MECHANICS. o7 
ferent direction. In depression the piston drives the air anto the receiver 
and compresses it ; in elevation the external air opens the piston valve and 
presses into the tube, while the air in the receiver is retained by the valve 
in the bottom. The receiver must be screwed down, else it will be forced 
up by the compressed air. Many condensing pumps are so arranged as to 
be applicable to various apparatus or receivers in which the air is to be 
condensed. One of this kind is represented in fig. 52. It consists of a 
tube or cylinder, and a piston, 6, without a valve. The receivers are 
screwed on to the lower end of the tube, either at c or d; a valve then 
attached admits only the ingress, not the egress of air. The receivers 
f andi may be closed when necessary by the cocks, e,h, and g. For 
admitting successive portions of air into the cylinder or tube, a lateral 
opening in the tube, or, as in the figure, a Jateral valve, may be used. The 
latter serves principally when a gas, not atmospheric air, is to be con- 
densed. 
The air-pump, in its application, is confined not merely to’ physical ex- 
periment, but is of the highest importance in the arts. It is there employed 
on the one hand for rarefying the air, as in the steam engine and sugar 
manufacture, and on the other for condensing air, as in driving of machines 
by condensed air, in the air-gun, &c. In the air-gun the air-vessel in 
which the condensed air is contained is either a bal! screwed on beneath 
the stock or it is the piston itself. This vessel has then a valve which 
prevents the escape of the included air, and upon which stands a pin con- 
nected with the discharge of the gun. Thus, when the trigger is pulled 
and the cock descends, this pin is pressed upon for an instant with such 
force as to open the valve sufficiently to allow the escape of enough air to 
propel the ball. 
To measure the pressure of gas contained in a certain apparatus, pres- 
sure valves are partly used, and partly manometers, to which latter belong 
the barometer gauge of the air-pump, as also the safety tube represented in 
pl. 18, fig. 58. The latter contains a liquid, standing at an equal height in 
the two legs when the pressure is equal to that of the atmosphere. When 
this is not the case the liquid cannot stand at an equal height in the two 
legs; and from the difference of level, knowing the density of the fluid em- 
ployed, the pressure in the interior of the inclosed space to which the tube 
is applied can easily be determined. 
For pressure valves the relation is somewhat different, since while in 
manometers the internal pressure is measured by the height of the mercury 
or other fluid, in those it is given directly in terms of weight. The wall 
of the compressing vessel is provided with an aperture of determi- 
nate size, a square inch for instance, which is so constructed by open- 
ing outwards as to form the bed of a conical valve. This valve is loaded 
with weights, either directly or by means of a lever, upon which, as in the 
steelyard, a shifting weight may be placed. In such cases the valve when 
raised gives directly the pressure exercised by the gas, upon every square 
inch of surface. All these valves, however, give indications only when the 
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