1893.] 



the Mechanical Equivalent of Heat. SfC. 



7 



be higher than that of the water surrounding it, and hence more heat 

 will be generated than there should be." 



A short account is given in this summary of the manner in which 

 this difficulty has been overcome, and I think it will be seen that 

 this objection to the electrical method of investigation is now 

 removed. 



I have denned the thermal unit as the quantity of heat required 

 to raise unit mass of water through 1° C. of the air thermometer at 

 15° C, and so much confusion has arisen from ambiguity as to the 

 value of the unit, as ordinarily defined, that I have given reasons in 

 support of the suggestion that this definition should be generally 

 adopted. 



Throughout the whole of this enquiry I have been ably assisted by 

 Mr. Gr. M. Clark, B.A., Sidney College, Cambridge, and this com- 

 munication should, in justice, be regarded as a joint contribution. 



The value of an investigation of this kind depends, in a great 

 measure, on the attention given to matters of detail. It is, therefore, 

 impossible, in a short abstract, to produce the evidence on which our 

 results are based, and we content ourselves with a brief outline of 

 the method adopted and the conclusions arrived at, without attempt- 

 ing to justify those conclusions. 



If a calorimeter is suspended in a chamber, the walls of which 

 are maintained at a constant temperature, we can, by observations 

 over a small range across that outside temperature, deduce the rate of 

 rise due to the mechanical work done in the calorimeter, when the 

 supply of heat is derived from stirring only. By repeating the 

 observations in a similar manner over ranges whose mean tempera- 

 ture 6 l differs from that of the surrounding walls O , we obtain the 

 change in temperature due to the combined effects of the stirring, 

 radiation, conduction, and convection at all points of oar whole range 

 of temperature. As the success of the method depends (1) on the 

 possibility of maintaining the exterior temperature unchanged, and 

 (2) on the regularity of the supply of heat due to the stirring, w^ 

 briefly indicate our method of securing those conditions. 



1. The calorimeter* was suspended within an air-tight steel 

 chamber. The walls and floor of this chamber were double, and the 

 space between them filled with mercury. The whole structure was 

 placed in a tank containing about 20 gallons of water, and was sup- 

 ported in such a manner that there were about 3 inches of water both 

 above and beneath it. The mercury was connected by a tube with a 



* The calorimeter was of cylindrical form, and suspended by three glass tubes. 

 It was made of " gilding metal," which both internally and externally was covered 

 with a considerable thickness of gold. All metal surfaces within the calorimeter 

 were thickly gilded. 



