460 
PNEUMATICS. 
ceiver is larger than that of the pump and 
pipes. If the receiver is large, one exhaus- 
tion will be sufficient to clear the pump so 
fai , that what remains can be of no conse- 
quence. If the receiver is small, the opera- 
tion should be repeated two or three times. 
In all mercurial experiments with the air- 
pump, a short pipe must be screwed into 
the hole of the pump-plate, so as to rise 
above it about half an inch, to prevent the 
quicksilver from getting into the air pipe 
and barrels, in case any should accidentally 
be spilt over the jar; for if it gets into the 
barrels, it spoils them, by loosening the sold- 
er, and corroding the brass. 
W ith respect to the leathers, if your pump- 
plate is not ground, they are absolutely ne- 
cessary ; they should be previously soaked 
in oil from which the moisture has been ex- 
pelled by boiling, or hog’s-Iard with a little 
bee’s-wax, which gives a clamminess very 
proper for the purpose. 
It is evident, that the vacuum in the re- 
ceiver of the air-pump, can never be per- 
tect, that is, the air can never be entirely 
exhausted ; for it is the spring of the air 
in the receiver, that raises the valve, and 
forces its way into the barrel ; and the bar- 
rel at each suction can only take away a 
certain part of the remaining air, which is 
in proportion to the quantity before the 
stroke, as the capacity of the barrel is to 
that of the barrel and receiver added toge- 
ther. 
This, however, is an imperfection that is 
seldom of much consequence in practice, 
because most air-pumps, at a -certain period 
of the exhaustion, cease to act, on account 
of their imperfect construction ; for the 
valves usually consist of a piece of oiled 
bladder tied over a hole, so that the air is 
at liberty to pass by lifting up the bladder, 
but cannot return again ; and thus there will 
unavoidably be a small space left between 
the lower valve and the piston, when down. 
Now, it will happen, when the air in the 
receiver is very rare, that its spring will 
not be strong enough to overcome the ad- 
hesion of the bladder forming the lower 
valve, which consequently will remain shut 
and the exhaustion cannot proceed. Or, 
before this period, it may happen, that the 
air between the valves when the piston is 
up, may be so small as to lie in the space 
between the two valves when the piston is 
down, without being sufficiently condensed 
for its spring to overcome the adhesion of 
the bladder forming the upper valve, and the 
weight of the atmosphere that presses it : in 
this case, the upper valve will remain shut, 
and the exhaustion cannot proceed. 
Various modern improvements in the air- 
pump, obviate these inconveniences in a 
great degree. Mr. Smeaton, a very ingeni- 
ous mechanic, enlarged the size of the lower 
valve ; and, to strengthen it, supported it on 
a brass grating, resembling a honey-comb. 
This allowed the valve to rise more easily. 
He also covered the top of the barrel, mak- 
ing the piston-rod work through a collar of 
leathers, by which he took off ttie pressure of 
the atmosphere from the piston-valve, which 
acted against the rarefied air in the receiver. 
Pumps on this construction have been made 
by Nairne, and other artists in London, and 
have answered extremely well. 
The air-gun. This pneumatical instru- 
ment is 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. 14. represents the con- 
denser for forcing the air into the ball. At 
the end a of this instrument is a male screw, 
on which the hollow ball b is screwed, in 
order to be filled with condensed air. In 
the inside of this ball is a valve, to hinder 
the air after it is injected from making its 
escape, until it is forced open by a pin, 
against which the hammer of t he lock strikes ; 
which then lets out as much air as will drive 
a ball with considerable force to a great 
distance. 
When you condense the air in the ball, 
place your feet on the iron cross h h, to which 
the piston-rod cl is fixed ; then lift up the 
barrel e a, by the handles i i, until the end 
of the piston is brought between e and c ; 
the barrel a c will then be filled with air 
through the hole e. Then thrust down the 
barrel ac by the handles i i, until the piston 
e joins with the neck of the iron ball at a: 
the air,“ being thus condensed between e 
and a, will force open the valve in the ball ; 
and when the handles i i are lifted up again, 
the valve will close, and keep in the air ; so 
by rapidly continuing the stroke up and 
down, the ball will presently be filled ; after 
which, unscrew the ball off the condenser, 
and screw it upon another male screw, which 
is connected with the barrel, and goes 
through the stock of the gun, as represented 
fig. 15. Twelve dwts. of air have been inject- 
ed into a ball of 3.75 inches diameter, which 
has discharged 15 bullets with considerable 
force. 
There are many contrivances in construct- 
ing air-guns ; some have a small barrel con- 
tained within a large one, and the space be- 
tween the two barrels serves for the reception 
of condensed air. In this sort, a valve is 
fixed at a, (fig. 15,) with a condenser fixed 
to the barrel, and continued through the 
butt-end to c, where the piston-rod may be 
always left in. Place your feet on the pin, 
and the whole gun serves instead of the 
handles ii (fig. 14.) to condense the air in 
the barrel. 
Fig. 17. is a section of the gun, by which 
the principle of its action may be fully un- 
derstood : the inside barrel K is of a small 
bore from which the bullets are shot, and 
a larger barrel CDSR is on the outside of it. 
In the stock of the gun is a syringe S, which 
forces in the air through the valve EP into 
the cavity between the two barrels. The 
ball K is put into its place in the same way 
as in another gun. There is a valve at SL, 
which, being opened by the trigger O, per- 
mits the air to come behind the bullet, so as 
to drive it out with great force. If the 
valve is suddenly opened and closed, one 
charge of condensed air may make several 
discharges of bullets ; because only part of 
the air will go out at a time, and a fresh bul- 
let may be put into the place K. 
The magazine air-gun differs from the com- 
mon one, only by having a serpentine bar- 
rel, which contains ten or twelve balls ; 
these are brought into the shooting barrel 
successively, by means of a lever ; and they 
may be discharged so fast as to be nearly of 
the same use as so many different guns. 
Fig. 18. shews a section of the gun, or at | 
least, as much of it as is necessary to give a 
complete idea of the whole. AE is part of jj 
the stock ; G the end of the injection syringe, j 
with its valve H, opening into the cavity I 
FF between the barrels. KK is the small ' 
or shooting barrel, which receives the bullets, 
one at a time, from the magazine ED, which 
is a serpentine cavity, where the bullets are ] 
lodged and closed at the end D ; the circu- ;• 
lar part is the key of a cock, having a cy- j 
lindrical hole, IK, through it, equal to the 
bore of the small barrel, and forming a part ! 
of it. When the lock is taken off, the several j 
parts come in view, by means of which the 
discharge is made, by pushing up the pin 
P p, which rises and opens a valve, ^ , to let 
in the air against the bullet I, from the ca- 
vity FF, which valve is immediately shut, 
down again by means of a long spring of 
brass, NN. This valve V, being a conical 
piece of brass, ground very true, will be suf- j 
ticient to coniine the air. To make a dis- j 
charge, pull the trigger ZZ, which throws up 
the seer y x, and disengages it from the notch 
.r, upon which the strong spring WW proves 
the tumbler T, to which the cock is’fixed. 
The end u of this tumbler, bears down the j 
end v of the tumbling lever R, which by its 
other end m raises the flat end, l, ot the ho- 
rizontal lever Q, by which means the pin 
P p is pushed up, and opening the valve V,. I 
discharges the bullet. To bring another 
bullet instantly to succeed to I, there is a 
part H called the hammer, represented in 
lig. 19. which turns the cock so as to place j 
the cylindric bore of the key I k in any si- 11 
tuation required. Thus when the bullet is J 
in the gun/ the bore of the key coincides 
with that of the barrel KK ; but when it is 
discharged, the hammer H is instantly 
brought down to shut the pan of the gun ; 
by which motion the bore ot the key is turn- 
ed into the situation ik, so as to coincide 
with the orifice of the magazine ; and upon 
lifting the gun upright, the ball next the 
key tumbles into its cavity, and falling be- 
hind two small springs s s fig. 18. is by them j 
detained. 
American air-pump. It would not come 
within the limits of this work, to enumerate 
all the improvements, and different modes 
of construction, used in this instrument... 
The latest are the air-pumps made by Haas 
and Hurter, Cuthbertson, and Prince, each, 
of which has particular advantages. 
We shall however give a perspective view of 
the air-pump invented by Mr. Cuthbertson,. 
which is so excellent in its structure, and so 
powerful in its effects, as to demand in the 
present improved state of science, a particular 
notice and description. See Plate II. Pneu- 
matics, Cuthbertson's air-pump. The two 
principal gages of this pump are screwed 
in their places ; but it is not necessary that 
these should be used together, except in- 
experiments that require great nicety, and 
very exact exhaustion. In common cases, 
either of them may be taken away, and a. 
stop-screw put into its place. \\ hen the 
pear-gage, which has been already describ- 
ed, is used, a small round plate, large enough 
for the receiver to stand upon, must first be| 
screwed into a hole at a, bat when this gage 
is not used this hole must be closed with a 
stop-screw. When all these gages are used,. 
