Procsedings of the PoLYTEcnxic Association^. 835 



absorbs instead of transmitting the power. Besides, it is difficult, on 

 a large scale, to maintain a constant friction for any length of time. 



The dynamometers of greatest practical value transmit, and at the 

 same time indicate the power, without in any way interfering with 

 the regular duty of the engine. For instance, if the power be trans- 

 mitted by means of a belt, and we can in any way measure the ten- 

 sion of the two parts, their dilJerence represents a force moving with 

 a given velocity, which may easily be reduced to units of power. 



A dynamometer on this principle has been used abroad, which was 

 re-invented by Horatio Allen, Esq., President of The Xovelty Iron 

 Works, in this city, and by him applied to the engines used in the IT. 

 S. Steam Expansion Experiments. In this case the driving and 

 receiving shaft lay in the same horizontal line. jSTear the contiguous 

 ends, large wheels w' ei'e placed with a V groove in the circumference 

 of each. An endless rope passed in both directions over the top of 

 one wheel, then under side pulleys over the top of the other wheel. 

 The side pulleys were below the center of the large M^heels, and were 

 of such size that the four parts of the rope leading to them hung verti- 

 cally. These pulleys ran in bearings free to slide vertically, and were 

 connected to platforms carrying adjustable weights. The motion of 

 the wheel, on the engine shaft, turned the other shaft in the oppo- 

 site direction, by means of the rope, but tended, at the same time, to 

 lift the side pulley. The opposite side pulley was weighted sufficiently 

 to keep the rope from slipping, and weights and a small spring were 

 adjusted on the driving side to balance the lifting force. Then half 

 the difference in weight on the two side-wheels equaled the tension 

 of the cord, or the driving force, which, together with the velocity of 

 the cord, furnished the only elements necessary to calculate the power. 

 This instrument had means attached to automatically record the 

 strain on the cord, and answered its purpose very perfectly and satis- 

 factorily. It was, however, too expensive and cumbersome for every 

 day use. Three beveled wheels, on the above principle, have been 

 used as a governor, and would doubtless make a good dynamometer 

 also. 



Steel springs, properly arranged, form, we believe, the best dyna- 

 mometer for practical use. As commonly constructed, a pulley, 

 through which the power is transmitted, is made loose on the shaft 

 and then is driven from it, through the intervention of springs ; or 

 one shaft is driven from another in the same manner. It is necessary, 

 then, in order to calculate the power, to ascertain the tension of the 



