es of this kind are called train remontoires. In
arranging such a remontoire it is obvious that the clock-train must be
provided with a stop to prevent it from overwinding the scape-wheel
weight or spring, and further, that there must be on the scape-wheel
some sort of stud or other contrivance to release the clock-train as
soon as the scape-wheel weight or spring has run down and needs
rewinding. We believe the first maker of a large clock with a train
remontoire was Thomas Reid of Edinburgh, who described his apparatus
in his book on _Horology_ (1819). The scape-wheel was driven by a
small weight hung by a Huygens's endless chain, of which one of the
pulleys was fixed to the arbor, and the other rode upon the arbor,
with the pinion attached to it, and the pinion was driven and the
weight wound up by the wheel below (which we will call the third
wheel), as follows. Assuming the scape-wheel to turn in a minute, its
arbor has a notch cut half through it on opposite sides in two places
near to each other; on the arbor of the wheel, which turns in ten
minutes, suppose, there is another wheel with 20 spikes sticking out
of its rim, but alternately in two different planes, so that one set
of spikes can only pass through one of the notches in the scape-wheel
arbor, and the other set only through the other. Whenever, then, the
scape-wheel completes a half-turn, one spike is let go, and the third
wheel is able to move, and with it the whole clock-train and the
hands, until the next spike of the other set is stopped by the
scape-wheel arbor; at the same time the pinion on that arbor is turned
half round, winding up the remontoire weight, but without taking its
pressure off the scape-wheel. Reid says that, so long as this
apparatus was kept in good order, the clock went better than it did
after it was removed in consequence of its getting out of order from
the constant banging of the spikes against the arbor.

[Illustration: FIG. 15.--Gravity Train Remontoire.]

A clock at the Royal Exchange, London, was made in 1844 on the same
principle, except that, instead of the endless chain, an internal
wheel was used, with the spikes set on it externally, which is one of
the modes by which an occasional secondary motion may be given to a
wheel without disturbing its primary and regular motion. The following
is a more simple arrangement of a gravity train remontoire, muc