THE STEAM-ENGINE, 361 



FIG. 267. 



cylinder, where it pushes down the piston as indicated by the 

 arrows. The steam below the piston escapes by B and N. As the 

 piston nears the opening of B in the cylinder, the sliding- valve is 

 raised, by means of the rod R, to the position indicated in Fig. 

 267. Steam now enters the cylinder by B and pushes up the piston. 

 The steam above the piston escapes by A and N". As the piston 

 nears the opening of A in the cylinder, the sliding- valve is pushed 

 down by R and the process is thus repeated. The piston-rod and 

 the sliding- valve rod work through steam-tight packing-boxes. 



573. The Eccentric. By means of a crank or 

 similar device, illustrated in common foot-power machinery 

 like the turning-lathe, scroll-saw, or sewing-machine, the 

 alternating rectilinear motion of the piston-rod is changed 

 into a continuous rotary motion. A circular shaft is thus 

 given a revolution for every to-and-fro movement of the 

 piston. This shaft generally carries an eccentric for work- 

 ing the sliding-valve rod R. The eccentric (Fig. 268) con- 

 sists of a circular piece of metal, e, rigidly attached to the 

 shaft of the engine S, in such a position that the centre of 

 the piece does not coincide with the centre of the shaft, 

 16 



