56 PHYSICS. 
means of the canal ev. The mercury at first completely fills the one 
leg of the bent barometer tube, but after a considerable rarefaction, begins to 
sink, and the difference of height of the two mercurial surfaces gives the 
pressure of the air in the receiver. If, for example, this difference amount 
to one inch, at a barometrical height of 28 inches, the air in the receiver 
will be rarefied 28 times. Fig. 46 represents a double-acting cock, y, in the 
canal between the receiver and cylinder, that is, a cock bored through in 
two directions: the one aperture is straight, and during exhaustion unites 
the receiver and cylinder; the other is bent into a knee, and opens into a 
lateral opening, which, during exhaustion, is closed by a metal stopper, 6. 
To admit air into the receiver after exhaustion, the stopper must be 
extracted and the cock turned in such a manner that the air can penetrate 
into the receiver through the side aperture. 
Air-pumps are divided into cock and valve pumps, and moreover into one 
and two-cylindered. Fig. 47 represents an air-pump provided with two 
cylinders. Here the two piston rods are toothed, and a piston interposed in 
such a manner that by the motion o. a handle they can be alternately 
elevated and depressed, the one ascending, the other descending at any given 
time. In this manner the exhaustion zoes on uninterruptedly, and is com- 
pleted in much shorter time. 
In the common air-pumps, however well they may be constructed, there 
is always a space intervening between the piston and its point of greatest 
depression and the bottom of the cylinder, which can never be exhausted. 
The air in it obstructs the rarefaction of air in the receiver, and sooner or 
later puts a stop to it. To obviate this difficulty, Babinet has constructed a 
cock of peculiar form, represented in figs. 48—50. In fig. 48, a and d are 
the two cylinders of a double cylinder air-pump, and 7 the cock attached 
between the two cylinders, a little below their base. This cock has four 
openings (figs. 49 and 50). The first and second, s and ¢, pass completely 
through and are perpendicular to each other; the third, v, is parallel to s, 
going, however, only to the centre of the cock, and opens in ¢. The same is 
the case with the fourth opening, wu, which runs parallel to the longitudinal 
axis of the cock. From the bottom of the two cylinders pass curved canals 
which terminate at b and e in the openings of the cock. At first, the cock 
is fixed in such a manner that the opening, ¢, unites both canals; and this 
position, in which it exerts no particular influence, is continued until the 
mercury will fall no longer in the gauge. The cock is now slightly turned, 
so that the bore, s, unites the two cylinders; and at the same time the 
opening v unites the cylinder @ with the receiver. If the piston in a be 
depressed, the rarefied air beneath it is driven over into the other cylinder ; 
when, however, the piston in d is depressed, the valve in the bottom of d is 
closed, and in the cylinder a the space above mentioned contains only 
rarefied air, so that the rarefaction in a is much greater than before. It is 
only after repeated strokes of the pistons that a new limit to rarefaction is 
attained. 
The condensing pump (fig. 51) serves to condense the air, and differs 
from the air-pump merely in having the valves to open and shut in a dif- 
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