A careful study of Fig. 150 will discover a simple transposition which it became necessary to make in the clocks, for the effectual adaptation of the pendulum to their 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.
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 Dutertre, 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.