The valve seats, and the inlet and exhaust pipes, are seen in section. The inlet valve here works automatically, being pulled in by suction; but on many engines--on all powerful engines--the inlet, like the exhaust valve, is lifted by a cam, lest it should stick or work irregularly. Three dotted circles show A, a cog on the crank shaft; B, a "lay" cog, which transmits motion to C, on a short shaft rotating the cam that lifts the exhaust valve. C, having twice as many teeth as A, revolves at half its rate. This ensures that the valve shall be lifted only once in two revolutions of the crank shaft to which it is geared. The cogs are timed, or arranged, so that the cam begins to lift the valve when the piston has made about seven-eighths of its explosion stroke, and closes the valve at the end of the exhaust stroke. THE CARBURETTER. A motor car generally uses petrol as its fuel. Petrol is one of the more volatile products of petroleum, and has a specific gravity of about 680--that is, volume for volume, its weight is to that of water in the proportion of 680 to 1,000. It is extremely dangerous, as it gives off an inflammable gas at ordinary temperatures. Benzine, which we use to clean clothes, is practically the same as petrol, and should be treated with equal care. The function of a _carburetter_ is to reduce petrol to a very fine spray and mix it with a due quantity of air. The device consists of two main parts (Fig. 44)--the _float chamber_ and the _jet chamber_. In the former is a contrivance for regulating the petrol supply. A float--a cork, or air-tight metal box--is arranged to move freely up and down the stem of a needle-valve, which closes the inlet from the tank. At the bottom of the chamber are two pivoted levers, W W, which, when the float rests on them, tip up and lift the valve. Petrol flows in and raises the float. This allows the valve to sink and cut off the supply. If the valve is a good fit and the float is of the correct weight, the petrol will never rise higher than the tip of the jet G. [Illustration: FIG. 44.--Section of a carburetter.] The suction of the engine makes petrol spirt through the jet (which has a very small hole in its end) and atomize itself against a spraying-cone, A. It then passes to the engine inlet pipe through a number of openings, after mixing with air entering from below. An extra air inlet, controllable by the driver, is generally added, unless the carburetter be of a type which automatically maintains constant proportions of air and vapour. The jet chamber is often surrounded by a jacket, through which part of the hot exhaust gases circulate. In cold weather especially this is a valuable aid to vaporization. [Illustration: FIG. 45.--Sketch of the electrical ignition arrangements on a motor car.] IGNITION OF THE CHARGE.