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Maker, Ferdinand Berthoud, Paris, France, c. 1785. Three train pinwheel escapement, 1/2 second gridiron compensated pendulum on knife edge suspension, 30 second Robin remontoire, full perpetual calendar, epicyclical equation of time, moon's age and phase, quarter strike on solar time Chime and strike can be silenced. 25.5"h x 12.25"w x 10.75"d, (65 x 31 x 27.5cm)

Below we examine the perpetual calendar.


The perpetual calendar follows a widely used design in clock making for this function. It consists of a wheel that turns once per year with for indentations similar in function to those found on a count wheel in a strike train. Two of those are shown by the arrows. The wheel has three permanent indentations with a fourth that has a small cam in that position. The circled area is a cock with a small pin protruding inward. This pin advances a four armed year cam when it passes by the pin as shown in the next photo below. This cam can present a uniquely shaped arm to the area of the fourth indentation which will result in a different output depending on the input, that is the every four years an arm that is different from the other three will result in February having 29 days instead of the usual 28 days the other three years. The extra day is known as an intercalary day. This type of cam is commonly used in strike repeat work, especially in quarter or minute repeat work in watches and is known as a 'surprise piece' in that a different option is presented to the readout mechanism depending upon the input.

The cam described above is shown by the arrow. Note that three of the four arms look identical with the fourth having a different profile (at the arrow's apex) resulting in a different output reading every four years, that is a February with 29 rather than 28 days. Notice how Berthoud chose to make the spokes a star shape to add to the diversity of wheel design within the movement.

The large dial at the top has the calendar date readout. Each month with 30 or 31 days will be accounted for. The perpetual calendar assures that the correct days for the extra day for February every four years is accurately shown.

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Continental and American Skeleton Clocks, Derek Roberts, p. 24