glass jar containing mercury held in a stirrup; the mercury
being of such a height that, as the pendulum rod lengthened with a rise
of temperature, the mercury expanded _upwards_ sufficiently to keep the
distance between the point of suspension and the centre of gravity of
the bob always the same. With a fall of temperature the rod shortened,
while the mercury sank in the jar. This device has not been improved
upon, and is still used in observatories and other places where
timekeepers of extreme precision are required. The milled nut S in Fig.
203 is fitted at the end of the pendulum rod to permit the exact
adjustment of the pendulum's length.

For watches, chronometers, and small clocks

THE SPRING BALANCE

takes the place of the pendulum. We still have an escape-wheel with
teeth of a suitable shape to give impulses to the controlling agent.
There are two forms of spring escapement, but as both employ a
hairspring and balance-wheel we will glance at these before going
further.

[Illustration: FIG. 203.]

The _hairspring_ is made of very fine steel ribbon, tempered to extreme
elasticity, and shaped to a spiral. The inner end is attached to the
arbor of the _balance-wheel_, the outer end to a stud projecting from
the plate of the watch. When the balance-wheel, impelled by the
escapement, rotates, it winds up the spring. The energy thus stored
helps the wheel to revolve the other way during the locking of a tooth
of the escape-wheel. The time occupied by the winding and the unwinding
depends upon the length of the spring. The strength of the impulse makes
no difference. A strong impulse causes the spring to coil itself up more
than a weak impulse would; but inasmuch as more energy is stored the
process of unwinding is hastened. To put the matter very simply--a
strong impulse moves the balance-wheel further, but rotates it quickly;
a weak impulse moves it a shorter distance, but rotates it slowly. In
fact, the principle of the pendulum is also that of the hairspring; and
the duration of a vibration depends on the length of the rod in the one
case, and of the spring in the other.

Motion is transmitted to the balance by one of two methods. Either (1)
directly, by a cylinder escapement; or (2) indirectly, through a lever.

[Illustration: FIG. 204.--"Cylinder" watch escapement.]

THE CYLINDER ESCAPEMENT

is seen in Fig. 204. The escape-wheel has sharp teeth set on stalks.
(One tooth is removed to show the stal