ur escape-wheel tooth by a right
angle line tangent to the circle _w_, from the point _b_; which circle
_w_ represents the curve of the outer circumference of the cylinder. We
shape the point of the tooth as shown to give it the proper stability,
and draw the full line _j_ to a curve from the center _A_. We have now
defined the form of the upper face of the tooth. How to delineate the U
arms will be taken up later on, as, in the present case, the necessary
lines would confuse our drawing.

We would here take the opportunity to say that there is a great latitude
taken by makers as regards the extent of angular impulse given to the
cylinder, or, as it is termed, the "actual lift." This latitude governs
to a great extent the angle _A b g_, which we gave as sixty-four degrees
in our drawing. It is well to understand that the use of sixty-four
degrees is based on no hard-and-fast rules, but varies back and forth,
according as a greater or lesser angle of impulse or lift is employed.

In practical workshop usage the impulse angle is probably more easily
estimated by the ratio between the diameter of the cylinder and the
measured (by lineal measure) height of the impulse plane. Or, to be more
explicit, we measure the radial extent from the center _A_ between the
arcs _a k_ on the line _A b_, and use this for comparison with the outer
diameter of the cylinder.

We can readily see that as we increase the height of the heel of the
impulse face of our tooth we must also increase the angle of impulse
imparted to the cylinder. With the advantages of accurate micrometer
calipers now possessed by the horological student it is an easy matter
to get at the angular extent of the real lift of any cylinder. The
advantage of such measuring instruments is also made manifest in
determining when the proper proportion of the cylinder is cut away for
the half shell.

[Illustration: Fig. 130]

In the older methods of watchmaking it was a very common rule to say,
let the height of the incline of the tooth be one-seventh of the outer
diameter of the cylinder, and at the same time the trade was furnished
with no tools except a clumsy douzieme gage; but with micrometer
calipers which read to one-thousandths of an inch such rules can be
definitely carried into effect and not left to guess work. Let us
compare the old method with the new: Suppose we have a new cylinder to
put in; we have the old escape wheel, but the former cylinder is gone.
The