The perpetual calendar watch is considered one of the most sophisticated and enchanting of all high complications by watch enthusiasts, and yet at its heart, it’s a fairly straightforward mechanism. In most cases, perpetual calendars “know” when to change the date from the 28th (February) 29th (February of a Leap Year) 30th (September, April, June, November) or the 31st (January, March, May, July, August, October, and December) thanks to a program wheel, which typically has 48 steps on its circumference. The height of each step interacts with a lever that controls when the date indicator jumps to the first of the month. Depending on what kind of watch or clock you would like, you can design it so that the date indication is digital, or shown by a rotating hand, or a retrograde hand, or what have you –the details will vary (sometimes a lot) but the basic principle is the same.
Which brings us, as questions of watchmaking inventions so often do, to one Abraham Louis Breguet.
Like a lot of people who are interested in this sort of thing, I’d heard that Breguet is usually credited with inventing the perpetual calendar and I more or less took it on faith that that was the case for many years. Lately, though, I’ve been trying to find out whether certain urban watchmaking legends are true and as far as I can tell, this one is –not only have I not been able to find any earlier examples of true perpetual calendars than pre-date two watches by Breguet, I have been unable to find any watches other than his first two perpetuals that use exactly the mechanism he invented. (NB. it has since come to my attention –see comments –that there are at least 2 perpetual calendar watches by Mudge that pre-date those by Breguet. Perpetual calendar pocket watches of the 18th century remain extremely rare, however, and it seems that the Breguet watches discussed here are very probably the first self-winding perpetual calendar watches.)
Reproduction by Montres Breguet of no. 160, the “Marie Antoinette” (no. 1160.)
The first two perpetual calendars made by Breguet are also two of his most famous –Breguets no. 92, the “Duc de Praslin” and no. 160, the “Marie Antoinette.” Intriguingly enough they appear to have been started around the same time –both in about 1783 (some sources give 1782 for the Marie Antoinette.) Equally intriguingly, these are the two most complicated watches Breguet ever made. No. 160’s features are too well known to repeat here but it’s worth mentioning that the “Duc de Praslin” was hardly less complicated:
Breguet No. 92 “Duc de Praslin” bequest of Sir David Salomons to the Musée des Arts et Métiers
The “Duc de Praslin” also has two faces –the back shows the moonphase, power reserve, and regulator indications for the balance regulation and the strike regulation, and has a lever escapement with temperature compensated balance. Incidentally, the Duc de Praslin was donated to the Musée des Arts et Métiers in Paris by Sir David Salomons and it is there to this day for anyone to see who wishes (and ought to be a must-see for any watch enthusiast visiting Paris; why the watch is not better known, and more often discussed, I have no idea. Certainly I had no idea it was there until two months ago, I’m ashamed to admit.)
Back of no. 92, showing moonphase, power reserve, strike and timekeeping regulation indicators
Now, if you want a perpetual calendar it is nice to have a wheel in the watch that turns once a year; conveniently enough both 92 and 106 have equation works. The equation of time is a charming complication but it is trivially complex mechanically; a mathematically correct cam turns once per year, a feeler lever follows the contour of the cam, and this controls, via a rack, a hand that shows how far ahead or behind mean solar time actual solar time will be (the difference between the two is the equation of time.)
Both 92 and 106 have retrograde date hands –the hand, in this design, is on the pivot of a rack, mounted on a spring that causes the rack (and thus the hand) to “fly back” to the 1st at the end of every month. When this happens is determined by a toothed pivot attached to the rack that lifts the restraining detent off the rack at the end of the month.
Breguet no. 1160 with dial removed; 24 hour wheel controlling date advance left; date advance lever center right; pivot for releasing flyback date hand to return to the 1st of the month, far right on the date hand rack. Also visible, minute snail for repeater mounted on cannon pinion, center left; helical balance spring with parachute shock absorber, lower center. Far left, equation of time cam.
This toothed pivot has a rounded foot on it that interacts with a very large, L-shaped lever that’s one of the most prominent features of the under-the-dial work of the Marie Antoinette.
Breguet no. 1160, equation works; visible under “Breguet” is the February star that turns once every four years. Also visible, pins for the 30 day months; large L-shaped lever that trips the date flyback on months shorter than 31 days.
The actual perpetual calendar “memory” is part of the equation of time cam. On the cam are four pins –one each for September, April, June, and November. As the cam turns, these pins –in the appropriate months –push on the foot of the big L-shaped lever. This causes the lever to pivot slightly towards the rounded foot of the toothed pivot that releases the rack for the date –allowing it, if the L-shaped lever is advanced by the EOT cam, to jump back to the 1st on the 30th rather than the 31st.
The issue of February is dealt with ingeniously. For that month, there is a four-pointed star wheel on EOT cam, that rotates one full revolution once every four years. There is a pin on each point of the star –3 years out of four, these pins cause the L-shaped lever to make the date jump back to the 1st on the 28th of February. On the fourth year –the Leap Year –the depth of engagement of the fourth pin with the L-shaped lever is a little less deep, and the date jumps back to the 1st on February 29th.
All this is very clever, and the very odd thing about all this is that these appear to be the only true perpetual calendar watches Breguet made —he may simply have found the complication intellectually uninteresting once the problem was solved, or it may be that the annoyance of having to deal, during the Revolution, with making watches with Republican decimal calendars put him off (certainly, as one who had to flee during the Revolution and who had many clients in the Ancien Regime he would likely have had little love for the Republicans or their calendar.) In some cases, his strategy seems to have been to actually irritate the owner into manually changing the date; some of his watches with a date hand have no 31st on the dial and the owner is expected to push the hand to the 1st with a wooden peg, and then remember that it’s actually the 31st, and then push the hand back to the 1st when the 1st of the month actually occurs, which seems like a tremendous amount of trouble to which to put a customer for the preeminent watchmaker of his day. In any event I have been unable to find any other true perpetual calendar watches by Breguet than no. 92 and no. 160.
Assembly of no. 1160 –shown, placement of the feeler lever for the equation of time. Visible to the right, rack for perpetual calendar hand.
The mechanism, however, is clearly the intellectual ancestor of today’s perpetual calendar program wheel and even anticipates the use in some perpetual calendar watches of a four-faced block that rotates once every four years, to allow for the making of a more compact program wheel (the block controls the change of date for February through an entire leap year cycle.)
The legendary Nicolas G. Hayek, under whose direction Montres Breguet created no. 1160.
As a coda I should mention that it is often repeated that Breguet, when asked by Louis XVI for a “perfect watch” replied, “Sire, give me a perfect oil, and I will make you a perfect watch!” I have, as yet, been utterly unable to find out if Breguet actually said it –perhaps the quote is in Sir David Salomon’s book on Breguet, which is long out of print and which I haven’t read –but if I ever find out, you, gentle reader, will be the first to know.
Many thanks to Montres Breguet, and Breguet North America, for their support in researching this story; also to Mr. Emmanuel Breguet, curator of the Breguet archives and museum in Paris. The late Dr. George Daniels’ The Art of Breguet was also an essential resource.