CH. XXVIII. 



NEWCOMEN'S ENGINE. 



247 



end of the lever-beam. Directly it reaches the bottom 

 the stopcock b has to be shut, and a opened again. Up 

 rises the steam directly from the boiler, driving up the piston, 

 and the whole thing begins again. In this way the lever- 

 beam is kept moving up and down by simply turning tljc 



Fig. 41, 



Newcomen Engine (Black). 



a. Stopcock between boiler and cylinder, h. Stopcock between cold-water tank and 

 cylinder, c. Valve closing air-vent, d. Valve closing the outlet for condensed 

 steam, e. Weight which drags down the beam, p, p. Piston which is pressed 

 down by the atmosphere when the cylinder is empty. ' 



two Stopcocks one after the other. These were at first 

 opened and shut by boys; but one day an ingenious lad 

 named Humphrey Potter, who wanted to save himself the 

 trouble of turning the cocks, found that by tying strings from 



' The boiler and cold-water tank both in this figure and in fig. 43 

 are draAvn much too small in proportion, in order to bring them into 

 the figure. 



