Page:Reflections on the Motive Power of Heat.djvu/285



the preceding data are to-day subject to modification.

Thus a duty of 150,000,000 ft.-lbs. per 100 lbs. good coal is to-day attainable, and two thirds that figure is extremely common. With engines of large size the coal-consumption has fallen to one half, sometimes even to one fourth, the figure in the text.

Hot-air engines are superseded by the gas-engine and the oil-vapor engine; which even threaten, in the opinion of many engineers, to ultimately displace the steam-engine.

Compound and other multiple-cylinder engines, with two, three, and even four cylinders in series, are now always employed where fuel is costly. The reason of their success is, in part, that given in Note H; but in only small part. The real cause of their general adoption is the fact that the internal thermal waste by "cylinder-condensation"—which in simple engines ordinarily amounts, according to size, to from 25 to 50 per cent, or 261