Rare pocket chronometer with 'duo-in-uno' spring by this exceptional watch springer.

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Heavy silver double back crystal case hallmarked 1877, casemaker JTW (James Thomas White, Clerkenwell). Fine A P Walsh-type caliper of a half-plate keyless fusee movement jewelled to the 3rd with cap jewels on balance (diamond), escape and 4th, with hard brass setting on the fusee, the 18-size fame stamped JP (Preston). Earnshaw form of spring-detent escapement, the detent with integral foot and gold passing-spring, the scape wheel paired with an eccentric steel locking screw that can be used to prevent the wheel from turning - always a sign of quality. Top quality compensation balance with quarter-timing nuts, the helical balance-spring of duo-in-uno form, as often used by and closely associated with Hammersley, and fitted with an amplitude-limiting device which appears not to have been described before. Signed enamel dial in perfect condition, original blued-steel hands. 53.5 mm diameter.

John Hammersley, a highly regarded springer of best quality watches is presumed to have mostly worked for various watchmakers (such as A P Walsh) and relatively few items are known bearing his name. Credited with inventing the 'duo-in-uno' spring by earlier authors, this accolade is now usually given to the watchmaker John McLennan. However, a letter by Hammersley published in the February 1860 Horological Journal clearly states his claim to have invented this form of spring, predating the earliest known use of it by McLennan in a miniature pocket chronometer, and others, shown at the 1862 London Exhibition. No claim for the spring being McLennan's invention is made in the Juror's Report (written by Charles Frodsham) and it is, in my opinion, far more likely that Hammersley sprung these watches for McLennan. See also the Horological Journal, May 1861, and The Time Museum Catalogue of Chronometers, item 81.

NB: Hammersley also experimented with the even more rare 'tria-in-uno' form of balance-spring in a series of Deck Watches he entered into the Greenwich Trials in the late 1860's. These were not overly successful and he seems mostly to have used and supplied the duo-in-uno form, as in this fine example.

Case back with what I believe is an 'AC' monogram and the watch showing just minor signs of use. Otherwise a very rare example of Hammersley's signed work in fine condition. Ex W Phillipson collection, sold Phillips November 1985. Serviced and guaranteed.