The purpose of this unloading device is to maintain a uniform air pressure in the receiver and a uniform speed of engine, notwithstanding the consumption of the air, and to do this without waste of power or attention on the part of the engineer.  A weighted valve of safety valve pattern is attached to the air cylinder, and is connected with the air 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 these valves are placed in each piston of a double-acting air cylinder, the piston being hollow and the free air being admitted through a tail-rod pipe, letter E, Fig. 15.  JJ are water jacket passages for cooling the air during compression.  Owing to the absence of inlet valves, large water jackets are provided, not only around the cylinder itself, but through the heads.  As the heat of compression is greater near the end of the stroke, the advantage of a cool head is manifest.  H H are the discharge valves through which the compressed air is forced.

[Illustration:  FIG. 16.  PISTON INLET VALVE OPERATED BY THE NATURAL LAWS OF MOMENTUM.]