56 SCIENCE OF SUCCESSFUL THRESHING. 



cylinder. This gives them some tension so that they fit the 

 inside of the cylinder closely, thus preventing leakage of the 

 steam. The cylinder is bored slightly larger at the ends 

 "counter-bored" as it is called. This is done to guard against 

 the wearing of a shoulder, at the points, near each end of 

 the cylinder, at which the outer edge of the piston ring 

 stops. The forming of such a shoulder (which would cause 

 the engine to pound), is prevented by allowing part of the 

 ring to pass into the counter-bore. The entire width of the 

 ring must not be permitted to enter the counter-bore, how- 

 ever, or the ring would expand and catch against the 

 shoulder. 



The Throttle. The throttle controls the flow of steani 

 from the boiler to the steam chest. It should be left open 

 after the engine is started, and the control of the speed 

 left to the governor. The only exception to this rule is 

 when the engine is working hard, as when traveling up a 

 hill, with its boiler showing a tendency to prime. In this 

 case, the engine should be made to run very slowly by 

 means of the throttle. The skill with which some operators 

 handle the throttle enables them to drive an engine up a 

 hill which one less skilled could not make the engine climb. 

 This applies principally to localities in which the water is so 

 bad that it makes all boilers liable to prime. The throttle 

 should be drained in cold weather to prevent damage by 

 frost. 



