A valve is held against its seat by a spring, which must be compressed when the valve is opened. If this spring is too weak, it will not hold the valve tightly on its seat, while if it is too stiff, the cam shaft and other parts will be needlessly strained in compressing it. Friction between the cam and the end of the valve stem or push rod would cause rapid wear if these parts were not of hardened steel, and kept well oiled. Still further to reduce wear, there is usually a roller on the end of the push rod, as shown in Figure 16 and some of the other illustrations. Figure 15 shows a construction in which the end of the push rod is a flat disk, which rotates as the cam comes into contact with it. [Illustration: FIG. 18.—“OIL-PULL” ENGINE] When the mixture burns, the top of the piston, the cylinder head, and the walls of the combustion chamber become heated, and if it is not prevented they will get so hot that they will expand sufficiently to cause the piston to stick, or _seize_. The upper part of the cylinder is, therefore, provided with a cooling system that keeps these parts from getting overheated. Channels are provided through which water is circulated; the water takes the heat from the metal parts, becoming heated itself, and then passes to a _cooler_, or _radiator_, where it gives up the heat to currents of air. In addition to the channels, or _water jackets_, around the cylinder, a cooling system includes the radiator, the connections, and usually a pump that keeps the water in motion. [Illustration: FIG. 19.—HORIZONTAL DOUBLE OPPOSED ENGINE] In some tractors, notably the Fordson, no pump is used; the water circulates because it is heated. This is called a _thermo-syphon_ system. When the engine runs, the water in the cylinder jackets becomes heated; as hot water is lighter than cold water, it rises and flows out of the jackets to the radiator, its place being taken by cool water from the bottom of the radiator. This circulation continues as long as the water in one part of the system is hotter than the water in some other part of the system. The lubrication of an engine is described and explained in Chapter X. [Illustration: FIG. 20.—“MONARCH” ENGINE] CHAPTER IV FUELS AND CARBURETION In order that a thing may burn, it must be provided with oxygen. Oxygen is found in air, so it is usual to say that air is necessary in order that anything may burn. To prove this, light a candle and place an empty bottle over it, upside down; in a very short time the oxygen in the bottle will be used up, and the flame will flicker and get smoky, and finally die out. If a card is laid on the chimney of a coal-oil lamp so that it covers the opening, that flame also will flicker, get smoky and go out.