Fig. 119. Change Mechanism behind the Four Year Wheel in Fig. 118
It will be easy to see that when the pin, o, rests on the contour of the disc the upper arm, N, of the bell-crank lever is as high as possible, and out of contact with the pin as it is dotted in the figure, and then the 31 teeth of the month wheel will each leap successively one division by the action of the click, G, as the lever, M, falls backward till the 31st day. But when the pin, o, is in one of the shallow notches of the plate, V, corresponding to the months of 30 days, the upper arm, N, of the bell-crank lever will take a lower position, and the inclination that it will have by the forward movement of the lever, M, will on the 30th bring the pin, i, in contact with the bottom of the notch, just as the lever, M, has accomplished two-thirds of its forward movement, so the last third will be employed to make the wheel 31 advance one tooth, and the hand of the dial by consequence marks the 31st, the quick return of the lever, M, as it falls putting this hand to the 1st by the action of the click, G. If we suppose the pin, o, is placed in the shallowest of the four deep notches, that one for February of leap year, the upper end of the arm, N, will take a position lower still, and on the 29th the pin, i, will be met by the bottom of the notch, just as the lever has made one-third of its forward course, so the other two-thirds of the forward movement will serve to make two teeth of the wheel of 31 jump. Then the hand of the dial, A, Figs. [116] and [118], will indicate 31, and the ordinary quick return of the lever, M, with its detent, G, will put it to the 1st. Lastly, if, as it is represented in the figure, the pin, o, is in one of the three deepest notches, corresponding to the months of February in ordinary years, the pin will be in the bottom of the notch on the 28th just at the moment the lever begins its movement, and three teeth will pass before the return of the lever makes the hand leap from the 31st to the 1st.
The pin, o, easily gets out of the shallow notches, which, as will be seen, are sloped away to facilitate its doing so. To help it out of the deeper notches there is a weighted finger (j) on the arbor of the annual wheel. This finger, having an angular movement much larger than the one of the disc, V, puts the pin, o, out of the notch before the notch has sensibly changed its position.
Phases of the Moon.—The phases of the moon are obtained by a pinion of 10, [Fig. 120], on the arbor, B, which gears with the wheel of 84 teeth, fixed on another of 75, which last gears with a wheel of 113, making one revolution in three lunations. By this means there is an error only of .00008 day per lunation. On the wheel of 113 is fixed a plate on which are three discs colored blue, having between them a distance equal to their diameter, as shown in [Fig. 117], these discs slipping under a circular aperture made in the dial, produce the successive appearance of the phases of the moon.
Equation of Time.—On the arbor of the annual wheel, C, Figs. [116], [118], [120], is fixed a brass cam, Y, on the edge of which leans the pin, s, fixed to a circular rack, R. This rack gears with the central wheel, K, which carries the hand for the equation. That hand faces XII the 15th of April, 14th of June, 1st of September and the 25th of December. At those dates the pin, s, is in the position of the four dots marked on the cam, Y. The shape of the cam, Y, must be such as will lead the hand to indicate the difference between solar and mean time, as given in the table of the Nautical almanac.
To set the calendar first see that the return of the lever, M, be made at the moment of midnight. To adjust the hand of the days of the week, B, look at an almanac and see what day before the actual date there was a full or new moon. If it was new moon on Thursday, it would be necessary, by means of a small button fixed at the back, on the arbor of the hand of the wheel, B, of the week, to make as many returns as requisite to obtain a new moon, this hand pointing to a Thursday; afterward bring back the hand to the actual date, passing the number of divisions corresponding to the days elapsed since the new moon. To adjust the hand of the day of the month, A, see if the pin, o, is in the proper notch. If for the leap year, it is in the month of February in the shallowest of the four deep notches (o); if for the same month of the first year after leap year, then the pin should be, of course, in the notch, I, and so on.
