By single-acting is meant that the air cylinders compress their respective volumes of air _once_ every revolution. The air is admitted to the large cylinder through the piston, is compressed to about 30 pounds, and on the return stroke the pressure is raised to almost any point required, and in proportion to the diameter of the smaller cylinder. Though single-acting, the capacity of one of these compressors is about equal to that of the double-acting machine of the same cost of construction. The initial air cylinder is made large enough to correspond with the capacity of the smaller double-acting cylinder. The strains are equalized because the area of the large cylinder multiplied by its low pressure is exactly equal to that of the small cylinder multiplied by its high pressure. The maximum strains are reduced considerably below those which exist in compressors that do not compound the air. [Illustration: FIG. 11.] 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.