DOUBLE-ACTING ENGINES. 401 



Watt, though greatly irritated, was not discouraged. 

 His engines were not, in the first place, like Newco- 

 men's, mere pumps, mere draining pumps. In a few 

 years he transformed them into universal motive pow- 

 ers, and of indefinite force. His first step in this line 

 was the invention of a double-acting engine (a double 

 effef). 



To conceive the principle of it, let my report of the 

 modified engine of which I have already treated (page 

 391,) be consulted. The cylinder is closed ; the ex- 

 ternal air has no access to it ; it is steam pressure, and 

 not atmospheric, that makes the piston descend ; the 

 ascending movement is due to a simple counterpoise, be- 

 cause at the moment when this takes place, the steam, 

 being enabled to circulate freely from the higher to the 

 lower portions of the cylinder, presses equally on the 

 piston in both directions. Every one will hence see, that 

 the modified engine, or Newcomen's, has power only 

 during the descending oscillation of the piston. 



A very simple change remedied this serious defect, and 

 produced the double-acting engine. 



In the engine known under this name, as well as in 

 the one which we denominated the modified engine, the 

 steam from the boiler, when the mechanic wishes it, goes 

 freely above the piston and presses it down without 

 meeting any obstacle ; because at that same moment, 

 the lower area of the cylinder is in communication with 

 the condenser. This movement once achieved, and a 

 certain cock having been opened, the steam from the 

 caldron can enter only below the piston, and elevates it ; 

 the steam above it, which had produced the descending 

 movement, then goes to regain its fluid state in the con- 

 denser, with which it has become, in its turn, in free 



