regulation. The verge _V_ was set up
horizontally and the pendulum _B_, suspended freely from a flexible
cord, received the impulses through the intermediation of the forked arm
_F_, which formed a part of the verge. At first this forked arm was not
thought of, for the pendulum itself formed a part of the verge. A
far-reaching step had been taken, but it soon became apparent that
perfection was still a long way off. The crown-wheel escapement forcibly
incited the pendulum to wider oscillations; these oscillations not being
as Galileo had believed, of unvaried durations, but they varied sensibly
with the intensity of the motive power.

THE ATTAINMENT OF ISOCHRONISM BY HUYGENS.

Huygens rendered his pendulum _isochronous_; that is, compelled it to
make its oscillations of equal duration, whatever might be the arc
described, by suspending the pendulum between two metallic curves _c
c'_, each one formed by an arc of a cycloid and against which the
suspending cord must lie upon each forward or backward oscillation. We
show this device in Fig. 151. In great oscillations, and by that we mean
oscillations under a greater impulse, the pendulum would thus be
shortened and the shortening would correct the time of the oscillation.
However, the application of an exact cycloidal arc was a matter of no
little difficulty, if not an impossibility in practice, and practical
men began to grope about in search of an escapement which would permit
the use of shorter arcs of oscillation. At London the horologist, G.
Clement, solved the problem in 1675 with his rack escapement and recoil
anchor. In the interval other means were invented, especially the
addition of a second pendulum to correct the irregularities of the
first. Such an escapement is pictured in Fig. 152. The verge is again
vertical and carries near its upper end two arms _D D_, which are each
connected by a cord with a pendulum. The two pendulums oscillate
constantly in the inverse sense the one to the other.

[Illustration: Fig. 154]

[Illustration: Fig. 155]

ANOTHER TWO-PENDULUM ESCAPEMENT.

We show another escapement with two pendulums in Fig. 153. These are
fixed directly upon two axes, each one carrying a pallet _P P'_ and a
segment of a toothed wheel _D D_, which produces the effect of
solidarity between them. The two pendulums oscillate inversely one to
the other, and one after the other receives an impulse. This escapement
was constructed by Jean Baptiste Dut