piston. In order to first start a two-cycle motor in its operation a charge of gas or vapor must be drawn into the base and forced up to the top of the cylinder and compressed there. One complete revolution of the shaft is necessary to accomplish this, and this revolving a shaft by hand is commonly known as "cranking." Unless a motor is out of order or some adjustment is wrong one revolution only should suffice to start the engine, and if it fails to start then further cranking is merely a waste of time and strength until the fault is corrected. Indeed most motors when properly adjusted will start by merely rocking the fly-wheel back and forth and "throwing it up" against compression. In cold weather, however, motors will often start very hard and at such times "priming" should be resorted to. This matter will, however, be fully dealt with in the succeeding chapters.

In Figs. 5 to 8 a section of a four-cycle engine is illustrated. In this type of motor the parts are far more numerous and the operation more complicated. The motor is shown in Fig. 5 with the piston P at the top of stroke or "firing stroke," with a compressed charge ready to explode, and with both the valves VI, VE closed. The explosion taking place the piston is forced down as in the two-cycle motor already described, and the shaft is turned half a revolution. In its downward passage the exhaust valve VE is opened through the action of a cam C and gear G connected to the main shaft, and the motor then appears as illustrated in Fig. 6. The piston now commences its upward stroke, thus forcing the burnt gas out through the exhaust valve