THE STEAM-ENGINE. 



condenser ; and, on the other hand, during the ascent of the piston, the lower 

 end communicated with the boiler, and the upper end with the condenser ; 

 then the piston would be driven continually, whether upward or downward, by 

 the power of steam acting against a vacuum. Watt obtained his third patent 

 for this contrivance, on the 12th of March, 1782. 



This change in the principle of the machine involved several other changes 

 in the details of its mechanism. 



It was necessary, in the first place, to provide means for admitting and with- 

 drawing the steam at either end of the cylinder. For this purpose let B and 

 B' fig. 14, be two steam-boxes, B the upper, and B' the lower, communicating 



Fig. 14. 



respectively with the top and bottom of the cylinder by proper passages D D'. 

 Let two valves be placed in B, one, S, above the passage D, and the other, C, 

 below it ; and in like manner two other valves in the lower valve-box, B 7 , one, 

 S', above the passage D 7 , and the other, C', below it, Above the valve S in 

 the upper steam-box is an opening at which the steam-pipe from the boiler 

 enters, and below the valve C is another opening, at which enters the ex- 

 hausting-pipe leading, to the condenser. In like manner, above the valve S' 

 in the lower steam-box enters a steam-pipe leading from the boiler, and below 

 the valve C 7 enters an exhausting-pipe leading to the condenser. It is evident, 

 therefore, that steam can always be admitted above the piston by opening the 

 valve S, and below it by opening the valve S' ; and, in like manner, steam can 

 be withdrawn from the cylinder above the piston, and allowed to pass to the 

 condenser, by opening the valve C, and from below it by opening the valve C'. 

 Supposing the piston P to be at the top of the cylinder, and the cylinder 

 below the piston to be filled with pure steam, let the valves S and C' be open- 

 ed, the valves C and S' being closed as represented in fig. 15. Steam from 

 the boiler will, therefore, flow in through the open valve S, and will press the 

 piston downward, while the steam that has filled the cylinder below the piston 

 will pass through the open valve C' into the exhausting-pipe leading to the 

 condenser, and being condensed will leave the cylinder below the piston a 

 racuum. The piston will, therefore, be pressed downward by the action of 

 the steam above it, as in the single-acting engine. Having arrived at the 

 bottom of the cylinder, let the valves S and C 7 be both closed, and the valves 

 S ; and C be opened, as represented in fig. 15. Steam will now be admitted 

 through the' open valve S' and through the passage D 7 below the piston, while 

 the steam which has just driven the piston downward, filling the cylinder above 

 the piston, will be drawn off through the open valve C, and the exhausting- 

 pipe, into the condenser, leaving the cylinder above the piston a vacuum. 



