nt.]

Bloxam's escapement was modified in form by Lord Grimthorpe, his chief
improvement being the addition of a fly vane, which, however, had
previously been used for remontoires to steady the motion. He tried
various modifications of construction, but finally adopted the
"four-legged" and "double-three-legged" forms as being the most
satisfactory, the former for regulators and the latter for large
clocks. Fig. 19 is a back view of the escapement part of an
astronomical clock with the four-legged wheel; seen from the front the
wheel would turn the other way. The long locking teeth are made about
2 in. long from the centre, and the lifting pins, of which four point
forwards while four other intermediate ones point backwards, are at
not more than 1/30 of the distance between the centres EC, of the
scape-wheel and pallets; or rather C is the top of the pendulum spring
to which the pallets Cs, Cs' converge, though the resultant of their
action is a little below C. It is not worth while to crank them as
Bloxam did, in order to make them coincide exactly with the top of the
pendulum, as the friction of the beat pins on the pendulum is
insignificant, and even then would not be quite destroyed. The pallets
are not in the same plane, but one is behind and the other in front of
the wheel, with one stop pointing backwards and the other forwards to
receive the teeth alternately--it does not matter which; in this
figure the stop s is behind and the stop s' forward. The pendulum is
now going to the right, and just beginning to lift the right pallet
and free the stop s'; then the wheel will begin to turn and lift the
other pallet by one of the pins which is now lowest, and which moves
through 45 deg. across the line of centres, and therefore lifts with
very little friction. It goes on till the tooth now below s reaches s
and is stopped there. Meanwhile the pallet Cs' goes on with the
pendulum as far as it may go, to the end of the arc which we have
called [alpha], starting from [gamma]; but it falls with the pendulum
again, not only to [gamma] but to -[gamma] on the other side of 0, so
that the impulse is due to the weight of each pallet alternately
falling through 2[gamma]; and the magnitude of the impulse also
depends on the obliqueness of the pallet on the whole, i.e. on the
distance of its centre of gravity from the vertical through C. The fly
KK' is set on w