One of the first successful experiments in determining the relation between work units and heat units was devised by Joule in England. (See portrait p. 217.) The experiment consisted in taking a can of metal containing water (Fig. 162) in which was placed a thermometer, and a rod carrying paddles. The rod was turned by a cord connected through suitable apparatus to heavy weights, _W_ and _W_. The energy represented by the downward motion of the weights through a given distance was compared with the heat energy developed in the water as shown by its rise in temperature. Careful experiments showed that when 778 ft.-lbs. of work had been done by the moving weights the heat produced at the same time would warm one pound of water 1 Fahrenheit degree. If the experiment was performed using metric units, it was found that the expenditure of 42,700 gram centimeters (427 gram meters) would result in producing enough heat to warm one gram of water one centigrade degree. The facts just given may be summarized as follows: _778 foot-pounds of energy are equivalent to 1 British thermal unit and 42,700 gram centimeters, or 427 gram meters, of energy are equivalent to 1 calorie_. This relation of work units to heat units is called the _mechanical equivalent of heat_. =195. The Heat Equivalent of Fuels and Efficiency Tests of Engines.=--To determine the efficiency of a steam engine it is necessary to know not only the mechanical equivalent of heat but also the heat produced by burning coal or gas; 1 lb. of average soft coal should produce about 12,600 B.t.u. Now since 778 ft.-lbs. are equivalent to one B.t.u. the energy produced when 2 lbs. of average soft coal is burned is 778 × 12,600 × 2 = 19,605,600 ft.-lbs. In actual practice 2 lbs. of average soft coal burned will develop about 1 horse-power for 1 hour. 1 horse-power-hour = 33,000 ft.-lbs. × 60 = 1,980,000 ft.-lbs. Now efficiency equals (work out)/(work in) 1,980,000/19,605,600 = 1/10 or 10 per cent.. This is the efficiency of a good steam engine. Ordinary ones require 3 lbs. of coal burned to each horse-power-hour produced or they are but 2/3 as efficient or have but about 7 per cent. efficiency. HEAT OF COMBUSTION OF VARIOUS FUELS Data in this table are taken from U. S. Geological Survey, Bulletin No. 332, and U. S. Bureau of Mines, Bulletin No. 23. --------------------------+---------+---------- | B.T.U. | Calories | per lb. | per gram --------------------------+---------+---------- Alcohol, denatured | 11,600 | 6,450 Coal, anthracite, average | 12,600 | 7,500 Coal, bituminous, average | 19,000 | 7,000 Gasoline | 19,000 | 10,550 Illuminating gas | 18,000 | 10,000 Kerosene | 19,990 | 11,050 --------------------------+---------+---------- CONSTANTS FOR HEAT TRANSMISSION Data from "Ideal Fitter," American Radiator Co.