The latter carries also a small rod, K, which is provided
with a cord made of braided silk. This cord runs over a pulley, N, whose
axle carries at its other end a still larger pulley, O. Toward the
middle of the latter is fixed a silken cord which hangs down on each
side, after making several turns around the pulley. To the front cord
is attached a slide, Q, moving in a vertical direction, and to which is
fixed an inscribing style, R. The other extremity of the thread enters
the hollow upright, and carries a weight which is greater than the
combined weights of the slide, the membrane, and the internal reservoir.
The upright serves as a guide to this counterpoise.
In order to use the apparatus there is affixed to the cylinder, A B, a
sheet of paper divided in a vertical direction into as many parts as
the cylinder takes hours to make one revolution. The divisions running
horizontally represent centimeters of water or of mercury, according to
the strength of the spring, M, which should be so constructed as to be
in relation with the pressure. The operation of the apparatus may be
readily understood.
[Illustration: GAS INDICATOR OF MANOMETER.]
When the gas reaches the pressure chamber, the spring, M, contracts, and
consequently the counterpoise descends, and causes the cord, O, which
carries the slide and writing style, to wind around the pulley.
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