Page:Elementary Text-book of Physics (Anthony, 1897).djvu/267

§ 234] the two cylinders, of which the smaller may be considered as a supply-purap for taking air from outside and forcing it through the furnace into the larger cylinder, where, in consequence of its expansion by the heat, it is enabled to perform work. Ou the return stroke this air is expelled into the external air, still hot, but at a lower temperature than it would have been had it not expanded and performed work. This case is exactly analogous to that of the steam-engine, in which water is forced, by a piston working in a small cylinder, into a boiler, is there converted into steam, and then, acting upon a much larger piston, performs work, and is rejected. In another form of the engine, known as the "ready motor," the air is forced into the large cylinder through a passage kept supplied with crude petroleum. The air becomes saturated with the vapor, forming a combustible mixture, which is burned in the cylinder itself.

The Stirling hot-air engine and the Eider "compression-engine" are interesting as realizing an approach to Carnot's cycle. These engines, like those described above, consist of two cylinders of different capacities, in which work air-tight pistons; but, unlike those, there are no valves communicating with the external atmosphere. Air is not taken in and rejected; but the same mass of air is alternately heated and cooled, alternately expands and contracts, moving the piston, and performing work at the expense of a portion of the heat imparted to it.

It is of interest to study a little more in detail the cycle of operations in these two forms of engines. The larger of the two cylinders is kept constantly at a high temperature by means of a furnace, while the smaller is kept cold by the circulation of water. The cylinders communicate freely with each other. The pistons are connected to cranks set on an axis, so as to make an angle of nearly ninety degrees with each other. Thus both pistons are moving for a short time in the same direction twice during the revolution of the axis. At the instant that the small piston reaches the top of its stroke, the large piston will be near the bottom of the cylinder, and descending. The small piston now descends, as well