globe were covered with water to the depth of 
34 feet, or with quicksilver to the depth of 29 
inches. ‘This pressure has been computed at 
12,022,560,000,000,000,000 pounds, or as equiva- 
lent to that of a globe of lead 60 miles in dia- 
meter; and if we suppose that a man’s body ex- 
poses a surface of nearly 15 square feet, he will 
sustain a pressure of 32,3433 pounds, or 143 tons. 
As the different strata of the air are subjected 
to the pressure only of the superincumbent strata, 
it follows, that the pressure must decrease from 
below upward, and as the elasticity of the air 
will cause it to expand, as the pressure is re- 
moved, its density will also decrease; but as we 
ascend in a regular arithmetic progression, the 
density of the air decreases in a geometrical pro- 
gression. The decrease in density is therefore 
much more rapid than the increase in height. 
So that for every 2°705 miles (11,556 feet), the 
density is about halved, as shown by the following 
table :— 
Height above the sea in miles. Density. 
0. 1 
2705 
5-41 4 
8-115 i 
10:82 a 
13-525 au 
16:23 ue 
It might be supposed from this, that the atmo- 
sphere would extend itself infinitely into space; 
but that it does not extend to several of the other 
planets, can be proved by astronomical observa- 
tions. It certainly has a limit, either from a 
limit in the elasticity or expansibility of gases, 
or from the counteraction of these properties by 
the cold prevailing in the higher regions. From 
calculations founded on the phenomena of refrac- 
tion, the sensible atmosphere would seem not to 
extend beyond 45 miles. 
When a ray of light enters the atmosphere, it 
is bent from its course by the same cause which 
refracts the rays of light when they pass through 
any dense medium, such as glass or water. The 
refraction sustained by light at its first entrance 
into the atmosphere must be very small, from 
the extreme rarity of the air. The deviation, 
however, will gradually increase as it penetrates 
the denser strata, and the ray will describe a 
path increasing in curvature as it approaches the 
earth. From this property of the atmosphere, 
the apparent altitude of the sun, moon, and stars, 
is greater than their real elevation, and they ap- 
pear to be raised above the horizon when they 
are actually below it. The refraction of the at- 
mosphere near the earth’s surface is liable to very 
considerable anomalies. But while the solar rays 
traverse the earth’s atmosphere, they suffer an- 
other change from the resisting medium which 
they encounter. When the sun, or any of the 
heavenly bodies, are considerably elevated above 
the horizon, their light is transmitted to the 
earth without any perceptible change; but when 
these bodies are near the horizon, their light 
ATMOSPHERE. 
must pass through a long tract of air, and is con- 
siderably modified before it reaches the eye of the 
observer. The momentum of the red, or greatest 
refrangible rays, being greater than the momen- 
tum of the violet, or least refrangible rays, the 
former will force their way through the resisting 
medium, while the latter will be either reflected 
or absorbed. A white beam of light, therefore, 
will be deprived of a portion of its blue rays by 
its horizontal passage through the atmosphere, 
and the resulting colour will be either orange or 
red, according to the quantity of the least re- 
frangible rays that have been stopped in their 
course. Hence the rich and brilliant hue with 
which nature is gilded by the setting sun; hence 
the glowing red which tinges the morning and 
evening clouds; and hence the sober purple of 
twilight which they assume when their ruddy 
glare is tempered by the reflected azure of the 
sky. We have already seen, that the red rays 
penetrate through the atmosphere, while the 
blue rays, less able to surmount the resistance 
which they meet, are reflected or absorbed in 
their passage. It is to this cause that we must 
ascribe the colour of the sky, and the bright 
azure which tinges the mountains of the distant 
landscape. As we ascend in the atmosphere, the 
deepness of the blue tinge gradually dies away; | 
and to the aeronaut who has soared above the 
denser strata, or to the traveller who has ascended 
the Alps or the Andes, the sky appears of a deep 
black, while the blue rays find a ready passage 
through the attenuated strata of the atmosphere. 
It is owing to the same cause that the diver, at 
the bottom of the sea, is surrounded with the red 
light which has pierced through the superincum- 
bent fluid, and that the blue rays are reflected 
from the surface of the ocean. Were it not for 
the reflecting power of the air, and of the clouds 
which float in the lower regions of the atmo- 
sphere, we should be involved in total darkness 
by the setting of the sun, and by every cloud 
that passes over his disc. It is to the multiplied 
reflections which the light of the sun suffers in | 
the atmosphere, that we are indebted for the 
light of day, when the earth is enveloped with 
impenetrable clouds. From the same cause arises 
the sober hue of the morning and evening | 
twilight, which increases as we recede from the 
equator, till it blesses with perpetual day the in- 
habitants of the polar regions. The absolute re- 
fractive power of the atmosphere is 0:0005891712, 
and its relative power considered as unity. Its 
specific heat is also assumed as unity for com- 
parison with other gases. Compared with that of 
water it is = 02669. By rarefaction its capacity 
for heat is increased, but not in a direct ratio. 
_ Like other gases, atmospheric air expands 
0:002028, or 4); of its bulk at 32°, for each de- 
gree of Fahrenheit. If, therefore, the air at any 
given place become heated, it expands, whereby 
it becomes lighter than the rest, and ascends, 
being replaced by colder and heavier air flowing 
