To set the calendar, raise the pawl, N, and revolve the year cylinder until M and K are at their narrowest width; that is, a leap year. Then give the year cylinder as many additional turns as there are years since the last leap year, stopping on the current month of the current year. For instance, if it is two years and four months since the 29th of February last occurred, give the cylinder 2 and ⁴⁄₁₂ turns which should bring you to the current month, raise the shifter, Q, and set the hand to the current day. Then raise the pawl, S, and set the week cylinder to the current day. Place the hour hand on the movement so that the cam will drop E at midnight.
[Fig. 116] shows the dial of Brocot’s calendar work, which, with or without the equation of time and the lunations, is to be met with in many grandfather, hall and astronomical clocks. We will assume that all of these features are present, in order to completely cover the subject. It consists of two circular plates of which the front plate is the dial and the rear plate carries the movement, arranged on both sides of it. All centers are therefore concentric and we have marked them all with the same letters for better identification in the various views as the inner plate is turned about to show the reverse side, thus reversing the position of right to left in one view of the inner plate.
[Fig. 117] shows the wheel for the phases of the moon, which is mounted on the outside of the inner plate immediately behind the opening in the dial. The dark circles have the same color as the sky of the dial and the rest is gilt, white or cream color to show the moon as in [Fig. 116]. The position of this plate is also shown in [Fig. 120]. By the dotted circles, about the center D.
Fig. 116. Dial of Brocot’s Calendar.
The inner side containing the mechanism for indicating the days of the week and the days of the month is shown in [Fig. 118]. The calendar is actuated by means of a pin, C, fixed to a wheel of the movement which turns once in twenty-four hours in the manner previously described with [Fig. 113]. Two clicks, G and H, are pivoted to the lever, M. G, by means of its weighted end, [see Fig. 119], is kept in contact with a ratchet wheel of 31 teeth, and H with a ratchet wheel of 7 teeth. As a part of these clicks and wheels is concealed in [Fig. 118], they are shown separately in [Fig. 119].
When the lever, M, is moved to the left as far as it will go by the pin, e, the clicks, G and H, slip under the teeth; their beaks pass on to the following tooth; when e has moved out of contact the lever, M, falls quickly by its own weight, and makes each click leap a tooth of the respective wheels, B of 7 and A of 31 teeth. The arbors of these wheels pass through the dial ([Fig. 116]), and have each an index which, at every leap of its own wheel, indicates on its special dial the day of the week and the day of the month. A roll, or click, kept in position by a sufficient spring, keeps each wheel in its place during the interval of time which separates two consecutive leaps.
This motion clearly provides for the indication of the day of the week, and would be also sufficient for the days of the month if the index were shifted by hand at the end of the short months.
To secure the proper registration of the months of 30 days, for February of 28 during three years, and of 29 in leap year, we have the following provision: The arbor, A, of the day of the month wheel goes through the circular plate, and on the other side is fixed ([see Fig. 120]) a pinion of 10 leaves. This pinion, by means of an intermediate wheel, I, works another wheel (centered at C) of 120 teeth, and consequently turning once in a year. The arbor of this last wheel bears an index indicating the name of the month, G, [Fig. 116]. The arbor, C, goes through the plate, and at the other end, C, [Fig. 118], is fixed a little wheel gearing with a wheel having four times as many teeth, and which is centered on a stud in the plate at F. This wheel is partly concealed in [Fig. 118] by a disc V, which is fixed to it, and with the wheel makes one turn in four years. On this disc, V, are made 20 notches, of which the 16 shallowest correspond to the months of 30 days; a deeper notch corresponds to the month of February of leap year, and the last three deepest to the month of February common years in each quarternary period. The uncut portions of the disc correspond to the months of 31 days in the same period. The wheel, A, of 31 teeth, has a pin (i) placed before the tooth which corresponds to the 28th of the month. On the lever, M, is pivoted freely a bell-crank lever (N), having at the extremity of the lower arm a pin (o) which leans its own weight upon the edge of the disc, V, or upon the bottom of one of the notches, according to the position of the month, and the upper arm of N is therefore higher or lower according to the position of the pin, o, upon the disc.