The advantage of the single-acting air cylinder over the double is that
it compresses a volume of free air only once every revolution, hence
there is a better chance to cool the air during compression. The
cylinders have time to impart to the water jackets the heat produced by
compression and are kept cooler. The large air head of the initial
cylinder is jacketed, also adding to isothermal economy.

[Illustration: FIG. 12.]

Fig. 12 illustrates the Ingersoll-Sergeant Piston Inlet Cold Air
Compressor. This a straight line direct-acting engine, with steam and
air pistons connected to a single rod through a crosshead which connects
with two fly wheels. The strains are direct and the power and resistance
are equalized by the inertia of the crosshead, piston, rods, and fly
wheels. The Meyer's adjustable cut-off is used on the steam cylinder.
The air cylinder is provided with a tail rod tube through which all the
air is admitted into the cylinder.

[Illustration: FIG. 13.--AN AUTOMATIC AND ADJUSTABLE REGULATOR AND
UNLOADING DEVICE APPLIED TO INGERSOLL-SERGEANT AIR COMPRESSORS.]

Fig. 13 illustrates an unloading device and regulator applied to the
Ingersoll-Sergeant compressor.

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