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Rh of each train; of the miles run, and the time gained or lost, from which the actual average speeds can be calculated; and of the amounts of coal burnt and oil used. The data obtained are tabulated each day, and the results are compared carefully. Minor alterations in the new engine may be made as a result of these trials. British engineers generally prefer this form of extended road test, since it gives results under commercial working conditions. The tests may be elaborated by including a dynamometer car in the train, attached, immediately behind the tender, so as to secure a continuous record of the pull of the engine on the tender draw bar. At the same time indicator diagrams are taken at intervals. The indicated horse power calculated from the indicator diagrams, when compared with the useful horse-power calculated from the draw-bar pull registered by the dynamometer car, gives a measure of the efficiency of the engine.

Tractive Force and Horsepower. It may be explained that the power of a locomotive is not, as in marine and stationary engines, estimated on a horsepower basis, but by the tractive force. The horse-power unit involves the speed of the train, which varies continually. The resistance due to the load also varies continually with the gradients and curves of the line, and with the wind. The horse-power at any given moment can, of