Excerpt from Isochronism of Balance-Springs

To regularly educated chronometer-makers, the Author does not pretend to offer anything new, beyond the scientific investigation and explanation of the principles of the isochronal adjustment, which, in a practical point of view, must be familiar to them. Vvatch-makers, however, properly so called, constitute a much more numerous class than that of ch'ronometer-makers; and, although the practical knowledge of the isochronal adjustment, is not less necessary to the former, than to the latter, at least if they aspire to the manufacture of watches of a superior class, yet few of them have attained a full knowledge of the subject, either in principle or practice. To another, and a far larger class, namely, the wearers of watches, the Author ventures to address himself, with a view to impart to them the requisite knowledge for dis criminating between good and indifferent watches.

Of all the adjustments necessary in the parts of a good watch, the most essential to its performance, is unquestionably that of the isochronism of the balance spring; for, if this adjustment be wanting, whatever may be the excellence of the machine in other respects, and however labored its workmanship and other adjust ments, it will assuredly disappoint the expectation of the artist, who will find it incapable of being regulated to preserve the same rate of going, in the various positions in which it is liable to be placed. An example is subjoined, illustrative of the effects of a non-isochronal spring upon an otherwise good watch.

Suppose, for instance, that by comparison with a good clock, the going of a well-made watch is tried during twelve hours, four vertical positions, wherein the friction is much increased, and the arc of vibration of the balance considerably diminished in extent (those positions being with the hours XII, VI, 1x, and III, consecutively upwards, during three hours each), and that it keeps correct time in all those positions but that in the horizontal posi tion, or with the face upwards, with longer arcs of vibration, the watch gains one hundred and twenty seconds in twelve hours, the friction is least in the horizontal position, and the arcs of vibra tion are consequently of the greatest extent.

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