2 Dr. W. S. Day on a Comparison of 



probably the best one in which the heat was produced by the 

 expenditure of mechanical energy. Within the last few 

 years, however, a number of other determinations have been 

 made by electrical methods, and these have given values 

 differing somewhat from Rowland's. The chief ones of these 

 electrical determinations are those of E. H. Griffiths (Phil. 

 Trans. 184 A, p. 361, 1893), and of Schuster and Gannon 

 (Phil. Trans. 186 a, p. 415, 1895). The following table, 

 taken from Schuster and Gannon's paper, gives some com- 

 parative values. The unit is foot-pounds at Greenwich 

 (# = 981-17 cm./sec. 2 ), and the temperature scale is that of the 

 Paris nitrogen thermometer. 





Joule. 



Rowland. 



Griffiths. 



Schuster & Gannon 



At 19°-1 C. . 



. 774 



776*1 



779-1 



778-5 



At 15°. C. . 



. . 775 



778-3 



780-2 



779-7 



dW 

 The measurement of the quantity -=— depends not only on 



the measurement of dW, but also on the scale of temperature 

 used in measuring dt°. In Rowland's experiments this 

 quantity was measured on the absolute scale, as determined 

 by means of the air-thermometer. It was the object of the 

 comparisons described here to compare this absolute scale, as 

 represented by Rowland's thermometers, with the hydrogen 

 scale of the International Bureau of Weights and Measures, 

 at Sevres, near Paris. For this purpose there were secured 

 three Tonnelot thermometers that had been studied at the 

 International Bureau, and compared with the standards of 

 the Bureau at several points of the scale. These three ther- 

 mometers were compared with three of Rowland's thermo- 

 meters used in his mechanical equivalent experiment. The 

 difference between Rowland's absolute scale and the Paris 

 hydrogen scale was thus determined, and the corresponding- 

 corrections to the value of the mechanical equivalent were 

 calculated. The general result is contained in Table IX. on 

 page 25. 



2. Thermometers used. 



Rowland's value of the mechanical equivalent is the result 

 of fourteen experiments, each of which was made with some 

 one of four different thermometers in the calorimeter. In 

 thirteen of these he used three thermometers made by Baudin 

 in 1876-77, and in one he used a thermometer made by 

 Welsh in [1853, and called, in Rowland's paper, the Kew 

 standard. This thermometer belongs to the University of 



