Fig. 1 shows the arrangement of a perpetual calendar dial. At the top is the month hand: on the right is the date hand; on the Left is the day-of-the-week hand. Inside the seconds dial is the moon disc, showing by observation or by the numbers the age of the moon. Fig. 2 shows the mechanism underneath the dial. D is the moon disc. It has two moons, and around its edge are fifty-eight teeth, going round once in two lunar months. It rides loose upon a central pipe, and is driven, one tooth each day. by a pin in the wheel E1, driven in its turn by the wheel F. F is on the hour wheel of the watch, and goes round once in twelve hours; it has forty teeth. It drives the wheels E1 and E2. having eighty teeth each, and going round once in twenty-four hours. The wheels E1 and E2, by means of pins projecting from them, as shown, drive the day-of-the-week wheel B and the date wheel C one tooth each day. B has seven and C bas thirty-one teeth. The day-of-the-week hand is fastened to the axis of B, and the date hand to the axis of C. A is the month wheel; it has forty-eight teeth, and goes round once in four years. It is driven by the inter-mediate wheel G, driven in its turn by the date wheel C. Upon A is mounted a steel disc having notches of varying depth in its circumference.
Thus, the space representing the month of January is high: February is a deep slot, as it is three days short; March, again, is high:
April is a shallow notch, being one day short; and so on. It will he noticed that three Februaries are deep notches (three days short, or twenty-eight days), and one February is not so deep, being two days short, or twenty-nine days in leap year. The lever H, a finger on which enters these notches, regulates the number of days shown for each month by operating on a projecting pin on the date wheel C. The position of the lever H with regard to the wheel C varies according to whether its finger piece rests in a deep or a shallow notch of A. Thus, when resting on ahigh space, or a thirty-one-day month, the cam shown on C passes the lever without disturbing it at the end of the month. But when the lever II is resting in a notch, it projects farther over C, and the cam comes in contact with it one, two, or three days, as the case may be, before the end of the month. The pressure on the cam causes the pin in C to rise and come in the path of the lever H, as the latter is drawn back each day by the impulse pin in El acting on the arm I. Each day when the arm I is released.
II springs forward again and ordinarily does nothing, as there is no projecting pin on C; but after the cam on C has come in contact with H, the impulse pin C is caused to rise, and the lever H coming forward forces C round for several teeth. The wheel C is a delicate piece of work. There is a connection between the cam upon it and the impulse pin upon which the lever H acts. The connection is underneath the wheel, and consists of a spring lever. The effect is that, as soon as the cam presses against the end of H, the impulse pin rises from the level of the wheel and stands up in the path of II. It remains in this position until about the middle of the month, when it comes into contact with a fixed stud under C, and is restored to its normal position level with the surface. The lever II is kept up to A, and caused to return, when drawn back each day, by a steel spring, as shown. The month wheel A,day-ot'-tlie-week wheel 15, date wheel 0, and moon disc D are all held in position by spring flirts resting between their teeth, and causing them to jump one tooth accurately each time they are moved. This is but one of many forms of perpetual calendar movements.
All are complicated and difficult to make, and even when properly made frequently give trouble.
Fig. 2. Mechanism of Perpetual Calendar Watch.