To express the foregoing results in terms of British thermal units (B.Th.U.) they are multiplied by 1.8. Thus for ethyl alcohol: - 7,237 8 gram-calories X 18 = 13,028 B.Th.U.

The factor 1.8 is = 9/5, the ratio of the Centigrade to the Fahrenheit degree of temperature. The British heat unit is defined as the quantity of heat required to raise the temperature of 1 lb. of water by 1° F. (from 60° F. to 61° F.). In absolute values one British heat unit = 252 gram-calories; but for conversion purposes we need consider only the relative temperature values, since the pound, or the gram, is taken as the unit of weight for both fuel burned and water heated, and the question is only one of a smaller unit of temperature in the British system - 1° C.= § ° F.= 18° F.

The metric unit of heat adopted for technical purposes is the " large Calorie " or "K.C.U." This is the quantity of heat required to raise 1 kilogram of water through 1° in the neighbourhood of 15°. One B.Th.U. = 0252 large Calorie, and for the reason given in the preceding paragraph, x large Calories = 1.8 x B.Th.U.