54 SCIENCE OF SUCCESSFUL THRESHING. 



is alternately admitted on opposite sides of a piston, which 

 is driven back 'and forth, thereby. This reciprocating mo- 

 tion of the piston is changed into the rotary motion of the 

 shaft, by the crank and connecting rod. The admission of 

 steam to the cylinder is controlled by the "slide-valve," 

 which slides upon a planed surface, called the "valve-seat," 



FIG. 15. SECTIONAL VIEW OF SIMPLE CYLINDER. 



in a chamber, called the "steam-chest," which is adjacent 

 to the cylinder. Passages, called "ports," lead from the 

 valve seat to the ends of the cylinder and to the outside air, 

 called the "exhaust." The valve alternately uncovers the 

 ports and allows the steam in the chest to flow into the ends 

 of the cylinder. The underside of the valve is chambered in 

 such a manner that when the piston is being driven away 



