uld
give forty degrees of angular motion to the roller--and such escapements
have been constructed.

WHY THIRTY DEGREES OF ROLLER ACTION IS ABOUT RIGHT.

Now we have two sound reasons why we should not extend the arc of
vibration of the balance: (_a_) If there is an advantage to be derived
from a detached escapement, it would surely be policy to have the arc of
contact, that is, for the jewel pin to engage the fork, as short an arc
as is compatible with a sound action. (_b_) It will be evident to any
thinking mechanic that the acting force of a fork which would carry the
jewel pin against the force exerted by the balance spring through an arc
of fifteen degrees, or half of an arc of thirty degrees, would fail to
do so through an arc of twenty degrees, which is the condition imposed
when we adopt forty degrees of roller action.

For the present we will accept thirty degrees of roller action as the
standard. Before we proceed to delineate our fork and roller we will
devote a brief consideration to the size and shape of a jewel pin to
perform well. In this matter there has been a broad field gone over,
both theoretically and in practical construction. Wide jewel pins, round
jewel pins, oval jewel pins have been employed, but practical
construction has now pretty well settled on a round jewel pin with about
two-fifths cut away. And as regards size, if we adopt the linear extent
of four degrees of fork or twelve degrees of roller action, we will find
it about right.

HOW TO SET A FORK AND ROLLER ACTION RIGHT.

As previously stated, frequently the true place to begin to set a lever
escapement right is with the roller and fork. But to do this properly we
should know when such fork and roller action is right and safe in all
respects. We will see on analysis of the actions involved that there are
three important actions in the fork and roller functions: (_a_) The fork
imparting perfect impulse through the jewel pin to the balance. (_b_)
Proper unlocking action. (_c_) Safety action. The last function is in
most instances sadly neglected and, we regret to add, by a large
majority of even practical workmen it is very imperfectly understood. In
most American watches we have ample opportunity afforded to inspect the
pallet action, but the fork and roller action is placed so that rigid
inspection is next to impossible.

The Vacheron concern of Swiss manufacturers were acute enough to see the
importance of such inspection, a