Museum Boerhaave is the National Museum of the History of Science and Medicine in Leiden. Their collection contains a number of historically important clocks. For example, clocks directly linked to the inventor of the pendulum clock, Christiaan Huygens, as well as time devices used in the observatory of Leiden between 1670 and 1970. This chronometer belongs to the latter category. • The Verification Service of Nautical Instruments bought this chronometer in 1887. The Verification Service of Nautical Services has its roots in 1787 when the Board of the Admiralty in Amsterdam set up a committee to look after the quality of maps and instruments on their ships. Only after 1858 did this committee really take quality measurement seriously by hiring the then director of the Leiden Observatory, Frederik Kaiser, as head of the Verification. Instruments such as chronometers as well as sextants were meticulously investigated. Sometimes it took a year before a chronometer was returned to the Admiralty. Probably due to the interaction between this service and the Leiden Observatory, the chronometer came to the Observatory in 1899. The chronometer was then adjusted to sidereal time by Abraham de Casseres. • Most marine chronometers started their life as timepieces. These timepieces, usually gimballed, were positioned near the centre of the ship to reduce the effect of motion upon them. They were not set during the voyage and were wound once a day. Most timepieces served between 30 and 40 years, but were regularly maintained and adjusted, and repaired if necessary. After their life as timepieces, they became observation watches. These watches were usually mounted in metal canisters and used for taking observations. They were set on the basis of regulators before and after their observation. With the arrival of radio signals the need for chronometers on board ships ceased and in 1937 some chronometers were given to Museum Boerhaave. • Most of the chronometers in the collection of the museum are of the type common in England, as is this one by J.P. Dupont & Zoon, who did not make chronometers themselves. They bought their chronometers from the clockmaker Victor Kullberg in London and had their name engraved on the dial as well: 'J. P. DUPONT & ZOON. Rotterdam. No. 116'. Kullberg's number 4042 is found on the back of the dial, on the back plate and in the case. It has a silvered dial with subsidiary up-and-down and seconds dials, a movement with spring barrel and fusee with Harrison's maintaining power, bimetallic balance with half-second period, helical blued-steel balance spring, Earnshaw-type spring-detent escapement, and jewelled bearings for both balance and escape wheel. The balance is fitted with a palladium spring. Movement and dial are enclosed in a brass bowl with a screw cover holding the glass. The dial has a central hour hand (I-XII) and minute hand (divided in 60 parts). The subsidiary second dial is below the centre and there is an up-and-down dial above the centre. The chronometer has a duration of two days. • Literature: R.H. van Gent & J.H. Leopold, The time keepers of Leiden Observatory, Leiden 1992, p. 40; Hans Hooijmaijers, Telling Time, Devices for Time Measurement in Museum Boerhaave, Leiden 2005, p. 58; H. Spek, Verificatie van de rijkszee- en luchtvaartinstrumenten, 1858-1978: de geschiedenis van een marinebedrijf, Oegstgeest 1979; A.G. Randall, The Time Museum Catalogue of Chronometers, Rockford, 1991, pp. 211-220. • Museum Boerhaave inventory number 10350.