PNEUMATICS. 
by 2,160 pounds (the pressure on each square 
foot), gives 12,043,468,800,000,000,000 
pounds for the pressure, or whole weight 
of the atmosphere. 
If the top of a small receiver be covered 
hy a piece of flat, thin glass, upon exhaust- 
ing it, the glass will be broke to pieces by 
the incumbent weight ; and this would hap- 
pen to the large receiver itself, but for the 
arched top, that resists the weight much 
more than a flat surface. 
This experiment may be varied, by tying 
a piece of wet bladder over the open mouth 
of the receiver, and leaving it to dry till it 
becomes as tight as a drum. Upon ex- 
hausting the receiver, you will perceive the 
bladder rendered concave, and it will yield 
more and more, until it break with a loud 
report, which is occasioned by the air strik- 
ing forcibly against the inside of the re- 
ceiver, upon being r'e-admitted. Air, as 
we have seen, is one of the most elastic bo- 
dies in nature ; that is, it is easily com- 
pressed into less compass, and when the 
pressure is removed it immediately regains 
its former bulk. 
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 is at some height above it ; in 
the same manner as if wool were 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 at- 
mosphere, that surrounds our earth, and 
accompanies it in its motion round the sun. 
On the tops of lofty buildings, but still 
more on those of mountains, the air is found 
to be considerably less dense than at the 
level of the sea. The height of the atmo- 
sphere has never yet been exactly ascer- 
tained ; indeed, on account of its great elas- 
ticity, it may extend to an immense dis- 
tance, becoming, however, rarer, in propor- 
tion to its distance from the earth. It is 
observed, that at a greater height than for- 
ty-five miles it does not refract the rays of 
light from the sun ; and this is usually con- 
sidered as the limit of the atmosphere. In 
a rarer state, however, it may extend much 
further. And this is by some thought to 
be the case, from the appearance of certain 
meteors which have been reckoned to be 
seventy or eighty miles distant, and whose 
light is thought to depend upon their com- 
ing through our atmosphere. Dr. Cotes has 
demonstrated, that if altitudes in the air be 
taken in arithmetical proportion, the rarity 
of the air will be in geometrical proportion. 
And hence it is easy to prove by calcula- 
tion, that a cubic inch of such air as we 
breathe, would be so much rarefied at the 
altitude of 500 miles, that it would All 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 were less, it would yield to the 
pressure, and be more compressed ; were 
it greater, it would not be so much re- 
duced ; for action and re-action are always 
equal, so that the elastic force of any small 
portion of the air we breathe, is equal to 
the weight of tlie incumbent part of the at- 
mosphere j that weight being the force 
which confines it .to the dimensions it pos- 
sesses. 
To prove this by an experiment, pour 
some quicksilver into the small bottle, A, 
(fig. r), and screw the brass collar, C, of 
the tube, B C, 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 covered by the receiver, G, 
and large tube, E F ; 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 of the air-pump, and the air will, by 
the same means, be exhausted out of the 
inner tube, B C, 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 in the barometer. 
There is a little machine, consisting of 
two vanes of equal weights, independent of 
each other, and turn equally free on then- 
axles in the frame. Each vane has four 
thin arms or sails fixed into the axis : those 
of the one have their planes at right angles 
to its axis, and those of the other have their 
planes parallel to it. Therefore, as the for- 
mer turns round in common air, it is but 
little resisted thereby, because its sails cut 
the air with their thin edges ; but the lat- 
ter is much resisted, because the broad 
