86 PHYSICAL GEOGRAPHY.

 

portion of watery vapor. The gaseous ingredients,
though of different densities, are found in the
_ same relative proportions at all heights, owing
to a property of gases called diffusion.

The oxygen and carbonic acid are the most important
of the gaseous constituents. Oxygen supports combustion
and respiration, and is thus necessary to the existence of
animal life. Carbonic acid, composed of carbon and oxy-
gen, is the source from which vegetation derives its woody
fibre, and is thus necessary to the existence of plant life.
In respiration, animals take in oxygen and give out car-
bonic acid; in sunlight, plants take in carbonic acid and
give out oxygen. In this way the relative proportions of
the substances necessary to the existence of animal and plant
life are kept nearly constant.

228. Elasticity.—The atmosphere is eminently
_ elastic; that is, when compressed, or made to oc-
cupy a smaller volume, it will regain its original
volume on the removal of the pressure. Air also
expands when heated and contracts when cooled.

229. Pressure.—So evenly does the atmosphere
press on all sides of objects that it was long be-
fore it was discovered that air possesses weight.
The discovery was made by Torricelli, an Italian
philosopher and pupil of the famous Galileo. The
instrument Torricelli employed is called a Ba-
rometer.

Fig. 79. Barometer,

230, The Barometer.—The principle of the barometer
is as follows: A glass tube, about 33 inches in length, is
closed at one end and filled with pure mercury. Placing
a finger over the open end, the tube is reversed and dipped
below the surface of mercury in a cup or other vessel.
On removing the finger, a column of mercury remains in
the tube, being sustained there by the pressure of the at-
mosphere. Near the sea-level this column is about 30
inches high; on mountains it is much lower; in all cases,
the weight of the mercurial column being equal to that
of an equally thick column of air, extending from the
level of the reservoir to the top of the atmosphere.

Any variation in the pressure of the atmosphere is
marked by a corresponding variation in the height of the
mercury in the barometer, the column rising with in-
creased, and falling with diminished, pressure.

The entire atmosphere presses on the earth

with the same weight as would a layer of mer-
cury about 80 inches in depth. A column of
mercury 80 inches high, and one square inch in
area of cross section, weighs about 15 pounds.
Therefore, the pressure which the atmosphere exerts
on the earth’s surface, at the level of the sea, is equal
to about 15 pounds for every square inch of surface.
The entire weight of the atmosphere, in pounds,
is equal to 15 times the number of square inches
in the earth’s surface.

The atmospheric pressure is not uniform on all parts of
the earth at the same level. From a few degrees beyond
the equator the pressure increases in each hemisphere up
to about lat. 35°, where it reaches its maximum, decreasing
in the northern hemisphere to lat. 65°, when it again in-

_ ereases toward the poles.

231, Height-of the Atmosphere.—If the air
were everywhere of the same density, its height
could be easily calculated ; but, on account of its
elasticity, the lower layers are denser than the
others, because they have to bear the weight of
those above them. The density must, therefore,
rapidly diminish as we ascend.

If by pressure on a gas we diminish its volume one-
half, its density will be doubled; conversely, if the den-
sity be diminished one-half, the volume will be doubled.
The following table, calculated from the law of increase
in volume with diminished pressure, gives the barometric
height, the volume, and the density of the air at different
elevations above the sea. The elevation of 3.4 miles is the
result of observation; the other distances are estimated.

Estimated Distance
ab. Sea, in Miles.

Barometric Vol. of Given

Height in Inches.} Weight of Air, | Density.

30.00
15.00
7.50
3.75
1.87
.93

 

 

It appears from the above table that by far the
greater part of the air by weight lies within a few
miles of the surface, nearly three-fourths being
below the level of the summits of the highest
mountain-ranges.

The height of the upper limit of the atmosphere
has been variously estimated. Calculations based
upon the. diminution of pressure with the height,
place it at from 45 to 50 miles above the level of
the sea; others, based on the duration of twilight,
place it at distances varying from 35 to 200 miles.

The form of the atmosphere is that of an ob-
late spheroid, the oblateness of which is greater
than that of the earth.