io Dugald Clerk, The Work and Discoveries of Joule 



Joule gives the following table : — 



No. of Material Equivalent Equivalent 



series, employed. in air. in vacuo. Mean, 



i. Water 773-640 772.692 772.692 



2. Mercury 773-762 772.814 



3. Mercury 776.303 775-35 2 



4. Cast iron 776.997 776.045 



5. Cast iron 774.880 773-930 



Joule considers the equivalent derived from water to be 

 the most correct, and accordingly he here adopts 772.692 ft. lbs. 

 as the mechanical equivalent of heat. 



He concludes this short but epoch-making paper by con- 

 sidering it as demonstrated by the experiments described : — 



1 st. That the quantity of heat produced by the friction 

 of bodies, whether solid or liquid, is always proportional to 

 the quantity of force expended. And 2nd. That the quan- 

 tity of heat capable of increasing the temperature of a pound 

 of water (weighed in vacuo, and taken at between 55 ° and 

 6o°) by i° Fahr. requires for its evolution the expenditure 

 of a mechanical force represented by the fall of 772 lbs. 

 through the space of one foot. 



The paper is dated June 4th, 1849, and it was read at the 

 Royal Society on June 21st and published in the Philoso- 

 phical Transactions in 1850. 



In the volume 1 of Joule's Scientific Papers, published in 

 1884, Joule added a note : — 



"A third proposition, suppressed in accordance with the 

 wish of the Committee to whom the paper was referred, 

 stated that friction consisted in the conversion of mechanical 

 power into heat." 



Here again we experience the effect of the tenaciously held 

 belief in the material theory of heat as affecting even a Com- 

 mittee of the Royal Society in 1849. Two years later another 

 important paper was read at the Royal Society, the date being 

 June 19, 1851, and the title " On the Air Engine." Here 

 Joule proposed what we should now call a constant pressure 

 air engine which operated by increase of volume by tempera- 



