ATMOSPHERE. 
to be borne without crushing the human 
frame : this indeed must be the case, if all 
the parts of our body were not endowed 
with some elastic spring, whether of air or 
other fluid, sufficient to counterbalance the 
weight of the atmosphere. Whatever this 
spring is, it is certain that it is just able to 
counteract the weight of the atmosphere, 
and no more ; of course it must alter in its 
force as the density of the atmosphere varies : 
for if any considerable pressure be superad- 
ded to that of the air, as by going into deep 
water, it is always severely felt; and if, on 
the other hand, the pressure of the at- 
mosphere be taken off from any part of the 
human body, by means of the apparatus be- 
longing to the air pump, the inconvenience 
is immediately perceived. 
The difference in the weight of the atmos- 
phere is very considerable, as has been ob- 
served, from the natural changes in the state 
of the air. These changes take place chiefly 
in countries at a distance from the equator. 
In Great Britain, for instance, the barome- 
ter varies from 28 4 to 30.7. On the in- 
crease of this natural weight, the weather 
is commonly clear and fine, and we feel our- 
selves alert and active ; but when the weight 
of the air diminishes, the weather is often 
bad, and we feel listlessness and inactivity. 
Hence invalids suffer in their healtli from 
very sudden changes in the atmosphere. In 
our observations on the barometer, we have 
known the mercury to vary a full inch, or 
even something more, in the course of a few 
hours. Such changes, however, are by no 
means frequent. Ascending to the tops of 
mountains, where the pressure of the air is 
very much diminished, the inconvenience is 
rarely felt, on account of the gradual change ; 
but when a person ascends in a balloon 
with great rapidity, he feels, we are told by 
Gamerin and other aeronauts, a difficulty of 
breathing, and many unpleasant sensations. 
So also, on the condensation of the air, we 
feel little or no alteration in ourselves, ex- 
cept when the variations are sudden in the 
state of the atmosphere, or by those who 
descend to great depths in a diving-bell. 
See Diving-Bell. 
It is not easy to assign the true reason for 
the changes that happen in the gravity of 
the atmosphere in the same place. One 
cause is, undoubtedly, the heat of the sun ; 
for where this is uniform, the changes are 
small and regular. Thus, between the tro- 
pics the barometer constantly sinks about 
half an inch every day, and rises to its for- 
mer station in the night. But in the tem- 
perate zones, the altitude of the mercury is 
subject to much more considerable varia- 
tions, as we have seen with respect to what 
is observable in our own country. 
As to the alteration of heat and cold, Dr. 
Darwin infers, that there is good reason to 
conclude that in all circumstances where 
air is mechanically expanded, it becomes 
capable of attracting the fluid matter of 
heat from other bodies in contact with it. 
Now, as tlie vast region of air which sur- 
rounds our globe is perpetually moving 
along its surface, climbing up the sides ol 
mountains, and descending into the valleys ; 
as it passes along it must be perpetually 
varying the degree of heat according to the 
elevation of the country it traverses : for, 
in rising to the summits of mountains, it 
becomes expanded, having so much of the 
pressure of the superincumbent atmosphere 
taken away ; and when thus expanded, it 
attracts or absorbs heat from the mountains 
in contiguity with it ; and, when it descends 
into the valleys and is compressed into less 
compass, it again gives out the heat it has 
acquired to the bodies it comes in contact 
with. The same tiling must happen in the 
higher regions of the atmosphere, which are 
regions of perpetual frost, as has lately been 
discovered by the aerial navigators. When 
large districts of air, from the lower parts 
of the atmosphere, are raised two or three 
miles high, they become so much expanded 
by the great diminution of the pressure over 
them, and thence become so cold, that hail 
or snow is produced by the precipitation 
of the vapour : and as there is, in these high 
regions of the atmosphere, nothing else for 
the expanded air to acquire heat from after 
it has parted with its vapour, the same de- 
gree of cold continues till the air, on des- 
cending to the earth, acquires its former 
state of condensation and of warmth. The 
Andes, almost under the line, rests its base 
on burning sands : about its middle height 
is a most pleasant and temperate climate 
covering an extensive plain, on which is 
built the city of Quito ; while its forehead 
is encircled with eternal snow, perhaps 
coeval with the mountain. \ et, according 
to the accounts of Don Ulloa, these three 
discordant climates seldom encroach much 
on each other’s territories. The hot winds 
below, if they ascend, become cooled by 
their expansion ; and hence they cannot 
affect the snow upon the summit ; and the 
cold winds that sweep the summit, become 
