150 PROCEEDINGS OF THE AMERICAN ACADEMY 



as the one here used, and would thus bring the value even as high 

 as 429. 



One very serious defect in Joule's experiments is the small range 

 of temperature used, this being only about half a degree Fahrenheit, 

 or about six divisions on bis tliermometex*. It would seem almost im- 

 possible to calibrate a tbermometer so accurately that six divisions 

 should be accurate to one per cent, and it would certainly need a very 

 skilful observer to read to that degree of accuracy. Further, the 

 same thermometer " A " was used throughout the whole experiment 

 with water, and so the error of calibration was hardly eliminated, the 

 temperature of the water being nearly the same. In the experiment 

 on quicksilver another thermometer was used, and he then finds a 

 higher result, 424.7, which, reduced as above, gives 427.0 at Baltimore. 



The experiments on the friction of iron should be probably re- 

 jected on account of the large and uncertain correction for the energy 

 given out in sound. 



The recent experiments of 1878 give a value of 772.55, which re- 

 duced gives at Baltimore 42G.2, the same as the other experiment. 



The agreement of these reduced values with my value at the 

 same temperature, namely 427.3, is certainly very remarkable, and 

 shows what an accurate experimenter Joule must be to get with his 

 simple apparatus results so near those from my elaborate apparatus, 

 which almost grinds out accurate results without labor except in re- 

 duction. Indeed, the quantity is the same as I find at about 20° C. 



The experiments of Ilirn of 18G0-GI seem to point to a value of 

 the equivaleut higher than that found by Joule, but the details of tlie 

 experiment do not seem to have been published, and they certainly 

 were not reduced to the air thermometer. 



The method used by Violle in 1870 does not seem capable of 

 accuracy, seeing that the heat lost by a disc in rapid rotation, and while 

 carried to the calorimeter, must have been uncertain. 



The experiments of llirn are of much interest from the methods 

 used, but can hardly have weight as accurate determinations. Somo 

 of the methods will be again referred to when I come to the descrip- 

 tion of apparatus. 



Method by Heat generated by Electric Current. 



The old experiments of Quintus Icilius or Lenz do not have any 

 except historical value, seeing that Weber's measure of absolute 

 resistance was certainly incorrect, and we now have no means of find- 

 ins its error. 



