352 
ME. J. P. JOULE AND PROFESSOR THOMSON ON THE 
preferable in the abstract, and must be adopted ultimately. In the mean time it 
becomes a question, what is the temperature of melting ice, if the difference between 
it and the standard boiling-point be called 100°? When this question is answered 
within a tenth of a degree or so, it may be convenient to alter the foundation on 
which the degree is defined, by assuming the temperature of melting ice to agree with 
that which has been found in terms of the old degree ; and then to make it an object 
of further experimental research, to determine by what minute fraction the range from 
freezing to the present standard boiling-point exceeds or falls short of 100. The 
experimental data at present available do not enable us to assign the temperature of 
melting ice, according to the new scale, to perfect certainty within less than two- or 
three-tenths of a degree ; but we shall see that its value is probably about 2/3*7, 
agreeing with the value of - at 0° found by the first method in Section III. From the 
very close approximation to equality between - and -4+^, which our experiments 
have established, we may be sure that temperature from the freezing-point by the 
new system must agree to a very minute fraction of a degree with Centigrade tempe- 
rature between the two prescribed points of agreement, 0° and 100°, and we may 
consider it as highly probable that there will also be a very close agreement through 
a wide range on each side of these limits. It becomes of course an object of the 
greatest importance, when the new system is adopted, to compare it with the old 
standard ; and this is in fact what is substituted for the problem, the evaluation of 
Carnot’s function, now that it is proposed to call the reciprocal of Carnot’s function, 
temperature. In the next Section we shall see by what kind of examination of the 
physical properties of air this is to be done, and investigate an empirical formula 
expressing them consistently with all the experimental data as yet to be had, so far 
as we know. The following Table, showing the indications of the constant-volume 
and constant-pressure air-thermometer in comparison for every twenty degrees of 
the new scale, from the freezing-point to 300° above it, has been calculated from the 
formulae (9), (10), and (39) of Section V. below. 
