ese go in, the glass will come asunder directly
pressure is applied.

Now, if you press too hard in cutting, another thing takes place.

Imagine a quantity of roofing-slates piled flat one on top of another,
all the piles being of equal height and arranged in two rows, side by
side, so close that the edges of the slates in one row touch the edges
of those in the other row, along a central line.

Wheel a wheelbarrow along that line over the edges of both.

What would happen?

The top layer of slates would all come cocking their outer edges up as
the barrow passed over their inner ones, would they not?

Now, just so, if you press hard on your glass-cutting wheel, it will
press down the edges of the groove, and though there are no layers
_already made_ in the glass, the pressure will _split off_ a thin layer
from the top surface of the glass on each side in flakes as it goes
along (Plate X., D, E).

This is what gives the _noise_ of the cut, c-r-r-r-r-r-; and as the
thing is no use the noise is no use; like a good many other things in
life, the less noise the better work, much cry generally meaning little
wool, as the man found out who shaved the pig.

But the wheel or the diamond is not quite the same as the wheel of the
wheelbarrow, for it has a _wedge-shaped_ edge. Imagine a barrow with
such a wheel; what _then_ would happen to your slates? besides being
cocked up by the wheel, they would also be _pushed out_, surely?

This happens in glass. You must not imagine that glass is a rigid thing;
it is very elastic, and the wedge-like pressure of the wheel pushes it
out just as the keel of a boat pushes the water aside in ripples (Plate
X., D, E).

All these observations seem to me to bear out the theory of the
inventor, and perhaps to some extent to explain it. I am much tempted to
carry them further, and ask the questions, why a penknife as well as a
wheel will not make a cut in glass, but will make a perfectly definite
scratch on it if the glass is placed under water? and why this line so
made will yet not serve for separating the glass? and why a piece of
glass can be cut in two (roughly, to be sure, but still cut in two) with
a pair of scissors under water, a thing otherwise quite impossible?

But I do not think that the knowledge of these questions will help the
reader to do better stained-glass windows, and therefore I will not
pursue them.

(2) The question of sharpening the tool is soon disposed of.