| Maker, E. Dent, London, England. Model Congreve
rolling ball. c.1970's. 14"h x 14"w x 10"d. excluding case. S/N
134. The Dent company made high quality replicas of earlier famous skeleton clocks and
sold limited editions of these in the 1970's and 80's. This is a replica loosely based
upon the original design by Sir William Congreve in 1808. Serial number 134 of 150
produced. Some were also produced under the Frodsham name. Inscribed on rear plate Ser.
No. 134 Andrew Fell del. 1973.
Congreve, never actually built a clock, but was the designer of the original built by Gravel and Tolkein. Congreve was attempting to make a detached escapement through this design and thus improve its' performance. While Congreve is credited with the invention of this design, he may have assimilated some of the details from a clock made by Johann Sayller, Ulm, Germany 1626 (see last picture below). The oscillator in a Congreve clock is the combination of the rolling ball and the tilting table. Congreve's 1808 patent conceived of a table with a period (half cycle) of one minute. He believed that this oscillator was equivalent to a pendulum with a one minute period; such a pendulum, he calculated, would be 11,783 feet 4.800 inches in length. The difficulty with this analysis is that the virtue of a pendulum is that it is not just an oscillator but a resonator - it is an oscillator with a natural (or "resonant") frequency. The tilting table of a Congreve is just an oscillator, not a resonator. In reality this was a total failure as the frictional resistance of the ball as it rolls down the inclined table varies greatly with the cleanliness of both the ball and the grooves. These are notoriously poor time keepers and often difficult to keep running. However, the design is beautiful; one of the more fascinating to watch in practice. It takes 15 seconds for the ball to traverse the table. At the end of its run the ball trips a catch and the table reverses its' incline to start the process over again. The far right hand dial indicates the 'seconds'. It actually jumps 15 seconds at a time to reflect the action of the incline table. The center dial is minutes and the left, hours. Click here for Restoration photos. Provenance: Sotheby's, Amsterdam, Netherlands, October 2, 1996, lot #199. Formerly from private collection.
The photos above are of Sayller's clock built in 1626. The first photo shows a side view with the second photo the top of the table where the ball would roll back and forth along the wire guide tracts. The painting depicting astronomers contemplating a celestial sphere is below a glass surface to protect it from the wear of the ball. In this movement the table does not move. Rather when the ball has reached the end of the table it drops into a bin below, triggering a lever that actuates a lifting device to load another ball at the upper, opposite end of the table; beginning the cycle again. Congreves' innovation was to have the table tip back and forth allowing the use of the same ball continuously. Perhaps Sayller was attempting a detached escapement, or long duration with this design. It was reputed to be made as his masterpiece. Could Congreve have had some of his inspiration of his design from this earlier piece? 1, 2 Click here for Restoration photos 1. Clockwork Universe, Maurice Mayr, pg. 224. Photo: Stuttgart, Wurttembergishes Landesmuseum, Germany. 2,3. Antiquarian Horology, cover photo, March 2012, and pg. 350.
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