in drilling, so that the shellac or wax will
not adhere to it. This little hole acts as an outlet for the air in the
tube; and as the hot shellac enters at the end of the tube the air is
expelled through this vent. It also helps to hold the cement firmly in
place. Now try your staff in the tube again, and be sure that it is quite
free, and that you will be able to work on the portions of it above and
below the hub, according as one end or the other is inserted.

You are now ready to insert your staff and proceed with your work. Hold
your shellac in the flame of your lamp a moment until it is quite liquid,
and then smear both the inside and outside of the tube with it. Heat the
shell or tube gently by means of the lamp, keeping the lathe revolving
slowly all the while, and taking the staff in your tweezers proceed to
insert it carefully into the tube. Press firmly back, making sure that it
has reached the bottom of the V-shaped center. Pack the cement well in
around the staff, and while centering remove the lamp and allow the
whole to cool, keeping the whole revolving until quite cool. Now remove
the superfluous cement by means of the graver, and heating the tube again
slightly, proceed to center exactly by means of a pointed peg-wood,
resting on your T rest to steady it. Turn slowly in the lathe and examine
with glass to see that it is quite true. Your completed instrument will
resemble Fig. 23.

[Illustration: _Fig. 23._]

The advantage of the device is that your center is always ready, and all
you have to do is to insert your chuck in the lathe, warm it, and you are
ready to insert your staff and proceed to work. As I said in the first
place, it is well to employ a taper chuck exclusively for this work, and
not attempt to use it for any other, for if you try to remove your taper
and replace it again, you will surely find that your work is out of
center, and you will be compelled to remove the brass shell and find a
new center each time you use it. You can avoid all this trouble, however,
by purchasing an extra chuck and devoting it exclusively to wax work. Of
course, the brass shell can be removed and placed in position again
without in any way affecting the truth of the center, and any number,
shape and size of shells can be made to fit the one taper, and these
shells will be found very useful for holding a variety of work, aside
from balance staffs.

CHAPTER VII.

The two popular methods of holding a b