124 SCIENCE OF. THRESHING. 



inder heads, through the latter of which runs or slides 

 the piston stem or rod. Steam is prevented from 

 escaping around the rod by means of a stuffing box, 

 K, in which expansive packing is placed and secured 

 by the stuffing box head. The piston rod is connected 

 at its inner end to the piston head which slides loosely 

 in the cylinder, and is provided with expansion 

 packing rings, M, which spring out and bear evenly 

 around against the cylinder wall and so make a 

 steam tight joint. 



Steam is admitted to the cylinder through a supply 

 pipe, S, which enters a steam chest, O; from thence 

 it passes through steam ports, P t which conduct it to 

 each end of the cylinder. The ports are formed in a 

 valve seat, Q, on which a slide valve, T, moves back- 

 ward and forward through the agency of the valve 

 stem, R, which passes out of the steam chest through 

 a stuffing box similar to the piston stem box and is 

 operated by an eccentric on the main shaft. 



The valve is so arranged as to simultaneously 

 cover in one position one of the steam ports and an 

 exhaust port, U. There is a steam passage through 

 the under side of the valve, so that when the ports 

 are so connected, the exhaust steam can escape under 

 the valve through the exhaust pipe. Only one steam 

 port is open at a time, and this enables the steam to 

 get in behind the piston and drive it forward; mean- 

 time the valve is shifting, so that the steam is soon 

 cut off and as the piston reaches the end of its stroke, 

 the opposite port is uncovered and the steam again 

 enters behind the piston at the other end. In the 



