close approximation. In fine, then, we may,
_a priori_, hope to reach an approximation of one hundred-thousandth
of the value of _pq_.

Independently of the use which may be made of it for measuring time in
absolute value, the apparatus described possesses peculiar properties.
It constitutes a kind of clock which indicates, registers, and, if
needful, corrects automatically its own variations of speed. The
apparatus being regulated so that the magnetic needle may be at zero,
if the speed of the commutator is slightly increased, the equilibrium
is disturbed and the magnetic needle deviates in the corresponding
direction; if on the contrary the speed diminishes, the action of the
antagonistic circuit predominates, and the needle deviates in the
contrary direction. These deviations, when small, are proportional to
the variations of speed. They may be, in the first place, observed.
They may, further, be registered, either photographically or by
employing a Redier apparatus, like that which M. Mascart has adapted
to his quadrant electrometer; finally, we may arrange the Redier to
react upon the speed so as to reduce its variations to zero. If these
variations are not completely annulled, they will still be registered
and can be taken into account.

As an indicator of variations this apparatus can be of remarkable
sensitiveness, which may be increased indefinitely by enlarging its
dimensions.

With a battery of 10 volts, a condenser of a microfarad, 10 discharges
per second, and a Thomson's differential galvanometer sensitive to
10^{-10} amperes, we obtain already a sensitiveness of 1/1000000,
i.e., a variation of 1/1000000 in the speed is shown after some
seconds of a deviation of one millimeter. Even the stroboscopic method
does not admit of such sensitiveness.

We may therefore find, with a very close approximation, a speed always
the same on condition that the solid parts of the apparatus (the
condenser and the resistance) are protected from causes of variation
and used always at the same temperature. Doubtless, a well-constructed
astronomical clock maintains a very uniform movement; but the electric
clock is placed in better conditions for invariability, for all the
parts are massive and immovable; they are merely required to remain
unchanged, and there is no question of the wear and tear of
wheel-work, the oxidation of oils, or the variations of weight. In
other words, the system formed by a condenser and a resista