THE ENGINE PROPER. 55 



from one end of the cylinder, this chamber connects the steam 

 port of the opposite end with the exhaust port, and allows the 

 steam to flow through the exhaust pipe into the air. The 

 valve does not admit steam to the cylinder during a complete 

 stroke of the piston, but only during a part, which is known 

 as "admission/* When the piston has traveled a certain 

 distance, the valve closes the port, shutting off the steam, 

 at what is called the point of "cut-off." Since steam is 

 elastic, it continues to act, with gradually decreasing pres- 

 sure, upon the piston until the end of the stroke is reached. 

 This part of the stroke and action of the steam is known as 

 "expansion." In the same manner in which the admission 

 of live steam is stopped before the piston completes its out- 

 ward stroke, the exhaust is closed shortly before the return 

 stroke is completed. The steam caught between the piston 

 and the end of the cylinder is compressed as the piston nears 

 the end, raising the pressure of the steam and forming what 

 is called the "cushion." The part of the stroke after the 

 exhaust has closed is called "compression." The steam is 

 carried from the boiler to the steam chest by means of the 

 steam pipe, in which the throttle and governor are located. 



The Piston. The piston is always a little smaller than 

 the inside diameter of the cylinder. It is made steam tight, 

 however, by rings which are fitted into grooves on its cir- 

 cumference. These rings are originally made slightly larger 

 than the bore of the cylinder, and are afterward cut apart, 

 so that they may be compressed sufficiently to enter the 



