*Project Gutenberg*. Public domain.

It is important the spring should be absolutely free
and not touch the detent except at its point of attachment
at *w* and to rest against the end of the horn
*k*, and the extreme end of *k*, where the
gold spring rests, should only be what we may term
a dull or thick edge. The end of the horn *k*
(shown at *y*) is best made, for convenience of
elegant construction, square—that is, the
part *y* turns at right angles to *k* and
is made thicker than *k* and at the same time
deeper; or, to make a comparison to a clumsy article,
*y* is like the head of a nail, which is all
on one side. Some makers bend the horn *k*
to a curve and allow the end of the horn to arrest
or stop the gold spring; but as it is important the
entire detent should be as light as possible, the
square end best answers this purpose. The banking
placed at *j* should arrest the detent as thrown
back by the spring *h* at the “point of
percussion.” This point of percussion is
a certain point in a moving mass where the greatest
effort is produced and would be somewhere near the
point *x*, in a bar *G* turning on a pivot
at *z*, Fig. 138. It will be evident, on
inspection of this figure, if the bar *G* was
turning on the center *z* it would not give the
hardest impact at the end *v*, as parts of its
force would be expended at the center *z*.

[Illustration: Fig. 138]

DECISIONS ARRIVED AT BY EXPERIENCE.

Experience has decided that the impulse roller should
be about half the diameter of the escape wheel, and
experience has also decided that an escape wheel of
fifteen teeth has the greatest number of advantages;
also, that the balance should make 14,400 vibrations
in one hour. We will accept these proportions
and conditions as best, from the fact that they are
now almost universally adopted by our best chronometer
makers. Although it would seem as if these proportions
should have established themselves earlier among practical
men, we shall in these drawings confine ourselves
to the graphic plan, considering it preferable.
In the practical detail drawing we advise the employment
of the scale given, *i.e*., delineating an escape
wheel 10” in diameter. The drawings which
accompany the description are one-fourth of this size,
for the sake of convenience in copying.

With an escape wheel of fifteen teeth the impulse
arc is exactly twenty-four degrees, and of course
the periphery of the impulse roller must intersect
the periphery of the escape wheel for this arc (24
deg.). The circles *A B*, Fig. 139, represent
the peripheries of these two mobiles, and the problem
in hand is to locate and define the position of the
two centers *a c*. These, of course, are
not separated, the sum of the two radii, *i.e*.,
5” + 21/2” (in the large drawing), as these
circles intersect, as shown at *d*. Arithmetically
considered, the problem is quite difficult, but graphically,
simple enough. After we have swept the circle
*A* with a radius of 5”, we draw the radial
line *a f*, said line extending beyond the circle
*A*.