the cylinder, and that is so that the center
of the pivot hole coincides exactly with the center of the chord to the
curve of the impulse face of the tooth or the point _e_, Fig. 130. Any
adjustment or moving back and forth of the chariot to change the depth
could only be demanded where there was some fault existing in the
cylinder or where it had been moved out of its proper place by some
genius as an experiment in cylinder depths. It will be evident on
observing the drawing at Fig. 131 that when the cylinder is performing
an arc of vibration, as soon as the entrance lip has passed the point
indicated by the radial line _e x_ the point of the escape-wheel tooth
will commence to act on the cylinder lip and continue to do so through
an arc of forty degrees, or from the lines _x_ to _l_.

MAKING A WORKING MODEL.

To practically study the action of the cylinder escapement it is well to
make a working model. It is not necessary that such a model should
contain an entire escape wheel; all that is really required is two teeth
cut out of brass of the proper forms and proportions and attached to the
end of an arm 4-7/8" long with studs riveted to the U arms to support
the teeth. This U arm is attached to the long arm we have just
mentioned. A flat ring of heavy sheet brass is shaped to represent a
short transverse section of a cylinder. This segment is mounted on a
yoke which turns on pivots. In making such a model we can employ all the
proportions and exact forms of the larger drawings made on a ten-inch
radius. Such a model becomes of great service in learning the
importance of properly shaping the lips of the cylinder. And right here
we beg to call attention to the fact that in the ordinary repair shop
the proper shape of cylinder lips is entirely neglected.

PROPER SHAPE OF CYLINDER LIPS.

The workman buys a cylinder and whether the proper amount is cut away
from the half shell, or the lips, the correct form is entirely ignored,
and still careful attention to the form of the cylinder lips adds full
ten per cent. to the efficiency of the motive force as applied to the
cylinder. In making study drawings of the cylinder escapement it is not
necessary to employ paper so large that we can establish upon it the
center of the arc which represents the periphery of our escape wheel, as
we have at our disposal two plans by which this can be obviated. First,
placing a bit of bristol board on our drawing-board in which we can s