The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

THE AIR-VALVE AND PISTON. 397
through the iron plate, and has fixed on it there a hollow
pulley. This pulley contains within it a friction apparatus,
Fig. 5.
R LL
consisting of an ordinary spiral clock spring-the inner end
of which is fastened to the shaft, and the outer pressing
against the inner side of the case. Thus the
spring is always about the same strength, and Fig. 6.
acts upon the escapement wheel, causing it to
revolve uniformly when released by the escapement. The pulley revolves constantly, while the
shaft and escapement wheel may be stopped.
The escapement wheel has fourteen teeth, each
one of which causes two motions of the escapement, which
will make twenty-eight for a single revolution of the wheel,
which is shown in fig. 7.
When in operation, the piston to which the escapement arm
8, fig. 5, is attached, is subjected, on one side or the other, to
a pressure of condensed air; therefore
the piston and escapement will only be Fig. 7.
moved by the escapement wheel when
the air is removed from one side or the
other of the piston. The position of the
valve, fig. 4, attached to the magnet,, A
regulates the pressure of air on either
side of the piston, by opening one or the other of the side
passages into the second chamber. By breaking and closing
the circuit, therefore, the piston and escapement move backward and forward; thus a single revolution of the circuit
wheel at one station opens and closes the circuit twenty-eight
times, causing an equal number of movements of the magnets
in another station; they carry the valve which alternately
changes the air on either side of the piston. This permits the