ertre, of Paris.

Fig. 154 shows another disposition of a double pendulum. While the
pendulum here is double, it has but one bob; it receives the impulse by
means of a double fork _F_. _C C_ represents the cycloidal curves and
are placed with a view of correcting the inequality in the duration of
the oscillations. In watches the circular balances did not afford any
better results than the regulating rods or rules of the clocks, and the
pendulum, of course, was out of the question altogether; it therefore
became imperative to invent some other regulating system.

[Illustration: Fig. 156]

[Illustration: Fig. 157]

It occured to the Abbe d'Hautefeuille to form a sort of resilient
mechanism by attaching one end of a hog's bristle to the plate and the
other to the balance near the axis. Though imperfect in results, this
was nevertheless a brilliant idea, and it was but a short step to
replace the bristle with a straight and very flexible spring, which
later was supplanted by one coiled up like a serpent; but in spite of
this advancement, the watches did not keep much better time. Harrison,
the celebrated English horologist, had recourse to two artifices, of
which the one consisted in giving to the pallets of the escapement such
a curvature that the balance could be led back with a velocity
corresponding to the extension of the oscillation; the second consisted
of an accessory piece, the resultant action of which was analogous to
that of the cycloidal curves in connection with the pendulum.

CORRECTING IRREGULARITIES IN THE VERGE ESCAPEMENT.

Huygens attempted to correct these irregularities in the verge
escapement in watches by amplifying the arc of oscillation of the
balance itself. He constructed for that purpose a pirouette escapement
shown in Fig. 155, in which a toothed wheel _A_ adjusted upon the verge
_V_ serves as an intermediary between that and the balance _B_, upon the
axis of which was fixed a pinion _D_. By this method he obtained
extended arcs of vibration, but the vibrations were, as a consequence,
very slow, and they still remained subject to all the irregularities
arising from the variation in the motive power as well as from shocks. A
little later, but about the same epoch, a certain Dr. Hook, of the Royal
Society of London, contrived another arrangement by means of which he
succeeded, so it appeared to him at least, in greatly diminishing the
influence of shock upon the escapement; but many other,