PNEUMATICS. 
■vvlll preponderate, and show itself to be hea- 
vier than tlie lead ; but upon letting in tlie 
air again, the equilibrium will be restored. 
The reason of this is, that since the air is 
a fluid, and alt bodies lose as much of their 
absolute weight in it as is equal to the weight 
of their bulk of the fluid, the cork, being the 
larger body, loses more of its real weight 
than the lead does ; and therefore must in 
fact be heavier, to balance it under the 
disadvantage of losing some of its weight, 
which disadvantage being taken otf by re- 
moving the air, the bodies then gravitate 
according to their real quantities of matter, 
and the cork, which balanced the lead in 
air, shews itself to be heavier when in 
vacuo. 
Set a lighted candle upon the pump, and 
cover it with a tall receiver. If the receiver 
holds a gallon, tlie candle will burn a mi- 
nute ; and then, after having gradually de- 
cayed from the first instant, it will go out ; 
which shows tliat a constant supply of fresh 
air is as necessary to feed flame, as animal 
life. 
The moment when the candle goes out, 
the smoke will be seen to ascend to the top 
of tlie receiver, and there it will form a 
sort of cloud ; but upon exhausting the air, 
the smoke will fall down to the bottom of 
the receiver, and leave it as clear at the 
top as it was before it was set upon tlie 
pump. Tliis shows that smoke does not 
ascend on account of its being positively 
light, but because it is ligiiter than air; 
and its falling to the bottom when the 
air is taken away, shows that it is not des- 
titute of weiglit. So most sorts of wood 
ascend or swim in water ; and yet tliere 
are none wlio doubt of the wood’s having 
gravity or weiglit. 
Set a receiver which is open at top, on 
the air-pump, and cover it with a brass 
plate and wet leather ; and having exhatist- 
ed it of air, let the air in again at top 
through an iron pipe, making it pass through 
a charcoal flame at the end of the pipe; 
and when the receiver is full of that air, 
lift up the cover, and let down a mouse or 
bird into the receiver, and the burnt air 
will immediately kill it. If a candle be let 
down into that air, it will go out directly ; 
but by letting it down gently, it will drive 
out the impure air, and good air will get in. 
Set a bell on the pump-plate, having a 
contrivance so as to ring it at pleasure, and 
cover it with a receiver; then make the 
clapper strike against the bell, and the 
sound will be very well heard ; but, ex- 
liaust the receiver of hir, and then, if the 
clapper be made to strike ever so bard 
against the bell, it will make no sound ; 
which shows that air is absolutely necessary 
for the propagation of sound. 
It hits been shown that air can be rare- 
tied, or made to expand : we now proceed 
to show that it can also be condensed, or 
pressed into less space than what it gene- 
rally occupies. The instrument used for 
this purpose is called a condenser : (fig. 9j 
represents a machine of this kind ; it con- 
sists of a brass barrel containing a piston, 
which has a valve opening downwards ; so 
that as tlie piston is raised,' the air passes 
through the valve ; but as the piston is 
pushed down tlie air cannot return, and is 
therefore forced through a valve at the bot- 
tom of the barrel, that allows it to pass 
through into the receiver, B, but prevents 
it from returning. Thus, at every stroke of 
the piston, more air is thrown into the 
receiver, which is of very thick and strong 
glass. The receiver is held down upon the 
plate, C, by the cross piece, D, and the 
screws EF. The air is let out of the re- 
ceiver by the cock, G, which communicates 
with it. 
The sound of a bell is much louder in 
condensed than in common air. A phial 
that would bear tlie pressure of the com- 
mon atmosphere, when the air is exhausted 
from the inside, will be broken by con- 
densing the air round it. These experi- 
ments may be made under the receiver B. 
A very beautiful fountain may be made 
by condensed air. Procure a strong cop. 
per vessel, (fig. 10) having a tube that 
screws into the neck of it, so as to be air- 
tight, and long enough to reach near to the 
bottom. Having poured a quantity of water 
into the vessel, but not enough to fill it, and 
screwed in the tube, adapt to it a conden- 
sing syringe, and condense the air in the 
vessel ; shut the stop cock, and unscrew 
the syringe, then, on opening the stop-cock, 
the air acting upon the water in the vessel, 
will force it out into a jet of very great 
height. A number of different kinds of 
jels may be screwed on the tube, such as 
stars, wheels, &c. forming a very pleasing 
appearance. 
The air-gun is a pneumatical instrument 
of an ingenious contrivance, which will 
drive a bullet with great violence, by means 
of condensed air, forced into an iron ball 
by a condenser. Fig. 11 . represents the 
condenser for forcing the air into the ball. 
At the end of this instrument is a male 
