roller it would not
only have twice, but four times more effect than on the smaller one. We
will notice that the smaller the impulse angle the larger the roller,
because the ruby pin is necessarily placed farther from the center. The
position of the ruby pin should, therefore, govern the size of the
roller, which should be as small as possible. There should only be
enough metal left between the circumference of the roller and the face
of the jewel to allow for a crescent or passing hollow of sufficient
depth and an efficient setting for the jewel. For this reason, as well
as securing the correct impulse radius and therefore angle, when
replacing the ruby pin, and having it set securely and mechanically in
the roller, it is necessary that the pin and the hole in the roller be
of the same form, and a good fit. Fig. 23 illustrates the difference in
size of rollers. In the smaller one the conditions imposed are
satisfied, while in the larger one they are not. In the single roller
the safety action is at the mercy of the impulse and pallet angles. We
have noticed that in order to favor the impulse we require a large
roller, and for the safety action a small one, therefore escapements
made on fine principles are supplied with two rollers, one for each
action.

It may be well to say that in our opinion a proportion between the fork
and impulse angles in 10deg. pallets of 3 or 3 1/2 to 1, _depending_
upon the size of the escapement, is the lowest which should be made in
single roller. We have seen them in proportions of 2 to 1 in single
roller--a scientific principle foolishly applied--resulting in an action
entirely unsatisfactory.

When the guard point is pressed against the roller the escape tooth
must still rest on the locking face of the pallet; if the total lock
is 2deg., by allowing 1 1/4deg. freedom for the guard point between
the bank and the roller the escapement will still be locked 3/4deg.
How much this shake actually amounts to depends upon the guard
radius. Suppose this to be 4 mm., then the freedom would equal
4 x 2 x 3.1416 / 360 x 1.25 = .0873 mm.

[Illustration: Fig. 24.]

[Illustration: Fig. 25.]

_The Crescent_ in the roller must be large and deep enough so it will be
impossible for the guard point to touch in or on the corners of it; at
the same time it must not be too large, as it would necessitate a longer
horn on the fork than is necessary.

Fig. 24 shows the slot _n_ of the fork standing at the