CHAP VI.] 



THE STEAM-ENGINE. 



419 



which, instead of communicating with the condenser, as in the single- 

 cylinder engines, goes into the steam-chest of the cylinder B. Thence 

 it enters by the lower valve-entrance port below the piston p' ; and 

 in expanding it also produces the elevation of the piston ; as to the 

 steam which is on the other side in the upper chamber of the cylinder, 

 it goes as usual to the condenser or the open air through the pipe 

 cc. 



The simultaneous motion of the two slide-valves in opposite 

 directions will give rise to an upward motion of both pistons, the 



FIG. 295. Woolff's system of distribution and expansion : the two cylinders. 



steam acting at full pressure in the small cylinder, while in the large 

 cylinder it acts only by expansion. In the more modern form of 

 double cylinder, more properly compound engines, the steam in the 

 high pressure cylinder is cut off at from three to four-tenths of the 

 stroke, and is allowed to work expansively throughout the remainder. 

 It is then admitted into the low-pressure cylinder, and having done 



E K 2 



