The specific heat of air at constant pressure has been proved by the experiments of Regnault to be 0.2378, or something less than one-fourth of that of water--a result arrived at by Rankine from totally different data. In the case we have taken, there have been expended 500 x 0.2378, or (say) 118.9[theta] to produce 26,450 f.p. Each unit has therefore produced (26,450 / 118.0) = 222.5 f.p., instead of 772 f.p., which would have been rendered if every unit had been converted into power. We therefore conclude that (222.5 / 772) = 29 per cent. of the total heat has been converted. The residue, or 71 per cent., remains unchanged as heat, and may be partly saved by a regenerator, or applied to other purposes for which a moderate heat is required. The quantity of heat necessary to raise the heat of air at a constant volume is only 71 per cent. of that required to raise to the same temperature the same weight of air under constant pressure. This is exactly the result which Laplace arrived at from observations on the velocity of sound, and may be stated thus-- Specific Foot- Per heat. pounds. cent. Kp = 1 lb. of air at constant pressure 0.2378 x 772 = 183.5 = 100 Kv = 1 lb. of air at constant volume 0.1688 x 772 = 130.3 = 71 ------ --- ----- --- Difference, being heat converted into power 0.0690 x 772 = 53.2 = 29 Or, in a hot-air engine without regeneration, the maximum effect of 1 lb. of air heated 1 deg. Fahr. would be 53.2 f.p. The quantity of heat Ky necessary to heat air under constant volume is to Kv, or that necessary to heat it under constant pressure, as 71:100, or as 1:1.408, or very nearly as 1:SQRT(2)--a result which was arrived at by Masson from theoretical considerations. The 71 per cent. escaping as heat may be utilized in place of other fuel; and with the first hot-air engine I ever saw, it was employed for drying blocks of wood. In the same way, the unconverted heat of the exhaust steam from a high-pressure engine, or the heated gases and water passing away from a gas-engine, may be employed. [Illustration]