$5C) 
curely, and you will find that you cannot 
shut the bellows, which seems to be filled 
with something that yields a little, like wool ; 
but if yoiq unstop the nozzle, the air will 
be expelled, and may be felt against the 
hand. ' b 
W hen the air is at rest, we can move in 
it with the utmost facility ; nor does it offer 
to us a sensible resistance, except the motion 
is quick, or the surface opposed to it con- 
siderable ; but when that is the case, its re- 
sistance is very sensible, as may be easily 
.perceived by the motion of a fan. 
VV hen air is in motion, it constitutes wind ; 
which is nothing more than a current or 
stream of air, varying in its force, according 
to the velocity with which it Hows. 
The invisibility of air, therefore, is only 
4he consequence of its transparency ; but it 
is possessed of ail the common properties of 
matter. \\ hen a vessel is empty, in the 
usual way of speaking, it is in fact 'still filled 
with air. 
But it is possible to empty a vessel even 
of the air which it contains, by which means 
Ave shall be able to discover several proper- 
ties of this fluid. I he instrument, or ma- 
chine, by which this operation is performed, 
is called an air-pump. As it is by means of 
this useful instrument that all the mechani- 
cal properties of air are demonstrated, it will 
be necessary to describe its construction, 
and the manner of using it, before we pro- 
ceed to the experiments that are made with 
it. 
Plate I. Pneumatics, fig. 1. is the air-pump 
that is now most in use. AA are two brass 
barrels, each containing a piston, with a 
valve opening upwards. They are worked 
By means of the winch B, which has a pinion 
that fits into the teeth of the racks CC, which 
are made upon the ends of the pistons, and 
by this means moves them up and down al- 
ternately. 
On the square wooden frame DR, there are 
placed a brass plate G, ground perfectly flat, 
and also a brass tube, let into the wood, 
communicating with the two barrels and the 
cock 1, and opening into the centre of 
the brass plate at a. The glass vessel K, to 
be emptied or exhausted of air, has its rim 
ground quite flat, and rubbed with a little 
pomatum, or hogVlard, to make it fit more 
closely upon the brass plate of the pump. 
These vessels are called receivers. Having 
shut the cock I, the pistons are worked by 
.the winch ; and the air being suffered to es- 
cape when tire piston is forced down, be- 
cause the valve opens upwards, but prevent- 
ed from returning into the vessel for the same 
reason, the receiver is gradually exhausted, 
and will then be fixed fast upon the pump- 
plate. _ By opening the cock I, the air rushes 
.again into the receiver. 
As light as air,” is a common saying; 
yet air can be shewn to have more weight 
than is generally supposed. Take a hollow 
copper ball, or other vessel, which holds a 
wine quart, having a neck to screw on the 
.plate of the air-pump 1 ;; and after weighing it 
when full of air, exhaust it, and weigh it 
when empty ; it will be found to have lost 
sixteen grains, which shews that this is the 
weight of a quart of air. But a quart of wa- 
ter weighs 14621 grains : this divided by 16, 
quotes 9.14 in round numbers ; so that water 
’PNEUMATICS. 
T .” 14 Um , es as heavy as air near the surface 
ot the eai th. I his supposes air at a medium 
temperature and density ; for these, as will 
be seen afterwards, are variable. 
When the receiver is placed upon the 
plate of the air-pump without exhausting it, 
it may be removed again with the utmost 
facility, because there is a mass of air under 
h, that resists by its elasticity the pressure 
on the outside ; hut exhaust the receiver, 
thus removmg the counter-pressure, and it 
will be held clown to the plate by the weight 
of the air upon it. ° 
A hat the pressure of the air amounts to, 
is exactly determined in the following man- 
ner : 3 
A\ hen the surface of a fluid is exposed to 
the an, it is pressed by the weight of the 
atmosphere equally on every part, and con- 
sequently remains at rest. But if the pres- 
sme is removed from any particular part, 
he fluid must yield in that part, and he 
forced out of its situation. 
fnto the receiverjA (fig. 2.) put a small 
vessel with qmcksiWr, or any other fluid 
and through the collar of leathers at B, sus- 
pend a glass tube, closed, or hermetically 
sealed, as it is called, over the small vessel. 
Having exhausted the receiver, let down 
the tube into toe quicksilver, which will 
not rise into the tube as long as the receiver 
continues empty. But re-admit the air, 
and the quicksilver will immediately ascend. 
T lie teason of this is, that upon exhausting 
the receiver, the tube is likewise emptied of 
an- ; and therefore, when it is immersed in 
the quicksilver, and the air re-admitted into 
the receiver, all the surface of the quick- 
silver is pressed upon by the air, except that 
portion which lies above the orifice of the 
tube ; consequently, it must rise in the tube, 
and continue so to do, until the weight of 
the elevated quicksilver presses as forcibly 
on that portion which lies beneath the tube, 
as the weight of the air does on every other 
equal portion without the tube. • 
Take a common syringe of any kind, and 
having pushed the piston to the farthest end, 
immeise it into water; then draw up the 
piston, and the water will follow it. This is 
owing to the same cause as the last : when 
the piston is pulled up, the air is drawn out 
ot the syringe with it, and the pressure of 
the atmosphere is removed from the part 
ot the water immediately under it ; conse- 
quently the water is obliged to yield in tiiat 
part to the pressure on the surface. 
It is upon this principle that all those 
pumps called sucking-pumps act : the piston 
fitting tightly the inside of the barrel, by 
being raised up, removes the pressure of the 
atmosphere from that part, and consequently 
the water is drawn up by the pressure upon 
the surface. 1 
In the beginning of the last century, phi- 
losophers were of opinion that the ascent 
o I water m pumps, was owing to what they 
called “ rsature’s abhorrence of a vacuum 
and that, by means of suction, fluids might 
be raised to any height whatever. 
Galileo was the first who discovered that 
it was impossible to raise water higher than 
thirty-three feet by suction only ; and thence 
concluded, that not the power of suction, 
hut the pressure of the atmosphere, was the 
cause of the ascent of water in pumps ; that 
a column of water thirty-three feet high was , 
a counterpoise to one as high as the atmo 
sphere ; and that, for this reason, the 'water 
would not follow the sucker any farther. 
Ilis pupil Torricelli, considered that as 
mercury was fourteen times as heavy as wa- 
ter, a column of that fluid need only' be one- 
fourteenth of the length of one of water, to 
form an equal counterpoise to the pressure 
of the air; and accordingly, having filled 
with mercury a glass tube about three feet 
Ion?, heidneticaliy sealed at one end, he in- 
verted it into a small bason of mercury, and 
found, as he expected, that the mercury 
subsided to the height of about twenty-nine 
inches and a half, and there remained sus- 
P en #d> leaving a space at the top of the 
tube a perfect vacuum; which has been call- 
edjwrom the inventor, the Torricellian 
vacuum. 
It was, however, some tinn^after this ex- 
periment had been made, and even after it 
had been universally agreed that the sus- 
pension of tin* mercury was owing to the 
weight of the atmosph^e, before i( was dis- 
cov i i lci that the column ot mercury varied 
in height, and consequently that the pres- 
sure of the air was different at different 
times. 
I his phenomenon was, however, too re- 
markable to 'he long unobserved. It was 
impossible to avoid observing also, that the 
changes in the height of the mercury were 
accompanied, or very quickly succeeded, by 
alterations in the weather. Hence the in- 
strument obtained the name of weather-glass ; 
and from its also measuring the weight of the 
atmosphere, it is called the barometer. It 
is merely a tube filled with mercury, and in- 
verted into a bason of the same, having a 
scale fixed at the top to ascertain the rising 
and falling of the mercury, by the changes 
m the weight of the atmosphere. A more 
particular account of the construction and 
use of this instrument is given under Baro- 
meter. 
These effects arising from the weight and 
pressme of the atmosphere, have been ab- 
surdly attributed to suction ; a word which 
ought to be ''exploded, as it conveys a false 
notion of the cause of these and similar phe- 
nomena. To prove that an exhausted re- 
ceiver is held down by the pressure of the 
atmosphere, take one open at top, and ground 
quite flat, as A fig. 3, and covered with a 
hi ass plate R, which has a brass rod pass- 
ing through it, working in a collar of leather, 
so as to be air-tight ; to tins rod suspend a 
small receiver within the large one, a little 
way from the bottom ; place the receiver 
A upon the pump-plate, and exhaust it- 
it will now be fixed first down ; but the small 
receiver may he pulled up or down with 
perfect ease, as it is itself exhausted, and all 
the air which surrounded it removed, con- 
sequently it cannot be exposed to any pres- 
s'- 11 e ; then let the. small one down upon the 
plate, but not over the hole by which the 
an is exti acted, and re-admit the air into 
the large receiver, which may then he re- 
moved ; it will he found that "the small one 
being itself exhausted, it held down fast by 
the air, which is now admitted round the 
outside. If the large receiver is again put 
over it and exhausted, the small one will 
be at liberty ; and so on, as often as the ex- 
periment is repeated. 
