466 
slider, which will disengage it from the pins, 
and allow the mills to turn round by the im- 
pulse of the spring ; and as there is no air in 
the receiver to make any sensible resistance 
against them, they will both move a consi- 
derable time longer than they did in the 
open air, and the moment that one stops the 
other will do so too. This shews that air 
resists bodies in motion ; and that equal bo- 
dies meet, with different degrees of resistance, 
according as they present greater or less 
surfaces to the air. 
"Fake a tall receiver A, covered at top 
by a brass plate, through which works a rod 
in a collar of leathers, (fig. 7.) and to the 
bottom of which there is a .particular con- 
trivance for supporting a guinea and a feather, 
and for letting them drop at the same in- 
stant. If they are let fall while the receiver 
is full of air, the guinea will fall much quicker 
than the feather; but if the receiver is first 
exhausted, it will be found that they both 
.arrive at the bottom at the same instant : 
which proves that all bodies would fall to the 
■ground with the same velocity, if it was not 
lor the resistance of the air, which impedes 
most of the motion of those bodies that have 
the least momentum. In this experiment 
tire observers ought not to look at the top, 
but at the bottom of the receiver ; otherwise, 
on account of the quickness of their motion, 
they will not be able to see whether the 
guinea and feather fall at the same instant. 
3. Take a receiver, having a brass cap fit- 
ted to the top with a hole in it ; tit one end 
of a dry hazel-branch about an inch long, 
tight into the hole, and the other end tight 
into a hole quite through the bottom of a 
small wooden cup ; then pour some quick- 
silver into the cup, and exhaust the receiver 
of air ; and the pressure of the outward air on 
the surface of the quicksilver, will force it 
through the pores of the hazel, whence it 
will descend in a beautiful shower, into a 
glass cup placed under the receiver to catch 
it. 
Put a wire through the collar of leathers 
on the top of the receiver, and fix a bit of 
dry wood on the end of the wire within the 
receiver ; then exhaust the air, and push the 
wire down, so as to immerse the wood into 
a jar of quicksilver on the pump-plate; this 
done, let in the air ; and upon taking the 
wood out of the jar, and splitting it, its 
pores will be found full of quicksilver, which 
the force of the a'r, upon being let into the 
quicksilver, drove into the wood. 
Join the two brass hemispherical cups A 
and B together (rig. 8,) witti a wet leather 
between them, having a hole in the middle 
of it ; tUen having screwed off the handle at 
C, put both the hemispheres together and 
screw them into the pump-plate, and turn 
the cock E, so that the pipe may be open 
all the way into the cavity of the hemi- 
spheres ; then exhaust the air out of them, 
and turn the cock; unscrew the hemi- 
spheres from the pump, and having put 
on the handle C, let two strong men try to 
pull the hemispheres asunder by the rings, 
which they will find hard to do ; for if the 
diameter of the hemispheres is four inches, 
they will be pressed together by the external 
air with a force equal to 190 pounds; and 
to shew that it is the pressure of the air 
that keeps them together, hang them by 
PNEUMATICS. 
either of the rings upon the hook of the 
wire in the receiver A (fig. 3.) and, upon 
exhausting the air out of the receiver, they 
will fall asunder of themselves. 
Screw the end A of the brass pipe AB (fig. 
9.) into the pump-plate, and turn the cock 
t until the pipe is open; then put a wet 
leather on the plate c a, fixed on the pipe, 
and cover it with the tall receiver GH, 
which is close at top ; then exhaust the air 
out of the receiver, and turn the cock e to 
keep it out; which done, unscrew the pipe 
from the pump, and set its end A into a 
bason of water, and turn the cock e to open 
the pipe ; on which, as there is no air in the 
receiver, the pressure of the atmosphere on 
the water in the bason will drive the water 
forcibly through the pipe, and make it play 
up iu a jet to the top of the receiver. 
Set a square phial upon the pump-plate, 
and having covered it with a wire cage, put 
a close receiver over it, and exhaust the 
air out of the receiver ; in doing which, the 
air will also make its way out of the phial, 
through a small valve in its neck. When 
the air is exhausted, turn the cock below' the 
plate to re-admit the air into the receiver ; 
and as it cannot get into the phial again, 
because of the valve, the phial will be broken 
into some thousands of pieces by the pres- 
sure of the air upon it. Had the phial been 
of a round form, it would have sustained this 
pressure like an arch, without breaking ; 
but as its sides are flat it cannot. 
To shew the elasticity or spring of the air : 
tie up a very small quantity of air in a blad- 
der, and put it under the receiver ; then ex- 
haust the air out of the receiver, and the air 
which is confined in the bladder (having 
nothing to act against it) will expand by the 
force of its spring, so as to fill the bladder 
completely. But upon letting the air into 
the receiver again, it will overpower that in 
the bladder, and press its sides close toge- 
ther. 
If the bladder so tied up is put into a 
wooden box, and has twenty or thirty pounds 
weight of lead placed upon it, and tne box 
is covered with a close receiver ; upon ex- 
hausting the air out of the receiver, that 
which is confined in the bladder will expand 
itself so as to raise up all the lead by the 
force of its spring. 
Screw the pipe AB (fig. 9,) into the 
pump-plate; place the tall receiver G1I upon 
the plate c a, as before, and exhaust the air 
out. of the receiver ; then turn the cock e to 
keep out the air, unscrew the pipe front the 
pump, and screw it into the mouth of the 
copper vessel C (fig. 10.), the vessel having 
been first about half rilled with waiter. Then 
open the cock e\ and the spring of the air 
which is confined in the copper vessel will 
force the waiter up through the pipe AB in 
a jet into the exhausted receiver, as strongly 
as it did by its pressure on the surface of 
the water. 
If a rat, mouse, or bird, is put under a re- 
ceiver, and the air is exhausted, the animal 
will be at first Oppressed as with a great 
weight, then grow convulsed, and at last ex- 
pire in all the agonies of a most bitter and 
cruel death. But as this experiment is too 
shocking to most spectators, it is common to 
substitute a machine called the lungs-glass 1 
in place of the animal. 
If a butterfly is suspended in a receiver, j 
by a fine thread tied to one of its horns, it j 
will fly about in the receiver as long as it 
continues full of air ; but if the air. is ex- I 
hausted, though the animal will not die,, i 
and will continue to flutter its wings, it can- 
not remove itself from the place where it- J 
hangs, in the middle of the receiver, until < 
the air is let in again, and then the animal 
will fly about as before. 
Put a cork into a square phial, and fix- 
it in with wax or cement ; and put the phial 
on the pump-plate with the wire cage, and 
cover it with a close receiver ; then exhaust; 
the air out of the receiver, and the air that 
was corked up in the phial will break it out- ; 
wards by the force of its spring, because 
there is no air left on the outside of the 
phial to act against that within it. 
Put a shrivelled apple under a close re- 
ceiver, and exhaust the air ; the spring of 
air within the apple will plump it out, so as 
to cause all the wrinkles to disappear ; but. 
upon letting the air into the receiver again, 
to press upon the apple, it will instantly re- 
turn to its former decayed and shrivelled 
state. 
r l ake a fresh egg, and cut off a little of ' 
the shell and film irom its smallest end, then 
put the egg under a receiver, and pump out 
the air ; upon which all the contents of the 
egg will be forced out into the receiver, by 
the expansion of a small bubble of air con- 
tained in the great end, between the shell \ 
and film. 
Put some warm beer into a glass, and hav- ’ 
ing set it on the pump, cover it with a close 
receiver, and then exhaust the air. Whilst 
this is doing, and the pressure more and ; 
more taken off from the beer in the glass, 
the air in it will expand itself, and rise up 
in innummerable bubbles to the surface of the J 
beer ; and thence it will be taken away with. > 
the other air in the receiver. When the j 
receiver is nearly exhausted, the air in the J 
beer, which could not disentangle itself quick 
enough to get off with the rest, will now 
expand itseltso, as to cause the beer to have 
all the appearance of boiling ; and the great- 
est part of it will go over the glass. 
Put some water into a glass, and a bit of 
dr.v wainscot or other wood into the water; 
then cover the glass with a close receiver, 
and exhaust the air ; upon which the air in 
the wood, having liberty to expand itself, 
will come out plentifully, and make all the 
■water to bubble about the wood, especially 
about the ends, because the pores lie length- 
wise. A cubic inch of dry wainscot has so 
much air in it that it will continue bubbling 
for near half an hour. 
Let a large piece of cork be suspended by 
a thread at one end of a balance, and coun- 
terpoised by a leaden weight, suspended in. 
the same manner, at the other. Let this 
balance be hung to the inside of the top of a 
large receiver ; which being set on the 
pump, and the air exhausted, the cork will 
preponderate, and shew itself to be heavier 
than the lead ; but upon letting in the air 
again the equilibrium will be restored. The 
reason of this is, that since the air is a fluid, 
and all bodies lose as much of their absolute 
weight in it as is equal to the weight of their 
bulk of she fluid, the cork being the larger. 
