Manchester Memoirs, Vol. Ixv. (1921), No. 3 9 



his very numerous experiments he arrived at the mechanical 

 equivalent of heat as 772 ft. lbs. to raise one pound of water 

 through one degree Fahrenheit. This value was used by 

 physicists and engineers for many years. 



He began his work in 1838, and for all engineering 

 purposes concluded it in his great paper " On the Mechanical 

 Equivalent of Heat," published in the Philosophical Tran- 

 sactions of the Royal Society in 1850. He had definitely 

 established the mechanical theory of heat, finally dismissed 

 the material theory and also educated himself in scientific 

 research ; all within twelve years. In that period he published 

 some twenty-five papers dealing with electro-magnetic 

 engines, the heating in electrical circuits, origin of heat of 

 combustion, specific heat, the rarefaction and condensation of 

 air, mechanical equivalent of heat, electrolytic heat, theory of 

 heat, velocity of sound in gases, and also shooting stars. 



The 1850 paper is a perfect model of what a scientific 

 paper should be. It opens with a short historical statement 

 in which the reasoning and experiments of Count Rumford, 

 James Watt, Sir Humphrey Davy, Dulong, Faraday, Grove 

 and Mayer are clearly described in a few words. Joule's 

 Royal Society Paper of 1844 is also mentioned, in which he 

 proved that the absorption and evolution of heat by the 

 rarefaction and condensation of air is proportional to the 

 " force " evolved and absorbed in those operations. Joule 

 also gives the values of the mechanical equivalent which he 

 had obtained prior to 1850 by the different modes of experi- 

 ment as follows : — 



Passing water through narrow tubes 770ft. lbs.; paddle 

 wheel to produce friction from the agitation of water, sperm 

 oil, and mercury, the respective values were 781.5, 782.1 and 

 787.6 ft. lbs., all for the heating of one pound of water through 

 one degree Fahrenheit. 



The apparatus used for the 1850 investigation is beauti- 

 fully simple and it is clearly described ; it is the method which 

 is illustrated in all the text books. 



Three sets of experiments were made : (1) paddle in water, 

 (2) paddle in mercury, and (3) friction of discs rotated against 

 each other. The results obtained were : — 



(1) Friction of water ... ... ... 773.64 ft. lbs 



(2) Friction of mercury (a) ... ... 773.762 ,, 



w 776.303 „ 



(3) Friction of cast iron (a) ... ... 776.997 „ 



( h ) 774-88 „ 



