66 PROCEEDINGS OF THE AMERICAN ACADEMY 



the slot begin to turn while the hub of the pulley is stationary, the piece 

 must move laterally within the slot, being forced by the screw. If 

 the pulley start a quarter of a turn later than the shaft, the piece will 

 move laterally three quarters of an inch ; if the pulley start a half a 

 turn later than the shaft, the piece will move laterally an inch and a 

 half. The lateral motion of the piece in the slot is proportional to the 

 retardation of the pulley, and this retardation is proportional to the 

 strain upon the belt which passes over the pulley, and conveys the 

 power to be used. To the movable piece in the slot is connected 

 a small round rod, which runs out through the centre of the main 

 shaft and projects some little distance beyond it. On the end of this 

 rod is a circular rack of teeth, in which plays a pinion, on whose shaft 

 is a hand moving over a dial-plate. By applying strains, measured by 

 standard scales, to the belt which passes. over the pulley, — as a strain 

 of ten pounds, fifty pounds, one hundred pounds, — it is easy to gradu- 

 ate the dial-plate into pounds, so that the number of pounds of strain 

 upon the belt may be read off at any instant by a mere inspection of 

 the dial. The mode of operation of this part of the apparatus is then 

 as follows : — when no power is being conveyed from the pulley, shaft 

 and pulley start simultaneously ; there is no lateral motion of the piece 

 within the slot and its connected rod, and the hand on the dial points 

 to zero. But the moment that power begins to be expended in driving 

 the machinery, the strain upon the belt will be first felt by the spring 

 which connects the pulley to the main shaft, and the spring will yield 

 in proportion to the strain ; the effect is to let the shaft make a small 

 part of a revolution in the hub of the pulley, before the pulley begins 

 to turn and keep pace with the shaft; the rod within the end of the' 

 shaft is thus drawn in a little, the hand moves over the dial-plate, and 

 points to the exact number of pounds of power which the belt is con- 

 veying from the pulley at the instant of observation. 



The registering of the total amount of power delivered from the 

 pulley is effected by means of two small belts running over the round 

 rod, which projects beyond the end of the main shaft and carries the 

 index hand above described. These two small belts communicate 

 the motion of the shaft to two parallel and equal wheels, one of 

 which bears a dial-plate, and the other an index hand which moves 

 over the dial-plate. When there is no strain upon the main belt 

 going over the pulley, the two wheels revolve at the same rate, — 

 neither gaining upon the other, and the hand remains constantly 



