Sol’ 
PROFESSOK 0. REYNOLDS AND MR. W. H. MOORBY 
this material standard is available. But the standard of temiDerature actuallv 
established by Joule is universally avadable wherever the British standard of length 
is available, together with pure Avater and tlie necessary means and skill of expending 
a. definite quantity of work in niising the temperature of Avater betAA'een 50° and 
60 Fain., since in this Avay the scale on any thermometer ma\’’ be compared AA'ith 
that on Joule’s. 
ibe difiiculty of access to Joule’s thermometer, and the inherent difficulty of 
making an accurate determination of the equivalent, have limited the number of such 
comparisons. 
I he most serious attempts have been made Avith the very desirable object 
of determining the mechanical equivalent of a thermal unit, measured on the scale 
of piessuies of gas at constant A-mlumes, first recognised by Joule as the nearest 
a[)proximation to absolute temperature. 
Ihe results of these comparisons have been various, all haAung apparently shoAAm 
that Joule’s standard degree of temperature is less than the one-hundred-and- 
eightieth part betAveen freezing and boiling points on the scale of ^Jressure of gas at 
constant A'olume, the differences being from O'l to I'O per cent. Joule himself 
contemplated comparing liis thermometer Avitli the scale of air pressures, but did not 
do so. bo that only indirect comparisons have been possible. 
Hikx, who Avas the first to folloAv Joule, in one of his researches introduced 
a- method of measuring the work done Avhrch afforded much greater facility for 
applying the Avork to the Avater than the falling vA'eights used by Joule in his first 
determination, and this Avas adopted by Joule in his second determination. But not- 
A\ ithstanding the greater facilities enjoyed by subsequent observers, oAving to the 
progress of physical appliances, the inherent difficulties remained. The losses from 
radiation and conduction could only be minimised by restricting the range of 
temperature, and this insured thermometric difficulties, particularly with the air 
thermometer, Avhich, it seems, does not admit of very close reading. This, together 
Avith certain criticisms, of Avhich some of the methods employed admit, apjiear to 
have left it still an ojien question Avhat exact rise in the temperature in the scale of 
air pressures corresponds to tlie 772 ft.-lbs. 
2. Ihe research, to the method and ajjpliances for which this paper relates, has 
lieen the result of the occurrence of circumstances which offered an opportunity, such 
as might not again occur, of obtaining the measure, in mechanical units, of the heat 
in Avater betAveen the tAvo physically fixed points of temperature to Avhich all 
thermometrical measurements are referred, and of thus placing the heat as defined 
in mechanical units, on the same footing as the unit of heat as defined by tem¬ 
perature, Avitliout the intervention of scales, the intervals of AAdiich depend on the 
relative expansions of different materials such as mercury and glass. 
It has been, so far as I am concerned, undertaken Avith considerable hesitation, on 
account of the responsibility even in attempting such a determination, and the harm 
