he usual
18,000 vibrations to the hour. We can also understand, after a little
thought, that such a balance would exert as much breaking force on its
pivots as a balance of the same weight, but 3/4" in diameter acting
against a very much stronger hairspring. There is another factor in the
balance problem which deserves our attention, which factor is
atmospheric resistance. This increases rapidly in proportion to the
velocity.

HOW BAROMETRIC PRESSURE AFFECTS A WATCH.

The most careful investigators in horological mechanics have decided
that a balance much above 75/100" in diameter, making 18,000 vibrations
per hour, is not desirable, because of the varying atmospheric
disturbances as indicated by barometric pressure. A balance with all of
its weight as near the periphery as is consistent with strength, is what
is to be desired for best results. It is the moving matter composing the
balance, pitted against the elastic force of the hairspring, which we
have to depend upon for the regularity of the timekeeping of a watch,
and if we can take two grains' weight of matter from our roller table
and place them in the rim or screws of the balance, so as to act to
better advantage against the hairspring, we have disposed of these two
grains so as to increase the efficiency of the controlling power and not
increase the stress on the pivots.

[Illustration: Fig. 79]

We have deduced from the facts set forth, two axioms: (_a_) That we
should keep the weight of our balance as much in the periphery as
possible, consistent with due strength; (_b_) avoid excessive size from
the disturbing effect of the air. We show at _A_, Fig. 79, the shape of
the piece which carries the jewel pin. As shown, it consists of three
parts: (1) The socket _A_, which receives the jewel pin _a_; (2) the
part _A''_ and hole _b_, which goes on the balance staff; (3) the
counterpoise _A'''_, which makes up for the weight of the jewel socket
_A_, neck _A'_ and jewel pin. This counterpoise also makes up for the
passing hollow _C_ in the guard roller _B_, Fig. 80. As the piece _A_
is always in the same relation to the roller _B_, the poise of the
balance must always remain the same, no matter how the roller action is
placed on the staff. We once saw a double roller of nearly the shape
shown at Fig. 79, which had a small gold screw placed at _d_, evidently
for the purpose of poising the double rollers; but, to our thinking, it
was a sort of hairsplitting hard