tation. We could employ, for
instance, the pendulum by adopting, as the unit of force, the force
which renders the constant of gravitation equal to unity. The unit of
time thus defined would be independent of the unit of length, and
would depend only on the substance which would give us the unit of
mass under the unit of volume.

It would be equally possible to utilize electrical phenomena, and one
might devise experiments perfectly easy of execution. Thus, by
charging a condenser by means of a battery, and discharging it a given
number of times in a given interval of time, so that the effect of the
current of discharge should be the same as the effect of the output of
the battery through a given resistance, we could estimate, by the
measurement of the electrical magnitudes, the duration of the interval
noted. A system of this kind must not be looked upon as a simple _jeu
d'esprit_, since this very practicable experiment would easily permit
us to check, with a precision which could be carried very far, the
constancy of an interval of time.

From the practical point of view, chronometry has made in these last
few years very sensible progress. The errors in the movements of
chronometers are corrected in a much more systematic way than
formerly, and certain inventions have enabled important improvements
to be effected in the construction of these instruments. Thus the
curious properties which steel combined with nickel--so admirably
studied by M.Ch.Ed. Guillaume--exhibits in the matter of dilatation
are now utilized so as to almost completely annihilate the influence
of variations of temperature.

Sec. 5. THE MEASURE OF TEMPERATURE

From the three mechanical units we derive secondary units; as, for
instance, the unit of work or mechanical energy. The kinetic theory
takes temperature, as well as heat itself, to be a quantity of energy,
and thus seems to connect this notion with the magnitudes of
mechanics. But the legitimacy of this theory cannot be admitted, and
the calorific movement should also be a phenomenon so strictly
confined in space that our most delicate means of investigation would
not enable us to perceive it. It is better, then, to continue to
regard the unit of difference of temperature as a distinct unit, to be
added to the fundamental units.

To define the measure of a certain temperature, we take, in practice,
some arbitrary property of a body. The only necessary condition of
this property is, tha