receiver, and with a discharge valve on each end of the air cylinder,
also with a balanced throttle valve in the steam pipe. When the pressure
of the air gets above the desired point in the receiver, the valve is
lifted and the air is exhausted from behind the discharge valves, thus
letting the compressed air at full receiver pressure into the cylinder
at both ends, and balancing the engine. At the same instant the
compressed air is exhausted from the little piston connected with the
balanced steam valve and the steam is automatically throttled, so that
only enough steam is admitted to keep the engine turning around, or to
overcome the friction, no work being done.

[Illustration: FIG. 14.]

When the compressor is unloaded, it is evident that the function of the
air piston is merely to force the compressed air through the discharge
valves and passages from one end to the other until more compressed air
is required, this being indicated by a fall in the receiver pressure.
The weighted valve now closes and the small connecting pipes are
instantly filled with compressed air; the steam valve automatically
opens, and the compression goes on in the regular way. Another function
of this device is to prevent the compressor from stopping or getting on
the center. Direct-acting compressors are liable to center when doing
work at slow speed.

[Illustration: FIG. 15. PISTON INLET VALVE OPERATED BY THE NATURAL LAWS
OF MOMENTUM.]

Fig. 15 illustrates the Ingersoll-Sergeant Air Cylinder and Piston.

Fig. 16 shows the piston inlet valve, situated at G in Fig. 15. Two of