420 POPULAR LECTURES AND ADDRESSES. 



Joule, in 1843, asserted that heat is the equivalent 

 obtained for work spent in agitating a fluid, and 

 both gave good reasons in support of their asser- 

 tion. Many observations have been cited to prove 

 that heat is not generated by the friction of fluids : 

 but that heat is generated by the friction of fluids 

 has been established beyond all doubt by the 

 powerful and refined tests applied by Joule in his 

 experimental investigation of the subject. 



An instrument was exhibited, by means of which 

 the temperature of a small quantity of water, 

 contained in a shallow circular case provided with 

 vanes in its top and bottom, and violently agitated 

 by a circular disc provided with similar vanes, and 

 made to turn rapidly round, could easily be raised 

 in temperature several degrees in a few minutes 

 by the power of a man, and by means of which 

 steam power applied to turn the disc had raised 

 the temperature of the water by 30 in half an 

 hour. The bearings of the shaft, to the end 

 of which the disc was attached, were entirely 

 external ; so that there was no friction of solids 

 under the water, and no way of accounting for the 



