THE STEAM ENGINE 331 



they swing nearer to it. This action of the balls is due to 

 centrifugal force. It opens and shuts the throttle valve by 

 raising and lowering the rod which leads from the governor 

 and in this manner the supply of steam is regulated. If the 

 engine moves too fast, the balls of the governor swing out, 

 and this pulls on the rod and partly closes the valve, shutting 

 off some of the steam; if it goes too slowly the balls swing 

 inward and thus open the valve and let in more steam. Thus 

 the speed of the engine is regulated by the governor. 



The speed of the governor should be carefully adjusted, 

 and all its parts kept clean and in perfect working order. 

 When this is done, the engine will always run at a uniform 

 speed, no matter what load or work is on at any time. If any 

 machine is suddenly thrown out of action, the governor should 

 at once control the speed of the engine by cutting off the 

 supply of steam. On the other hand, when a heavy load 

 comes on more steam is admitted by the governor, and thus 

 the speed of the engine is kept nearly constant or uniform. 



370. Crank. The " crank" is a 'mechanical device em- 

 ployed for converting the parallel or reciprocating motion of 

 the piston into a rotary motion. It is connected by a key to 

 the shaft, which carries the fly-wheel. The power trans- 

 mitted to the crank exactly represents that exerted by 

 the steam in the cylinder against the piston, minus the 

 friction. 



Between the piston and crank, connection is made by 

 means of a cross-head and connecting rod; the cross-head 

 runs to and fro between guides. This motion of the cross- 

 head is necessary to prevent the piston rod from being broken 

 or bent by the oblique positions of the connecting rod when 

 the crank is at mid-travel. The distance from the center of 



