MOTIVE POWER OF HEAT. 189 



Mr. Joule's experiments were all conducted at 

 temperatures from 50 to about GO Fahr., or from 

 10 to 16 Cent.; and consequently, although some 

 irregular differences in the results, attributable to 

 errors of observation inseparable from experiments 

 of such a very difficult nature, are presented, 

 no regular dependence on the temperature is ob- 

 servable. From three separate series of experi- 

 ments, Mr. Joule deduces the following numbers 

 for the work, in foot-pounds, necessary to produce 

 a thermic unit Fahrenheit by the compression of 

 a gas. 



820, 814, 760. 



Multiplying these by 1.8, to get the corresponding 

 number for a thermic unit Centigrade, we find 



1476, 1465, and 1368. 



The largest of these numbers is most nearly 

 conformable with Mr. Joule's views of the relation 

 between such experimental '''equivalents," and 

 others which he obtained in his electro-magnetic 

 researches ; but the smallest agrees almost perfect- 

 ly with the indications of Carnot's theory ; from 

 which, as exhibited in the preceding table, we 

 should expect, from the temperature in Mrc Joule's 

 experiments, to find a number between 1369 and 

 1379 as the result.* 



* The best figure (1896) is J - 778 ft.-lbs. = 1 B.T.U., or 

 J = 426.8 kgin. = 1 calorie, aud probably with great ac- 

 curacy. 



