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Rh course, be calculated from the indicator diagrams, but it varies so greatly throughout the journey that this unit is not a convenient one for locomotives.

Coal Consumption. The coal consumption varies greatly with the class of engine, weight of train, gradients, weather, and speed. An express engine may burn anything from about 23 to 50 lbs. of coal per mile. Experiments on the Midland Railway, the main line of which has heavy gradients, showed an average consumption during certain tests of from 0·07 to 0·16 lbs. per ton-mile with different engines and trains.

Testing Plant. Another and modern form of test may be mentioned briefly. This is made in the works on a test plant, the conditions being such as to approximate as nearly as possible to a road test. The engine is placed over a special pit in such a way that its coupled wheels drive by friction another set of wheels and axles fixed across the pit. The engine can be run under its own steam at any speed desired, and the draw-bar pull is measured by a special apparatus at the back. Coal, water and oil consumptions at certain speeds over a definite length of time are measured and recorded. This form of test plant, which is extremely valuable for experimental purposes is of American origin, but there is only one in this country, at the Great Western works at Swindon. An illustration of this is given in Fig. 45.