66 



SCIENCE OF SUCCESSFUL THRESHING 



change of heat into motion takes place. Here the steam 

 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. Ip. 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 alternatelv uncovers 

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



