■ mils who v entered to breathe it.” See Che- 
mistry. 
Atmosphere, figure of. As the atmo- 
[ sphere envelopes all parts of the surface 
j i t our globe, it they both continued at rest, 
and were not endowed with a diurnal motion 
; about their common axis, then the atmo- 
sphere would be exactly globular, according 
[ to the laws of gravity ; for all the parts of the 
| surface of a fluid in a state of rest must be 
I equally removed from its centre, But as the 
f earth and the ambient parts of the atmo- 
sphere revolve uniformly together about their 
axis, the different parts of both have a centri- 
I fugal force ; the tendency of which is more 
J considerable, and that of the centripetal less, 
I as (lie parts are more remote from tlni axis ; 
and hence the figure of the atmosphere must 
I become an oblate spheroid, since the parts 
i that correspond to the equator are farther re- 
; moved from the axis than the parts which 
■ correspond to the poles. Besides, the figure 
I ot the atmosphere must, on another account, 
represent a flattened spheriod ; namely, be- 
cause the sun strikes more directly the air 
f which encompasses the equator, and is com- 
prehended between the two tropics, than that 
J which pertains to the polar regions ; for from 
j this it follows, that the mass of air, or part of 
I the atmosphere, adjoining to the poles, being 
; less heated, cannot expand so much, nor 
reach so high. And yet, notwithstanding, as 
the same force which contributes to elevate 
the air diminishes its gravity and pressure 
I on the surface of the earth, higher columns 
I of it about the equatorial parts, ail other cir- 
cumstances being the same, may not be hea- 
| vier than those about the poles. 
fn the transactions of the Royal Irish Aca- 
demy for 1 7 S8, Mr. Kirwin has an ingenious 
j dissertation on the figure, height, weight, &c. 
ot the atmosphere. He observes, that in the 
natural state of the atmosphere, that is, when 
the barometer would every where, at the 
level of the sea, stand ait 30 inches, the 
weight of the atmosphere, at the surface of 
the sea, must he equal all over the globe; 
j and in order to produce this equality, as the 
weight proceeds from its density and Height, 
it must be low est w here the density is great- 
] est, and highest where the density is. least; 
that is, highest at the equator and lowest at 
the poles, with several intermediate grada- 
tions. s 
Atmosphere, weight or pressure of . 
It is evident that the mass of the atmosphere, 
in common with all other matter, must be 
endowed with weight and pressure; and this 
principle was asserted by almost a!! philoso- 
phers, both antient and modern. But it was 
only by means of the experiments made witli 
pumps and the barometrical tube, by Galileo 
and Torricelli, that we came to the proof, 
not only that the atmosphere is endued with 
a pressure, but also what the measure and 
quantity of that pressure is. 4 Thus it is found 
that the pressure of the atmosphere sustains a 
column of quicksilver in the tube of the 'ba- 
rometer, of about 30 inches in height ; it 
therefore follows, that the whole pressure of 
the atmosphere is equal to the weight of n 
column ot quicksilver of an equal base, and" 
30 inches height ; and because a cubical inch 
of quicksilver is found to weigh nearly half a 
pound avoirdupoise, therefore the whole 30 
uithes, or the weight ot the atmosphere on 
cveiy square inch of surface, is equal to 15 
ATMOSPHERE. 
pounds. Again, it lias been found that the 
pressure of the atmosphere balances, in the 
case of pumps, Ac. a column of water of 
about 34+ feet high ; and the cubical foot of 
water weighing just 1000 ounces, or 62+ 
pounds, 34+ times 62+, or 21581b., will be 
the weight of the column of water, or of the 
atmosphere, on a base of a square fool ; and 
consequently the 1 44th part of this, or 151b., 
is the weight of the atmosphere on a square 
inch, the same as before. Hence Mr. Cotes 
computed, that the pressure of this ambient 
fluid on die whole surface of the earth, is 
equivalent to that of a globe of lead of 60 
miles in diameter. And hence also it ap- 
pears, that the pressure upon the human 
body must be very considerable ; for as every 
square inch of surface sustains a presureof 15 
Pounds, every square foot will sustain j 44 
times as much, or 2160 pounds ; then if the 
whole surface of a maids body is supposed to 
contain 15 square feet, which is pretty near 
the truth, he must sustain 15 times 2160, or 
32400 pounds, that is, nearly 14+ tons weight, 
for his ordinary load. By this enormous pres- 
sure vve should undoubtedly be crushed in a 
moment, if all parts of our bodies were not 
filled either with air or some other elastic 
fluid, the spring of which is just sufficient to 
counterbalance the weight of the atmosphere. 
But whatever this fluid may be, it is certain 
that it is just able to counteract the weight of 
the atmosphere, and no more ; for if any con- 
siderable pressure is superadded to that of 
the air, as by going into deep water, it is al- 
ways severely felt, let it be ever so equable, 
at least when the change is made suddenly ; 
and if, on the other hand, the pressure of the 
atmosphere is taken oil from any part of the 
human body, as the hand for instance, when 
put over an open receiver, whence the air is 
afterward extracted, the weight of the ex- 
ternal atmosphere then prevails, and we ima- 
gine the hand strongly sucked down into the 
glass. 
7 difference in the weight of the air 
which our bodies sustain at one time more 
than another, is also very considerable, from 
the natural changes in the state of the atmo- 
sphere. This change takes place chiefly in 
countries at some distance from the equator ; 
and as the barometer varies at times from 28 
to 31 inches, or about one-tenth of the 
whole quantity, it follows that this difference 
amounts to about a ton and a half on the 
whole body of a man, which he therefore 
sustains at one time more than at another. 
On the increase of this natural weight, the 
weather is commonly fine, and we feel our- 
selves what we call braced and more alert 
and active; but on the contrary, when the 
weight of the air diminishes, the weather 
is bad, and people feel a listlessness and in- 
activity about them. And hence it is no 
wonder that persons suffer very much in 
their health, from such changes in the atmo- 
sphere, especially when they take place very 
suddenly, for it is to this circumstance chiefly 
that a sensation of uneasiness and indisposi- 
tion is to be attributed : thus, when the varia- 
tions of the barometer and atmosphere are 
sudden- and great, we feel the alteration and 
effect on our bodies and spirits very much ; 
bfit when the charge takes place by very 
slow degrees, and bv a long continuance, we 
are scarcely sensible of it ; owing, undoubt- 
edly, . to the power with which the body is 
181 
naturally endowed, of accommodating itself 
to this change in the state of the air, as well 
as to the change of many other circumstances 
of life ; the body requiring a certain interval 
of time to effect the alteration in its state, 
proper to that of the air, & c. Thus, in going 
up to the tops of mountains, where the pres- 
sure of the atmosphere is diminished two or 
three times more than on the plain below, 
little or no inconvenience is felt from the ra- 
rity of the air, if it is not mixed with other 
noxious vapours, &c. because in the ascent 
the body has had sufficient time to accom- 
modate itself gradually to the slow varia- 
tion in the state of the atmosphere ; but when 
a person ascends with a balloon very rapidly 
to a great height in the atmosphere', he feel's 
a difficulty in. breathing and an uneasiness of 
body; and the same is soon felt by an animal 
when inclosed in a receiver, and ’the air sud- 
denly drawn or pumped out of it. So also, 
on the condensation of the air, we feel little 
or no alteration in ourselves, except when 
the change happens suddenly ; as in very ra- 
pid changes in the weather, and in descend- 
ing to great depths in a diving bell. 
It is not easy to assign the true reason for 
the variations that happen in the gravity of 
the atmosphere in the same place. One 
cause of it however, either immediate or 
otherwise, it. seems, is the heat of the sun ; 
for where this is uniform, the changes are 
small and regular : thus between the'tropics 
it seems the change depends on the heat of 
the sun, as the barometer constantly sinks 
about half an inch every day, anil rises a^ain 
to its former station in the night time ; but in 
the temperate zones the barometer ranges 
from 28 to near 31 inches, shewing by its 
various altitudes the changes that are about 
to take place in the weather. If we could 
know, therefore,* the causes by which the 
weather is influenced, we should also know 
those by which the gravity of the atmosphere 
is affected. These may perhaps be reduced 
to immediate ones, viz. an emission of latent 
heat from the vapour contained in the atmo- 
sphere, or of electric fluid from the same, or 
from the earth ; as it is observed that both 
produce the same effect with the solar heat in 
the tropical climates, viz. to rarefy the air, 
by mixing with it, or setting loose a lighter 
fluid, which did not before act in such lame 
proportion in any particular place. 
„ Atmosphere, height and density of. 
+ arious attempts have been made to ascer- 
tain the height to which the atmosphere is 
extended all round the earth. These com- 
menced soon after it was discovered, by 
means of the Torricellanian tube, that air is 
endued with weight and pressure. And had 
not the air an elastic power, but was it every 
where of the same density, from the surface 
of the earth to the extreme limit of the at- 
mosphere, like water, which is equally dense 
at all depths, it would be a very easy mat-, 
ter to determine its height from its density, 
ami the column of mercury which it would 
counterbalance in the barometer tube ; for it 
having been observed that the weight of the 
atmosphere is equivalent to a column of 30 
inches, or 2+ feet of quicksilver, and the 
density of the former to that of the latter as 
1 to 11040; therefore the Height of the uni- 
form atmosphere would bell040 times 2+ 
feet, that is,‘ 27600 feet, or little more than 
five miles and a quarter. But the air, by its 
