# Page:Popular Science Monthly Volume 22.djvu/224

Assuming, then, in an explosive gas-engine a temperature of 1,500° C., at a pressure of four atmospheres, we should, in accordance with the second law of thermo-dynamics, find a temperature after expansion to atmospheric pressure of 600° C., and therefore a working range of 1500° - 600° ${\displaystyle =}$ 900°, and a theoretical efficiency of 9001,500 ${\displaystyle +}$ 274 ${\displaystyle =}$ about one half, contrasting very favorably with that of a good expansive condensing steam-engine, in which the range is 150 ${\displaystyle -}$ 30 ${\displaystyle =}$ 120° C, and the efficiency 120150 ${\displaystyle +}$ 274 ${\displaystyle =}$ 27. A good expansive steam-engine is therefore capable of yielding as mechanical work two-seventh part of the heat communicated to the boiler, which does not include the heat lost by imperfect combustion, and that carried away in the chimney. Adding to these the losses by friction and radiation in the engine, we find that the best steam-engine yet constructed does not yield in mechanical effect more than one seventh part of the heat-energy residing in the fuel consumed. In the gas engine we have also to make reductions from the theoretical efficiency, on account of the rather serious loss of heat by absorption into the