index
_D_. Suppose, for instance, we place the jaws _B'_ inside the pallet
staff, we then place the index arc so the hand reads to the arc
indicated by the dotted arc _m_, Fig. 78, and if set outside of the
pallet staff, read by the arc _o_.

[Illustration: Fig. 78]

HOW A BALANCE CONTROLS THE TIMEKEEPING OF A WATCH.

We think a majority of the fine lever escapements made abroad in this
day have what is termed double-roller safety action. The chief gains to
be derived from this form of safety action are: (1) Reducing the arc of
fork and roller action; (2) reducing the friction of the guard point to
a minimum. While it is entirely practicable to use a table roller for
holding the jewel pin with a double-roller action, still a departure
from that form is desirable, both for looks and because as much of the
aggregate weight of a balance should be kept as far from the axis of
rotation as possible.

We might as well consider here as elsewhere, the relation the balance
bears to the train as a controlling power. Strictly speaking, _the
balance and hairspring are the time measurers_, the train serving only
two purposes: (_a_) To keep the balance in motion; (_b_) to classify and
record the number of vibrations of the balance. Hence, it is of
paramount importance that the vibrations of the balance should be as
untrammeled as possible; this is why we urge reducing the arc of
connection between the balance and fork to one as brief as is consistent
with sound results. With a double-roller safety action we can easily
reduce the fork action to eight degrees and the roller action to
twenty-four degrees.

Inasmuch as satisfactory results in adjustment depend very much on the
perfection of construction, we shall now dwell to some extent on the
necessity of the several parts being made on correct principles. For
instance, by reducing the arc of engagement between the fork and roller,
we lessen the duration of any disturbing influence of escapement action.

To resume the explanation of why it is desirable to make the staff and
all parts near the axis of the balance as light as possible, we would
say it is the moving portion of the balance which controls the
regularity of the intervals of vibration. To illustrate, suppose we have
a balance only 3/8" in diameter, but of the same weight as one in an
ordinary eighteen-size movement. We can readily see that such a balance
would require but a very light hairspring to cause it to give t