(115.)
This change in the principle of the machine involved several other changes in the details of its mechanism.
Fig. 33.
(116.)
Fig. 34.
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 opened, the valves C and S′ being closed as represented in [fig. 34.] Steam from the boiler will, therefore, flow in through the open valve S, and will press the piston downwards, 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 vacuum. The piston will, therefore, be pressed downwards 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′ be both closed, and the valves S′ and C be opened, as represented in [fig. 34.] Steam will now be admitted through the open valve S′ and through the passage D′ below the piston, while the steam which has just driven the piston downwards, 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. The piston will, therefore, be pressed upwards by the action of the steam below it, against the vacuum above it, and will ascend with the same force as that with which it had descended.