Fig. 4 shows the fly-wheel, still turning, forcing the piston up and thus expelling the burned gases from the cylinder through the exhaust valve, held open for this purpose. From this position the engine goes again to that of Fig. 1, and through 2, 3, and 4, continuously, exploding every second revolution, and giving a regular rotary motion to the fly-wheel. [Illustration: FIG. 2.] [Illustration: FIG. 3.] [Illustration: FIG. 4.] The illustrations show a one-cylinder motor, but these engines can be built with two or more cylinders, arranged to explode at different times, thus giving very smooth action to the fly-wheel and main shaft. Aeroplanes, almost all automobiles, various pumps and other machinery are driven by gasoline engines. The rotary motion can readily be transmitted by chains or gears to the propellor of an aeroplane or motor boat, or the wheels of an automobile. It is only in the past few years that the gasoline engine has reached its present high state of perfection. [Illustration: THE BEGINNING OF AN AUTOMOBILE CRANKCASE SHOWING BEARINGS. The heart of the automobile is the engine. It is built around the crankcase, which is its foundation or base.] [Illustration: CRANKCASE WITH CRANKSHAFT AND FLY-WHEEL ADDED. The crankshaft serves the same purpose in an automobile as the pedals do on a bicycle. The fly-wheel on the end helps it to keep turning at an even speed.] [Illustration: Gasoline vapor is exploded in the cylinders. This pushes the piston down, and as the piston is connected to the crankshaft it starts the crankshaft turning. The piston and the rod that connect it to the crankshaft are just like the feet and limbs of any one riding a bicycle. Cylinders showing piston in place and connected to crankshaft.] [Illustration: The gears or “cog-wheels” are for running the fan, the pump and other parts.] [Illustration: THE HEART OF THE AUTOMOBILE Cylinder added to crankcase. The cylinders are next bolted down to the crankcase, the pistons and crankshaft having been connected, as shown in Fig. 3. A cover is placed over the gears to keep them clean.] [Illustration: An oil pan or reservoir is attached to the bottom of the crankcase to hold oil for the engine.] [Illustration: The carburetor furnishes the gasoline vapor for the cylinders. It is connected to the engine by a crooked pipe called the intake manifold. After the gasoline has been exploded a valve opens and allows the burned gases to escape through another pipe, called the exhaust manifold.] [Illustration: Oil is poured in the spout which is at the left of the carburetor. It runs down into the reservoir and is pumped up through the engine a little at a time. Oil pump and filler added to motor.] [Illustration: THE POWER PLANT OF AN AUTOMOBILE The electric generator makes electricity to be used for starting the engine and lighting the car.] [Illustration: The magneto gives an electric spark, which explodes the gasoline in the cylinders.