FIRST USE OF THE PADDLE. 97 



Having his attention thus directed to the determination 

 of specific heats, he invented two new methods of making 

 these determinations, which he described in the Manchester 

 Memoirs, 1845 and 1847. 



In the meantime Joule had made a fresh determination 

 of the mechanical equivalent of heat by turning a paddle 

 in a can of water, the result being " that for each degree 

 of heat evolved by the friction of water a mechanical power 

 equal to that which can raise a weight of 890IDS. to the height 

 of one foot had been expended." 



His description of this apparatus and these results is 

 given in a letter to the Philosophical Magazine, August 6th, 

 1845. This letter is interesting in consequence of this 

 method, which in spite of its having given him, in this his first 

 attempt, the least accurate result of any of his methods, 

 is the one by which he subsequently accomplished the deter- 

 mination with such extreme accuracy. The letter is also 

 interesting because it contains distinct evidence as to his 

 having now acquainted himself with the principles of 

 mechanics, and this by means of his dynamical hypothesis 

 of gases. He uses " mechanical power " instead of " force,'* 

 which he has been using ever since his first statement of his 

 equivalent in 1843. And he then goes on to discuss 

 the result of his hypothesis, as showing the enormous 

 quantities of heat that exist in matter at ordinary tempera- 

 tures. After discussing the various values of his equivalents as 

 derived 1st, from magneto-electrical experiments (8231b.); 

 2nd, from the cold produced by rarefaction of air (795 lb.) ; 

 3rd, from experiments on the motion of water through 

 narrow tubes (7741b.), and with the paddle 8901b., and 

 obtaining as the mean of the three classes of experiments 

 817, he says: — 

 H 



