3G3 
^HE MECHANICAL EQUIVALENT OF HEAT. 
of close attention to the thermometry is shown by the fact that the difierence between 
the ah’ and mercury readings, even at low temperatures, will more than account for a 
change of decrease into increase in the specific heat of water as the temperature rises. 
An indirect result of my present investigation is to add further proof (if that is 
necessary) to the accuracy of the measurements of temperature by platinum thermo¬ 
meters, and it is remarkable that so exact and easy a method is not more generally 
applied to that most difficult subject, thermometry. 
Another difficulty which meets any enquirer into calorimelric measurements, is the 
uncertain nature of the thermal unit ordinarily adopted. I take it that the thermal 
unit is usually defined as the amount of heat required to raise unit mass of water 
from 0° to i° C.” The obvious objections to this unit are that no one has more than 
a vague idea as to its magnitude, and that in choosing such a range of temperature 
we have selected, without doubt, the most difficult of all temperatures at which 
to ascertain it. Again, it is not improbable that over sucli a range water may 
- exhibit greater changes in its properties than it does at other temperatures. The 
consequence is that investigators, such as Howland, are compelled, to express their 
results in terms of a variable unit, the theoretical one being an unascertained and 
indefinite quantity. To escape this difficulty I have taken as my standard the 
quantity of head required to raise unit mass of water through 1° C. of the air- 
thermometer at 15° C. {i.e., from 14°*5 to 15°'5 C.) ; in other words, I assume the 
specific heat of water at 15° C. to be 1, and I venture to suggest that (for want of a 
better) the above definition be accepted as that of the thermal unit.'^ 
I have not included in this paper any prolonged analysis of the work of previous 
observers. Howland has given a most complete summary, with references, of all 
results anterior to 1880, and the agreement between later observers is not sufficient 
to warrant any modification of his conclusions.t 
The main portion of this communication is devoted to an account of the experi¬ 
ments performed in my laboratory during 1892, and, in order to avoid the introduction 
of redundant matter into that account, I here give a brief summary of my previous 
investigations. 
I commenced this work in 1887, and the general principle on which I proposed to 
proceed was that of eliminating the effects of conduction, radiation, Ac., rather than 
of ascertaining the actual loss or gain due to such causes. If a calorimeter is sus¬ 
pended in a chamber whose walls are kept at a constant temperature, it is obvious 
that if the initial temperature of the calorimeter is below that of the external 
envelope, and if the calorimeter has its temperature steadily raised by means of a 
* Personally I sbonkl have pieferred to select a higher temperature, since a temperature above that 
of our ordinary surroundings is, at all times, more easily obtainable than a lower one ; but as 15° C. has 
become so commonly accepted as a standard of temperature in electrical measuremeaits, I have adopted 
it as my standard. 
t MicuLiiSCU gives all results published before January, 1892. See ‘ Annales de Ohiinie ct do 
Physique,’ vol. 27, p. 20G. 
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