Let us suppose that the air is blown from the engine in the usual way, all the valves closed, and the engine ready to start, the pistons being at the top of the cylinders. Open the valves C, E, and F. The steam which occupies the greater cylinder below the piston will now pass into the condenser through F, leaving a vacuum below the piston. The steam which is in the lesser cylinder below the piston will pass through D and open the valve E, and will press down the greater piston. The steam from the boiler will flow in at C, and press on the lesser piston. At first the whole motion will proceed from the pressure upon the greater piston, since the steam, both above and below the lesser piston, has the same pressure. But, as the pistons descend, the steam below the less passing into the greater cylinder, expands into a greater space, and consequently exerts a diminished pressure, and, therefore, the steam on the other side exerting an undiminished pressure, acquires an impelling force exactly equal to the pressure lost in the expansion of the steam between the two pistons. Thus both pistons will be pressed to the bottoms of their respective cylinders. It will be observed that in the descent the greater piston is urged by a continually decreasing force, while the lesser is urged by continually increasing force. Upon the arrival of the pistons at the bottoms of the cylinders, let the valves, C, E, F be closed, and C´, E´, F´ be opened, as in fig. 44. The steam which is above the greater piston now flows through F´ into the condenser, leaving the space above the piston a vacuum. The steam which is above the lesser piston passes through E´ and D´ below the greater, while the steam from the boiler is admitted through C´ below the lesser piston. The pressure of the steam entering through E´ below the greater piston, pressing on it against the vacuum above it, commences the ascent. In the mean time the steam above the lesser piston passing into the enlarged space of the greater cylinder, loses gradually its elastic force, so that the steam entering from the boiler at C´ becomes in part effective, and the ascent is completed under exactly the same circumstances as the descent, and in this way the process is continued. It is evident that the valves may be easily worked by the mechanism of the engine itself.