18 Value of the Mechanical Equivalent of Heat, fyc. [Feb. 16, 



equivalents of the calorimeter, and of the changes in it and in the 

 capacity for heat of the water. If we reject Group B (and we have 

 already stated that it has little value), the results are practically 

 identical. 



Hence (the thermometry depending on comparisons with platinum 

 thermometers) if we assume 



1. The unit of resistance as defined in the ' B.A. Report,' 1892; 



2. That the E.M.F. of the Cavendish Standard Clark cell at 15° C. 

 , = 1-4342 volts;* 



3. That the thermal unit = quantity of heat required to raise 



1 gram of water through 1° C. at 15° C, 



the most probable value of 



J = 41940xl0'.f 



This, by reduction, gives the following : — 



J = 42 7 "45 kilogramme-metres in latitude of Greenwich (g = 

 98117). 



J = 1402'2 ft.-lbs. per thermal unit C. in latitude of Greenwich 

 (g = 32195). 



J = 778*99 ft.-lbs. per thermal unit F. in latitude of Greenwich 

 (g = 32-195). 



The length of this abstract is already unduly great, and we will, 

 therefore, not enter on any discussion of the results beyond remark- 

 ing that if we express Rowland's value of J in terms of our thermal 

 unit we exceed his value by 1 part in 930, and we exceed the mean of 

 Joule's determination by 1 part in 3504 



The difference between Rowland's value of the temperature coeffi- 

 cient of the specific heat of water and ours would, however, cause 

 both his and our values of J to be identical if expressed in terms of a 

 thermal unit at 11-5° C. 



* If we assume the E.M.F. of otlr Clark cells to be the same as that of the 

 Cavendish standard (and we are inclined to think we have over-estimated the 

 difference), we get J = 4-1930 x 10 7 . 



f The value obtained by us in 1891 = (4-192 + ) x 10?. 



X Rowland obtained the mean value of Joule's determinations by assigning 

 values to different experiments, and the above comparison refers to the numbers 

 thus obtained. If, however, we attach equal weight to all Joule's results, as given 

 by Rowland, the mean exceeds our value by 1 in 4280, assuming our expression 

 for the temperature coefficient of the specific heat of water. 



