n this way. Of course
the directions for using a cement chuck on the upper part of a staff are
equally applicable to the lower. Before going further I think it
advisable to consider the requirements of a pivot, but will reserve this
for another chapter.

CHAPTER II.

The chief requirements of a pivot are that it shall be round and well
polished. Avoid the burnish file at all hazards; it will not leave the
pivot round, for the pressure is unequal at various points in the
revolution. A pivot that was not perfectly round might act fairly well in
a jewel hole that was round, but unfortunately the greater proportion of
jewel holes are not as they should be, and we must therefore take every
precaution to guard against untrue pivots. Let us examine just what the
effect will be if an imperfect pivot is fitted into an unround hole
jewel, and to demonstrate its action more clearly let us exaggerate the
defects. Suppose we pick a perfectly round jewel and insert into the
opening a three-cornered piece of steel wire, in shape somewhat
resembling the taper of a triangular file. We find that this triangular
piece of steel will turn in the jewel with the same ease that the most
perfect cylindrical pivot will. Now suppose we change the jewel for one
that is out of round and repeat the experiment. We now find that the
triangular steel soon finds the hollow spots in the jewel hole and comes
to a stand-still as it is inserted in the hole. The action of a pivot
that is not true, when in contact with a jewel whose hole is out of
round, is very similar, though in a less marked degree. If the pivot
inclines toward the elliptical and the jewel hole has a like failing,
which is often the case, it is very evident that this want of truth in
both the pivot and hole is very detrimental to the good going of a watch.

[Illustration: _Fig. 4._]

[Illustration: _Fig. 5._]

There are two kinds of pivots, known respectively as straight and conical
pivots, but for the balance staff there is but one kind and that is the
conical, which is illustrated in Fig. 4. The conical pivot has at least
one advantage over the straight one, _i. e._, it can be made much smaller
than a straight pivot, as it is much stronger in proportion, owing to its
shape. All pivots have a tendency to draw the oil away from the jewels,
and particularly the conically formed variety, which develops a strong
capillary attraction. To prevent this capillary attraction of the oi