124 HEAT. 



Writing after the result the units in which it is expressed, we 

 have 



772-55 foot-lbs. at Greenwich per Ib. of water per 1 F. at 60 F. ; 



g 



multiplying by - for the Centigrade scale, 

 5 



1390'6 foot-lbs. at Greenwich per Ib. of water per 1 C. at 15 ; 



multiplying by 3O48 to convert to centimetres, and remembering that 

 we may use any unit of mass, if we use it on both sides of the transac- 

 tion : 



42385 centimetre grammes at Greenwich per gramme of water 

 per 1 0. at 15 C. ; 



multiplying by 981*17 the value of g at Greenwich, 



4'155 x 10 7 ergs per gramme of water per 1 0. at 15 0. 



Later Researches. Joule's work has been followed by other 

 researches on the value of the mechanical equivalent of heat. We 

 shall give a brief account of some of the more important of these. 



Rowland's Experiment. In 1877-78 Rowland repeated Joule's water- 

 friction experiment on a large scale at Baltimore (Proc. An>. Ac., xv., 

 1879, p. 75). The calorimeter and its contents had a water equivalent 

 about 9 kgm. The paddles were worked by a steam engine, and the 

 spindle to which they were fixed passed up through the bottom of the 

 calorimeter. The calorimeter was hung on a wire, which of course tended 

 to twist when the paddles revolved, but a measured couple was put on 

 to bring the calorimeter back to its undisturbed position. When the rate 

 of working was about 2200 kgm. metres per minute, the rise in tempera- 

 ture was about 25 in 40 minutes. The temperature was measured by 

 mercury-in-glass thermometers, which were standardised in terms of 

 the air thermometer, and the air thermometer temperatures were then 

 reduced to absolute temperatures by means of Thomson and Joule's 

 experiment (chap, xviii.). 



More recently Dr. Day has determined Rowland's temperatures in 

 terms of the hydrogen scale, which is now generally used as the standard 

 (Phil. Mag., xlvi., 1898, p. 1). In the table below are given Rowland's 

 values in ergs per gramme degree of water on the absolute scale, and 

 the values corrected to tlie hydrogen scale, by Dr. Day : 



Absolute Kowland's Values Corrected ergs 



Temperature in ergs and in degrees on 



above 0. absolute degrees. hydrogen scale. 



5 4-212 xlO 7 



6 4-209 4-203 x 10 7 



7 4-207 4-201 



8 4-204 4-199 



9 4-202 4-198 



10 4-200 4-196 



11 4-198 4-194 



12 4-196 4-192 



13 4-194 4-191 



14 4-192 4-189 



15 4-189 4-188 



