132 SCIENCE OF THRESHING. 



remain constant, and it is necessary to have some 

 means for varying the amount of steam to the 

 cylinder automatically to suit the work to be done. 

 A device for doing this is called the "governor." 

 One type of governor is shown in the figure. 

 Two or more controlling weights are so hung 

 as to revolve around a stem and to move outwardly 

 therefrom under centrifugal force against the ten- 

 sion of the springs. The spindle is rotated by a 

 pulley and a belt connection or the like from the 

 main shaft of the engine. The side motion of the 

 balls moves the valve through which the steam 

 must pass on its way to the steam chest. This is a 

 "balanced valve" so that its closure moves easily. 

 It is so connected to the balls by appropriate 

 mechanism that the valve admits more steam when 

 the balls are turning slowly, as when the engine is 

 running slowly, than when the balls are rotating 

 rapidly and so moving away from the spindle. 

 Thus steam is admitted in proportion to the work 

 to be done, as the increase in load and slowing 

 down of the engine is met by an increase in the 

 steam admitted to the cylinder, while a decrease in 

 the load and consequent increase of the engine speed 

 is counteracted by the shutting off of the steam. 



SPEED CHANGER. 



Speed changers are now provided for traction en- 

 gines which permit of the speed being changed from 

 the foot board while the engine is in motion. 



