such engines are known as single-acting engines. In other forms work is done on both sides of the piston, and these are known as double-acting engines.
3. The construction of the single-acting engine is illustrated in Fig. 1, in which a is the cylinder; b, the piston;
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c, the cylinder head; and d, the rotating shaft, known as the main, or crank-shaft. In order to convert the to-and-fro, or reciprocating, motion of the piston into a rotary motion of the crank, a connecting-rod e and a crank f are used. The connecting-rod is attached to the crank by means of the pin g y called the crankpin, and to the piston by means of the pin h, called the piston pin, or wristpin. The frame i supports the cylinder a and the main-shaft bearing j, and forms the rigid connection between them.
4. The construction of the double-acting engine is shown in Fig. 2, corresponding parts being represented by the same
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letters as those used in Fig. 1. It will be seen that the principal points of difference between these two constructions are
