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Thomas Earnshaw
Born	4 February 1749(1749-02-04) Ashton-under-Lyne
Died	1 March 1829 (aged 80) London
Occupation	Watchmaker

Thomas Earnshaw (4 February 1749 in Ashton-under-Lyne – 1 March 1829 in London) was an English watchmaker who first simplified the process of marine chronometer production, making them available to the general public. He's also known for his improvements to the transit clock at the Royal Greenwich Observatory in London and his invention of the chronometer escapement and the bimetallic compensation balance.^[1]

Diagram of Earnshow's standard chronometer detent escapement.

In 1783, he invented and patented the standard spring detent escapement which became standard in marine chronometers,^[2] following the invention of the detent escapement by Pierre Le Roy in 1748.^[3] John Arnold also invented a similar escapement in 1779. In 1805, Earnshaw and Arnold were granted awards by the Board of Longitude for their improvements to chronometers; Earnshaw received £2500 and John Arnold's son John Roger Arnold received £1672. The bimetallic compensation balance and the spring detent escapement in the forms designed by Earnshaw have been used essentially universally in marine chronometers since then, and for this reason Earnshaw is generally regarded as the "father of the chronometer".^[4]

Although, he was principly a watchmaker, he didn't shy away from building clocks. When asked by Nevil Maskelyne, he produced a clock for the Armagh Observatory, which is recognised by horologists today as one of the world's most important clocks.

This clock incorporated Earnshaw's new design of escapement and had a number of novel features including an air-tight case (designed to reduce dust and draughts). It was highly praised by John Thomas Romney Robinson in the 19th century who at that time believed it to be the most accurate clock in the world. In 1794, its purchase price was £100 and Earnshaw charged £100 to travel with it to Armagh and set it up in the new Observatory.^[5]

The Observatory also purchased Earnshaw's second clock ^[6] which was operated at sidereal rate with Edward Troughton's Equatorial Telescope.

In June 1801, Matthew Flinders' ship, HMS Investigator, carried two boxed Timekeepers by Earnshaw E520 and E543, at a cost of 100 Guineas each. Investigator also carried two of Arnled's timekeepers on the first circumnavigation and mapping of the coastline of Australia. The chronometer, E520, is mounted in a wooden box with gimbals to compensate for the motion of the ship. Flinders went to shore regularly to check the settings of the chronometers against the stars. The Earnshaw chronometer was the only one working at the end of the journey, causing Flinders to refer to it in his book A Voyage to Terra Australis as "this excellent timekeeper". Flinders was taken prisoner-of-war by the French in Mauritius. In 1805 Captain Aken, a fellow prisoner, was released and returned to England. Flinders gave him the chronometers to return to the Greenwich Observatory. By some unknown sequence of events Earnshaw 520 was sold to an Australian collector and in 1937 became the property of the Museum of Applied Arts and Sciences (Powerhouse Museum) in Sydney. It was not until 1976 that it was identified as the chronometer of Flinders' historic journey.^[7]

References

1. ^ Thomas Earnshaw at Encyclopaedia Britannica Online.
2. ^ Details on Earnshaw's spring indent escapement
3. ^ Britten's Watch & Clock Makers' Handbook Dictionary & Guide Fifteenth Edition p.122 [1]
4. ^ Gould, Rupert T. (1923). The Marine Chronometer. Its History and Development. London: J. D. Potter. pp. 116–128. ISBN 0-907462-05-7.  
5. ^ Information on Earnshaw's first clock at the Armagh Observatory website.
6. ^ Information on Earnshaw's second clock at the Armagh Observatory website.
7. ^ Thomas Tooth, Matthew Flinders Marine Chronometer, 1790