English fine gilt metal pocket chronometer Circa 1803

Gilt metal pair cased pocket chronometer
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An English fine gilt metal pair cased pocket chronometer with sugar tongs compensation, circa 1803. Full plate movement, spring detent escapement, flat three-arm steel balance, spiral hairspring, sugar tongs bimetallic compensation, fusee and chain, cylindrical pillars, signed T. Earnshaw, Invt. et Fecit.
White enamel dial, Roman numerals, outer minute ring, subsidiary seconds, gold hands, subsidiary seconds with Arabic numerals, signed Thomas Earnshaw, Invt. et Fecit.
Plain gilt metal cases, the inner with winding aperture to back, inner case with maker’s mark TC with axe above incuse for Thomas Carpenter.
Later deck box three-tier mahogany box containing keys
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Diameter: 59 mm
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Signed: T. Earnshaw, Invt. et Fecit
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Studying the numbering sequence prepared by Anthony Randall and published in his book, The Time Museum Catalogue of Chronometers, the present watch can be dated more or less exactly to 1803. Thomas Earnshaw claimed to have invented the spring detent escapement in 1781 after he discovered inherent issues with the oiled pivots of the pivoted detent escapement. Previously collaborating with Brockbanks for the construction of chronometers with the pivoted detent escapement, Earnshaw and Brockbanks had a disagreement and Earnshaw approached Thomas Wright with the designs of his new escapement. At the time, Thomas Wright was watchmaker to King George III and, as Earnshaw lacked funds to apply for the one hundred guinea patent for his invention, the pair entered an agreement whereby Wright would apply and pay for the patent in his name. Earnshaw would then make movements for any other watchmakers who ordered them, adding a one guinea fee payable to Wright to cover the patent cost. The patent, no. 1354, was filed in 1783. Earnshaw listed names of makers for whom watches were made under this agreement and these included: Barraud, Frodsham, Margetts and Vulliamy. For a discussion of Earnshaw’s patent see: Rupert T. Gould, The Marine Chronometer.

From around the mid-1790s, Earnshaw fitted his chronometers with plain steel balances, spiral springs and ‘sugar tongs’ bimetallic compensation (although he also continued to use the combination of compensation balance and helical spring). The arms of the ‘sugar tongs’ embrace the outer coil of the spring, altering their shape and length, the curved ends moving closer together or further apart with changes in temperature, thus compensating for the effect caused by these changes on the balance and spring. The timekeeping may not have been so constant, but the balance was lighter and the pivots therefore less susceptible to damage. This set-up would also have made the watch easier to regulate as compared to chronometer balances with timing screws. Shortly before 1800, Earnshaw ceased gilding the plates and blueing the steel parts of many, although not all, of his movements.

Thomas Earnshaw 1749-1829 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. He is also known for his improvements to the transit clock at the Royal Greenwich Observatory in London and 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.