
the range it has lost a great deal of moisture in
the shape of rain, and as it descends on the lee
side as dry air, it gains heat at the. rate of If*"
per 300 feet, or, in other words, it gains an extra
1° for every 300 feet it descends. Consequently,
by the time it reaches the lee valleys it appears
as a hot, dry wind. The higher the mountain
chain the hotter the wind. It used to be thought
that the famous hot nor'-wester of the Canterbury
Plains in New Zealand derived its heat from
Australia, but it is found that when it is blow-
ing hot and dry in Christchurch it is rainy and
cool on the western side of the southern Alps.

CHAPTER X.

THE SOUNDS OF THE ATMOSPHERE.

As inhabitants of this earth planet we are far
more dependent for our happiness on sound,
even inharmonious noises, than we are inclined
to admit. A world without the voices of men

THB SOUNDS OF THE ATMOSPHERB. 147

and animals, without music and song, wrapped
in profound silence would be insupportable.
And of all phenomena, sound is one which
peculiarly belongs to the atmosphere, since
while light can travel wherever aether exists,
even through vacuum, sound cannot exist apart
from air. Every sudden movement of the air
propagates a series of waves from the point of
origin of the motion similar to what occurs in
water when a boy throws a stone into a pond.

A sudden meeting of two solid objects gives
a blow to the adjoining air which suffices to
originate a series of such air waves. These
waves differ from those on the surface of
water in one essential point — ^viz., that whereas
in water waves, the "water moves up and down,
while the wave motion is propagated horizon-
tally; in the case of air waves, the air moves
bacKwards and forwards in the same direction
as that in which the wave is transmitted. The
air is thus alternately compressed and dilated,
and as such conditions travel forward in all
directions from the origin of the disturbance,
the sensation of sound which is produced when
such waves meet the ear is propagated through
considerable distances. When these wives enter
the human ear they beat up against a delicate
plate, or tympanum ad it is termed, of hard
skin, and cause it to shake backwards and for-
wards. The movements of the tympanum are
passed on by a series of bones in loose contact^
which filter out irregularities and pass the waves
into a kind of aural piano fitted with a number
of delicate filaments instead of keys, each of

148 THE STORY OF THE EARTH'S ATMOSPHERE.

which is attached to a separate nerve. Upon
this piano tones are phijed, as in the case of
an ordinary piano, while from our brains we
experience the sensation of high and low notes,
harmonies and discords, just as similar effects
can be produced on the artificial instrument.
We can only distinguish such waves as sound,
when they follow one another more rapidly than
16 times per second, or less rapidly than 38,000.
Waves exist beyond these limits, but to us they
are inaudible. A deep bass voice causes about
100, and the highest soprano has reached about
2000 waves per second.

All sounds travel at about the same rate —
1120 feet every second in air of ordinary tem-
perature. Consequently when we hear thunder
follow about five seconds after a flash of lightning,
we know it is a mile distant.

The dense air near sea-level is a better medium
for transmission of sound than the more rare
air at great elevations. Sound rises upwards
easier than it descends, and travels better
through damp than dry air. In a balloon
Mr Glaisher heard the noise of a railway train
at four miles high when in the clouds. When
clouds were far below him no sound was
heard.

Echoes or reflections of sound are often a very
curious atmospheric phenomenon. To echo the
last word spoken distinctly, the reflecting surface
must be at least 110 feet away. A full sentence
requires a much greater distance. A dome-shaped
roof often produces a multiple echo, the reflected
waves undergoing continual reflections between

THE SOUNDS OF THE ATMOSPHERE. 149

the floor and the roof, until the wave motion is
finally converted into heat.

In the Taj Mahal, at Agra, the incomparable
marble mausoleum, erected by Shah Jehan to the
memory of his wife, the central dome gives a
beautiful multiple echo.

In buildings of a paraboloid form, such as the
Mormon tabernacle at Salt Lake City, Utah, the
slightest sound, such as a pin dropped at one end
of the building, can be heard near a certain point
at the other end. Tet this building can seat
11,000 people. The Whispering Gallery at St
Paul's is another example of the same kind. In
the first case the sound waves are all reflected
toward the same point, and therefore reinforce
each other enough to render their continued
eflect audible. In the latter, their continued
reflection between the walls of the gallery
prevents the loss they would usually experience
by spreading out in all directions.

Thunder can be heard at 30 miles, explosions
at 100 miles or over. Thus the firing at Waterloo
was heard at Dover. The sounds of volcanic
eruptions, however, have been heard at immense
distances. In 1883 the eruption of Krakatoa, a
volcanic island in the Sunda Straits, was heard
over an area equal to one-thirteenth of the entire
globe. In one direction the sounds were heard
at Rodriguez in the Indian Ocean, 3000 miles
away (they took four hours to reach it), and in
another, at Alice Springs, in the very centre of
Australia. At intermediate points every place
thought a vessel was firing distress guns, and
search was made for the supposed vessel over an

150 THE STORY OF THE EARTH'S ATMOSPHERE.

area as large as Europe. Besides sounds, lar^e
air waves were propagated, which expanded m
circles until they girdled the earth and then
converged upon the Antipodes of Krakatoa,
whence they were reflected back again to
Krakatoa, and so on no less than seven times.
Every recording barometer in the world shews
little notches in its record for August 27 and
following days. Each notch shews the passage
of the wave backwards and forwards from
Krakatoa. These waves travelled with the same
velocity as the sound waves, and took thirty-six
hours to perform each circuit of the globe.

CHAPTER XI.

THE COLOURS AND OPTICAL PHENOMENA OF
THE ATMOSPHERE.

The story of our advance in the knowledge of
Light, life that in most other branches of
physical knowledge, is one of gradual dispersal
of error and perplexity, and the dawning of
truth and harmony.

Even the great Newton's emission theory, by
which light was supposed to be due to a kind of
bombardment of minute corpuscles, broke down
when subjected to the keen analysis of modem
science, and another generation, led by Huyghens,
Euler, Young, and Fresnel, was required to
formulate and develop the modem theory of wave
motion of the invisible aether which surrounds

OPTICAL PHENOMENA OF THE ATMOSPHERE 151

and penetrates all matter. This theory of aether-
wave motion accords with all the observed facts,
and enables discovery to march forwards with
certainty and power.

Light and heat are simply effects of the same
wave motion. When the waves of aether are
between ayiDiytli and s^J^^th of an inch in length,
they produce the effect of light upon our eyes,
and at the same time heat upon our faces.

When the rays of other lengths between these
extreme limits reach us, they appear of certain
colours corresponding to their wave-length or
position in the so-called spectrum which is pro-
duced when white light is passed through a
glass prism. The longest waves produce red
Bght, and the shortest blue or violet, the order
of colours corresponding to decreasing wave
length, and therefore greater rapidity of wave
succession, being red, orange, yellow, green, blue,
violet. White light is made up of aJl these rays
mixed. When these rays either singly or mixed,
as generally happens, come in contact with air,
or matter floating in it, they set up small oscilla-
tory motions in the tiny molecules of which it
is composed. The effect of these motions con-
stitutes a condition which we term heat or light,
according as it affects certain nerves. A portion
of the radiant energy is used up in this generous
performance, or in other words is said to be
absorbed. Kelatively, more heat can be derivedi
from the long wave rays of red colour, and more
light from the short wave rays of violet colour,
but both are produced all through the spectrum.
Heat and lignt, therefore, are simply effects of

152 THE STORY OF THE EARTH'S ATMOSPHERE.

the same wave motion according as it specially
affects our senses of sight or feeling, an<J they both
inseparably belong to the same radiant energy
of wave motion of the ether, started by a body
already in a state of incandescence like our sun.
Eays near the violet end of the spectrum pro-
duce the chemical action noticed in photography
besides being converted into heat and light.
Davm and ttoilight have ever formed expansive
themes to the poet. " Eosy-fingered dawn" is
a familiar metaphor of the immortal Homer.
These half lights are the result of reflection of
the sun's rays when below the horizon, chiefly
by the small dust and water particles at great
heights in the atmosphere. The fingered appear-
ance alluded to by Homer is due to the light
passing between clouds or mountains below the
honzon. The reddish colours of the clouds at
both times are chiefly due to the selective
scattering which is exerted by the dust arid
vapour suspended in the air. The smaller waves
corresponding to the blue rays, as we have
already remarked in Chap. VI., are more easily
tumea aside than the larger ones corresponding
to the red rays, and this dispersion reaches its
greatest eflect when the sun is shining through
a great thickness of air at its rising and setting.
Consequently red lays predominate at these
times and tint the clouds as they successively
receive its parting or coming rays. Occasionally
when a sunset has disappeared below the western
horizon it is brilliant enough to cause a second
sunset on clouds near the western horizon by
reflection, just as though it were the sun itself.

OPTICAL PHENOMENA OF THE ATMOSPHERE. 153

When the dust ejected by the volcano of Kra-
katoa Island in 1883 had spread in a layer above
50,000 feet all over the world we had such bril-
liant primary and reflected sunsets, which often
lasted 1| hours after the sun had disappeared.

The reflection was assisted by a peculiar action
called diffraction, by which white light meeting
fine dust is split up into its coloured elements,
just as though it passed through a glass prism.

In this way a huge coloured ring called a corona
was produced round the sun. iShis ring is blue
inside, and exhibits thence all the spectrsd colours
in turn, ending with red at its border.

Inside this ring a white central glow was pro-
duced even when the sun was high up in the
sky. When it was setting this diffraction glow
became pink and finally red through the extinc-
tion of all other colours except the reds owing to
the great length of air traversed by the rays. Such
glows are always present to some extent, due to
diffraction by suspended water particles, but when
the Krakatoa dust was still in the upper atmos-
phere they were intensified, and the ordinary re-
flections prolonged far beyond their usual limits.

Similar small coronse are produced when small
clouds pass over the sun and moon. On a small
scale they can be frequently observed when we
look through our eyelashes at the flame of a
candle or gas lamp.

The smaller the particles of cloud the larger the
corona. Hence the large corona seen round the
sun after Krakatoa, called Bishop's ring from its
discoverer in Honolulu, showed that the material
was composed of very small particles.

154 THE STORY OF THE EARTH'S ATMOSPHERE.

A hah is a large ring seen when the sun and
moon shine through a thin sheet of cirrus or
cirro-stratus, and can only be produced by re-
fraction through ice-prisms. Consequently its
presence is one indication of the ascent of vapour
into very lofty regions, such as occurs in cyclones.
It is thus a signed of the approach of rainy and
stormy weather.

A primary halo is always the same size — 45*"
diameter. Sometimes, however, secondary halos
are formed by more complicated refractions and
reflections of light through the ice prisms.

For example, outside the ordinary halo, and
concentric with it, an extraordinary halo is occa-
sionally seen of 90° diameter. Intersecting these
halos, a huge circle passing through the sun and
parallel to the horizon makes its appearance. At
the points of intersection of these halos, the light
is so reinforced that the patches look like separate
suns, and form what are termed mock-suns or
parhelia. Similar appearances round the moon or
mock-moons are termed paraselense. At the oppo-
site points of the sky similar mock-suns are occa-
sionally formed. Some years back the author saw
four mock-suns at the same time. Two in front
where the primary halo intersected the large
horizontal halo, 22|° on each side of the sun,
and two behind him, making angles of 1571*"
with the sun on each side.

The mirage, or serab (illusion), as the Arabs
term it, is a phenomena which has often formed
a subject for the poet as well as the artist.

The thirsty traveller in the dreary and parch-
ing wastes of the Sahara and Arabian deserts

OP*nOAL PHENOMENA OF THE ATMOSPHERE. 155

frequently sees looming up in the distance a
beautiful lake dotted over apparently with
islands and trees. This lake is an illusion pro-
duced by the bending or reflection of the light
that occurs at the boundary of two strata of air
of different temperatures. In this case a layer
of cool air overlies one of very hot air just above
the heated sand. Any object, such as a tree or
mound above this layer, has its image inverted by
reflection, while the light from wie ground is
thrown back by what is termed internal reflec-
tion. Consequently the effect is just the same as
though a layer of water were really present. A
special kind of mirage is termed "looming."
In this case objects which are ordinarily below
the horizon are seen raised above it, sometimes
inverted and sometimes erect. These effects are
due to a great increase in the ordinary refraction
which takes place near the horizon, due, probably,
to a cold and dense layer of air over the sea,
overlain by a warmer layer derived from the
neighbouring land. The famous Fata Morgana or
castles of 'the witch Morgana of Eeggio are an
instance of this kind of mu*age. During certain
conditions of the air the inhabitants of Eeggio
see castles and men and trees, etc., suspended
above the sea in the direction of Messina,
whose reflected image they really are. A
southern imagination converts them into en-
chantments.

A curious effect of looming occurred once at
Malta, where the top of Etna appeared by re-
fraction like an islana in the sea. Several ships
sailed out to take possession of this supposed

156 THK 8T0BT OF THE EABTH^ A33fOSFHERE.

new irimd, but toon the image Taimihed and
the quest was seen to be tsul

TUs tUxy was paraHded more reeeaHlj whra
the gorgeooB Krakatoa stmsets first made their
appearance in Ammea. A local fire Mgade in
a raw Western township, seeing the sky so red,
with more zeal than wisdom harnessed np and
set forth with all speed to put it out When
they tdtimatelv fotmd oat their mistake they
were not a little, pat oat themselves.

In the poliu- r^ons, where the sea is osoally
colder than the air, the images of objects below
the horizon are frequently reflected to the observer
from the top warm layer and appear inverted.
If the upper warm layer is of no great thickness,
there is thus often both a direct and inverted
image. Scoresby once recognised his father's
ship, the Fame^ by observing its inverted image
through a telescope. The real ship was after-
wards found to have been thirty-five miles
distant.

The rai/nbaw has always been a majestic
symbol of the union between earth and heaven.

Iris, the goddess of the rainbow, was one of
the most graceful of the Grecian deities. She
was represented as the messenger between
Olympus and his earthly subjects.

According to the Teutonic mythology the
rainbow was the bridge over which the heroes
passed to the festive abode of Walhalla.

Bobbed of its fanciful mysticism, the rain-
bow loses nothing of its beauty when we know
that it is the result of the refraction of the
white light from the sun as it enters the rain-

OPTICAL PHENOMENA OF THE ATMOSPHERE. 157

drop subsequently reflected from the back of
the drop to our eyes. The whole operation is
so wonderful The different coloured rays which
make up the white ray when they meet the new
surface, part company according to their wave
frequency, and travelling along separate paths
are reflected by the mirror back as though they
were painted in the sky. The tiny violet waves
being more bent inwaras, appear inside the bow,
while the longer red waves form the external
boundary. Ordinarily the earth cuts off the
lower half of the bow, and when the sun is
more than 40° above the horizon, the entire
phenomenon disappears.

Inside the bow the violet is occasionally seen
repeated in what are termed supernumerary
bows, while the external bow is often visible
in which the colours are reversed. The explana-
tion of these belongs rather to a book on optics.
The "Spectre of the Brocken" is simply a
shadow of the spectator projected on 1x) a
screen of vapour rising up from the surround-
ing valleys, and may be seen on any mountain
where the conditions are favourable.

The "ignis fatuus," or wandering flame oc-
casionally seen in marshy land, or over church-
yards, where it is called the "corpse candle,"
is believed to be merely a distillation from the
soil of phosphoretted hydrogen gas which has
the property of self-ignition on emerging into
the atmosphere.

The "aurora polaris" or "northern lights"
are a manifestation of quiet electrical discharge
round either pole, attaining its greatest brilliancy

158 THE STORY OF THE EARTfl'S ATMOSPHERE.

and frequency near the magnetic poles, which
are at some distance from uie true geo^aphic
poles. In the northern hemisphere the oelt of
greatest frequency (80 auroras per annum) occurs
from latitude 50' to 62* in America, and from
latitude 66° to 75° over Siberia. From thence
they diminish both north and south.

The Aurora exhibits various forms. Streamers,
curtains, bands, and rays, and it frequently corus-
cates, whence the name " Merry Dancers." It is
believed that the Aurora is a sheet of rays which
converge downwards towards the magnetic axis
of the earth, a kind of luminous collar, the top of
whose arch is as much as 130 miles above the
earth, though parts of it are believed to be quite
near the earth. It is therefore an electrical
discharge taking place in highly rarified air or
vacuum. Lemstrom of Finland recently suc-
ceeded in causing an artificial aurora by suitably
imitating what is believed to occur in Nature.
The Aurora is certainly closely connected with
the magnetic condition of the earth and also of
the sun. When any great sun-spot appears on
the latter orb, the magnetic balance of the earth
is affected, as shewn by the irregular movements
of the magnetic needles and me simultaneous
appearance of aurorae at both poles.

CHAPTER XII.

WHIRLWINDS, WATERSPOUTS, TORNADOES, AND
THUNDERSTORMS OF THE ATMOSPHERE.

Besides the large cyclones, there is a peculiar
group of local disturbances or storms of the
atmosphere which, according to their violence,
occur in one or other of the above forms. The
harmless dust-whirl we see arise on a still
day in early summer, and sweep across the
young com, is but the embryo of the terrible
tornado of the Middle United States.

The dreaded simoom of the Arabian Desert
is simply a larger whirlwind laden with the
dust of the desert. Where the whirl is broader
and higher, and the air is moist, we have the
common thunderstorm of Europe with or with-
out hail, the " nor'-wester " of India, the " pam-
pero" of the Argentine, and the so-called
" arched squall " and " bull's eye squall " of the
tropical seas.

When the action is very intense and concen-
trated, we have the " tornado " which is common
in the Mississippi Valley. The freaks of some
of these tornadoes, while generally of the tragic
ofrder, occasionally border on the ridiculous.
Thus — even in India where they occasionally
occur in a mild form — it is stated that in the
district of the Brahmaputra, on March 26, 1875,
after a tornado had passed the village of Uladah
a dead cow was found stuck in the branches
of a tree some 30 feet from the ground.

In America, in the tornado of June 4, 1877,

169

160 THE STORY OF THE EARTH'S ATMOSPHBEE.

at Mount Carmel, Illinois, the spire, vane, and
gilded ball of the Methodist Church were carried
fifteen miles to the north-eastward. In other cases
ploughshares and even houses (generally of wood)
have been carried up into the air, and, so to
speak, transplanted. In the recent terrible
visitation at St Louis, in June 1896, it was
stated that a carriage was lifted from the road
up into the air and gently let down again 100
yards off without damage, while at the end of
this remarkable performance the coachman's hat
was declared to have remained securely attached
to his head. This last circumstance sounds a
little tall, but there is no obvious exaggeration
in that given by one spectator who informed
the writer that he looked up a street in St Louis
and saw everything — horses, carriages, people,
and furniture being whisked along in tumultuous
chaos towards him as the centre of the tornado
passed over it.

When the centre of a tornado passes it seems
to sweep everything movable along with it,
often destroys the most substantial buildings
and cuts a clear lane through a forest. In all
these cases, the prime cause appears to be a
local instability of the air due to an abrogation
of heat near the suriace, combined with an in-
cursion of cold air in the stratum above. These
together cause a rapid fall of temperature in a
vertical direction.

In such a case even dry air may temporarily
ascend in a narrow column and burst througn
the upper layers.

When once this has taken place the surround-

WHIRLWINDS, ETC., OF THE ATMOSPHERE. 161

ing air rushes in to supply its place, and there
ensues a whirling round just as in the case of
water running down through a sink.

In Tornadoes the whirling round of the air
is not due as in that of the large cyclones to
the deflection caused by the rotation of the earth,
since this would be practically insensible for
movements within such limited areas. It is
due to the rapid development of gyration as
the air is forced inwards towards the centre
when once such gyration has started. The
slightest deviation to one side of the direct
path to the centre is enough to start a gyration,
and any slight irregularity in the flow suffices to
cause a deviation.

After the whirling has once started, the gyi'a-
tions near the centre become so rapid that
ultimately a funnel shaped column of highly
rarefied air is produced, which is marked by the
appearance of a sheath of cloud or water within
which, in extreme cases, is a nearly complete
vacuum. Eound and up the sides of this the air
ascends, flows out above, and again quietly
descends over a wider area.

When the air is dry, the action, as we know,
cannot continue very long, since the uprising air
soon reduces the vertical temperature differences
below V in 180 feet. Dust whirls and sand
stonns are consequently short-lived and never of
destructive violence

When, however, the lower air is very damp as
well as hot, the action can go on for a much
longer time and with far greater energy.

Lieut. Finley of the U.S. Navy, who has made

162 THE STORY OF THE EARTH'S ATMOSPHERE

a special study of American tornadoes, estimates
that the velocity of the wind rotating near the
centre of a tornado may reach as much as 500
miles an hour, and exert a pressure of 250 lbs.
to the square foot. Even the upward velocity
near the vortex probably amounts, in many cases,
to over 100 miles an hour, otherwise it could not
sustain the objects it visibly does.

The awful effects frequently produced by the
arrival of such a piece of what may be termed
metewological dynamite can therefore be under-
stood. The central column of rarefied air by
reason of its expansion is cooled below dew
point. Hence, whatever vapour exists there,
becomes condensed into a visible sheath. This
is the cause of what are termed waterspouts,
which are only a mild form of tornado. In
the real tornadoes, the black funnel shaped cloud,
which forms one of their most marked features,
is due to the same causes. The popular notion
of a waterspout accounts for the water by
imagining it to be drawn up from the sea.
But this is erroneous. When waterspouts pass
over the sea, they cause a disturbance and slight
upward rise round their bases, but the long
visible column, often half a mile in length, which
dips down from the clouds, is entirely composed
of vapour, condensed out of the inflowing air.
As Ferrel puts it " the cloud (or rather the con-
ditions which favour the production of cloud) is
here drawn down towards the earth by the
reduction of pressure produced by the rapid
whirling of the air."

At the same time, the downward dip is only

WHIRLWINDS, ETC., OF THE ATMOSPHERE. 163

an apparent and not a real descent of water. As
long ago as 1753, indeed, the great Franklin
correctly explained this where he says —

" The spout appears to drop or descend from
the cloud though the materials of which it is
composed are all the while ascending, for the
moisture is condensed faster in a right line doum-
wardSf than the vajHmrs themselves can climb in a
^rai line upwards"

The freshness of the water in a marine spout
is clearly testified to in a story quoted by Prof.
Davis in his Meteorology :

A water spout had fallen upon a vessel and
poured its contents so freely over the captain,
that he was nearly washed overboard. He was
asked afterwards, rather jocularly, if he had
tasted the water ? " Taste it," said he, " I could
not help tasting it. It ran into my mouth, nose,
eyes and ears." Was it then salt or fresh asked
his querist? "As fresh," said the captain, "as
ever I tasted spring water in my life."

Waterspouts occur mostly in the tropics, and '
during the day hours. They are children of the
sunshine.

The prevailing funnel shape, tapering down-
wards, of the waterspout or tornado cloud, is a
consequence of the increased pressure of the
air near the surface. Above the surface the
absence of friction and the lower pressure allows
the central area of rarefaction, produced by the
rapidly whirling air, to extend for some space
laterally. Lower down the centrifugal tendency
of the rotating air is met by increased inward
pressure and is thus confined to a narrower

164 THE STORY OF THE EARTH'S ATMOSPHERE.

space. Outside the central core the air moves
gently towards the centre. When water in a
basin is descending through a hole, a similar
gentle flow may be observed, the rapid whirling
only extending for a short distance immediately
around the hole.

Even in destructive tornadoes the area of
dangerous damage and violent wind is confined
to comparatively narrow limits. The width of
the destructive path of the tornadoes in America
has been found by Finley to vary from 20 feet
to about 2 miles, the average being about 1369
feet.

The length of their paths is usuallj^ not more
than 20 miles, since the forces which give rise
to them, unlike those of cyclones, depend en-
tirely on specially marked vertical gradients of
temperature which seldom prevail simultaneously
over large areas.

The mode in which the air travels up into
and round these phenomena, may be gathered
from the adjoining Fig. (36). Instead of rising
up vertically it travels along the lines which
are represented as winding spirally round the
funnel until it becomes cooled partly by ascent
and partly by expansion into the tornado-
core and its vapour becomes visible at a point
considerably below the ordinary cloud level FH.

Tornadoes may be regarded as a kind of
atmospheric eruption analogous to those by which
the volcanic energy of the earth's interior is
expended in one spot.

They prevail where the local conditions favour
the establishment of explosive heat conditions.

WHIRLWINDS, ETC., OF THE ATMOSPHERE. 165

For example, where the geographical conditions
are favourable to the facile movement of cold
air from the north alongside or above warm air
from the south.

Such an area exists par excellence over the
flat river basins of the Mississippi, Missouri,
and Ohio. The states lying in these basins are

Fia. 36.— ToRWADO Funnel Clour

those in which tornadoes are found to be most
prevalent.

The general north and south trend of the
mountains and hills in America favours the
flow of air of such contrasted conditions, while
the prevalent east and .west ranges in the old
world make them act as preventive barriers.

The time of year most favourable to the pro-
duction of tornadoes is Spring or early Summer,

166 THE STORY OF THE EARTH'S ATMOSPHERK

when the earth is heating up rapidly, and the
air above it is still cold from the effects of the
preceding winter. Lieut. Finley found May to
be the month of greatest frequency of tornadoes,
while during autumn and winter they are almost
absent. The time of day at which they mostly
occur is in the afternoon when the accumulation
of heat in the lower layers has reached its
greatest amount. When the gun is loaded it
only requires the slightest pull on the trigger
to release an immense potential of energy.
Half a degree more temperature and the tornado
is born and starts off on its wayward journey.

The destruction caused by these tornadoes in
America is hardly realised in Europe which is so
happily exempt from them. At the same time
the deaths from this cause in the U.S. are
estimated to be less than those caused by
fire and flood.

Thunderstorms, like tornadoes, originate from
the uprise of a mass of warm moist air, but the
width of the column of uprising air is much
greater, and the whole action is much less con-
centrated and violent.

The vertical anatomy of a thunderstorm is
shewn in Fig. (37) where the spectator is
supposed to be standing to the right and viewing
an advancing storm. First of all he sees a
layer of cirro-stratus cloud (c)* commonly in
the western sky in the afternoon. Gradually
this grows thicker, and. from its under surface

* This layer should extend as far again as the width of
the figure to the right. The exigencies of space have
necessitated its curtailment in the Sijoining figure.

WHIRLWINDS, ETC., OF THE ATMOSPHERE. 167

festoons (/) similar to those in the "Festooned
Cumulus," Fig. (28), appear.

The cirro-stratus may extend from 10 to 50
miles in advance of the storm. In this way as
soon as they are visible, thunderstorms may
readily be forecasted within a lew hours by
experts such as the late Rev. Clement Ley.
Then follow the thunderheads (t) of cumulo-
nimbus (as in frontispiece) which represent the
front portion of the uprising current. Below
these, a low level base (h) of similar cloud is
seen, underneath which is a rain curtain (r).

Fig. 87.— Tht7Ndb£sto&m in Section.

A ragged squall cloud (s) rolls beneath the
dark cloud mass, a little behind its forward
edge, and the whole structure moves over the
land at the rate of from 20 to 50 miles an
hour. As the squall cloud comes overhead
the wind changes suddenly from an in-flow to
an out-flow, represented in the figure by two
arrows near the surface, with heads to the right.
In contrast with the hot muggy air preceding
the storm, this squall is deliciously cool, especially
in a Bengal north-wester. Simultaneously with
the arrival of the squall, the barometer rises

168 THE STORY OF THE EARTH'S ATMOSPHERE.

about J^th of an inch, the rain or hail begins
to fall, the lightning flashes and the thunder
crashes right overhead, until the centre passes,
and everything gradually resumes its former
aspect, except the temperature which has
been permanently lowered. The outflowing
squall is believed to be very similar to the
recoil of a gun when it is discharged. The
humid air in the centre of the storm expands
so suddenly in rising, that it actually kicks
against the surface air, and drives it outwards in
the direction where the pressure is least, that is
towards the front of the storm.

Thunderstorms travel along with the move-
ment of the air near' their tops, while the pre-
ceding inflow in front occurs as in the figure in
the contrary direction. This has given rise to
the saying that they travel against the wind.

There are at least two kinds of thunderstorms.
One is chiefly confined to the equatorial regions
and the summer in high latitudes, and the other
occurs in connection with cyclones in their south-
east quadrants.

They are both due to the convectional ascent
of warm moist air, but in the former case it is
locally manufactured during the daytime. In
the latter, it is often imported from a distance.
In the former storms, the cloud is isolated and
continuous often from 1000 feet up to the cirrus
level at 30,000 feet. When it ceases to ascend
it spreads out in a sheet in all directions, so that
a thunderstorm cloud of this kind often

E resents in the distance the appearance of a
uge anvil.

WHIRLWINDS, ETC., OF THE ATMOSPHERE. 169

The cyclonic thunderstorms are not so depen-
dent on local sun heat, and frequently occur at
night, and in the winter season in Scotland,
Norway and Iceland. In this case the cooling of
the upper air produces the same effect as the
heating of the lower.

Like the tornadoes they travel mostly east-
wards, and their occurrence generally betokens
the existence of a cyclone centre to the K.W.
in Europe, and to the S.W. in Australasia.

As long ago as 1752, Franklin proved by his
memorable kite experiment at Philadelphia, the
identity of lightning with electricity artificially
produced on the earth. There is, however, still
very little known as to the exact cause of the
accumulation of electrical potential which finds
vent in the lightning discharge.

The air is ordinarily found to be charged with
a certain amount of positive electricity, while the
earth is usually negative. The concentration
observed in thunderstorms, is believed to be due
to the increase in electrical quantity, and rapid
increase in electric potential (or power of doing
work) caused by the masses of damp air which
rise up, form towering cumulus clouds, and dis-
charge their vapour in drops, by condensation.

As the tiny droplets of vapour in the cloud
unite to form single large water drops, the
electrical charges which always exist to some
degree on their surfaces, become added together.
I^ot so the surfaces ; since the surface of a single
globe is always smaller than that of two globes
which unite together to form it. Consequently,
as more and more droplets unite together, the

170 THE STORY OF THE EARTH'S ATMOSPHERE.

electricity has less room over which to spread
itself. It consequently increases in thickness,
or in electrical language, density. It takes
300 trillions of droplets to form a single
rain drop, and it thereupon results that the
surface of the rain drop is one 8-millionth of
the area made up of all the surfaces of its
component droplets. Therefore the density of
electricity on the resulting raindrop is 8 million
times increased and by a simple electrical la\r
its potential or power to discharge, is increased
50 billion times.

We can thus understand how it is that so long
as masses of damp air are ascending in sufficient
quantity to cause the great condensation and
rainfall which usually accompanies thunder-
storms, \;he tremendous discharges of lightning
may be produced and accounted for without
recourse to any special theory of its origin.

Lightning destroys about 250 persons per
annum in America chiefly between April and
September.

Lightning conductors act by equalising the
flow of electricity between the air and earth
and preventing a disruptive discharge.

They are now generally made of iron and
must always be in contact with damp earth
since they act not by drawing the atmospheric
electricity down, but by allowing the earth
electricity to flow upwards.

Even in perfectly clear weather there is a con-
stant difference of electrical condition between
the air and earth. In flying kites at Blue Hill
near Boston with steel wire, a conductor has to

WHIRLWINDS, ETC., OF THE ATMOSPHERE. 171

be attached to the earth, otherwise the observers
even on a cloudless day experience severe shocks.

Lightning is of various kinds. Sometimes it
branches out in all directions from cloud to
cloud and is too far above the earth to strike
through the intervening space. This frequently
happens in the tropics where the author has
often witnessed a beautiful electrical storm right
overhead, the thunder of which was inaudible.
At other times, especially in cyclonic thunder-
storms, it occurs in lower clouds and strikes
down to earth in what is termed forked lightning
accompanied by loud thunder. Thunder is pro-
duced by the rapid heating and expansion of
air by the discharge passing through it.

The noise is occasioned precisely in the same
way as the sudden generation and expansion of
gas which ensues upon the ignition of gunpowder
in a confined space such as a gun.

The destruction of a tree or house is occasioned
in like manner by the expansion of air or material
which is unable to conduct the discharge. Upon
a human being the etFect is partly caused by heat
and partly by shock to the nervous system.

A peculiar form of lightning is occasionally
witnessed in which it descends from the clouds
in a globular form.

These isolated globes of electricity play peculiar
pranks, meandering slowly along in the most
wayward and capricious manner, and apparently
doing little damage until they burst. They are
believed to be somewhat of the nature of Leyden
jars in which a layer of air takes the place of
the glass.

172 THE STOBT OF THE EAKTH's AIMOSPHKRS.

St Ehnc/s fire is an appeanmee sometames
seen cm the masts of ships in stormj weather.
Each mast head is smroonded hy a fiunt lomin-
cms ball of electric li^t. It is reallj a brosh
discharge which takes place between the top of
the mast and the hig^j charged atmosphere
orerhead.

The most violent storms of lightningand thunder
in the world are probably to be foond in the north
westers of Bengal where the ligjbtning is con-
tinuous for more than an hour at a time. This
is due to the enormous condensation caused by
the upward oonyection of the yery damp air of
that region. The most awe-inspiring electrical
manifestations, however, frequently occur when
a thunderstorm occurs in a region like Ck)lorado
where the air is usually dry. The author once
experienced a storm at the Colorado Springs rail-
way station in which every time a flash of light-
ning appeared, a miniature flash and loud report
were simultaneously observed in the telegraph
office. The wire of the conductor outside was
fused, and upon one of the party venturing out
with an umbrella up he returned declaring it
was raining lead.

At the summit of Pike's Peak, 14,000 feet
high in the same district, the observers in the
now discontinued observatory used occasionally
to experience most disagreeable shocks even in
the simple act of shutting the door, while after
walking across the room they could light the
gas with their fingers. In Canada, in winter
when the air is very dry and frosty, the same,
phenomena are frequently observed.

SUSPENSION AND FLIGHT IN ATMOSPHERE. 173

It was formerly supposed that thunder and
hail were unknown in the Arctic regions, but
Mr Harries of the Meteorological Office has
recently shewn that they both occur right up
to Spitzbergen and are fairly frequent in the
Barents Sea. It seems possible that the warm
ocean currents bring enough warmth and moisture
to these cold regions to cause the vertical in-
stability of the atmosphere which originates
them.

The peculiar arched appearance of the clouds
in norwesters, pamperos, and the arched squalls
of tropical seas and higher latitudes is simply
an effect of perspective caused by a long roll of
cloud advancing athwart the spectator.

CHAPTEE Xm.

SUSPENSION AND FLIGHT IN THE ATMOSPHERE.

The conquest of the earth by man may be looked
upon as tolerably complete. The conquest of the
air has so far eluded all his efforts. Only for
short periods and with great trouble and risk has
he been able to mount into the air by the aid of
balloons.

The balloon itself, old though it may appear
to most of us, dates back only 100 years.

Lichtenberg of Gdttingen, in 1781, was among
the first to experiment, and made a small balloon
of goat-skin, which ascended in the air when
filled with hydrogen. Thomas Cavallo, an Italian

174 THE STORY OF THE EARTH'S ATMOSPHERE.

refugee, about the same time began by blowing
soap bubbles tilled with hydrogen, and watching
them mount as the school-boy does to-day. Be-
fore he got much further, a step in advance was
made in France by two brothers, Montgolfier,
who curiously enough started by trying to make
a cloud of steam ascend in a silk bag. On light-
ing a fire to increase the "cloud" they accident-
ally struck on the " hot air balloon," which has
rendered their names famous.

The first human being to actually ascend in a
balloon was Pilatre de Rozier on Nov. 21, 1783 ;
but in this case ordinary coal gas was employed,
and has ever since been generally adopted.

Soon after this, in 1 785, Blanchard safely crossed
the English channel in a balloon, and thence-
forward ballooning came into fashion, though at
first it was frequently attended with mishaps and
loss of life. The parachute, which, is now so
familiar to the world through the recent beauti-
ful descents effected by Baldwin, was first
employed by Garnerin on Oct. 21, 1797. He
then descended safely from a balloon, but experi-
enced violent oscillations. These are now
obviated by means of a central aperture through
which the imprisoned air flows quietly upwards.
The history of the balloon ascents of Lunardi,
Tissandier, Fonvielle, Gay Lussac, Green, Nadar,
Glaisher, and Coxwell is that of continual im-
provement, success, and safety. Their voyages,
particularly those of the two last, have added
considerably to our knowledge of the conditio as
of the up])er air. Within quite recent years
great strides have been made in the construction

SUSPENSION AND FLIGHT IN ATMOSPHERE. 175

of balloons, chiefly in relation to their use in
operations of war, by the English military balloon
department at Chatham.

The material employed is oxgut, which is
capable of holding pure hydrogen without leak-
age. Since pure hydrogen is nearly 2J times as
light as coal gas, balloons filled with it have
greater buoyancy and are better fitted to with-
stand the depressing influence of the wind when
captive. A balloon of this material, which con-
tains 10,000 cubic feet of gas, weighs only 170
lbs. The top valve is made of aluminium, and
a telephone conductor is arranged for communica-
tion between the occupant of the car and those
below. Men can readily be seen at a distance of
two miles from the car, and general military
reconnaisance, including photography can be con-
ducted with considerable accuracy.

By the aid of balloons man has ceitainly
succeeded in attaining suspension in mid air.
They have not, however, aided him in travelling
through the air towards some definite point.
If he commits himself to them he must needs
go nolens volens whither the wind may carry
him. Far from having conquered the air as he
has conquered the earth and the sea, he has
hardly more power to guide himself in a balloon
than a piece of straw hurled along by a whirl-
wind.

Some few years back Messrs Krebs and Kenard
in France were supposed to have solved the
problem of the dirigible balloon by means of a
cigar shaped balloon and a motor which drove a
rotary fan screw at one end ; but though in

176 THE STORY OF THE EARTH'S ATMOSPHERE.

calm weather progi^ess at some few miles an
hour was obtained, it was found to be useless
against the wind which ordinarily prevails at
any considerable height above the earth's surface.

The late Prof. Helmholtz dealt a death-blow
to the practical realisation of the dirigible balloon
by shewing on theoretical principles that a
balloon could not be driven against the air at a
rate of more than twenty miles an hour without
destroying its framework. To accomplish aerial
locomotion therefore, we must look elsewher^.

From the earliest times the flight of birds has
attracted the admiration and envy of mankind.

The ancient legend of Icarus who made a pair
of wings and singed them off by flying too near
to the radiant Phoebus, was evidently based on
the desire man has always shewn, to be able to
fly like a bird.

As long ago as 1470, that "preternatural
genius,'' Leonardo da Vinci, in the intervals of
painting the holy family, etc., amused himself by
planning amongst other things flying machines.
Moreover, he appears from his remarks, even
then, to have realised that the main difficulty
to be met with apart from elevating and motive
power, was the question of balance.

The recent accident by which that enthusiastic
soarer Herr lilienthal of Steglitz lost his life,
occurred through his inability to accommodate
his balance to a sudden gust of wind.

The early history of the attempts of mian to
fly is not calculated to inspire the human race
with a belief in its intuitive sagacity. For the
most part it is a history of miserable failures

SUSPENSION AND FLIGHT IN ATMOSPHERE. 177

and fatuous inability to realise the feebleness of
human muscular power. The first serious attempt
to grapple scientifically with the problem was
inaugurated by Wenham in 1866 in a paper
before the Aeronautical Society, in which the
principle of suspension by soaring as well as
flapping was alluded to.

Since that time great progress has been made
in the development of what are termed flying
machines by Prof. Langley of Pittsburgh, Hiram
Maxim of England, Octave Chanute of Chicago,
and Hargrave of Sydney.

In these machines no attempt is made to
imitate the flapping by which birds mount into
the air, but only of those principles by which
many of them are enabled to soar or sail with
outstretched wings when sufficient speed has
been attained.

Although it is a fairly safe rule to follow
Nature, exact imitation is by no means in every
case necessary or advisable. Thus, just as in
travel on the earth's surface, it has been foimd
more convenient to employ the wheel than
rapidly moving artificial legs, so in the atmo-
sphere, it is better from an aerial engineering
point of view to analyse the compound move-
ment of a bird's wing into the two distinct
elements, support and forward propulsion, and
deal with them quite separately. In the case of
the bird, the wing thrusts backwards, and also
acts as an inclined plane, which, when it is
forced horizontally through the air, converts the
pressure into support. In the artificial flpng
machine, the back thrust is given by the fan

M

178 THE STORY OF THE EARTH'S ATMOSPHERE.

screw or aerial wheel at the rear of the plane,
and the plane itself remains fixed at a certain
an^e.

The principle of the inclined plane is strictly
analogous to that by which a kite is suspended
when moored in a breeze. When the breeze
fails, the boy converts his kite into a flying
machine by running with it, and restoring sup-
port by the relative breeze thus created. If
we cut the string of the kite and supply it with
a motor and propelling fan, it will fly itself
without the boy's aid, and become a veritable free
flying machine. The kite, therefore, is the basis
of the flying machine. A flying machine is a
self-propelled kite.

There are two actions of the wind on a kite
or inclined plane. Partly it tends to make it
drift to leeward, and partly to lift it upward.
Certain birds, such as the Kestrel hawk,
shewn in fig. (38), the eagle, vulture, and
albatross, (especially the two latter), possess the
power of obviating the tendency to drift, and of
keeping themselves poised, or of sailing for long
periods without flapping by the action of the
wind on their wing planes. The precise way in
which this is accomplished is not yet fully
determined Maxim regards it as effected by
an intuitive utilisation on the part of the birds
of local upward currents which exist naturally,
or else artificially up declivities.

The albatross of the southern seas which the
author has frequently watched for hours and
days together, ^undoubtedly makes use of the
wind blowing up a wave to restore its lift.

SUSPENSION AND FLIGHT IN ATMOSPHERE. 179

after it has descended nearly to the surface of the
water.

Prof. Langley, on the other hand, attributes
the suspension in both hovering and sailing,
more generally to a like intuitive adjustment
on the part of the bird to certain rapid changes
which are found to occur in the speed of the
wind. When a strong gust comes, he slides
down a little to meet it, and overcoming the

FiQ. 88.— Ebstbbl Hawk HoVBRiNa.

back drift entirely by his forward momentum, is
able to utilise it simply for lifting him vertically
to the same height he was at before. When
the lull occurs, by lying flatter, he is able in this
way to derive a larger proportion of lift from the
lighter wind, and therefore maintains nearly the
same elevation, and so on.

In the circular sailing so commonly seen when
vultures sight a piece of carrion, the inclination

180 THE STORY OF THE EARTH'S ATMOSPHERE.

of the wing planes is similarly increased on the
windward half and decreased on the leeward half
of the circle.

The soaring and sailing of birds is only pos-
sible while the air is in motion. Directly there
is a calm, even the Albatross is obliged to flap.

It is therefore only when a wind is blowing,
that soaring can be exactly imitated by an
intelligently controlled flying-machine. In any
other case an artificial wind must be created by
means of the rotating fan-screw in order to
ensure support, and the plane must be kept
constantly inclined upwards.

It will be long before man will be able to gain
such a sense of flight as to be able to dispense
with the motor of his flying machine and sail
like the albatross without any apparent wing
motion, but such a sense will doubtless gradually
be developed as soon as he is fairly launched
into the air, on what is termed the motor
aeroplane, and future generations will witness
the ascent of man.

The present position of human flight stands
thus. Mr Maxim has built a large machine on
the aeroplane principle, which on being propelled
forward, has lifted itself and seversJ people a
few feet from the ground.

Professor Langley has made a small model
machine actuated by a petroleum motor which
has flown for a considerable distance while the
motive power held out.

Mr ilargrave of Sydney is making a machine
but no actual flight has yet been announced.

The basis of this machine is the so-called

SUSPENSION AND FLIGHT IN ATMOSPHERE. 181

cellular or double plane kite of which Mr Har-
grave is the inventor, and which has recently
-been shown to be the most efficient and stable
kite yet made.

Though a slavish imitation of bird architecture
has never found favour with flying machinists, a
study of birds, especially the large soaring and
saiUng birds, shows, what the Duke of Argyll in
his * Reign of Law ' has so lucidly demonstrated,
that birds fly " not because they are lighter, but
because they are immensely heavier than the air. i
If they were lighter than the air they might
float, but they could not fly. This is the
difference between a bird and a balloon/*

Any machine to travel through the air can
only do so in consequence of its superior
momentum. Consequently a flying machine
must be heavy in proportion to the resistance
it offers to the air.

Another important point is deduced from the
circumstance that a bird's wing presents a great
length (from tip to tip) and narrow width to the
wind.

For example, the wings of that king of flight
the Albatross {Diomedea emlans) measure 15
feet from tip to tip and only 8 inches across.

There is a reason for this. When a plane
surface is forced through the air, the upward
pressure of the air is mostly concentrated near
its front edge. If the surface extended far back
from the edge, its weight would act at some
distance from the front edge. Consequently the
unbalanced pressure of the air would tend to
turn the plane over backwards. If, however, its

182 THE STORY OF THE EARTH'S ATMOSPHERE.

width were small, the weight would act so close
to where the resistance acts in the opposite
direction that the forces would neutralise each
other and stability ensue.

Mr Hargrave has adopted this principle in his
cellular or box kite in fig. (39), whose construc-

tion is sufficiently obvious from the figure to
render detailed description unnecessary.

The dimensions in the figure are in inches.
The length of each cell (from right to left in
figure) is 30 inches, and the width and- height
and opening between are about II inches; but
these dimensions may vary, so long as the two

SUSPENSION AND FLIGHT IN ATMOSPHERE. 183

cells together form a nearly square area. An
important feature of this peculiar tailless kite
consists of the covered-in sides. These ensure
stability even better than two planes, bent
upwards in V shape, such as the wings of the
kestrel when hovering, and they prevent the
kite from upsetting, very much as the sides of
a ship give it stability.

Mr Maxim once showed the advantage of such
side planes by a simple experiment, in which a
piece of paper, when held horizontally and let
fall to the floor, is seen to execute a series of
zigzags in the air, frequently ending in its com-
plete overthrow ; whereas, when the same piece
of paper is folded up round the edges like a boat,
it sails to the floor quite evenly, and in a straight
line. The flying machine of the future seems
destined to be built somewhat after this pattern.

The prime problem is to laimch a stable
aeroplane into the air, provided with an engine
and screwfan powerful enough to drive it
forward at the velocity required. Mr Maxim
places his planes at a slope of 1 in 13, and
his practical experiments have shown that
the support gained by the pressure of the
air on such planes is more than twenty
times, and the motive power of the fanscrew
thirteen times what had formerly been sup-
posed. The engine which drives the fan is a
very light one, actuated by petroleum. Har-
grave estimates the entire weight of an engine
to generate 3J horse power at 30 lbs. It is
placed in the hollow between the two cells in
flg. (39).

184 THE STORY OF THE EARTH's ATMOSPHERE.

Prof. Langlev's recent experiment with his
model over the Potomac showed that the
elevating power derived from such an engine
is sufficient The main difficulty will be to
ensure stability under all conditions, and to
accommodate the apparatus to the varying
currents, by the aid of movable front and side
wings. To essay a journey except in a dead
calm, without considerable practice, would at
first probably end in mishaps. An era of pre-
liminary misadventure, in fact, appears to be
almost a necessary corollary to the establishment
of every new form of locomotion. That success,
however, will eventually be achieved is now
the firm belief of all those who have studied
the question.

The development of the flying machine will
also be much assisted by improvements in the
kite. The most efficient kite will be the most
suitable aeroplane basis for the flying machine.
The kite was first invented bv the Chinese
general, Han Sin, in 206 B.C., for use in war,
and was frequently employed after that date in
China, by the inhabitants of a besieged town, to
communicate with the outside world. After this
kites appear to have degenerated into mere toys.

At the middle of the present century, how-
ever, Pocock of Bristol employed them to draw
carriages, and is said to nave travelled from
Bristol to London in a carriage drawn by kites.
They were also occasionally employed to elevate
thermometers to measure the temperature of the
upper air, by Admiral Back on the T&iror, and
Mr Birt at Kew in 1847.

SUSPENSION AND FLIGHT IN ATMOSPHERE. 185

These observations had been quite forgotten
when the author first suggested the employment
of kites for systematic observations in 1883. It
has since been discovered that Dr Wilson of
Glasgow, as long ago as 1749, resuscitated kites
from their long burial with a similar idea of
employing them to measure temperature.

In the author's experiments, steel wire was
first employed to fly them with. Two kites of
diamond pattern made of tussore silk and bamboo
frames were flown tandem, and four self-recording
Biram anemometers weighing 1 J lbs. each were
attached at various points up the wire. Heights
from 200 to 1500 feet were reached by the
instruments, and the increase of the average
motion of the atmosphere was measured on
several occasions for three years. Kites were
also employed, first by the author in 1887,
to photograph objects below by means of a
camera attached to the kite wire, the shutter
being released by explosion. Since that time
kite photography has leapt into popularity, and
has been successfully practised by M. Batut in
France, Capt. Baden Powell in England, and
Eddy in New Jersey.

The figure following represents a recent photo-
graph of Middleton Hall, Tamworth, taken by
Capt. Powell with a kite-suspended camera at a
height of about 400 feet above the ground.

At the Blue Hill Meteorological Observatory,
near Boston, Mass., which is carried on by Mr
A. L. Eotch, tandems of kites are used to
elevate a box of self-recording instruments,
cameras, etc.

186 THE STORY OF THE EARTH'S ATMOSPHERE.

The adjoining fig. (41) shows the building,
which is 630 feet above sea level, and a tandem
of Hargrave kites supporting a camera with the
adjustment involving the use of an extra cord
for slipping the shutter, devised by Mr W. A.

Fia. 40.

Eddy. The height of the camera is determined
by simultaneous observations of theodolites at
the end of a base line.

By attaching several kites to the same main
wire great altitudes have been reached at Blue
Hill, and complete records of the pressure and

SUSPENSION AND FLIGHT IN ATMOSPHERE. 187

temperature recorded on a revolving drum of a
Eichard's thermograph and barograph.

The highest point attained so far was 9385 feet

Fig. 41.

above sea level, in October 1896. In order to
accomplish this, nine kites (of moderate size)
and three miles of steel ware were required. At
the highest point the temperature fell to 20',

188 THE STORY OF THE EARTH'S ATMOSPHERE.

while at the observatory, 8755 feet below, it was

On other occasions when the author was present
heights of 6079 and 7333 feet were attained.

t'or all such purposes, therefore, kites are able
to do as much as free balloons up to about
three miles. They are also cheaper and more
portable than captive balloons, and possess far
greater elevating power, especially in windy
weather, when such balloons are nearly useless.

It was further suggested by the author in
1888,* that kites could be used for various pur-
poses in war as well as science.

Since then Capt. Baden Powell, in May 1895,
read a paper on " Kites, their uses in War." In
both these publications it was pointed out that
kites possessed several distinct advantages over
balloons 5 next, that they could be applied to
all the purposes for which balloons could be em-
ployed, such as signalling, photography, torpedo
projection, carrying despatches between vessels,
and lastly, they could be employed to raise a
Tnan for purposes of reconnaissance.

This question of "man raising" was long
scouted as impossible, but both Capt. Powell
:ind Mr Hargrave have practically proved its
possibility by elevating themselves by kites, the
former having reached a height of 100 feet.

To give an idea of the size of kite required
for such a purpose, Capt. Powell was lifted by
a single large kite spreading 500 square feet,
weighing 60 lbs., and capable of folding into a

* Lea Cerf Volants Militaires. Bibliotheque des Con-
naissances Militaires. Paris, 1888.

SUSPENSION AND FLIGHT IN ATMOSPHERE. 189

package 12 feet long. Mr Hargrave, at Stanwell
Park, N. S. Wales, on Nov. 12th, 1894, was raised
16 feet by four kit.es flown tandem which spread
together an area of 232 square feet, the wind
blowing about 21 miles an hour. The total
weight supported was 208 lbs. An ounce of fact
is said to be worth a ton of theory. Here we
see that in an ordinary 20 mile an hour wind a
kite area amounting to 250 square feet is ample
to support a man.

FiQ. 42.

For a speed of only 10 miles an hour a larger
surface would be required, but if the system of
tandem kites redommended by Hargrave is
followed, this could be readily attained by the
addition of more kites. Under these circum-
stances, by two or more Hargrave kites a man
could be raised, as in fig. 42, and effect a recon-
naissance of an enemy's fortifications and dis-
positions, especially in mountainous country, with

190 THE STORY OF THE EARTH'S ATMOSPHERE.

considerable ease and far greater immunity than
in a captive balloon.

The portability of such a series of kites even for
man lifting may be guessed from the remark by
Mr Hargrave, in his latest paper dated August 5,
1896, that "a nineteen square feet kite has been
made, that weighs only 19 ounces, and folds to
about the size of an umbrella. Ten of these
could be tucked under one's arm, and with a coil
of line and a decent breeze, an ascent could be
made from the bridge of a torpedo boat or the
top of an omnibus."

The torpedo boat certainly sounds more heroic,
and probably less dangerous than the omnibus.

Numerous possibilities have been suggested by
Capt. Baden-Powell, and there seems no reason
why kites should not enter in as a regular part
of the paraphernalia of naval and military
operations.

Some few years back, the author, with a kite
of the ordinary diamond pattern, 18 feet by 14
feet, was able to carry up 600 feet of steel rope
cable, by which Col. Templer tethered his large
war balloon in Egypt.

This weighed 50 lbs., and as an additional
test, a man's kit weighing 10 lbs. was suspended
to its tail. Two such kites could lift a man and
pack away like fishing rods.

Quite recently (July 1896) a brochure by
Prof. Marvin, dealing with the whole science of
kites, has been published by the U. S. Weather
Bureau. This represents the most complete
discussion of kite-flying up to date, and one or
two of the results are worthy of special record.

SUSPENSION AND FLIGHT IN ATMOSPHERE. 191

The best kites are double plane Hargraves,
with certain improvements in details. Tandems
of two kites only, with 9000 feet of wire out, have
several times reached over 6000 feet in height.

Kites can be made to fly at angles of 60" or
more, and utilise most of the wind pressure in
lifting.

By adjusting the point of suspension or alter-
ing the kite, we can make it fly in the ideal
position. This is found to occur when the
direction of string or wire is inclined at an angle
of 66° to the horizon, and cuts the kite plane at
right angles, so that the latter is inclined at 24**
to the horizon.

Also theory shews that, in order to gain the
greatest effect when kites are flown tandem, the
largest kite or a bunch of two ought to be
placed at the top of the main wire.

In conclusion. — By balloons alone, man will
never be able to complete the conquest of the
air. For travel through the air, or as Prof.
Langley terms it " aero dromics," steam propelled
kites will be the future vehicle. For rest in the
air, it is not impossible that kites will again be a
serious rival of balloons. In fine, we may look
upon kites as likely to take a very much more
important place in the future than in the past
story of our atmosphere.

Before closing this chapter, it is worthy of
notice that the principle of the inclined plane is
made use of in two other important applications
of the motion of the atmosphere besides that of
supporting kites — viz., in the sails of shios, and
in windmills.

192 THE STORY OF THE EARTH'S ATMOSPHERE.

In the former, the wind meets the sail at a
certain angle, and produces effects analogous to
those on a kite, especially when the latter forges
overhead, under the influence of a freshening
breeze.

The water here acts like the controlling string,
except that it allows the sail and boat to move
through it, and, so to speak, form fresh attach-
ments every instant. The slip to leeward is
analogous to the lift in the kite, which is
checked by the inextensibility of its string.
The back drift is prevented by the pressure of the
water, and the shape of the main-sail, which tends
to make its forward part, and therefore the boat^
turn continually towards the wind. The shape
of the boat, the jib-sail, and the action of the
rudder convert this turning-round force into
continuous motion ahead.

As in the case of the kite, there is one position
(different for each combination of sails and boat,
and varying with the force of the wind) in which
the greatest advantage or speed is attained for a
given direction. To find this and maintain it is
the object of the steersman.

In practice it appears to be very similar to
the best inclination for a kite, so that for any
wind between head and beam, the sail should
not be inclined more than 24** to the keel. In
the case of a windmill, " the angle of weather," as
it is termed, or the angle which the sails make
with the plane of rotation, answers to the angle
between the keel and the boat-sail, and varies,
according to circumstances, round an average
of 24^

at'

ch

ii
ii

li-
lt

ii

le

SUSPENSION AND FLIGHT IN ATMOSPHERE. 1 93

Windmills are a means of converting the
motion of the wind into mechanical energy,
which may be employed either for pumping up
water, gnndiog corn, or, as Lord Kelvin sug-
gested in 1881, for generating electricity. Be-
fore the present coal-burning epoch, windmills

Fia. 48.— Tachtino in Stdbbt Harbocb.

used to be extensively employed for corn-grind-
ing. To-day they are mostly employed in raising
water for drainage, storage, or irrigation. Most
railway stations, every farm-house, and almost
every private country house in the Middle
United States and Australia, have their windmill
and tank. Labelled "cyclone" or "eclipse,"

N

194 THE STORY OF THE EAETH'S ATMOSPHERE.

according to their particular make, they form
quite a feature of the landscape, and it is
estimated that there are more than a million
such mills in the United States alone.

The " useful efficiency " of windmills, especially
in the modem geared form, is comparable with
that of the best simple steam-engines.

A geared modern wheel, 20 feet in diameter,
will develop 5 horse-power in an 18 mile an
hour breeze, and can be applied to work agri-
cultural machinery and djnaamos for electric
lighting. With a single wheel of this size, Mr
M'Questen of Marblehead Neck, Mass., U.SA.,
works an installation of 137 electric lights, for
which he formerly used a steam-engine. As a
result, he finds that he effects a saving of more
than 50 per cent.

According to Lord Kelvin, wind still supplies
a large part of the energy used by man. Out of
40,000 of the British shipping, 30,000 are sailing
ships, and as coal gets scarcer, "wind will do
man's work on land, at least in proportion com-
parable to its present doing of work at sea, and
windmills or wind motors will again be in the
ascendant."

LIFE IN THE ATMOSPHERE. 195

CHAPTER XIV

LIFE IN THE ATMOSPHERE

The limits of space warn us abruptly that we
must bring our story to a close. And yet,
facing us in the book of nature, there is a
large unwritten story of how the atmosphere
affects the lives of men and plants, embracing
questions connected with weather, climate, dis-
ease, hygiene, agriculture, sanitation.

The chief elements of climate have abeady
been dwelt upon in the chapter on temperature
and rainfall.

Hygiene and sanitation open out points in
which other factors, such as soil enter as well
as air.

The relations of the atmosphere to agriculture,
though a subject of immense interest to the agri-
culturist, is not a fascinating one to the general
public. Prof. Hilgard, of the University of Cali-
fornia, has exhaustively discussed this theme
in a bulletin published by the U.S. Weather
Bureau, 1892, and Sir J. B. Lawes and Pro-
fessor Gilbert hav6 carried out experiments in
England, at Eothampsted, all of which show
that in order to derive our maximum subsistence
from the soil, we must have a thorough know-
ledge of the actions which take place between it
and out atmosphere.

The relation of climate to life, health, and
disease is a very wide one, and though it has
attracted man's attention for years, it has only

196 THE STORY OF THE EARTH'S ATMOSPHERE.

recently been studied with anything like scientific
accuracy. An excellent summary of the prin-
cipal modem results will be found in Moore's
Meteorology,

As an example of how disease is dependent on
season, the following table will suffice :—

Development measured by Mortality.
Disease. MAximnm. ^ Minimum.

Enteric fever, Oct., Nov. May, June.

Smallpox, . Jan. to May. Sept., Oct.

Measles, . June, Dec. Mar., Oct.

Scarlet fever, Oct., Nov. Mar. to May.

The opposition between enteric and smallpox,
in regard to season, shows clearly that seasonal
conditions have a great deal to answer for in the
development of disease.

There is little doubt that besides the regular
effects of seasonal changes, the quality of the
^.ir of a place is a potent factor in relation to
health.

We talk of going away for a change of air, and
we know that beneficial effects usually follow if
we choose our fresh locality aright.

The air of cities, as we have seen, contains
vastly more dust particles than that of the
country, and it is full of other impurities, thrown
off by the multitudes of human beings crowded
together in a small space.

The pallor of children in cities compared to
the ruddy health of those who dwell in the
comparatively unpolluted country air is well
known. Similarly the air on mountains and
high plateaux is less dusty and vastly purer
than that near sea-level.

LIFE IN THE ATMOSPHERE. 197

In certain parts where vegetation decays in
presence of water, noxious exhalations arise
called significantly malaria (bad air), and cause
fevers not only in the Mangrove Swamps of the
tropics, but formerly even in the undrained fen-
districts of England.

This bad air usually remains quite close to the
ground, and its effects can often be obviated in
the tropics by sleeping on an upper floor.

The atmosphere undoubtedly acts in many
cases as a disease propagator by conveying germs
from one place to another.

For example the mysterious influenza, which
has of late years so afilicted the whole world, is
evidently propagated through the air. As a
rule, however, water is a far more effective
disseminator of disease than air, and where a
good water supply has been established, in
many parts of India, where formerly cholera
was rife, it now occurs very rarely and in a
milder form.

In general, the atmosphere acts as a health
and life giver^

The more fresh air we breathe, the Bg^ore we
dilute the poisons which would otherwise harm
our systems.

We are no doubt temporarily and permanently
affected by the particular climate we live in, as
well as by the air we breathe.

Climate is an average of the general weather condi-
tions, and is chiefly determined by the temperature,
rainfall, humidity, sunshine, and winds which
prevail in a district.

All the regular and irregular variations men-

198 THE STORY OF THE EARTH'S ATMOSPHERE.

tioned in chapters (IV.) and (VIIL) are involved,
particularly annual and daily temperature ranges.

At some seasons a change to a drier and
warmer climate such as that of Egypt or
Colorado is desirable.

Sometimes a mild one like that of Madeira or
New Zealand is recommended, while a return to
England or Europe is often indispensable to the
Anglo-Indian who has endured years of Indian heat.

Peimanent residence in different climates tends
to develop certain national characteristics.

Thus the dry, rapidly changeable, continental
climate of North America, accounts for the
activity and impulsive go-aheadness by which
the Americans are characterised. At the same
time it accounts for their liability to neuralgia.

The debilitating, nerveless lassitude of the
natives of tropical coasts is directly due to
the moisture and heat.

The dry heat of central India and Arabia
developes the martial energy of the Sikh and
the Bedouin, while the mild but cool and
temperate climate of England and Western
Europe is distinctly accountable for the well-
balanced mental and physical development of
the races which have hitherto ruled the world.

Climates may be hot or cold, moderate or
extreme {ie,, of small or large range), dry or
damp, calm or boisterous.

It was formerly deemed suflScient to pay atten-
tion to the temperature alone, but it has now been
found that the other factors are equally important.

Even in regard to temperature, the average
for the year is no safe criterion. The average

LIFE IN THE ATMOSPHERE. 199

is an artificial centre, round which the values
oscillate, and may be very seldom experienced.
The ranges are far more important.

Thus Calcutta, in Bengal, has the same mean
temperature of 77*7' F., as Agra, in the North-
West Provinces, but their climates are very
difierent when the ranges of temperature are con-
sidered. The difierence of average temperature
between the hottest and coldest months at Agra
is 34', at Calcutta only 20^ The average daily
range at Agra is about 30°, at Calcutta only 16**.

When we touch rainfall and humidity we
find Agra has only 29 inches to Calcutta 65
inches ; while if 5 represents the humidity at
Agra, 8 represents the amount at Calcutta.
Agra also has half the cloud, and therefore about
double the bright sunshine of Calcutta. Such
instances could be multiplied indefinitely.

Here, therefore, we have two places situated
in the same river valley, only 4° of latitude
apart, and yet with totally different climates.

To attempt to group climates together over large
areas is therefore impossible, except very roughly.

The old divisions of one torrid, two temperate
and two arctic zones served as a rough outline.
They are totally inadequate to explain the
variations found at places not far apart within
the same zone.

The only way to gain an idea of the climate
of a place, apart from a study of actual figures,
is to have a clear idea of the effects of all the
different factors, such as —

(1) Latitude.

(2) Hemisphere, north or south.

200 THE STORY OF THE EARTH'S ATMOSPHERE.

This makes a great difference. The tempera-
ture ranges are far smaller in the Southern
hemisphere.

(3) Situation with respect to large continents,
particularly east or west. If on the east, as the
U.S. or China, the temperature ranges, daily and
seasonal, are much greater than on the west

(4) Position, oceanic, coastal, or continental.
This affects both temperature range, and
humidity very largely.

(5) Elevation above the sea, and whether
isolated or on a tableland. If the former, the
climate is moderate ; if the latter, extreme. In
both cases the general temperature diminishes
about IT. for every 300 feet of elevation above
sea-level.

(6) Situation with respect to neighbouring
mountain ranges, especially leeward or wind-
ward, with reference to prevailing winds. If
on the windward side, such as Mull, Coimbra
in Portugal, Vancouver, Bombay, Colombo,
Valdivia in Chili, Brisbane, and Chirrapunji in
Assam, the rainfall is often over 75 inches,
while correspondingly on the lee sides of the
adjacent ranges we find, Aberdeen, Salamanca
(less than ten inches). Cariboo (east of the Coast
range), Poona, Bandarawela, Bahia Blanca, Eoma,
and Shillong, with amounts varying from 20 to
30 inches only.

(7) Situation with respect to prevalent winds,
trades, anti-trades, monsoons. This determines
the season of rain, such as the monsoon rains in
the Indian summer, whereas the summer in

LIFE IN THE ATMOSPHERE.

201

Australia, exposed to the trades, is the dry
season. The temperature conditions are thus
considerably modified.

(8) The neighbouring oceanic currents. The
effects of these have already been alluded to
on p. 64.

(9) The nature and covering of the adjacent
land.

(10) Situation with respect to the tropical
or circumpolar rain and wind-belts.

As types of various general climates at sea-
level, the following may serve as illustrations.

Climates.

l^pe.

Examples.

Batavia,
Colombo,
Singapore,
Cumana,
(2) Tropical, lat. lO** to 23*^—

/Calcutta,

Description.

(1) Equatorial,
lat. 0' to lat.
10^

(a) Coastal,

(b) Inland,

\Hong Kong,

'Lahofe,
Delhi,
Mandalay,
Timbuktu,

( Riviera,
(3) Sub - Trop- S. California,
ical, lat. 23* < Cape Colony,

to lat. 35^

Southern
Australia,

Hot, moist, equa-
ble, salubrious.

Similar to (1) but
less equable and
salubrious.

Hot, dry, and
extreme, try-
ing, except in
winter.

Temperate and
dry, owing to
position be-
tween tropical
and polar rain-
belts, very salu-
brious.

202 THE STORY OF THE EARTH'S ATMOSPHERE.

Type.

(4) Temperate-

(a) North, lat.
35' to lat. 60"

(J) South, lat.
35"tolat.50^

(5) Polar, lat.
60° to poles,

Examples.

/'England,
Europe,
UnitedStates
Central Si-
beria, and
China.

(New Zea-
land,
Tasmania,

DescripdoD.

Cool, moist, and
equable near
sea, dry and ex-
treme inland.

'Cool, moist, and
equable, most
salubrious in
the world.

/N.Siberia, i^}^ and fairly
{Greenland, \ J^J. extreme m-

Judged by averages alone, a climate with an
annual average temperature between

75° and 85° is hot.
65° „ 75° is warm.
55° „ 65° is mild.
60° „ 55° is temperate.
. 40° „ 50° is cold.
Below 40° is arctic.

These adjectives are, however, only applicable
when the range is small between summer and
winter.

Man can never hope to control or sensibly
alter the climate of the countries in which he
is placed. Nature works on too vast a scale.
He can, however, by studying the different
kinds of climate and their properties, discover

LIFE IN THE ATMOSPHERK 203

which are suitable for eertain diseases and ages,
and by utilising this knowledge, to some extent
shelter himself against influences which are re-
cognised to be hostile, and which lead not merely
to loss of individual life and health, but tK)
degeneration of the human race.

INDEX.

A.

Atmosphere, origin of, 9.
„ height, 14.

of uie sun, 14.
„ nature, 17.

„ composition, 17.

,, pressure, 27.

„ weight, 27.

,, temperature, 83.

68.

„ motion of, 95.

•,, laws which rule,

100.
„ cyclones, 133.

„ rain, snow, and

hail, 127.
,, sound of, 147.

,, optical pheno-

mena, 150.
„ whirlwinds,

waterspouts,
tornadoes,
thunderstorms
of, 169.
„ suspension and

flight in the,

itI.

„ life in, 195.

Aitken, John, 24.
Ammonia, 19.
Argon, 19.
Aluminium. 19
a04

Actinometer, 38.
Arago, 58.
AbM, Prof., 112.
Abercromby, Ralph, 118.
Anti-trade current, 89.
Anti-cyclone, 135, 140, 143.
Aurora polans, 157.
Albatross, flight of, 178-180,

181.
ArgyU, Duke of, 181.
Aero-dromics, 191.

B.

Berson, Dr, 16.

Brown, J. Allan, 31.

Ben Nevis, 43.

Buchan, 52.

Bezold, Prof., 67, 119.

Barometric pressure, chap. iii.

distribu-
tion of,
75, 78.
,, ,, vertical,

82.
, , , , Antarctic

depres-
sion of,
84.

Boyle's law, 101.

Buys Ballot's law, 137. .

Bora, the, 145.

Balloons, 173-176, 43.

INDEX.

205

Blanohard, 174.
Baldwin, 174.
Balloonists, list of, 174.
Bird flight, 178.
Birt, 184.

Baden-PoweU, Capt., 185, 188.
190.

C.

Capper, 136.

Courant, ascendant, 63.

Currents, oceanic, and tem-
perature, 64, 65.

Currents, convection. 111.

Curve, inertia, 72, 73.

Clayton, 98.

Clouds, speed at different
heights, 98.
,, general, 113 et teg.
,, formation of, 24,
120 a sea.
varieties of. 118, 126.
, , round a cyclone, 124.

C*harles and Gkty-Lussac's law,
102.

Colours of sky, 109, 150.

Cyclones, 137 et teq,

dfiinook, 145.

Corona, 153.

Cavallo, Thos., 173.

Chanute. 177.

Climate, 197-203.

D.

Davis, Prof., 72.

Dobb€, Prof., 21.

Dust particles, 25.

Diurnal variation of pressure,

81.
Doldrums, 87.
Drought areas, 92.
Diffusion of gases of the

atmosphere. 107. ~

Dew, 148.
Dampness, 115.
DovC Prof., 136.
Dawn, 152.
Disease, 196.

K

Eliot, J. . 30.

Edison, 39.

Ekholm, Dr, 98.

Euler, 150.

Electricity of atmosphere

169-173.
Elmo's, St, fire, 172.
Eddy, W. A., 185-186.

Pridlander. R D., 24.
Ferrel, 51, 67, 70, 79, 84.
Fritz, Prof., 67.
Fog, 114.
Fresnel, 150.
Fata Morgana, 155.
Franklin on tornadoes, 163.
Finley, Lieut., on tornadoes,

162, 166.
Funnel cloud, 165.
Flying machine, 180.

G.

Glaisher, 16, 148.
Gulf Stream, 47.
Gilbert, Prof., 195.

H.

Hail, 128.
Huxley, Prof., 22
Herschel, Sir John, 28.

206

INDEX.

Hawk, hovering,
Hypsometry = measurement

of altitude by barometer,

82.
Halos, 154.

Himalaya, temperature, 44
Heat, effect on l^md and
water, 49.

,, effect of enclosure on,
46.

,, local accession of, 59.

,, and light through at-
morohere, 109, 110.
Hargrave, 177, 180, 182, 186,

lo«7, xVtU.
Hann, Dr, 67.
Hadley, 69.

Hehnholt25, von, 84, 89, 176.
Hagstrom, 98.
Howard, Luke, 117.
Hildebrandson, 118.
Button, Dr, 119.
Hilgard, Prof., 195.
Harding, C, 144.
Harries, Mr, 173.

Isobars, 31, 78.
Isothermals, 47.
Inertia curve, 73.
Ignis fatuus, 157.
Icarus, 176.

Joule's, Dr, law, 105.

K.

Kelvin, Lord, on power, 89

193,194.
Kites, 43, 170, 178, 182. etc.

K5ppen, 57. .

Krakatoa, eruption of, 149,
153. . '

Krebs and Renard, 175, 193.

Lawes, Sir J. B., 195.

La Place, 10.

Lowell, Mr, 11.

Lambert, 86.

Langley, Prof., 38, 109, 177,

179, 180.
Land and soa breezes, 60.
Law, Boyle and Marriotte's

„ Charles and Gay-Lus-
sac's, 102.

„ Poiseon's, 102.

„ Joule's, 105.

,, Buys Ballot's, 137.
Light and heat, passage of

through atmosphere, 109,

150aseq.
Ley, Clement, 118.
Leste, 144.
Leveche, 144.
Looming, 155.
Lightning, 170, 173.
Lichtenburg, 173.
Leonardo da Vinci, 176.
lilienthal, 176.

Mai de Montague, 29.

Meech, 36.

Migration of sun, effect of, 49.

Monsoons, 61, 62, 85.

Maury, 68.

MoUer, 84, 98.

Marriotte's and Boyle's law.

101.
Mist, 116.

INDEX.

207

Meldram, Dr, 138.
Mirage, 154.
Montgolfier Bros., 174.
Maxim. 177, 183, 178, 180.
Machine, flying, 183.
Marvin, Prof., 190.
M*Queston, 194.

N.

Nebular theory, 10.
Nitrogen, 19.
Nitragin, 21.
Newton, 150.

Oberbeck, Prof..84.

Oxygen, 19.

Ozone, 19.

Oceanic currents, effect

temperature, 64, 65.
Optical phenomena, 150.

Priestley, 18.

Poisson, 36, 70, 71.

Poisson's law, 102.

Power of heat absorbed by

atmosphere, 39.
Piazzi Smyth, Prof., 89.
Piddington, 136.
Peri-cyclone, 140.
Pilatre de Rozier, 174.
Parachute, 174.
Pocock, 184.

Baylei^h, Lord, 26.
Badiation of heat, 34,
Elays, incidence of sun's, 35,
37.

Rainfall, belts, 91.

,, general, 130.

, , highest inworld, 131 .

,, highest in 24 hours,
134, note.
bow, 156.
Redfield, 136.
Beid, 136.
Reye, Prof., 139.
Rotch, A. L., 185.

S.

Sailing, 192.

Siemens, Dr, 84.

Silicon, 19.

Simoom, 159.

Sirocco, 144.

Solar heat, intensity of, 38.

Snowline, height of, 45.

Sunspots, effect on tempera-
ture, 57.

Smyth, Piazzi, 57.

Stone, 57.

Sprung, Dr, 84.

Stevenson, T., 97.

Snow, 127.

Solano, 144.

Sounds of the atmosphere,
147.

Sunsets, 15Z

Templer, Col., 190.
Thiesen, Dr, 14.
TorriceUi, 27.

,, vacuum of, 27.
Tyndall, Prof., 33, 109.
Thermometer, 41, 42.
Trade winds, 69.
Tornadoes, 133, 159.
Thunderstorm, 167.
Temperature, definition of, 33.

208

INDEX.

Temperature, decrease of, in

ascending air,

105, 111.

„ epochs, 40.

,1 . vertical distri-

budon of, 43,

.44.
„ vertical, cause

of, 46.
y, horizontal, dis-

tribution of,

47.
,, Land and water,

49.
,, distribution in

latitude, 50.
,, daily range, 52.

,, daily epochs, 55.

, , annual range,

56.
, , sunspots, effect

on, 57.
,, oceanic eur-

rents, 64. 65.
„ extremes, 66,

67.

face, 94; above, 95; of
clouds, 98.
Vettin, Dr, 9a

W.

Water, vapour, 23.

weight of, 28.
Waterloo tunnel, 29.
Winds, trade, 69.
„ general, 75.
,, shift of . general sys-
tem, 87.
,, velocity of general,
94, 96; at great
heights, 98 ; special.
144.
Wells, Dr, 115.
Whirlwinds, 159 a «eq.
Waterspouts, 159, 1^2.
Wilson, Dr, 185.
Windmills, 192.

V. I Y.

Velocity of air motion on sur-l Young, 160.

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