“ 
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 
effet). 
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 
