ith a friction spring like the common striking-part
fly, and should be as long as there is room for, length being much
more effective than width.

The double three-legged gravity escapement, which was first used in
the Westminster clock, is shown in fig. 20. The principle of it is the
same as of the four-legs; but instead of the pallets being one behind
and the other in front of the wheel, with two sets of lifting pins,
there are two wheels ABC, abc, with the three lifting pins and the two
pallets between them like a lantern pinion. One stop B points forward
and the other A backward. The two wheels have their teeth set
intermediately or 60 deg. apart, though that is not essential, and the
angle of 120 deg. may be divided between them in any other
proportions, as 70 deg. and 50 deg., and in that way the pallets may
be still more oblique than 30 deg. from the vertical, which, however,
is found enough to prevent tripping even if the fly gets loose, which
is more likely to happen from carelessness in large clocks than in
astronomical ones.

Of course the fly for those escapements in large clocks, with weights
heavy enough to drive the hands in all weather, must be much larger
than in small ones. For average church clocks with 1-1/4 sec. pendulum
the legs of the scape-wheels are generally made 4 in. long and the fly
from 6 to 7 in. long in each vane by 1-1/4 or 1-1/2 wide. For 1-1/2 sec.
pendulums the scape-wheels are generally made 4-1/2 radius. At
Westminster they are 6 in.

Lord Grimthorpe considered that these escapements act better,
especially in regulators, if the pallets do not fall quite on the
lifting pins, but on a banking, or stop at any convenient place, so as
to leave the wheel free at the moment of starting; just as the
striking of a common house clock will sometimes fail to start unless
the wheel with the pins has a little run before a pin begins to lift
the hammer. The best way to manage the banking is to make the
beat-pins long enough to reach a little way behind the pendulum, and
let the banking be a thin plate of any metal screwed adjustably to the
back of the case. This plate cannot well be shown in the drawings
together with the pendulum, which, it may be added, should take up one
pallet just when it leaves the other.

[Illustration: FIG. 21.--Chronometer Spring Remontoire.]

Chronometer spring remontoire.

In chronome