=199. Basis of Computations for Duty.=—It was formerly necessary in giving the duty of a pumping-engine to state whether the 100 pounds of coal was actually coal as shovelled into the furnace, or whether it was that coal less the weight of ash remaining after combustion. It was also necessary to specify the quality of coal used, because the heating capacity of different coals may vary materially. For these different reasons the statement of the duty of a pumping-engine in terms of a given weight of coal consumed involved considerable uncertainty, hence in 1891 a committee of the American Society of Mechanical Engineers, appointed for the purpose, took into consideration the best method of determining and stating the duty of a pumping-engine. The report of that committee may be found in vol. XII of the Transactions of that Society. The committee recommended that in a duty test 1,000,000 heat-units (called British Thermal Units or, as abbreviated, frequently B.T.U.) should be substituted for 100 pounds of coal. In other words, that the following should be the expression for the duty: foot-pounds of work done Duty = ----------------------------------- × 1,000,000. total number of heat-units consumed For some grades of coal in which 1,000,000 heat-units would be available for every 100 pounds the numerical value of the duty expressed in the new terms would be unchanged, but for other grades of coal the new expression of the duty might be considerably different. =200. Heat-units and Ash in 100 Pounds of Coal, and Amount of Work Equivalent to a Heat-unit.=—The following table exhibits results determined by Mr. George H. Barrus (Trans. A. S. M. E., vol. XIV. page 816), giving an approximate idea of the total number of heat-units which are made available by the combustion of 100 pounds of coal of the kinds indicated: Semibitumintous: George’s Creek Cumberland, Percentage of Ash. 1,287,400 to 1,421,700 6.1 to 8.6 Pocahontas, 1,360,800 to 1,460,300 3.2 to 6.2 New River, 1,385,800 to 1,392,200 3.5 to 5.7 Bituminous: Youghiogheny, Pa., lump, 1,294,100 5.9 Youghiogheny, Pa., slack, 1,166,400 10.2 Frontenac, Kan., 1,050,600 17.7 Cape Breton Caledonia, 1,242,000 8.7 Anthracite: 1,152,100 to 1,318,900 9.1 to 10.5 [Illustration: Worthington Pump.]