ch as one
would operate a bradawl against a hard piece of wood. The surface of
the glass will shortly be ground away, and then the file bradawl will
make much quicker progress than might be expected. Two or three
minutes should suffice to bore a bit of sheet window-glass.

The following points require attention:

(1) Use any quantity of oil.

(2) After getting through the skin reduce the pressure on the file.

(3) Be sure to turn the file backwards and forwards through a complete
revolution at least.

(4) When the hole is nearly through reduce the pressure.

(5) When the hole is through the glass be exceedingly careful not to
force the file through too rapidly, otherwise it will simply act as a
wedge and cause a complete fracture.

(6) In many cases it is better to harden the file in mercury before
commencing operations; both files and glass differ so much in hardness
that this point can only be decided by a trial. If it is found
necessary to harden the file, use either a large blow-pipe and a coke
or charcoal bed, or else a small forge. A small blowpipe, such as is
generally found in laboratories, does more harm than good, either by
burning the end of the file or raising it to an insufficient
temperature.

(7) To sharpen the file, which is often necessary after passing
through the "skin" of the glass, put it in a vice so that the point
just protrudes clear of the jaws. Then, using a bit of waste iron as
an intermediary anvil or punch, knock off the least bit from the
point, so as to expose a fresh natural surface. The same result may
be brought about by the use of a pair of pliers.

If several holes have to be bored, it is convenient to mount the file
in the lathe and use a bit of flat hard wood to press up the glass by
means of the back rest. A drilling machine, if not too heavy, does
very well, and has the advantage of allowing the glass to remain
horizontal so that plenty of oil can be kept in the hole.

Use a very slow speed in either case--much slower than would be used
for drilling wrought iron. It is essential that the lubricant should
flow on to the end of the file very freely, either from a pipette or
from the regular oil-feed. If a little chipping where the file
pierces the back surface is inadmissible, it is better, on the whole,
to finish the bore by hand, using a very taper file. It is not
necessary to use a special file for the lathe, for a well-handled file
can be chucked very con