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
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.