The physician, mathematician and musician Lotharius Zumbag de Koesfelt (1661-1727) invented this sea clock in 1714. Just as Huygens before he wanted to make a clock which could be the solution to the longitude problem on ships. His son Conrad (1697-1780), who worked in Leiden as a mathematician and astronomer, made adjustments to its design and described the clock in a pamphlet in 1749. In that year he also had this 'horologum autobarum' (self weighted clock) constructed by the Leiden clockmaker Franciscus le Dieu. A 1762 painting (now in the Lakenhal, Leiden) by Pieter van Zanten shows Zumbag de Koesfelt jr. with this clock.
The main advantage of the clock had to be its constant temperature. To accomplish that Zumbach de Koesfelt put the clock in a (now missing) glass bowl and a heater underneath the clock (also missing). The movement itself was not special, only that it was driven by its own weight. This was the main reason why the Leiden astronomer Johan Lulofs gave an unfavourable judgement of the clock, even before it went to sea. Lulofs claimed that such a clock would be deregulated by the up and down movements of ships. In 1752 Zumbag de Koesfelt offered his 'horologium autobarum' to the Dutch admiralty, who tested the clock on board of the war vessel 'Haarlemmerhoud'. However the disappointing results this did not prevent De Koesfelt to advertise his invention. It never became a big success though.
The clock is mounted on a dish with 6 screws which allow the clock to be adjusted horizontally, which is indicated by the plummet. The movement is housed in a brass bowl, which descends on two guides between four pillars, one of which has a rack. On top is a small wooden decoration. The brass dial plate has a silvered brass dial ring with a central hour hand (I-XII, also indicating half hours) and a minute hand (5-60, divided in 60 parts). One subsidiary silvered brass ring inside the central ring above the centre indicates the seconds (5-60, divided in 60 parts). Duration is one day.
The movement is driven by the weight of the clock itself along a rack. The verge escapement is regulated by a balance wheel which is missing. The movement has a going barrel construction with six wheels. Duration is one day.
h 22; ø 14
Signed on dial 'L & C. Zumbag de Koesfelt in ventores'
and 'Fr. le Dieu fecit LUGD. BATAV'
By Franciscus le Dieu, Leiden