The valve in Figure 13 is held on its seat by a spring. The cam bears against the end of the valve stem, and as it revolves its bulge forces the valve stem and valve to move endways and thus to uncover the valve opening. As the movement of the piston depends on the crank shaft, the valve can be made to open at the right time by a proper setting of the gears that drive the cam shaft. The length of time that the cam will hold the valve open depends on the shape of the bulge of the cam. It can be seen that the pointed cam of Figure 13 will not hold the valve open for as long a time as the flat-end cam of Figure 14. [Illustration: FIG. 15.—“TWIN CITY” TRACTOR ENGINE] In the design shown in Figure 13 the cam bears directly against the end of the valve stem, the cam shaft in this case lying along the cylinder head. In the construction shown in Figure 15 the valves are not placed in the cylinder head, but are in an extension or _valve pocket_ projecting from the combustion chamber; this cam shaft is near the crank shaft. It would not be practicable to make the valve stem long enough to reach down to the cam, so a length of rod, called a _push rod_, or _tappet_, is placed between them; the cam moves the push rod and the push rod in turn moves the valve. This is a construction frequently used for automobile engines. [Illustration: FIG. 16.—“HART-PARR” VALVE MECHANISM] In tractor engines the cam shaft is usually placed near the crank shaft, as in Figure 15, and the valves are in the head, so that a valve moves in the opposite direction to the movement of the push rod. This requires still another part to be used, called the _rocker arm_. It is shown in Figure 16. It is a short bar, pivoted at or near the center, with one end at the push rod and the other at the valve stem. When it is moved by the push rod it in turn moves the valve. Valves operated by push rods and rocker arms are also shown in Figures 17, 18 and 19; Figure 18 is a single-cylinder horizontal engine, while Figure 19 is a horizontal double opposed engine, in which one cam operates a valve in each cylinder. Figure 20 shows the valve mechanism of a vertical engine in which all parts, including the rocker arm, are enclosed to protect them from dust, and so they can run in oil. A small space is always left somewhere between the cam and the valve stem, to give the valve stem room to lengthen, which it will do when it gets hot. If this space were not left, the valve stem, in lengthening as it became hot, would strike the part next to it, and the valve would be lifted from its seat. This would cause the engine to lose power. This space must be kept properly adjusted, and instructions for this will be found in Chapter XII. [Illustration: FIG. 17.—“HART-PARR” ENGINE]