3G2 
MR. E. H. GRIFFITHS OR THE VALUE OF 
remembered that even Joule’s later determinations differ by as much as 1 part in 
100 *; and that marvellous as is the agreement, amongst themselves, of the results 
obtained by Rowland, they, since his method of investigation was unaltered 
throughout, stand in need of confirmation by different methods of observation. 
Again, Rowla.nd, as far as I have been able to ascertain, stands practically alone in 
his conclusion that the specific heat of water diminishes as the temperature rises 
from 0° to 30° C. It is difficult to conceive of a more important investigation (for 
the purposes of accurate physical measurements) than the determination of the 
capacity for heat of water at different temperatures, and it is to me a matter of 
extreme surprise that greater efforts have not been made to trace the variation 
(if any) in its value. The science of calorimetry must be regarded as in its infancy 
so long’ as its fundamental unit is a matter of doubt. 
Other observers who have attempted to obtain the value of the mechanical 
equivalent, by means of the work done by an electric current, have been hampered 
by constant perplexities as to the absolute values of the electrical units adopted. 
The science of electrical measurements has now arrived at such a stage that its units 
may be regarded as sufficiently established,! and, therefore, the time seems parti¬ 
cularly appropriate for an enquiry into the relation between those units and the 
mechanical ones. 
The difficulties of such an investigation are, of course, great, as is shown by the 
divergence of the results obtained by those observers who, in recent years, have 
adopted the electrical method. One cause of inaccuracy has been present in all 
determinations I have examined, viz., the increase in temperature of the conductor 
above the temperature of the medium in which it was placed, and the consequent 
alteration in its resistance. Rowland;]; writes as follows :—“ There can be no doubt 
that experiments depending on the heating of a wire give too small a value of the 
equivalent, seeing that the temperature of the wire during heating must alwa^^s be 
higher than that of the water surrounding it, and hence more heat will be generated 
than there should be.” 
An account is given in the following pages 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. 
The difficulties with regard to measurement of temperature are not peculiar to the 
electrical method of investigation, and, therefore, I need not dwell upon them. I 
would, however, venture to add my expressions of astonishment to those of Rowland 
that so many enquirers have attached so little importance to this point; many 
investigators, whose methods have otherwise been of a high order of accuracy, having 
contented themselves with the mercurial thermometer as a standard. The importance 
* See Table XLV., infra. 
t ‘ Report Brit. Assoc.,’ 1892. 
X ‘ Proceedings American Academy,’ June, 1879, p. 153. 
