English chronometer with spring detent escapement Circa 1787

Rare open faced pocket chronometer by John Arnold & Son
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An English rare pocket chronometer “Of The Second Kind” with John Arnold’s spring detent escapement and Arnold’s OZ balance, circa 1787.
Silver case with case maker’s punch mark “TH (Thomas Hardy)”; white enamel dial with Roman hours, Arabic minutes and subsidiary dial for the Arabic seconds; full plate movement, chain/fusee, Arnold’s spring detent escapement, OZ balance, blued helical balance spring.
Movement signed and dial numbered.
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Diameter: 56 mm
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Weight: 136 g
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Signed: John Arnold & Son, London , Invt. et Fecit
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This smaller size of pocket chronometer, often now known as the ‘301’ series because this is the difference between the two serial numbers, quickly became the standard output of John Arnold and his son, John Roger, and was the foundation of his worldwide reputation. As working watches in constant and important use, they were prone to accidents and to be updated both by Arnold himself and later repairers. This watch is a rare survival in original condition.
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The spring detent escapement, as first invented by Arnold, had the delicate steel detent fitted into a slot in the top plate. Being so fragile, many original detents have not survived and most of Arnold’s watches have later detents, the fitting of which can deface the watch. This detent is original to the watch and is of typical Arnold’s shape and finish.
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Mentioned in “100 Jahre Präzisionsuhren von John Arnold bis Arnold & Frodsham” by Hans Staeger, Filderstadt 1997, pages 176/176 and in “John Arnold & Son Chronometer Makers” byon Vaudrey Mercer, London 1972.