English gold quarter repeating pocket chronometer Circa 1819

Gold pair cased pocket watch
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A large English 18k gold pair cased quarter repeating pocket chronometer with “Z” balance, circa 1819. The cases with London hallmarks for 1819-1820. The outer case is made by Richard Grove (mastermark RG), and the inner case is polished. White enamel dial with radial Roman numerals, outer minute track, large subsidiary seconds and gold spade and leaf hands.
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Frosted gilt full plate movement with cylindrical pillars, fusee and chain with Harrison’s maintaining power, Earnshaw spring detent escapement, bimetallic “Z” balance with wedge-shaped sliding temperature weights and two gold meantime screws set with a special and unusual device to prevent a too important compensation for the high temperatures, single footed cock with diamond endstone, escapement jewelled, blued steel free-sprung helical balance spring with outer terminal curves. Dial and movement signed.
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Diameter: 65 mm
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Signed: Thomas Earnshaw, Invt. Et Fecit  London
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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 had to be riveted.