132 JOULE'S FINAL EQUIVALENT. 



friction of water, is the most correct, both on account of 

 the number of experiments tried and the great capacity of 

 the apparatus for heat, and since, even in the friction 

 of fluids, it is impossible to avoid vibration, and the 

 production of slight sound, it is probable that the above 

 number is slightly in excess. I will, therefore, conclude by 

 considering it as demonstrated by the experiments con- 

 tained in this paper, — 



ist. That the quantity of heat produced by bodies, whether 

 solid or liquid, is ahvays proportional to the qtiantity of force 

 expended. 



2nd. That the quantity of heat capable of increasing the 

 temperature of lib. of water (between 8o° and 6o°) by i° 

 Fahrenheit, requires for its evolution the expenditure of a 

 mechanical force represented by the fall of Jj2lb. through one 

 foot." 



This was Joule's final result, and is still accepted, as 

 expressing the true value within the limits of accuracy of 

 any known means of determination ; while it now enters 

 into almost all physical calculations as well as those which 

 guide the practical conversion of heat into " work." 



The paper in which this result is given contains a very 

 interesting preamble, in which Joule gives a full sketch of 

 the mechanical theory of heat, commencing with Count 

 Rumford. 



In dealing with Rumford, he does what Rumford had 

 not done, and what had not been done in the meantime. 

 By applying to Rumford's definite statement of the heat 

 generated in 2h. 30m. (26*5 81bs. of water raised 180° 

 Fahrenheit) by the " work " that could have been done by 

 one horse, the recognised measure of 1 horse power, 

 33,ooolbs. raised one foot per minute, he shows that the 



