ley interposed
between the barrel, or circular drum containing the mainspring, and the
train of wheels which the spring has to drive. The principle of the
"drum and fusee" action will be understood from Fig. 201. The mainspring
is a long steel ribbon fixed at one end to an arbor (the watchmaker's
name for a spindle or axle), round which it is tightly wound. The arbor
and spring are inserted in the barrel. The arbor is prevented from
turning by a ratchet, B, and click, and therefore the spring in its
effort to uncoil causes the barrel to rotate.

[Illustration: FIG. 201.]

A string of catgut (or a very fine chain) is connected at one end to
the circumference of the drum, and wound round it, the other end being
fixed to the larger end of the fusee, which is attached to the
driving-wheel of the watch or clock by the intervention of a ratchet and
click (not shown). To wind the spring the fusee is turned backward by
means of a key applied to the square end A of the fusee arbor, and this
draws the string from off the drum on to the fusee. The force of the
spring causes the fusee to rotate by pulling the string off it, coil by
coil, and so drives the train of wheels. But while the mainspring, when
fully wound, turns the fusee by uncoiling the string from the smallest
part of the fusee, it gets the advantage of the larger radius as its
energy becomes lessened.

The fusee is still used for marine chronometers, for some clocks that
have a mainspring and pendulum, and occasionally for watches. In the
latter it has been rendered unnecessary by the introduction of the
_going-barrel_ by Swiss watchmakers, who formed teeth on the edge of the
mainspring barrel to drive the train of wheels. This kind of drum is
called "going" because it drives the watch during the operation of
winding, which is performed by rotating the drum arbor to which the
inner end of the spring is attached. A ratchet prevents the arbor from
being turned backwards by the spring. The adoption of the going-barrel
has been made satisfactory by the improvements in the various escapement
actions.

THE ESCAPEMENT.

[Illustration: FIG. 202.]

The spring or weight transmits its power through a train of cogs to the
_escapement_, or device for regulating the rate at which the wheels are
to revolve. In clocks a _pendulum_ is generally used as the controlling
agent. Galileo, when a student at Pisa, noticed that certain hanging
lamps in the cathedral there swung on th