[Illustration: Fig. 21.]

[Illustration: Fig. 23.]

[Illustration: Fig. 22.]

English watches are met with having a cylindrical or round ruby pin.
Such a pin should never be put into a watch. The law of the
parallelogram of forces is completely ignored by using such a pin; the
friction during the unlocking and impulse actions is too severe, as it
is, without the addition of so unmechanical an arrangement. Fig. 21
illustrates the action of a round ruby pin; _ii_ is the path of the ruby
pin; _cc_ that of the acting length of the fork. It is shown at the
moment the impulse is transmitted. It will be seen that the impact takes
place _below_ the center of the ruby pin, whereas it should take place
at the center, as the motion of the fork is _upwards_ and that of the
ruby pin _downwards_ until the line of the centers has been reached.
The same rule applies to the flat-faced pin and it is important that the
right quantity be ground off. We find that 3/7 is approximately the
amount which should be ground away. Fig. 22 illustrates the fork
standing against the bank. The ruby pin touches the side of the slot but
has not as yet begun to act; _ri_ is the real impulse circle for which
we allow 1 1/4deg. of freedom at the acting edge of the fork; the face
of the ruby pin is therefore on this line. The next thing to do is to
find the center of the pin. From the side _n_ of the slot we construct
the right angle _o n t_; from _n_, we transmit 1/2 the width of the pin,
and plant the center _x_ on the line _n t_. We can have the center of
the pin slightly below this line, but in no case above it; but if we put
it below, the pin will be thinner and therefore more easily broken.

[Illustration: Fig. 14.]

_The Safety Action._ Although this action is separate from the impulse
and unlocking actions, it is still very closely connected with them,
much more so in the single than in the double roller escapement. If we
were to place the ruby pin at _X_, Fig. 14, we could have a much
smaller roller than by placing it at _P_. With the small roller the
safety action is more secure, as the intersection at _m_ is greater than
at _k_. It is not as liable to "butt" and the friction is less when the
guard point is thrown against the small roller. Suppose we take two
rollers, one with a diameter of 2.5 mm., the other just twice this
amount, of 5 mm. By having the guard radius and pressure the same in
each case, if the guard point touched the larger