portions of our delineations more
distinctly. If we give the subject a moment's consideration we will see
that the length of the prong _E_ of the lever fork is limited to such a
length as will allow the jewel pin _D_ to pass it.

HOW TO DELINEATE THE PRONGS OF A LEVER FORK.

[Illustration: Fig. 58]

[Illustration: Fig. 59]

To delineate this length, from _B_ as a center we sweep the short arc
_f_ so it passes through the outer angle _n_, Fig. 59, of the jewel pin.
This arc, carried across the jewel pin _D_, limits the length of the
opposite prong of the fork. The outer face of the prong of the fork can
be drawn as a line tangent to a circle drawn from _A_ as a center
through the angle _n_ of the jewel pin. Such a circle or arc is shown at
_o_, Figs. 58 and 59. There has been a good deal said as to whether the
outer edge of the prong of a fork should be straight or curved.

To the writer's mind, a straight-faced prong, like from _s_ to _m_, is
what is required for a fork with a single roller, while a fork with a
curved prong will be best adapted for a double roller. This subject will
be taken up again when we consider double-roller action. The extent or
length of the outer face of the prong is also an open subject, but as
there is but one factor of the problem of lever escapement construction
depending on it, when we name this and see this requirement satisfied we
have made an end of this question. The function performed by the outer
face of the prong of a fork is to prevent the engaged pallet from
unlocking while the guard pin is opposite to the passing hollow.

The inner angle _s_ of the horn of the fork must be so shaped and
located that the jewel pin will just clear it as it passes out of the
fork, or when it passes into the fork in the act of unlocking the
escapement. In escapements with solid bankings a trifle is allowed, that
is, the fork is made enough shorter than the absolute theoretical length
to allow for safety in this respect.

THE PROPER LENGTH OF A LEVER.

We will now see how long a lever must be to perform its functions
perfectly. Now let us determine at what point on the inner face of the
prong _E'_ the jewel pin parts from the fork, or engages on its return.
To do this we draw a line from the center _r_ (Fig. 59) of the jewel
pin, so as to meet the line _e_ at right angles, and the point _t_ so
established on the line _e_ is where contact will take place between the
jewel pin and fork.