taken as being 4.180 joules per gramme-degree-centigrade on the scale
of the platinum thermometer, corrected to the absolute scale as
explained in the article THERMOMETRY, which has been shown to be
practically equivalent to the hydrogen scale. The value 4.180 joules
at 20 deg. C. is the mean between Rowland's corrected result 4.181 and
the value 4.179, deduced from the experiments of Reynolds and Moorby
on the assumption that the ratio of the mean specific heat 0 deg. to
100 deg. to that at 20 deg. is 1.0016, as given by the formulae
representing the results of Callendar and Barnes. This would indicate
that Rowland's corrected values should, if anything, be lowered. In
any case the value of the mechanical equivalent is uncertain to at
least 1 in 2000.

The mean specific heat, over any range of temperature, may be obtained
by integrating the formulae between the limits required, or by taking
the difference of the corresponding values of the total heat h, and
dividing by the range of temperature. The quantity actually observed
by Rowland was the total heat. It may be remarked that starting from
the same value at 5 deg., for the sake of comparison, Rowland's values
of the total heat agree to 1 in 5000 with those calculated from the
formulae. The values of the total heat observed by Regnault, as
reduced by Shaw, also show a very fair agreement, considering the
uncertainty of the units. It must be admitted that it is desirable to
redetermine the variation of the specific heat above 100 deg. C. This
is very difficult on account of the steam-pressure, and could not
easily be accomplished by the electrical method. Callendar has,
however, devised a continuous method of mixture, which appears to be
peculiarly adapted to the purpose, and promises to give more certain
results. In any case it may be remarked that formulae such as those of
Jamin, Henrichsen, Baumgartner, Winkelmann or Dieterici, which give
far more rapid rates of increase than that of Regnault, cannot
possibly be reconciled with his observations, or with those of
Reynolds and Moorby, or Callendar and Barnes, and are certainly
inapplicable above 100 deg. C.

S 16. _On the Choice of the Thermal Unit._--So much uncertainty still
prevails on this fundamental point that it cannot be passed over without
reference. There are three possible kinds of unit, depending on the
three fundamental methods already g