In the cleaner shown in Figure 37, the air is passed through loose wool, which filters out the dust. Another type of cleaner works on the same principle as a cream separator; the air is given a whirling motion, which throws the dirt out at the sides, and it is collected in a glass jar. [Illustration: FIG. 34.—“AVERY” FUEL CONNECTIONS] [Illustration: FIG. 35.—“OIL-PULL” FUEL SYSTEM] These air cleaners must be emptied frequently, for if they are not kept clean it cannot be expected that they will do their work. A tractor engine is built to develop its maximum power at a certain speed; if it runs at greater speed, it will not operate efficiently, and there will be unnecessary wear of its parts. These engines are therefore usually fitted with _governors_ which hold them at their most efficient speed. A governor operates by _centrifugal force_. Anything in motion tries to move in a straight line; if it is forced to move in a circle, it will exert force in trying to move away from its center. It is this that is called centrifugal force. It is centrifugal force that holds water in a pail that is being swung around the head, and that makes the pail fly off if it is released. [Illustration: FIG. 36.—AIR WASHER] In applying this principle to a governor, weights are attached to a plate and made to revolve; springs hold them together, but in spite of this, centrifugal force throws them outward. In moving, they act on a rod that operates the throttle; as the speed increases, the move outward more and more, and it is a simple matter of adjustment to cause them to close the throttle when the speed reaches a desired point. [Illustration: FIG. 37.—AIR STRAINER] [Illustration: FIG. 38.—“E-B” GOVERNOR] A governor and its connections are shown in Figure 38. The weights, R, are L-shaped, and pivoted at the angle to a plate driven by the engine. The shaft that drives the plate also supports a collar, P, that is loose on it and can slide endways; the collar rests against the short bar of the L-shaped weights. The other end of the collar touches the lever, E, which is moved when the collar moves. As the lever is connected with the throttle, a movement of the collar will control the position of the throttle. [Illustration: FIG. 39.—“CASE” GOVERNOR] When the shaft revolves, the long arms of the L-shaped weights tend to fly outward; this moves them on their pivots, and the short arms thereupon force the collar to slide on the shaft, which moves the lever and operates the throttle. The speed at which the throttle will begin to close is determined by the setting of the spring that holds the weights in. [Illustration: FIG. 40.—“HART-PARR” GOVERNOR] Governors and governor connections are shown in Figures 39 and 40. The governor shown in Figure 41 is enclosed in a housing that can be locked or sealed. This prevents the unauthorized changing of the adjustment. [Illustration: FIG. 41.—VERTICAL GOVERNOR] CHAPTER VI IGNITION