164 VERIFICATION OF EQUIVALENT. 



1*84 divisions or o""i4 Fan., the value of the divisions was 

 unaltered — the new determination giving o :, 077.22; Fan. as 

 the value of a division, while the old was 0^077,2 14. 



The apparatus for agitating the water and measuring- 

 the power used in these experiments was entirely new, and 

 although in designing it Joule had availed himself to the 

 utmost of his previous experience, the new apparatus 

 differed essentially from the old. 



The most important difference was in the method of 

 applying and measuring the work In his experiments of 

 1849 Joule had used descending weights, like clock weights, 

 to turn the paddle, the power so employed being determined 

 as the product of the weight multiplied by the fall. The 

 disadvantage of this method arose from the necessity of 

 winding up the weight some 20 times for each experiment, 

 and the time thus taken up increasing the time and radia- 

 tion effects during the experiments. To obviate this it 

 was necessary to realize the principle that with a steady 

 motion of the paddle, the turning moment on the paddle 

 would be exactly equal to the resistance necessary to hold 

 the vessel. This principle Joule realized for himself, and 

 delicately poising his vessel on a pivot, by passing silk 

 threads round a pulley on the pivots, he determined the 

 resistance of the vessel, and then by turning the handle of 

 the paddle so as just to balance the fixed resistance, and 

 counting the revolutions he measured the work done. This 

 invention was precisely the same as had previously been 

 made by Hirn, and used by him in verifying Joule's Equiva- 

 lent in 1865, and was shortly after almost simultaneously 

 employed by Froude and others for measuring the work 

 of steam engines. With respect to it, Joule says : " The plan 

 I accptei was, in regard to the measurement of work. 



