intersection of the radial _a c_ with the peripheral line _a_. It will
be noticed that the inner angle of the entrance pallet _A_ seems to
extend inward, beyond the radial line _a j_, that is, toward the pallet
center _g_, and gives the appearance of being much thicker than the exit
pallet _A'_; but we will see on examination that the extreme angle _x_
of the entrance pallet must move on the arc _i_ and, consequently, cross
the peripheral line _a_ at the point _u_. If we measure the impulse
faces of the two pallets _A A'_, we will find them nearly alike in
linear extent.

Mr. Grossmann, in delineating his exit pallet, brings the extreme angle
(shown at _4_) down to the periphery of the escape, as shown in the
drawing, where it extends beyond the intersection of the line _g f_ with
the radial line _a 3_. The correct form for the entrance pallet should
be to the dotted line _z x y_.

[Illustration: Fig. 17]

We have spoken of engaging and disengaging frictions; we do not know how
we can better explain this term than by illustrating the idea with a
grindstone. Suppose two men are grinding on the same stone; each has,
say, a cold chisel to grind, as shown at Fig. 17, where _G_ represents
the grindstone and _N N'_ the cold chisels. The grindstone is supposed
to be revolving in the direction of the arrow. The chisels _N_ and _N'_
are both being ground, but the chisel _N'_ is being cut much the more
rapidly, as each particle of grit of the stone as it catches on the
steel causes the chisel to hug the stone and bite in deeper and deeper;
while the chisel shown at _N_ is thrust away by the action of the grit.
Now, friction of any kind is only a sort of grinding operation, and the
same principles hold good.