English 18 carat gold eight day duration lever watch Circa 1840

Gold eight day duration watch 
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An extremely rare English precision pocket watch with eight day movement in an 18 carat gold case, circa 1840. Jewelled three quarter plate gilt brass, chain/fusee movement with a pointed tooth lever escapement with a round polished three arm steel balance, round pillars.
Engine turned 18 carat gold case with maker’s hallmark HW/JW. The back opens by depressing the button on top of the pendant.
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White enamel dial with black Roman numerals, outer minute track, seconds indication respectively at 6 o’clock, blue steel spade hands. The dial proudly displays the signature of the renowned London based watchmaker, Tho’s Earnshaw, whose legacy is synonymous with precision and quality.
Dial signed and movement signed: Tho’s Earnshaw, London 119 High Holborn.
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Diameter: 54 mm
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Weight 132 g
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Signed: Tho’s Earnshaw,  London
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Thomas Earnshaw was born on February 4th, 1749 in Ashton-under-Lyne and died on March 1st, 1829 in London. He was an English watchmaker who, following John Arnold’s earlier work, further simplified the process of marine chronometer production, making them available to the general public. Earnshaw is also known for his improvements to the transit clock at the Royal Greenwich Observatory in London, his invention of a chronometer escapement and a form of bimetallic compensation balance. 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, following the invention of the detent escapement by Pierre Le Roy in 1748. John Arnold also invented a similar escapement in 1782. 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.
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Reference 0013500