All About Engines

Power and Its Measurement 311 of steam at 2120 Fahr—that is, the number of B.T.U.s required to convert 1 lb. of water at that temperature into steam at that temperature. To find it for any other temperature the engineer uses a formula or refers to “ Steam Tables,” which give the figures for all temperatures likely to occur in practice. If the temperatures are measured in degrees Centi- grade, the unit of heat is called the pound-calorie, and the latent heat of steam at ioo° C. is 537 pound-calories. The total heat of steam at ioo° C., starting from water at o° C., is then 100 + 537 or 637 pound-calories. No accurate measurement of the relation between heat and work was made until 1849, when James Prescot Joule, after six years of the most careful experiment, came to the conclusion that one B.T.U. was capable of being converted into 772 ft.-lb. of mechanical work. That is, it would be capable of raising 772 lb. 1 foot high, or 1 lb. 772 high, or 386 lb. 2 feet high, or any other product of pounds and feet which comes to the magic number 772. Later experiments by Professor H. A. Rowland and others give 778 and 774, but they are all so near together that for practical purposes it does not matter much which is used. The corresponding value in Centigrade units is 1,400. Now i lb. of fairly good coal produces, when burnt, 14,000 B.T.U.’s, so that the energy stored up in it is 10,892,000 ft.-lb. Since there are 2,240 lb. in i ton, this means that by burning 1 lb. of coal enough energy would be produced to raise a weight of 4,862 tons i foot from the ground.