3. The =PISTON= is a plunger or movable plug fitting the bore closely enough to prevent the escape of gas, but at the same time is capable of sliding freely to and fro. When pressure is established in the cylinder from the combustion, pressure is also exerted on the end of the piston tending to force it out of the cylinder. The extent of this force is governed by the area of the end of the piston and also by the pressure of the gas. Thus the purpose of the piston is to convert the pressure of the expanding gas into direct mechanical force, and also to transform the increasing volume of gas into motion. Another, and no less important function of the piston is to compress the combustible gas in the upper end of the cylinder for ignition. [Illustration: Piston and Connecting Rod of the Sturtevant Aero Motor, Showing Three Piston Rings. ] 4. The =CONNECTING ROD= (Sometimes called the Pitman) transmits the pressure on the piston to the crank, the connecting rod being the means through which the to and fro motion of the piston is transmitted into the rotary motion of the crank; its action being similar to that of the human arm turning the crank of a pump or windlass. 5. The =CRANK= receives the pressure and motion of the piston from the connecting rod, changing the reciprocating motion of the piston into the rotary motion required by the machinery which the engine drives. In the majority of cases the crank revolves, while the cylinder stands still, but in some of the recently developed aeronautic motors this is reversed, the cylinders revolving with the crank stationary. The relative motion, however, is the same in both cases. (6.) The =CRANK SHAFT=, of which the crank is an integral part, transmits the rotary motion of the crank to the driving pulley. (7.) The admission and release of the gases to and from the cylinder are controlled by the =INLET VALVE= and =EXHAUST VALVE=, respectively, in a four stroke cycle engine. The valves are merely gates, allowing the gas to flow, or stopping it, at the proper intervals, depending on the event taking place at that time in the cylinder. In the two stroke cycle engine there are no valves, the admission and release of the gas being controlled by the position of the piston, and the openings cut in the cylinder walls. 6. =IGNITION= or the firing of the combustible charge is accomplished by the =IGNITION SYSTEM=. In most modern engines the mixture is ignited when it is under the greatest pressure or at the end of the stroke. For maximum efficiency the mixture should be ignited when it is under the greatest pressure or compression. The time at which ignition occurs is also controlled by the ignition system. 7. The =GOVERNOR= regulates the speed of the engine; either by changing the richness of the mixture, by changing the number of working strokes in a given time or by altering the quantity of gas admitted to the cylinder, or sometimes by a combination of these methods.