Thomas Earnshaw

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Thomas Earnshaw
Born(1749-02-04)4 February 1749
Ashton-under-Lyne
Died1 March 1829(1829-03-01) (aged 80)
London
OccupationWatchmaker

Thomas Earnshaw (4 February 1749 in

Royal Greenwich Observatory in London and his invention of a chronometer escapement and a form of bimetallic compensation balance.[1]

Diagram of Earnshaw's standard chronometer detent escapement.

In 1780, he devised a modification to the detached chronometer escapement, the detent being mounted on a spring instead of on pivots. This spring detent escapement was patented by Thomas Wright (for whom he worked) in 1783. Whilst initially the design was crude and unsuccessful, with modifications it later became the standard form 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 1782.

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 one of the pioneers of chronometer development.[4]

Although he was principally a watchmaker, he did not shy away from building clocks. When asked 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.

Chronometers on notable voyages

Earnshaw chronometer No. 506

In July, 1791, Captain

Guineas for an expedition he was to command in HMS Providence to transport breadfruit plants from Tahiti to the West Indies. This was the second expedition that Bligh had undertaken with this mission. The first expedition was with HMS Bounty which had ended in the infamous mutiny led by Fletcher Christian and from which Bligh returned to England only with the greatest of difficulties. The second expedition, however, was entirely successful.[4]

Around 1796 Earnshaw, who had temporarily lost interest in the Board of Longitude reward, was tempted back by the failure of Josiah Emery to win it. Earnshaw had a low opinion of Emery's chronometers and claimed that an already existing chronometer of his, no. 265, could outperform Emery's even though it had just returned from a voyage to the West Indies and had not been cleaned (dirty oil has an adverse effect on the going of a chronometer). Earnshaw was easily proved right in this and his chronometer had a good average rate over a 12-month trial. However, he failed to win any reward because the method of rating required that the rate of the piece in the first month be used as the baseline for the trial rather than the absolute timekeeping of the instrument. Ironically, this method had originally been proposed by Earnshaw's ally Maskelyne.[4]

In June 1801,

Greenwich Observatory. By some unknown sequence of events, the 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][8]

Between 1831 and 1836,

Meteorological Office. This was only one of many chronometers carried by the Beagle since great accuracy was required for this task. This was also the voyage that carried Charles Darwin who was afterwards inspired to write his book about the theory of evolution, On the Origin of Species.[9] The chronometer is object 91 in the BBC Radio 4 series A History of the World in 100 Objects.[10][11]

References

Wikisource has the text of the 1885–1900 Dictionary of National Biography's article about Earnshaw, Thomas.
  1. ^ Thomas Earnshaw at Encyclopædia Britannica Online.
  2. ^ Details on Earnshaw's spring indent escapement Archived 4 February 2012 at the Wayback Machine
  3. ^ Britten's Watch & Clock Makers' Handbook Dictionary & Guide Fifteenth Edition p.122 [1]
  4. ^
    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
  8. ISBN 0-646-15386-2
  9. ISBN 1-84486-066-3
    .
  10. ^ "Ship's chronometer from HMS Beagle" Archived 10 October 2010 at the Wayback Machine
  11. ^ "Ship's chronometer from HMS Beagle"

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