OPERATION OF THE STEAM IN THE CYLINDERS.
As the work performed by a steam-engine is in direct proportion to the pressure exerted by the steam on the side of the piston which is pulling or pushing on the crank-pin, it is important that the steam should press only on one side of the piston at once. Hence, good engines have the valves operated so that, by the time a stroke is completed, the steam, which was pushing the piston, shall escape, and not obstruct the piston during the return stroke, and so neutralize the steam pressing upon the other side. When an engine is working properly, the steam is admitted alternately to each side of the piston; and its work is done against a pressure on the other side not much higher than that of the atmosphere.