Page:Elementary Text-book of Physics (Anthony, 1897).djvu/268

254 as the large one, the air in both cylinders is compressed, and there is but little transfer from one to the other. There is, therefore, comparatively little heat given up. The large piston, reaching its lowest point, begins to ascend, while the descent of the smaller continues. The air is rapidly transferred to the larger heated cylinder, and expands while taking heat from the highly heated surface. After the small piston has reached its lowest point there is a short time during which both the pistons are rising and the air expanding, with but little transfer from one cylinder to the other, and with a relatively small absorption of heat. When the descent of the large piston begins, the small one still rising, the air is rapidly transferred to the smaller cylinder: its volume is diminished, and its heat is given up to the cold surface with which it is brought in contact. The completion of this operation brings the air back to the condition from which it started. It will be seen that there are here four operations, which, while not presenting the simplicity of the four operations of Carnot,—since the first and third are not performed without transfer of heat, and the second and fourth not without change of temperature,—still furnish an example of work done by heat through a series of changes in the working substance, which brings it back, at the end of each revolution, to the same condition as at the beginning.

Gas-engines derive their power from the force developed by the combustion, within the cylinder, of a mixture of illuminating gas and air.

As compared with steam-engines, hot-air and gas engines use the working substance at a much higher temperature. $$S - R$$ is, therefore, greater, and the theoretical efficiency higher. There are, however, practical diflBculties connected with the lubrication of the sliding surfaces at such high temperatures that have so far prevented the use of large engines of this class.

235. Sources of Terrestrial Energy.—Water flowing from a higher to a lower level furnishes energy for driving machinery. The energy theoretically available in a given time is the weight of the water that flows during that time multiplied by the height of