MECHANICS. oo 
the volume of the body examined will be = 15.07 — 11.72 = 3.35 cubic 
centimetres. 
Next to the barometer comes the air-pump, invented by Otto von 
Guericke, one of the most important instruments for elucidating the proper- 
ties of the air. It serves to produce by successive rarefaction as complete a 
vacuum as possible, although this can never become so perfect as the Torri- 
cellian vacuum. Imagine a cylinder in which a piston moves air-tight, and 
closed below, then on raising the piston a vacuum will be produced. If, 
now, the cylinder be united with another inclosed space by a tube, so that 
the air can pass from the latter into the former, then, on raising the piston, 
the air would make this transit, but on depressing the piston it would return 
again. Suppose, however, a cock to be placed in the tube, by means of 
which the return of the air can be prevented, while its egress is allowed ; 
then by the alternating action of the piston and turning of the cock, theair 
in the vessel may be reduced to a minimum, even if a perfect vacuum may 
not be attainable on account of the infinite expansion of air. This is the 
simplest construction of the air-pump ; it has, however, since its invention, 
received various modifications and improvements. 
Pl. 18, fig. 44, represents a small hand air-pump, according to the con- 
struction of Gay Lussac. The main part consists of a hollow cylinder or 
tube of brass, in which an air-tight piston plays up and down. In the latter 
is a valve opening upwards; thus shut during the ascent of the piston, and 
open during its depression. At bd is attached the receiver, the vessel in 
which the vacuum is to be made, consisting generally of a plate and glass 
bell. The screws a and f serve to screw the air-pump to a table or board ; 
at da ccck is attached, as also at s._ If, now, the latter cock be opened and 
the former closed, and the piston elevated, a part of the air in the receiver 
wiil pass out through the first horizontal and then vertical canal, ad, into the 
cylinder, and the air in the receiver will become rarefied. Depress the 
piston after closing the cock s, and the air under the piston passes out 
through it by means of the valve in the piston head. To let the air again 
into the receiver, the cock at d must be opened. 
A sectional view of a larger air-pump is shown in jig. 45, pl. 18. Here 
a is the cylinder, in which works the air-tight piston, 0, which contains a 
valve opening upwards, and is moved by the piston rod, c. The rod ed 
opens and closes the valve for the cylinder ; at its lower end is a truncated 
cone, é, fitting in a conical opening. At f/ is seen the glass bell to he 
exhausted, whose edge must be ground perfectly plane, in order that it may 
fit air-tight upon the ground plane, pp. In the centre of this plate is a 
female screw, v, for screwing on any other form of receiver ; and from this 
goes a canal to the conical opening at e. If, now, the piston resting on the 
bottom of the cylinder be elevated, the valve at e opens until the shoulder 
at d strikes against the upper plate of the cylinder, and the air in part 
rushes from the receiver into the cylinder: on depressing the piston, the 
valve at e is closed, and the air in the cylinder escapes through the valve in 
the piston. At 7 is the barometer gauge, or contracted barometer, inclosed 
in a long narrow bell, and in communication with the air in the receiver by 
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