curs 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 con-
densation takes place, and a second stratum of
cloud is formed and so on. Ultimately a point is
reached at which the vapour-sphere nearly van-
ishes, 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 role. Mr. John Aitken of Glasgow has
made this question the subject of special investi-
gation, and has found that the atmosphere, espe-
cially in its lower parts over land, contains thou-
sands 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 coun-
ter in various parts of the world, embracing the
Atlantic and Pacific Oceans, New Zealand, Cali-
fornia, 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, and espe-
cially in towns. They are, however, less numerous
at sea, especially in the Pacific and Indian Oceans.
Thus comparing all three- oceans we have at sea-
level.
24 THE STORY OF THE EARTH'S ATMOSPHERE.
Number of
dust particles per
cubic centimetre.*
Atlantic Ocean . . . 2053
Pacific . . . 613
Indian " . . . 5 12
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 pol-
luted 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 regu-
larly with the ascent. From observations on the
Bieshorn, Fridlander found the number gradually
diminish in the following ratio.
Height above Number of
sea-level. particles per cc.
6,700 feet 950
8,20O
8,400
10,665
II.OOO
13,200
13,600
4 80
513
406
257
2I 9
157
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
$ths of what it was at the lower level. The bear-
ing of this fact on the question of the beneficial
influence of high mountain resorts on pulmonary
and other diseases is obvious.
* About 15 cubic centimetres are equal to i cubic inch.
PRESSURE AND WEIGHT OF ATMOSPHERE. 25
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 Rayleigh 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 bar-
ometer 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 an
accepted dogma that " Nature abhorred a vacuum,"
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 discov-
eries, he cynically remarked that Nature evi-
dently abhorred a vacuum up to thirty-three feet.
His pupil, Torricelli, however, was not content
with this perfunctory explanation, and applying
Z 6 THE STORY OK THE EARTH'S ATMOSPHERE.
his genius to the question, conjectured that the
column of thirty-three feet of water exactly bal-
anced a similar column of air stretching to the
limits of the atmosphere. Remembering that
mercury was about thirteen times as heavy as
water, he inferred that if this were true, a mer-
cury 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, leav-
ing what is known as the Torricellian vacuum in
the upper part of the tube.
This is substantially the mercurial barometer
by which to-day we measure what we term atmos-
pheric pressure.
The reason the itvm pressure is employed and
not weight 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
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 Ibs.
PRESSURE AND WEIGHT OF ATMOSPHERE. 27
It is easy from this, knowing that mercury is 13.6
times as dense as water, and air only xoV^ths 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 moun-
tain 18,000 feet high it would only weigh half as
much. The weight of a cubic foot of water va-
pour 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 evi-
dently exercise a very considerable pressure on
the surface. Westminster Hall alone contains 75
tons of air, while the entire weight of air resting
on the earth has been estimated by Sir John
Herschel to amount to uf trillions of pounds.
Sudden alterations of this pressure, which are in-
dicated by the rise and fall of the barometer, un-
doubtedly affect some persons of a sensitive tem-
perament, while the steady fall of pressure which
occurs when we ascend a mountain or rise in a
balloon occasions what is termed mal de montagne
in both men and animals.
On the other hand, the excessive pressure ex-
perienced in diving-bells or caissons, or in the
digging of tunnels, where the men work under 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 pres-
sure with the height when the barometer marks
30 inches at sea-level, we find the following rela-
28 THE STORY OF THE EARTH'S ATMOSPHERE.
tive scale for air of an average temperature and
dampness.
Pressure. Altitude.
30 inches ..... o
910
1,850
2,820
3,820
4,850
5,910
7,oio
8,150
9,330
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 eleva-
tions the height in feet corresponding to one
inch in the barometer is less than at the higher.
The atmosphere is in fact more tightly packed
near the earth, so that while i inch of mercury
represents the weight of the first 900 feet of
ascent, i inch at 16,000 feet represents the weight
of about 1500 feet, and the proportion increases
at greater heights.
Were the scale i 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 i, while
its mass bears to that of the latter the ratio of
only i to 300.
The pressure at the average depth of the
PRESSURE A^D WEIGHT OF ATMOSPHERE. 29
ocean viz., two miles, is as much as 320 atmos-
pheres.
The barometric pressure undergoes changes,
some of which are irregular, and due to the pas-
sage of what are termed cyclones and anticy-
clones, in which the air is moving round moving
centres, while others, such as those which complete
their period in a year, are connected with seasonal
transfers of 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 mysteries that require
explanation. The seasonal changes and the gen-
eral 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
30 THE STORY OF THE EARTH'S ATMOSPHERE.
fall of the mercury column is a double one, reach-
ing 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, al-
though it undoubtedly depends on the action of
the sun, the total effect is made up of a combina-
tion 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 introduced by the
passage of cyclones, which frequently amount to
i or 2 inches' rise and fall of the mercury.
Barometric charts in which isobars, or lines of
equal barometric pressure, are drawn over the
representation of different parts of the earth, will
be referred to in chap. V. These charts are simi-
lar to those employed in weather bureaux in or-
der to forecast the probable weather for the ensu-
ing 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, metron, measure). We have already seen
that the pressure descends in a certain proportion
as we ascend in the atmosphere, and formulae
have been determined by which the height may be
calculated under certain conditions of tempera-
ture, humidity, etc. For rough and ready pur-
poses, 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 pres-
sures observed at each in inches divided by their sum
and multiplied by the number 55,76*, when the ai>er-
age of the temperatures at the two places is 60 f."
THE TEMPERATURE OF THE ATMOSPHERE. 31
When the average temperature of the two sta-
tions 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 arith-
metic will shew that the difference of their heights
is 1409 feet.
CHAPTER IV.
THE TEMPERATURE 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 conduction, we de-
rive 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 Tyndall 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 neighbour-
hood, and set all their molecules in a similar state
3
32 THE STORY OF THE EARTH'S ATMOSPHERE.
of oscillation. The process, however, is an ex-
change all round, and the temperature of any body
measures the rate at which it loses heat to or gains
heat from surrounding bodies. This rate depends
upon its capacity for heat, and its power of ab-
sorbing 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 com-
ing 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 al-
lows 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 tempera-
ture it is necessary to see how radiation affects
the atmosphere, since the conditions which regu-
late radiation, affect the temperature of the at-
mosphere in a somewhat similar manner.
When the sun's radiations have reached the
earth's surface from which the lowest stratum of
THE TEMPERATURE OF THE ATMOSPHERE. 33
the atmosphere chiefly derives its temperature,
their heating effect on a given area is modified by
two circumstances, (i) 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-
sidered it will be found to cover a smaller area in
proportion as they fall more vertically or less in-
clined. Thus in the accompanying diagram the
Sun vertical ( at noon
Equinox on Equator).
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 hori-
zon, 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 latitude we can, by travel-
34 THE STORY OF THE EARTH'S ATMOSPHERE.
ling 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 radia-
tion. We should not, however, be able to secure
the same average temperature since the direct ef-
fects of the radiation on temperature are modified
by what goes on over entire hemispheres. More-
over the effect of changing our latitude introduces
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 hemisphere which is en-
joying summer. There are thus two influences
which work in opposite directions, one, the gen-
eral 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 there-
fore 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 yth, the zone of greatest heat reception
lies in 41 N., while from May 3ist to July i6th,
more heat falls on the North Pole than on any
other part of the earth. The temperature of the
THE TEMPERATURE OF THE ATMOSPHERE. 35
Pole does not of course 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 melt-
ing the Arctic ice floes, and in raising the temp-
erature of the water. At the same time this
beneficial arrangement 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
FIG. 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.
36 THE STORY OF THE EARTH'S ATMOSPHERE.
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 sur-
face 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 sunrise and sunset the sun
has only -j-g^th part of the brilliancy it possesses
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 deriv-
ing 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
Ibs. o^f water, they would raise it i degree on a
Fahrenheit thermometer in i minute.
By the time these rays have reached the earth,
as we have seen, about th of the original radiation
has been absorbed or scattered by the atmosphere,
and therefore only about 7 Ibs. of water could be
raised i degree per minute. This however gives
us some faint idea of the enormous quantity of
heat which is continually falling on either the
earth or the clouds. If we take the heat which
falls on a square mile of the earth's surface per
THE TEMPERATURE OF THE ATMOSPHERE. 37
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 popularly put by
one writer in the following graphic manner, in
which the different amount of heat 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 angle
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 converted into
horse power, would be always represented 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 dratfn
upon when our coal supply shows symptoms of
giving out.
One effect has not been alluded to viz., the
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
38 THE STORY OF THE EARTH'S ATMOSPHERE.
by the large mass of water in the southern hemi-
sphere, 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 ^th to what would occur if the dis-
tance were invariable.
Since the temperature of the atmosphere re-
sults 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 Feb-
ruary 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 i degree Fahr. Con-
sequently, not only does the temperature of the
air over oceans rise and fall less daily and sea-
sonally than that over the continents, but the
highest temperature of the year in maritime