CHAP. VI.] 



THE STEAM-ENGINE. 



415 



while the two others communicate, the upper with the steam-pipe, 

 the lower with the outer air or the condenser. 



FIG. 291. Phases of the reciprocating motion of the piston and slide-valve. 



The piston slide-valve is so called because it consists of two pistons, 

 moved by one rod in a cylindrical space adjoining the cylinder,, which 

 first gives the steam free access to one of the steam- entrance ports, 

 and to the corresponding chamber of the cylinder, and then puts 



FJG. 292. Distribution of the steam : D valve. 



that chamber and the steam which has done its work in communica- 

 tion with the condenser. 



Lastly, the D valve (Fig. 292) is a hollow piece moving in the 

 steam-chest, which is applied to and slides along the face of the 



