HEAT OF WATER, WITH EXPERIMENTS BY A NEW METHOD. 
!) 
constructed with washed mercurous sulphate according to the specification then 
adopted. 
The idea that the absolute values of the mechanical equivalent deduced from the 
continuous-electric method were uncertain to the extent of more than 1 in 1,000, and 
had recently required correction, appears to have arisen from the fact that some of 
the residts were origdnally expressed in terms of the conventional, or legal, value, 
I'4342 volts at 15° C., of the Clark cell, adopted twenty years ago. This value was 
known at the time to l)e inaccurate, and was employed merely for the purpose of 
comparing results with those of other observers who had also assumed the conven¬ 
tional value. 
The resistance of the standard platinum-silver coil employed in the current 
measurements was probably known to 1 in 10,000 in terms of then existing standards, 
but it has been suggested that some uncertainty existed with regard to the heating 
effect of the current. The coil was designed with eight wires in parallel, so that its 
rise of temperature in a well-stirred oil-bath when carrying 8 amperes (4 watts in 
each wire), should not exceed 1° C., or 1 in 4,000 increase of resistance. The currents 
actually employed in the specific-lieat determinations gave about 2‘5 watts in each 
wire. It is, therefore, unlikely that the increase of resistance could have been greater 
than 1 in 5,000, even if allowance is made for slight differences between the experi¬ 
mental coil on which the design was based and the actual standard. 
The experiments, made by Profs. Viriamu Jones and W. E. Ayrton, with the 
Lorenz apparatus, which I ordered for McGill College in 1895, had shown (Callen- 
DAR, loc. cit., p. 7l) that the Board of Trade Standard Ohm was probably 2 or 3 parts 
in 10,000 larger than 10® C.G.S. Since the absolute values of the Clark cells as well 
as the platinum-silver resistance were determined witli reference to this standard, the 
absolute values of the mechanical equivalent woidd require to be raised on this 
account by 2 or 3 parts in 10,000. Correction for the heating effect of the current 
would require the results to be lowered to nearly the same extent. Since these 
corrections were nearly equal and of opposite sign, and since both were so small and 
uncertain, it did not appear desirable at the time to correct for either. 
Formula for the Variation from 0° C. to 200° C. 
When the preliminary results of the continuous-electric method were first announced 
at the meeting of the British Association at Dover in 1899, I suggested two simple 
formidae to represent the variation empirically, one covering the range of miiiimum 
specific heat from 20° C. to 60° G., and the other, a simple modification of Regnault’s, 
fitting the first at 60° C. and representing Regnault’s corrected and reduced results 
up to 200° C. The first formula was subsequently modified by the addition of a small 
term below 20° C. to represent more accurately the rapid increase of specific heat as 
the freezing-point was approached. It is for many purposes inconvenient to have to 
deal with different formula covering limited ranges, however carefully they may have 
VOL. ccxii.—A. c 
