THE ENGINE PROPER 67 



ends of the cylinder. The underside of the valve is cham- 

 bered in such a manner that when the piston is being driven 

 away 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 grad- 

 ually decreasing pressure, 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 man- 

 ner in which the admission of live steam is stopped before 

 the piston completes its outward stroke, the exhaust is 

 closed shortly before the return stroke is completed. The 

 steam caught between the piston and the end of the cylin- 

 der is compressed as the piston nears the end, raising the 

 pressure of the steam and forming what is called the "cush- 

 ion." 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. 



Wide End of Valve. Sometimes the widths between 

 the outside edges and the exhaust chamber edges of the 

 valve are different for the two ends of the valve. The ob- 



