highest development in connexion with the determination of the
mechanical equivalent of heat, but they may be applied with great
advantage in connexion with other problems, such as the measurement of
the variation of specific heat, or of latent heats of fusion or
vaporization.

S 7. _Mechanical Equivalent of Heat._--The phrase "mechanical equivalent
of heat" is somewhat vague, but has been sanctioned by long usage. It is
generally employed to denote the number of units of mechanical work or
energy which, when completely converted into heat without loss, would be
required to produce one heat unit. The numerical value of the mechanical
equivalent necessarily depends on the particular units of heat and work
employed in the comparison. The British engineer prefers to state
results in terms of foot-pounds of work in any convenient latitude per
pound-degree-Fahrenheit of heat. The continental engineer prefers
kilogrammetres per kilogramme-degree-centigrade. For scientific use the
C.G.S. system of expression in ergs per gramme-degree-centigrade, or
"calorie," is the most appropriate, as being independent of the value of
gravity. A more convenient unit of work or energy, in practice, on
account of the smallness of the erg, is the _joule_, which is equal to
10.7 ergs, or one _watt-second_ of electrical energy. On account of its
practical convenience, and its close relation to the international
electrical units, the _joule_ has been recommended by the British
Association for adoption as the absolute unit of heat. Other convenient
practical units of the same kind would be the _watt-hour_, 3600 joules,
which is of the same order of magnitude as the kilo-calorie, and the
_kilowatt-hour_, which is the ordinary commercial unit of electrical
energy.

S 8. _Joule_.--The earlier work of Joule is now chiefly of historical
interest, but his later measurements in 1878, which were undertaken on
a larger scale, adopting G.A. Hirn's method of measuring the work
expended in terms of the torque and the number of revolutions, still
possess value as experimental evidence. In these experiments (see fig.
4) the paddles were revolved by hand at such a speed as to produce a
constant torque on the calorimeter _h_, which was supported on a float
_w_ in a vessel of water _v_, but was kept at rest by the couple due
to a pair of equal weights _k_ suspended from fine strings passing
round the circumference of a horizontal wheel attach