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.