The marine chronometer

A marine chronometer keeps the time of a reference meridian at sea, so that the ship’s longitude can be worked out from observations of the Sun or stars.^1, 2

Rating made it usable: a chronometer’s average daily gain or loss was recorded on a certificate, and the navigator corrected its readings by that rate.^1, 5

Christiaan Huygens attempted sea clocks from 1673; Robert Hooke said he had conceived a spring-driven clock earlier, and that priority is still disputed.^1, 6

Royal Museums Greenwich calls Harrison’s timekeepers, made between 1735 and 1759, the first successful marine timekeepers, but they were too complex to copy cheaply: Kendall’s exact copy of H4 cost £450.^2, 7

In Paris, Pierre Le Roy combined a detached, or detent, escapement with a temperature-compensated balance and a balance spring that kept time whatever the size of the swing, and won the Académie prize after trials at sea.^6, 8

Ferdinand Berthoud’s sea clocks were tried at sea in 1768–69, and in 1770 he was named clockmaker to the King and the Navy.^8, 9, 10

In London, John Arnold patented a spring detent escapement in 1782; Wikipedia’s account also dates a patent to 1775 for a bimetallic compensation balance and a helical balance spring.^11, 12

Wikipedia’s account of Arnold says his No. 36, tested at Greenwich in 1779–80, was the watch he called a chronometer, helping give the word its modern sense; some at the time suspected its result was luck.¹¹

Thomas Earnshaw’s spring detent of 1780, patented through Thomas Wright in 1783, and his compensation balance became the usual form of the marine chronometer. Whether Arnold or Earnshaw had the spring detent first, and how far they were rivals, is debated.^11, 13

Royal Museums Greenwich says Earnshaw made the chronometer a relatively simple design that others could reproduce. Arnold made them in quantity at Well Hall, Eltham.^{2, 3, 11, 14}

Arnold and Breguet

According to Wikipedia’s account of Arnold, he let Breguet make broad use of his inventions and sent his son to train with him, and Breguet later credited Arnold with the idea behind the tourbillon. Breguet became chronometer-maker to the French Navy in 1815.^{11, 15, 16}

In 1821 the Royal Observatory began testing chronometers for the Royal Navy under John Pond, its Superintendent of Chronometers.^1, 17

A good trial meant sales: Porthouse No. 233 was bought by the Navy straight after its trial of 1823 to 1825 and served for 112 years.^1, 5

Admiralty chronometers went on the long voyages: Royal Museums Greenwich records twenty-two on the Beagle’s second voyage, and more with Franklin in 1845.²

The London trade of the nineteenth century included makers such as Molyneux, French and Mercer. Mechanical chronometers were made well into the twentieth century, until radio time signals and, later, electronic navigation took over their work.^1, 2, 18