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.
THE NECESSITY FOR GOOD INSTRUMENTS.
It is to be hoped the reader who intends to profit by this treatise has
fitted up such a pair of dividers as those we have described, because it
is only with accurate instruments he can hope to produce drawings on
which any reliance can be placed.
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