n-metal, with the teeth
cut in an engine. The French clockmakers had begun to use cast iron
striking parts, and cast iron wheels had been occasionally used in the
going part of inferior clocks for the sake of cheapness; but they had
never been used in any clock making pretensions to accuracy. But in
consequence of the success of a clock shown in the 1851 Exhibition, it
was determined by Sir G. Airy and Lord Grimthorpe (then E. Denison),
who were jointly consulted by the Board of Works about the great
Westminster clock in 1852, to alter the original requisition for
gun-metal wheels there to cast iron. But cast iron wheels must drive
cast iron pinions, for they will wear out steel.

[Illustration: FIG. 17.--Mudge's Gravity Escapement.]

[Illustration: FIG. 18.--Bloxam's Gravity Escapement.]

Gravity escapements.

The next kind of remontoire still leaves the scape-wheel linked up
with the clock-train, but makes it wind up the pallets which are held
raised up till their action is wanted, when they are allowed to drop
gently on the crutch or the pendulum rod. In this case the two arms of
the anchor are usually divided and mounted on separate shafts so as to
act independently. This idea was first started by Thomas Mudge
(1717-1794) and Alexander Cumming (1733-1814). Mudge's escapement is
shown in fig. 17. The tooth A of the scape-wheel is resting against
the stop or detent a at the end of the pallet CA, from the axis or
arbor of which descends the half-fork CP to touch the pendulum. From
the other pallet CB descends the other half-fork CO. The two arbors
are set as near the point of suspension, or top of the pendulum
spring, as possible. The pendulum, as here represented, must be moving
to the right, and just leaving contact with the left pallet and going
to take up the right one; as soon as it has raised that pallet a
little it will evidently unlock the wheel and let it turn, and then
the tooth B will raise the left pallet until it is caught by the stop
b on that pallet, and then it will stay until the pendulum returns and
releases it by raising that pallet still higher. Each pallet therefore
descends with the pendulum to a lower point than that where it is
taken up, and the difference between them is supplied by the lifting
of each pallet by the clock, which does not act on the pendulum at
all; so that the pendulum is independent of all variation