s of force
and friction in the train. This escapement is said by Lord Grimthorpe,
in his _Rudimentary Treatise on Clocks_, first published in 1850, to
be liable to trip, the pallets being apt to be jerked by the pendulum,
so that the teeth slip past the hook, and the wheel flies round. This,
however, appears entirely a matter of construction. The really weak
point is that while the impulses on the pendulum due to the
gravitational fall of the arms are uniform, the force which has to be
exercised by the pendulum in unlocking them from the scape-wheel
varies with the pressure of the clock-train. Hence we miss the
compensation which is so beautiful a result of Graham's escapement. To
avoid this, J. M. Bloxam, a barrister, proposed about the middle of
the 19th century his legged gravity escapement (fig. 18). By this
arrangement the parts of the scape-wheel which lifted the gravity
arms were brought as near to the axis of the scape-wheel as possible,
while the locking arms were brought as far from the axis as possible
so that the pressure should be light. The pallet arbors were cranked,
to embrace the pendulum-spring, so that their centres of motion might
coincide with that of the pendulum as nearly as possible--perhaps an
unnecessary refinement; at least the three-legged and four-legged
gravity escapements answer very well with the pallet arbors set on
each side of the top of the spring. The size of the wheel determines
the length of the pallets, as they must be at such an angle to each
other that the radii of the wheel when in contact with each stop may
be at right angles to the pallet arm; and therefore, for a wheel of
this size, the depth of locking can only be very small. The pinion in
Bloxam's clock only raises the pallet through 40' at each beat; i.e.
the angle which we call [gamma], viz. the amplitude of the pendulum
when it begins to lift the pallet, is only 20'; and probably, if it
were increased to anything like a/sqrt(2), where a is the semiarc of
swing, the escapement would trip immediately. The two broad pins
marked E, F, are the fork-pins, and A and B are the stops. The clock
which Bloxam had went very well; but it had an extremely fine train,
with pinions of 18; and nobody else appears to have been able to make
one to answer.

[Illustration: FIG. 19.--Four-legged Gravity Escapement.]

[Illustration: FIG. 20.--Double Three-legged Escapeme