SECTION III, 1902 [ 141 ] Trans. R. S$. C. 
XVIL— On the Absolute Value of the Mechanical Equivalent of Heat. 
By H. T. Barnes, M.A.Sc., D.So. 
Assistant Professor of Physics, McGill University, Montreal. 
(Communicated by Professor John Cox, M.A.) 
In the very careful and elaborate reports submitted to the Paris 
Congress in 1900, on the Mechanical Equivalent of Heat and the 
Specific Heat of Water, by J. S. Ames and E. H. Griffiths, a general 
summary of the work of previous observers was made. An examina- 
tion of this summary shows what a wide divergence exists in the 
absolute value of this important constant. 
This has already been pointed out by the author,’ who has also 
shown that the results of Rowland and of Reynolds and Moorby, 
which are the best direct mechanical measurements we have, can be 
compared, and that they are in good accord with each other. ‘ 
The author ‘desires at this time to give what he considers to be 
the most probable value of the mechanical equivalent. 
In selecting a value of the thermal unit to which to refer the 
mechanical equivalent of heat we have the one recommended by 
Griffiths in his report. This defines the unit as the heat required 
to raise the temperature of one gramme of water from 15° C to 16° C. 
We have also the one recommended by the author in his paper on the 
Specific Heat of Water, which defines the limit between 15-5° and 
16-5° C. giving a mean temperature of 16° C. Since the specific heat 
decreases with rise in temperature these units differ a little, but their 
mean is almost exactly equal to the mean value over the entire range 
of temperature between the freezing and boiling points. It was this 
important fact which originally decided the author to recommend the 
unit at 16° C. Some time ago, before the variation curve for the spe- 
cific heat of water was accurately known over the entire range, Griffiths 
was led to believe from certain of his measurements that the mean 
value of the specific heat between 0° C. and 100° C. was not far from 
the value at 15° C. 
The absolute value of the specific heat at this temperature which 
was recommended by Griffiths in his report is 4187 X 107 ergs. 
This value in terms of the 16° C. unit becomes 4:186 X 107 ergs. 
Rowland’s value for this temperature, corrected by Waidner and 
Mallory,? comes 4:186 X 107, which is the value given by Griffiths. 

1 Proc. Roy. Soc., 67, 238 (1900). Phil. Trans., A., Vol. 199 (1902). 
* Physical Review, 8, 193, (1899). 
