To resume the explanation of why it is desirable to make the staff and all parts near the axis of the balance as light as possible, we would say it is the moving portion of the balance which controls the regularity of the intervals of vibration.  To illustrate, suppose we have a balance only 3/8” in diameter, but of the same weight as one in an ordinary eighteen-size movement.  We can readily see that such a balance would require but a very light hairspring to cause it to give the 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 hardly worth the extra trouble.  Rollers for very fine watches should be poised on the staff before the balance is placed upon it.