VARIOUS MODIFICATIONS
Some of these modifications we shall show. For the first example, then, let Fig. 162 illustrate. In this arrangement the fork is carried upon the axis of the pallet P', which effectually does away with the counter-weight C, as shown. Somewhat more complicated, but of the same intrinsic nature, is the arrangement displayed in Fig. 163. We should not imagine that it enjoyed a very extensive application. Here the two levers are completely independent of each other; they act upon the piece B B upon the axis V of the fork. The counter-weights C C' maintain the arms carrying the rollers D D' in contact with the piece B B' which thus receives the impulse from the wheel R. Two adjusting screws serve to place the escapement upon the center. By degrees these fantastic constructions were abandoned to make way for the anchor recoil escapement, which was invented, as we have said, in 1675, by G. Clement, a horologist, of London. In Fig. 164 we have the disposition of the parts as first arranged by this artist. Here the pallets are replaced by the inclines A and B of the anchor, which is pivoted at V upon an axis to which is fixed also the fork. The tooth a escapes from the incline or lever A, and the tooth b immediately rests upon the lever B; by the action of the pendulum the escape wheel suffers a recoil as in the pallet escapement, and on the return of the pendulum the tooth c gives out its impulse in the contrary direction. With this new system it became possible to increase the weight of the bob and at the same time lessen the effective motor power. The travel of the pendulum, or arc of oscillation, being reduced in a marked degree, an accuracy of rate was obtained far superior to that of the crown-wheel escapement. However, this new application of the recoil escapement was not adopted in France until 1695.
The travel of the pendulum, though greatly reduced, still surpassed in breadth the arc in which it is isochronous, and repeated efforts were made to give such shape to the levers as would compel its oscillation within the arc of equal time; a motion which is, as was recognized even at that epoch, the prime requisite to a precise rating. Thus, in 1720, Julien Leroy occupied himself working out the proper shapes for the inclines to produce this desired isochronism. Searching along the same path, Ferd. Berthoud constructed an escapement represented by the Fig. 165. In it we see the same inclines A B of the former construction, but the locking is effected against the slides C and D, the curved faces of which produce isochronous oscillations of the pendulum. The tooth b imparts its lift and the tooth c will lock against the face C; after having passed through its recoil motion this tooth c will butt against the incline A and work out its lift or impulse upon it.
THE GABLE ESCAPEMENT.
