u can readily see that when you come
to reverse your staff and use the wax chuck, that by starting at the top
of staff your wax has a much larger surface of metal to cling to, and
again the shape of the balance seat is such as to secure the work firmly
in the wax, while if the reverse method is employed, the larger portion
of the balance seat is exposed and the staff is more liable to loosen
from the motion of the lathe and pressure of the graver and polishers.

CHAPTER IV.

By the aid of the pinion calipers and the old staff, the diameter of the
roller seat and the balance and hair-spring collet seats may be readily
taken, but it is perhaps better to gauge the holes, as the old staff may
not have been perfect in this respect. A round broach will answer
admirably for this purpose, and the size may be taken from the broach by
means of the calipers. In fitting our pivots, we can not be too exact;
and as yet no instrument has been placed upon the market for this purpose
which is moderate in price and yet thoroughly reliable. The majority of
watchmakers use what is termed the pivot-gauge, a neat little instrument
which accompanies the Jacot lathe, and which may be obtained from any
material house. This tool, which is shown in Fig. 11, is, however, open
to one objection in the measurement of pivots, and that is that it may be
pressed down at one time with greater force than at another, and
consequently will show a variation in two measurements of the same pivot.
Some of my readers may think that I am over-particular on this point, and
that the difference in measurement on two occasions is too trivial to be
worthy of attention, but I do not think that too much care can be
bestowed upon this part of the work, and neglect in this particular is,
I think, the cause of poor performance in many otherwise good
timepieces. The ordinarily accepted rule among watchmakers is that a
pivot should be made 1/2500 of an inch smaller than the hole in the jewel
to allow for the proper lubrication. I am acquainted with watchmakers,
and men who are termed good workmen, too, who invariably allow 1/2500 of
an inch side shake, no matter whether the pivot is 12/2500 or 16/2500 of
an inch in diameter. Now if 1/2500 of an inch is the proper side shake
for a pivot measuring 12/2500 of an inch in diameter, it is certainly not
sufficient for a pivot which is one-third larger. Of course it is
understood that side shakes do not increase in proporti