English silver pair-cased pocket chronometer watch Circa 1785

Silver pocket chronometer by Thomas Earnshaw
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A rare English silver pair-cased pocket Chronometer with Earnshaw’s spring detent escapement, circa 1785. The outer and inner case are both polished. White enamel dial, radial Roman numerals, outer minute track, subsidiary seconds and gold heart and poker hands. Secured by screws, gilt full-plate movement with cylindrical pillars, fusee and chain with Harrison’s maintaining power. Earnshaw spring detent escapement, escape wheel jewelled with endstones in screwed chatons, Earnshaw’s “Z” bimetallic compensation balance with sliding timing weights and two steel mean time screws, single-footed clock with endstone in screwed chaton, free-sprung blued steel helical balance spring.
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Diameter: 61 mm
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Thickness: 27 mm
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Signed: Thomas Earnshaw  London
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Using Earnshaw’s numbering system, the first number is the number of watches made at that point using Earnshaw’s spring detent escapement. The second number, in this case, 2660, is the total number of watches produced at that point. Thomas Earnshaw (1749-1829) Born in Ashton-Under-Lyne in Lancashire, by 1780 he had the reputation of being a very fine watch finisher. He invented the modern Chronometer spring detent escapement towards the end of 1780, a design that he insisted was stolen by Arnold. However, Earnshaw and Arnold likely arrived at their conclusions separately. Earnshaw perfected the fusing together of brass and steel to make laminae for compensation balances which previously had to be riveted.