one-fifth of the space between _A_ and _B_, and the ratio of fork to
roller action would be four to one, and ten degrees of fork action would
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.