MECHANICS AND USEFUL ARTS. 43 



shaft is a hand moving ovei* a dial-plate. By appljiug strains, 

 measured by standard scales, to the belt which passes over the 

 pulley, — as a strain often pounds, fifty pounds, one hundred 

 pounds, — it is easy to graduate tlie 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 oper- 

 ation of this part of the apparatus is then as follows : When no 

 power is being conveyed from the pulley, shaft and pulley start 

 simultaneous!}' ; 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 tliat power begins to be expended in driving- the 

 machinery, the strain upon the belt Avill be lirst 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 numl^er of pounds of power 

 which the belt is conveying from the pulley at the instant of 

 observation. 



The registering of the total amount of jDower delivered from 

 the palley 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 jiarallel 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 

 sti-ain 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 over the same figure on the dial- 

 plate ; but Avhen a strain is put upon the belt, and the round rod 

 moves laterally, as above described, the lateral motion brings a 

 conical enlargement of the rod under the little belt which moves 

 the wheel bearing the dial. The dial-wheel now goes faster than 

 the wheel carrying the hand, and begins to count up the power 

 used. The greater the latei-al motion of the rod, or, in other 

 words, the greater the power transmitted to the working-ma- 

 chines, the larger the diameter of the cone which comes under 

 the belt of the dial-wheel, and the greater the gain of the dial 

 npon the hand. The wheels of both dial and hand are constantly 

 revolving in the direction opposite to that of the motion of the 

 hands of a watch. The belt of the hand-wheel runs always upon 

 the rod where its diameter is constant, and as the rod moves later- 

 ally under the little belts, guides are necessary to keep the belts 

 themselves from moving laterally also. The proportions of the 

 cones on the rod and of the two wheels which carry the dial and 

 the hand, can lie so adjusted as to make a difference of one com- 

 plete revolution between the motions of the iiands and of the dial, 

 indicating a delivery of ten thousand foot-pounds, or of ten million, 

 or of any other convenient number, and by a system of gearing 

 analogous to that used in gas-metres, any desired amount of 

 power could be consecutively registered. It is obvious that the 



