60 
PROFESSOR HUGH L. CALLENDAR OX 
presenting the results from two distinct and independent points of view. It was the 
more necessary in the present instance owing to the comparative independence of 
our several shares in the work, and to the impossibility of satisfactory collaboration 
at such a distance. I had hoped at one time that it might be possible by some 
rearrangement of the matter to weld the separately written portions into a 
continuous whole, but as the part written by Dr. Barnes had already been accepted 
by the Royal Society, and the Abstract had been already published, it appeared 
desirable that it should be printed without alteration as nearly as possible as it was 
received, subject only to a rearrangement of the Tables of Results, and the addition 
of one or two samples of the original observations. 
The delay in publication has been partly due to the necessity of this rearrange¬ 
ment, and partly to the difficulty of obtaining satisfactory determinations of the 
resistance of the manganin standard ohm, on which the absolute values of the results 
depended. I have taken advantage of this delay to verify the calculations as far as 
possible, and to subject the whole work to as complete and careful a revision as the 
time at my disposal would allow. The final results do not materially differ from 
those previously published in the ‘Report of the British Association, Dover,’ 1899, 
and in the ‘Physical Review.’ There was, therefore, no need for haste so far as the 
numerical results of the work were concerned, but it was important in an investiga¬ 
tion of this character that all the details of the apparatus, and the theoretical and 
practical difficulties of the work should be adequately explained and illustrated. 
[Added March 1 1th, 1903.—Frequent references are made in the following pages 
to the paper by Dr. Barnes, infra, pp. 149-263, describing his experimental results. 
These references are generally indicated by the name (Barnes) in brackets, with 
the addition of the page, table, or section referred to. 
It is hardly necessary that I should say anything here in praise of the con¬ 
scientious accuracy with which Dr. Barnes has carried out his share of the work. 
In re-arranging the tabular summary of observations (Barnes, Table XVIII., p. 243), I 
have endeavoured to indicate clearly the order of accuracy attained, and it must be 
evident to anyone who studies the paper, that it would be difficult to make any 
improvement in this respect.] 
Part II. Electrical Measurements. —(A.) Potential. 
(2.) Adrantajes of the Potentiometer Method . 
The simplest method of observing the electrical energy expended in the calori¬ 
meter would be to measure the current C, and to assume the value of the resistance 
It to be that corresponding to the observed mean temperature of the calorimeter, 
fhe watts expended woidd then be given by the formula, W = C~R. This method was 
adopted by the majority of the earlier experimentalists. An equally simple method, 
