scapes 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.

[Illustration: Fig. 166]

[Illustration: Fig. 167]

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.

[Illustration: Fig. 168]

[Illustration: Fig. 169]

The _gable escapement_, shown in Fig. 166, allows the use of a heavier
pendulum, at the same time the anchor embraces within its jaws a greater
number of the escape-wheel teeth; an arrangement after this manner leads
to the conclusion that with these long levers of the anchor the friction
will be considerably increased and the recoil faces will, as a
consequence, be quickly worn away. Without doubt, this was invented to
permit of opening and closing the contact points of the anchor more
easily. Under the name of the _English recoil anchor_ there came into
use an escapement with a _reduced gable_, which embraced fewer teeth
between the pallets or incli