A square column of quicksilver twenty- 
line and a half inches high, and an inch thick, 
weighs just fifteen pounds, consequently, the 
air presses with a weight equal to fifteen 
pounds upon every square inch of the earth’s 
surface: and 144 times as much, or 2160 
pounds, upon every square foot. 
d he earth’s surface contains in round num- 
■bers, 200,000,000 square miles ; and as every 
Square mile contains 27,876,400 square feet, 
[■there must be 5,575,080,000,000,000 square 
[feet on the earth’s surface ; which number 
(-multiplied by 21 60 pounds (the pressure 
on each square foot), gives 12,043,468,800, 
-000,000,000 pounds lor the pressure, or 
•whole weight, of the atmosphere. 
Reckoning the surface of a middle-sized 
inan to be about 14 square feet, he sustains 
a pressure from the air equal to 30,240 pounds 
[Troy, or 1 1 tons 2 cvvt. and 18£ lbs. It may- 
be asked, how it happens that we are not 
[sensible of so great a pressure? The rea- 
son is, that such pressures only are perceived 
by us, as move our fibres, and put them, out 
of their natural situations. Now the pres- 
sure of the air being equal on all parts of 
the body, it cannot possibly displace any of 
tiie fibres, but on the contrary, braces, and 
keeps them all in their relative situations. 
Rut if the pressure is removed from any 
'p irticular part, the pressure on the neigh- 
bouring parts immediately becomes sensible. 
Thus, if you take a receiver open at the top, 
and cover it with your hand, upon exhaust- 
ing the receiver, and so taking off the pres- 
sure from the palm of the hand, you will 
■eel it pressed down by an immense weight, 
[so as to give pain that would be insupport- 
able, and endanger the breaking of your 
•hand. 
If the top of the receiver is covered by a 
[piece of flat glass, upon exhausting it, the 
[glass will be broken to pieces by the incum- 
bent weight ; and this would happen to the 
receiver itself, but for the arched top, that 
resists the weight much more than a flat sur- 
face. 
This experiment may be varied, by tying 
►a piece of wet bladder over the open mouth 
j©f the receiver, and leaving it to dry till it 
becomes as tight as a drum. Upon exhaust- 
big the receiver, you will perceive the blad- 
der rendered concave, and it wiil yield more 
iand more, until it breaks with a loud report, 
■which is occasioned by the air striking for- 
Icibly against the inside of the receiver, 
•upon being re-admitted. 
- Air is one of the most elastic bodies in 
mature; that is, it is easily compressed into 
dess compass, and when the pressure is re- 
moved, it immediately regains its former 
bulk. 
Let mercury be poured into a bent tube 
ABCD (fig. 4,) open at both ends, to a 
small height as BC ; then stopping the end 
D with a cork, or otherwise, air-tight, mea- 
sure the length of confined air DC, and pour 
mercury into the other leg AB, till the 
height above the surface of that in CD is 
[equal to the height at which it stands in the 
barometer at the time. Then it is plain, that 
the air in the shorter leg will be compressed 
[With a force twice as great as at first, when 
lit possessed the whole space CD ; for then 
it was compressed only with the weight of 
Vo*. II. 
PNEUMATICS. 
457 
the atmosphere, but mow -it is compressed 
by that weight, and the additional equal 
weight of the column of mercury. The sur- 
face of the mercury will now be at E; and 
it will be found, upon measuring it, that 
the space DE, into which the air is com- 
pressed, is just half the former CD. If 
another column of mercury was added, 
equal to the former, it would be reduced 
into one-third of the space it formerly oc- 
cupied. 
Hence the density of the air is proportional 
to the fo: - ce that compresses it. 
As all the parts of the atmosphere gravi- 
tate, or press upon each other, it is easy to 
conceive, that the air next the surface of 
the earth is more compressed and denser 
than what it is at some height above it ; in the 
same manner as if wool was thrown into a 
deep pit until it reached the top. The 
wool at the bottom having all the weight of 
what was above it, would be squeezed into a 
less compass; the layer, or stratum above 
it, would not be pressed quite so much, 
the one above that still less, and so on; till 
the upper one, having no weight over it, 
would be in its natural state. This is the 
case with the air, or atmosphere, that sur- 
rounds our earth, and accompanies it in its 
motion round the sun. On the tops of lofty 
buildings, but still more on those of moun- 
tains, the air is found to be considerably 
less dense than at the level of the sea. 
The height of the atmosphere has never 
yet been exactly ascertained ; indeed, on 
account of it.-, great elasticity, it may extend 
to an immense distance, becoming, however, 
rarer, in proportion to its distance from the 
earth. 
It is observed, that at a greater height 
than forty-five miles, it does not refract the 
rays of light from the sun ; and this is usu- 
ally considered as the limit of the atmo- 
sphere. In a rarer state, however, it may 
extend much farther. And this is by some 
thought to be the case, from the appear- 
ance of certain meteors which have been 
reckoned to be 70 or 80 miles distant- 
Dr. Cotes has demonstrated, that if al- 
titudes in the air are taken in arithmetical 
proportion, the rarity of the air will be in 
geometrical proportion. For instance. 
7 
\n 
4 
14 
16 
21 
64 
28 
V 
256 
35 
1024 
42 
-C 
- 4096 
49 
V 
- 16384 
56 
- 65536 
63 
262144 
70 
c 
1048576 
77 
V 
u 
4194304 
84 
t: 
16777216 
91 
5 
67108864 
98 
V 
268435456 
105 
Za 
V 
1073741824 
112 
o 
4294967296 
119 
-O 
17179869184 
126 
68719476736 
133 
- - 274877906944 
140 
1099511627776 
And hence it is easy to prove by calcula- 
tion, that a cubic inch of such air as we 
3 M 
breathe, would be so much rarefied at the 
altitude ot 500 miles, that it would fill a 
sphere equal in diameter to the orbit of Sa- 
turn. 
The elastic power of the air is always 
equivalent to the force which compresses it: 
for if it was less, it would yield to the pres- 
sure, and be more compressed ; was it 
greater, it would not be so much reduced ; 
lor action and re-action are always equal • 
so that the elastic fo»ce of any small portion 
of the air we breathe, is equal to the weight 
of the incumbent part of the atmosphere ; 
that weight being the force which confines it 
to the dimensions it possesses. 
To prove this by an experiment, pour 
some quicksilver into the small bottle A (fig. 
5.) and screw the brass collar c of the tube 
BC into the brass neck of the bottle, and 
the lower end of the tube will be immersed 
into the quicksilver, so that the air above 
the quicksilver in the bottle will be confined 
there. This tube is open at top, and is co- 
vered by the receiver G, and large tube 
EE; which tube is fixed by brass collars to 
the receiver, and is closed at top. This 
preparation being made, exhaust the air out 
of the receiver G, and its tube, by putting 
it upon the plate eff the air-pump, and the 
air will, by the same means, be exhausted 
out of the inner tube BC, through its open 
top at C. As the receiver and tubes are 
exhausting, the air that is confined in the 
glass bottle A, will press so by its spring, as 
to raise the quicksilver in the inner tube to 
the same height as it stands at in the baro- 
meter. 
M iscellaneous experiments. 
There is a little machine, consisting of 
two mills, a and b, which are of equal 
weights, independant of each other, and 
turn equally free on their axles in the frame. 
Eacli mill has four thin arms or sails, (fig. 6.) 
fixed into the axis: those of the mill a, have 
their planes at right angles to its axis, and 
those of b have their planes parallel to it. 
As the mill a therefore turns round in com- 
mon air, it is but little resisted by it, be- 
cause its sails cut the air with their thin 
edges ; but the mill b is much resisted, be- 
cause the broad side of its sails moves against 
the air when it turns round. In each axle 
is a fine pin near the middle of the frame, 
which goes quite through the axle, and 
stands out a little on each side of it : under 
these pins a slider may be made to bear, and 
so hinder the mills from going, when a 
strong spring is set or bent against the oppo- 
site ends of the pins. 
Having set this machine upon the pump- 
plate, fig. 1 . draw' up the slider to the pins on 
one side, and set the spring at bend on the op- 
posite ends of the pins ; then push down the 
slider, and the spring acting equally strong 
upon each mill, will set them both going 
with equal forces and velocities ; but the 
mill a will run much longer than the mill 
b, because the air makes much less resist- 
ance against the edges of its sails than 
against the sides of the sails of b. 
Draw up the slider again, and set the 
spring upon the pins as before ; then cover 
the machine with the receiver upon the 
pump-plate : and having exhausted the re- 
ceiver of air, push down the wire (through 
the cottar of leathers in the neck) upon the 
