Date: 1894
When he finds a difference in the respective " duties " of two pumping engines at his mine, there is nothing to tell him whether the fault of the less effec tive plant lies in the coal, the engine, the boilers, the transmitting arrangements, or the pumps themselves.  It is important, there fore, that the engines should be indicated, and that the indicated horse-power of the engine should be compared with the actual useful effect in water raised.  Hammer * has found that the power consumed in some cases by the mere friction of the guides in the shaft is as much as 24 to 30 per cent, of the total power given out by the engine.