on according as
the pivot increases in size, for if they did a six-inch shaft would
require at this rate a side shake of 1/2 inch, or 1/4 inch on each side,
which would be ridiculously out of all proportion, as the 1/64 of an inch
would be ample under any circumstances. Neither can we arrive at the
proper end shake for a pivot by reducing in proportion from the end shake
allowed on a six-inch shaft, because if we followed out the same course
of reasoning we would arrive at a point where a pivot measuring 12/2500
of an inch would require an end shake so infinitely small that it would
require six figures to express the denominator of the fraction, and the
most minute measuring instrument yet invented would be incapable of
recording the measurement. We must leave sufficient side shake, however,
on the smallest pivot and jewel for the globules of the oil to move
freely, and experiments have shown conclusively that 1/2500 of an inch or
1/5000 on each side of the pivot, is as little space as it is desirable
to leave for that purpose, as the globules of the best chronometer oil
will refuse to enter spaces that are very much more minute. But to return
to our pivot gauge.

[Illustration: _Fig. 11._]

[Illustration: _Fig. 12._]

Each division on the gauge represents 1/2500 of an inch, which is all
that we require. The diameter that the pivot should be, can be
ascertained by inserting a round pivot broach into the jewel and taking
the measurement with the pivot gauge, and then making the necessary
deduction for side shake. Slip the jewel on the broach as far as it will
go, as shown in Fig. 12, and then with the pivot gauge, take the size of
the broach, as close up to the jewel as you can measure, and the taper of
the broach will be about right for the side shake of the pivot. If,
however, you prefer to make the measurement still more accurate, you can
do so by dipping the broach into rouge before slipping on the jewel and
then remove the jewel and the place which is occupied on the broach can
be plainly discerned and the exact measurement taken and an allowance of
1/2500 of an inch made for the side shake. Another method, and one which
is particularly applicable to Swiss watches, where the jewel is burnished
into the cock or plate, is to first slip on to the broach a small flat
piece of cork and as the broach enters the jewel the cork is forced
farther on to the broach, and when the jewel is removed it marks the
place on the br