Page:Passages from the Life of a Philosopher.djvu/394

378 Turn the glass over and let the slight cavity thus formed rest upon the fixed centre-punch. Repeat the light blows upon this side of the glass, and after turning it two or three times, a very small hole will be made through the glass. It not unfrequently happens that a small crack occurs in the glass; but with a little skill this can be cut out with the pane of the hammer.

The next process is to enlarge the hole and cut it into the required shape with the pane of the hammer. This is accomplished by supporting the glass upon the point of the fixed centre-punch, very close to the edge required to be cut. A light blow must then be struck with the pane of the hammer upon the edge to be broken. This must be repeated until the required shape is obtained.

The principles on which it depends are, that glass is a material breaking in every direction with a conchoidal fracture, and that the vibrations which would have caused cracking or fracture are checked by the support of the fixed centre-punch in close contiguity with the part to be broken off.

When by hastily performing this operation I have caused the glass to crack, I have frequently, by using more care, cut an opening all round the cracked part, and so let it drop out without spreading.

This process is rendered still more valuable by the use of the diamond. I usually carried in my travels a diamond mounted on a small circle of wood, so that I could easily cut out circles of glass with small holes in the centre. The description of this process is sufficient to explain it to an experienced workman; but if the reader should wish to employ it, his readiest plan would be to ask such a person to show him how to do it.

The above technical description will doubtless be rather