All About Engines

Power and Its Measurement 321 friction of sliding, turning, or rolling between sur- faces in contact. In a well-designed and well-made engine these sources of loss will not be more than, say, 10 per cent., so that the i.h.p. does give some idea of the rate at which the engine will perform external work. As the real test of an engine, however, is in the rate at which it gives out work rather than at which Fig. 179.—Prony brake it takes work in, it is very necessary to have some means of measuring the output directly ; and with an engine of moderate size this is not a difficult matter. Perhaps the simplest piece of apparatus is a Prony brake (Fig. 179). The rim of a small pulley on the engine shaft is gripped by two blocks of wood, held together by two bolts and nuts so that the grip can be adjusted. To the upper block is fixed an iron bar with a spring balance at one end and a small weight at the other to counterbalance the extra v