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The necessity for a re-determination of the value of the mechanical equivalent may not be obvious at first sight. The classic determinations by Joule have undergone but little alteration at the hands of succeeding observers, and the researches of Rowland (1879) into this matter were of such an exhaustive nature that there would appear to be little room left for farther investigation. It should, however, remembered that even Joule’s later determinations differ by as much as1 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, Rowland, 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. I t 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,t and, therefore, the time seems parti­cularly appropriate for an enquiry into the relation between those units and the mechanical ones.

philosophical transactions of the royal society of london. a

VI. The value of the mechanical equivalent of heat, deduced from some experiments performed with the vieiv of establishing the relation between the electrical and mechanical units; together with an investigation into the capacity for heat of water at different temperatures