The governor proper depends upon centrifugal force for its action, and generally consists of two weights which are pivoted at one end to a rotating shaft driven by the engine. When these weights are rotated rapidly the bottoms are thrown outwardly by the centrifugal force and tend to assume a horizontal position. The faster the weights are rotated, the greater will be the tendency for the bottoms of the weights to come into the horizontal, and the greater will be the pressure exerted by them on the controlling levers connected to the throttle. It is evident that the centrifugal pull on the weights varies directly with the speed of rotation and consequently with the speed of the engine. The exact relation between the travel of the weights and the speed of the engine is controlled by a spring that acts between arms cast on the weights and the spindle. If a heavy spring is used, greater speed must be attained to move the weights a given distance than with a weak spring, as the centrifugal force must be greater. [Illustration: Fig. 124-d. Governor and Governor Mechanism of Fairbanks-Morse Type “R E” Engine. The Fly-Balls, Springs, and Control Rods Are Shown on the Governor Staff. The Upper End of the Bell Crank Goes to the Throttle. ] The throttle valve of the engine is connected by a rod to the governor through a sliding collar in such a way that the movement of the governor weights due to an =INCREASE= of speed partially closes the valve until the speed of the engine is reduced. Should the speed of the engines =DECREASE=, owing to a heavy load coming on, the spring will force the balls to occupy a lower position which will increase the valve opening until the engine again reaches the normal speed for which the tension of the spring is adjusted. Thus the speed of the engine is kept practically constant by the action of the governor in opening and closing the throttle, which in turn, varies the =QUANTITY= of mixture admitted to the cylinder. The =QUALITY= of the mixture is varied by hand, in the engine by means of cocks in both the air and gas pipes. The =GOVERNOR PROPER= is of practically the same construction in the hit and miss engine, the difference of the two types lying in the method of connecting it to the controlling system. In one case (hit and miss) the governor controls the exhaust valve, and in the other (throttling) it controls the quantity of gas admitted by the throttle valve. The speed of the engine may be varied within certain limits by a lever connected to the valve controlling rod. (130) Types of Governors. The types of governors used on the leading makes of engines will be found described and illustrated in Chapter V which treats of each engine in detail. (131) Governor Troubles. Hit and miss governor troubles may be due to the following defects: =BINDING GOVERNOR COLLAR=, stuck with dirt or gummy oil, will cause the engine to die under load, and overspeed on light load.