244. A dynamometer, or instrument used for ascertaining the amount of useful effect given out by any motive-power. It is used as follows:—A is a smoothly-turned pulley, secured on a shaft as near as possible to the motive-power. Two blocks of wood are fitted to this pulley, or one block of wood and a series of straps fastened to a band or chain, as in the drawing, instead of a common block. The blocks or block and straps are so arranged that they may be made to bite or press upon the pulley by means of the screws and nuts on the top of the lever, D. To estimate the amount of power transmitted through the shaft, it is only necessary to ascertain the amount of friction of the drum, A, when it is in motion, and the number of revolutions made. At the end of the lever, D, is hung a scale, B, in which weights are placed. The two stops, C, C', are to maintain the lever as nearly as possible in a horizontal position. Now, suppose the shaft to be in motion, the screws are to be tightened and weights added in B, until the lever takes the position shown in the drawing at the required number of revolutions. Therefore the useful effect would be equal to the product of the weights multiplied by the velocity at which the point of suspension of the weights would revolve if the lever were attached to the shaft.