It is important the spring should be absolutely free and not touch the detent except at its point of attachment at w and to rest against the end of the horn k, and the extreme end of k, where the gold spring rests, should only be what we may term a dull or thick edge. The end of the horn k (shown at y) is best made, for convenience of elegant construction, square—that is, the part y turns at right angles to k and is made thicker than k and at the same time deeper; or, to make a comparison to a clumsy article, y is like the head of a nail, which is all on one side. Some makers bend the horn k to a curve and allow the end of the horn to arrest or stop the gold spring; but as it is important the entire detent should be as light as possible, the square end best answers this purpose. The banking placed at j should arrest the detent as thrown back by the spring h at the “point of percussion.” This point of percussion is a certain point in a moving mass where the greatest effort is produced and would be somewhere near the point x, in a bar G turning on a pivot at z, Fig. 138. It will be evident, on inspection of this figure, if the bar G was turning on the center z it would not give the hardest impact at the end v, as parts of its force would be expended at the center z.
[Illustration: Fig. 138]
Experience has decided that the impulse roller should be about half the diameter of the escape wheel, and experience has also decided that an escape wheel of fifteen teeth has the greatest number of advantages; also, that the balance should make 14,400 vibrations in one hour. We will accept these proportions and conditions as best, from the fact that they are now almost universally adopted by our best chronometer makers. Although it would seem as if these proportions should have established themselves earlier among practical men, we shall in these drawings confine ourselves to the graphic plan, considering it preferable. In the practical detail drawing we advise the employment of the scale given, i.e., delineating an escape wheel 10” in diameter. The drawings which accompany the description are one-fourth of this size, for the sake of convenience in copying.
With an escape wheel of fifteen teeth the impulse arc is exactly twenty-four degrees, and of course the periphery of the impulse roller must intersect the periphery of the escape wheel for this arc (24 deg.). The circles A B, Fig. 139, represent the peripheries of these two mobiles, and the problem in hand is to locate and define the position of the two centers a c. These, of course, are not separated, the sum of the two radii, i.e., 5” + 21/2” (in the large drawing), as these circles intersect, as shown at d. Arithmetically considered, the problem is quite difficult, but graphically, simple enough. After we have swept the circle A with a radius of 5”, we draw the radial line a f, said line extending beyond the circle A.