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A marine chronometer was a portable clock, which kept extremely accurate time, and was used in finding a ship’s longitude at sea, by comparing the clock’s standard time (set to that at Greenwich) with the ship’s local time. The local noon was determined by using a sextant to pinpoint the moment of the maximum elevation of the sun above the horizon.

This example was made around 1820, probably made in Edinburgh by Robert Bryson (1778-1852), one of the most important Scottish clock and instrument makers of the 19th century.

After the major breakthrough in horological design by John Harrison and the production of the marine chronometer, the instrument was further developed by the London makers John Arnold and Thomas Earnshaw. This example, made by Bryson, is a two-day marine chronometer with an Earnshaw-type spring detente escapement. The clock face is inscribed ‘ROBT. BRYSON EDINBURGH 34’, and is housed in a brass-bound mahogany box, slung in gimbals.

When this chronometer was ‘rated’ by the Greenock instrument makers D. McGregor, who were appointed by the Admiralty, in 1880 it was gaining five-tenths of a second per day; and by 1932 gaining four-tenths of a second per day. It is not clear when this example ceased to be used at sea.

©SCRAN/National Museums of Scotland
Chronometer, 1820