locks where each particular
second has always to be true. In clocks with only three wheels in the
train it is best to make the scape-wheel turn in two minutes. In that
case four notches and four remontoire arms are required, and the fly
makes only a quarter of a turn. Lord Grimthorpe made the following
provision for diminishing the friction of the letting-off work. The
fly pinion f has only half the number of teeth of the scape-wheel
pinion, being a lantern pinion of 7 or 8, while the other is a leaved
pinion of 14 or 16, and therefore the same wheel D will properly drive
both, as will be seen hereafter. The scape-wheel arbor ends in a
cylinder about 5/8 in. in diameter, with two notches at right angles
cut in its face, one of them narrow and deep, and the other broad and
shallow, so that a long and thin pin B can pass only through one, and
a broad and short pin A through the other. Consequently, at each
quarter of a turn of the scape-wheel, the remontoire fly, on which the
pins A, B are set on springs, as in fig. 15, can turn half round. It
is set on its arbor f by a square ratchet and click, which enables the
spring to be adjusted to the requisite tension to obtain the proper
vibration of the pendulum. A better construction, afterwards
introduced, is to make the fly separate from the letting-off arms,
whereby the blow on the cylinder is diminished, the fly being allowed
to go on as in the gravity escapement. It should be observed, however,
that even a spring remontoire requires a larger weight than the same
clock without one; but as none of that additional force reaches the
pendulum, that is of no consequence. The variation of force of the
remontoire spring from temperature, as it only affects the pendulum
through the medium of the dead escapement, is far too small to produce
any appreciable effect; and it is found that clocks of this kind, with
a compensated pendulum 8 ft. long, and weighing about 2 cwt., will not
vary above a second a month, if the pallets are kept clean and well
oiled. No turret clock without either a train remontoire or a gravity
escapement will approach that degree of accuracy.

The introduction of this remontoire led to another very important
alteration in the construction of large clocks. Hitherto it had always
been considered necessary, with a view to diminish the friction as far
as possible, to make the wheels of brass or gu