ts a single roller. It will
be noticed that the corner of the crescent has passed the guard pin _u_
by a considerable angle, and although this is so, in case of an accident
the _acting edge_ of the fork would come in contact with the ruby pin;
this proves that a well made single roller escapement really requires
but little horn, only enough to ensure the safe entry of the ruby pin in
case the guard point at that moment be thrown against the roller. We
will now examine the question from the standpoint of the double roller;
S2, Fig. 25, is the safety roller; the corner of the crescent has safely
passed the dart _h_; the centers of the ruby pin _o_ and of the crescent
being on the line A' A2, we plant the compass on the pallet center and
the center of the face of the ruby pin and draw _k k_, which will be the
path described by the horn. The end of the horn is therefore planted
upon it from 1 1/2deg. to 1 3/4deg. from the ruby pin; this freedom at
the end of the horn is therefore from 1/4deg. to 1/2deg. more than we
allow for the guard point; it depends upon the size of the escapement
and locking angles which we would choose. It must in any case be less
than the lock on the pallets, so that the fork will be drawn back
against the bank in case the horn be thrown against the ruby pin.

When treating on the width of the ruby pin, we mentioned the Savage pin
roller escapement, which we illustrate in Figs. 26 and 27. This
ingenious arrangement was designed with the view of combining the
advantages of both wide and narrow pins and at the same time without any
of their disadvantages.

In Fig. 26 we show the unlocking pins _u_ beginning their action on the
line of centers--the best possible point--in unlocking the escapement.
These pins were made of gold in all which we examined, although it is
recorded that wide ruby pins and ruby rollers have been used in this
escapement, which would be preferable.

The functions of the two pins in the roller are simply to unlock the
escapement; the impulse is not transmitted to them as is the case in the
ordinary fork and roller action. In this action the guard pin _i_ also
acts as the impulse pin. We will notice that the passing hollow in this
roller is a rectangular slot the same as in the ordinary fork. When the
escapement is being unlocked the guard pin _i_ enters the hollow and
when the escape tooth comes into contact with the lifting plane of the
pallet the pin _i_, Fig. 27, transmits th