French marine chronometer with power reserve Circa 1851

Brass ship’s chronometer by Onésime Dumas
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A very fine and rare French mahogany 8-day-going marine chronometer with a power reserve indicator, circa 1851.
Gilt-brass three-quarter plate movement with main frame and sub-frame, conical pillars, fusee with chain and maintaining power, spring detent escapement, entirely fitted including the balance on a spotted finished platform, two-arm compensation balance with cylindrical weights and timing screws, free-sprung blued steel helical balance spring with terminal curves. Silvered dial with Roman radial hours, outer minutes track, subsidiary dial for the seconds,  power reserve indicator and original Breguet-style hands. Mahogany box with brass inlays, hinged handles at the side, lid and glass, case key, brass gimbals and bowl and a three-body.
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Dimensions: 180 x 175 x 175 mm
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Signed: O. Dumas,  Fr. de Grannery
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According to the serial number, this chronometer is certainly one of the first produced by Onésime Dumas as the successor of Gannery. As opposed to 8-day-going marine chronometer movements made in England, with a large frame for the fusee and barrel and a small sub-frame for the escapement, this chronometer is remarkable for its escapement, which is mounted on the platform of the sub-frame, almost isolated from the rest of the movement and, as in Breguet chronometers, it can be removed as easily as the platform escapement of a carriage clock.