Little "ways and methods" like the above have long been known to the trade, but for some reason are never mentioned in our text books. A detent spring 2/1000" thick and 80/1000" wide will stand the thrust for any well-constructed marine chronometer in existence, and yet it will not require half a pennyweight to deflect it one-fourth of an inch. It is a good rule to make the length of the detent from the foot F to the center of the locking jewel pipe j equal to the diameter of the escape wheel, and the length of the detent spring h two-sevenths of this distance. The length of the horn k is determined by the graphic plan and can be taken from the plotted plan. The end, however, should approach as near to the discharging jewel as possible and not absolutely touch. The discharging (gold) spring m is attached to the blade i of the detent with a small screw l cut in a No. 18 hole of a Swiss plate. While there should be a slight increase in thickness in the detent blade at w, where the gold spring is attached, still it should be no more than to separate the gold spring m from the detent blade i.
IMPORTANT CONSIDERATIONS.
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.
DECISIONS ARRIVED AT BY EXPERIENCE.
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°). 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" + 2-1/2" (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.
LOCATING THE CENTER OF THE BALANCE STAFF.
Somewhere on this line is located the center of the balance staff, and it is the problem in hand to locate or establish this center. Now, it is known the circles which define the peripheries of the escape wheel and the impulse roller intersect at e e2. We can establish on our circle A where these intersections take place by laying off twelve degrees, one-half of the impulse arc on each side of the line of centers a f on this circle and establishing the points e e2. These points e e2 being located at the intersection of the circles A and B, must be at the respective distances of 5" and 2-1/2" distance from the center of the circles A B; consequently, if we set our dividers at 2-1/2" and place one leg at e and sweep the short arc g2, and repeat this process when one leg of the dividers is set at e2, the intersection of the short arcs g and g2 will locate the center of our balance staff. We have now our two centers established, whose peripheries are in the relation of 2 to 1.
To know, in the chronometer which we are supposed to be constructing, the exact distance apart at which to plant the hole jewels for our two mobiles, i.e., escape wheel and balance staff, we measure carefully on our drawing the distance from a to c (the latter we having just established) and make our statement in the rule of three, as follows: As (10) the diameter of drawn escape wheel is to our real escape wheel so is the measured distance on our drawing to the real distance in the chronometer we are constructing.