But before we further consider the action of the tooth _D_ in its action
as it passes the exit lip of the cylinder we must finish with the action
of the tooth on the entrance lip. A very little thought and study of
Fig. 130 will convince us that the incline of the tooth as it enters the
cylinder will commence at _t_, Fig. 130, but at the close of the action
the tooth parts from the lip on the inner angle. Now it is evident that
it would require greater force to propel the cylinder by its inner angle
than by the outer one. To compensate for this we round the edge of the
entrance lip so that the 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.
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