Determining the Mechanical Equivalent of Heat. 169 



1892 gives 4*186 x 10 7 on the mercury scale of hard glass, or 

 4*187 on the hydrogen scale, at about 12° C. The latter 

 number expressed in foot-lbs. at Greenwich per lb. degree 0. 

 is 1400, and in foot-lbs. per lb. degree F. is 778. A platinum 

 non-thermal junction was used in these experiments, and was 

 calibrated against a Gonnelot standard mercury thermometer. 



Griffiths, using the electrical method, gives as his final 

 result (Proceedings Koyal Society, vol. iv. p. 26), 4*198 X 10 7 , 

 the corresponding numbers in the other units being 1404 and 

 780. 



The recent determination of Schuster and Gannon, also by 

 the electrical method, gives 4*180 X 10 7 on the mercury scale 

 of hard French glass, or 4*192 on the hydrogen scale, at a 

 temperature of 19°*1 C. The latter value expressed in foot- 

 lbs, at Greenwich gives 1402 and 779. 



Joule's values obtained by friction of water in 1878 gave 

 the numbers 4*159 X 10 7 , 1390*5, and 772*5 respectively 

 (mercury scale) ; while his values obtained by the electrical 

 method in 1867, when corrected for the error in the B.A. ohm, 

 give 4*155 X 10 7 , 1389, and 772, the average temperature of 

 the water being 19° C. The following table gives the results 

 obtained by various methods since 1867, the numbers havino- 

 been recalculated from those given in a table at the end of 

 M. Miculescu's paper (Ann. de Chimie, vol. xxvii. p. 202) : — 



Date. 



Observer. 



Method. 



Ergs per 

 gramme- 

 degree C. 



Foot-lbs 



per 1b.- 



degree F. 



1867. 

 1870. 



1875. 

 1878. 

 1878. 

 1879. 



1888. 



1889. 

 1891. 



1892. 

 1893. 



1894. 



Joule. 

 Violle. 



Puluj. 

 Joule. 

 Weber. 

 Eowland. 



Perot. 



Dieterici. 

 D'Arsouval 



Miculescu. 



Griffiths. 



Schuster 



and 

 Grannon. 



Electrical. 



Heating of a disk between the 



poles of a magnet. 



Friction of metals. 



Friction of water. 



Electrical. 



Friction of water 15°. 



dp 

 By the relation li=T(v 2 —v 1 )-f, . 



Electrical. 



Heating of a cylinder in a mag- 

 netic field. 

 Friction of water. 

 Electrical. 



Electrical. 



4-155 xlO 7 

 4-269 xl0 T 



4-179 xlO 7 

 4-159 xlO 7 

 4-145 xlO 7 

 4-189 xlO 7 



4167 xlO 7 



4-232 xlO 7 

 4-161 xlO 7 



4-186 xlO 7 

 4-198 xlO 7 



4-192 xlO 7 



772 

 793 



776 



772 

 770 

 778 



774 



786 

 773 



778 

 780 



779 



The results obtained since 1879 by stirring water and by 

 electrical methods mav be tabulated thus : — 



