All About Engines

3io All About Engines two blocks of ice to melt by rubbing them together ; and probably every boy knows that if a brass button be rubbed vigorously on a board it soon becomes too hot to hold in the hand. Before the exact relation between heat and work can be considered, the unit of heat must be defined. This is the amount of heat which is necessary to raise the temperature of i lb. of water through one degree Fahrenheit, and as this quantity is not quite the same at all temperatures it has been measured at a temperature of 39°Fahr., although it is sufficiently accurate for most engineering purposes to assume that one unit of heat will raise the temperature of I lb. of water one degree at any temperature. It is called a British Thermal unit or B.T.U. Since the freezing point on Fahrenheit’s thermometer is 320, and the boiling point is 2120, 180 units are required to raise a pound of water at the melting point of ice to the boiling point. But to convert i lb. of water at 2120 Fahr, into steam at the same temperature requires 967 units of heat. This quantity, therefore, is necessary to change the state from liquid to vapour. It cannot be detected by a thermometer, so it is said to be not sensible heat, but latent heat, and the total heat of the steam is obtained by adding together the sensible and latent heats, or H = h + L. Measured from water at 320 Fahr, the total heat of steam at 2120 Fahr, is, therefore, 180 + 967 = 1,147 B.T.U.s. The number 967 only expresses the latent heat