he action of the tooth instead of commencing on
the outer angle commences on the center of the edge of the entrance lip
and also ends its action on the center of the entrance lip. To give
angular extent enough to the shell of the cylinder to allow for rounding
and also to afford a secure rest for the tooth inside the cylinder, we
add six degrees to the angular extent of the entrance lip of the
cylinder shell, as indicated on the arc _o'_, Fig. 131, three of these
degrees being absorbed for rounding and three to insure a dead rest for
the tooth when it enters the cylinder.

WHY THE ANGULAR EXTENT IS INCREASED.

Without rounding the exit lip the action of the tooth on its exit would
be entirely on the inner angle of the shell. To obviate this it is the
usual practice to increase the angular extent of the cylinder ten
degrees, as shown on the arc _o'_ between the lines _f_ and _p_, Fig.
131. Why we should allow ten degrees on the exit lip and but six degrees
on the entrance lip will be understood by observing Fig. 130, where the
radial lines _s_ and _r_ show the extent of angular motion of the
cylinder, which would be lost if the tooth commenced to act on the inner
angle and ended on the outer angle of the exit lip. This arc is a little
over six degrees, and if we add a trifle over three degrees for rounding
we would account for the ten degrees between the lines _f_ and _p_, Fig.
131. It will now be seen that the angular extent is 196 degrees. If we
draw the line _w_ we can see in what proportion the measurement should
be made between the outer diameter of the cylinder and the measure of
the half shell. It will be seen on measurement that the distance between
the center _e_ and the line _w_ is about one-fifteenth part of the outer
diameter of the cylinder and consequently with a cylinder which measures
45/1000 of an inch in diameter, now the half shell should measure half
of the entire diameter of the cylinder plus one-fifteenth part of such
diameter, or 251/2 thousandths of an inch.

After these proportions are understood and the drawing made, the eye
will get accustomed to judging pretty near what is required; but much
the safer plan is to measure, where we have the proper tools for doing
so. Most workmen have an idea that the depth or distance at which the
cylinder is set from the escape wheel is a matter of adjustment; while
this is true to a certain extent, still there is really only one
position for the center of