Chronometer no. 508 by John Roger Arnold

Catalogue description — Royal Museums Greenwich

John Roger Arnold, London c.1818 No.508 1 day marine chronometer in mahogany box For notes on John Roger Arnold, see pp.xxx Box/Mounting Three-tier plain mahogany box measuring 124mm high, 138mm wide, and 128mm deep. The mahogany lid has ebony edging to the top and brass butt- hinges, opening to 90° to reveal a glazed panel retained with narrow wooden beading (now missing). Pasted inside the lid is a cut-out paper advert for chronometer manufacturer J.Sewill of London and Liverpool. The front of the upper half has a slotted, brass push-button lid catch. The lock on the lower half has an oval ebony inlaid escutcheon. The box is of fine, concealed dovetailed construction at all four corners, and the upper half is now restricted to 90° opening by a brass strut on the right hand side in the box. There is an upstanding beading running round the centre of the upper edge of the lower half, mating with a groove in the upper half and forming a dust seal when closed. The box fittings are standard, with brass drop handles on the sides, the gimbal screws threaded directly into the wood of the sides with domed washers behind. The narrow, lacquered brass gimbal ring supports a straight-sided brass bowl (cast tube with base soldered in), with two brass poising weights inside, one unused hole and one plugged hole probably where earlier poising weights have been. The flat base of the bowl is stamped: “508”, has a circular brass winding shutter centred on the base, and a narrow brass bayonet-fitting bezel (2 prongs: 1 peg, 1 screw), with a thin convex glass over the dial. The screw hole for the gimbal locking piece on the bowl has been plugged and moved down slightly, probably from manufacture. The interior fittings are otherwise of standard layout except that the J.R.Arnold-type heart-shaped winding key, which is stamped on both sides: “508”, is mounted on a shelf at the rear left corner, and the Arnold-type gimbal lock, on the front right hand side in the box consists of a small curved brass swiveling bar mounted on the gimbal ring and locating simultaneously in slots in brass pieces on the bowl and under a thumb screw, mounted on a triangular brass plate in the corner of the box. The underside of the box has a green baize covering. Dial and hands The 63.3 mm Ø, engraved and wax-filled, silvered-brass dial has three small riveted feet which fix to the pillar plate with pins and the dial itself seats in the recess in the bowl, a pin at XII and a notch in the bowl orientating it correctly, The dial has roman hour numerals and there is a large seconds dial at VI o’clock having arabic ten-second figures with straight batons at alternate five-second intervals. The dial is signed below XII: “Arnold / London/ No. 508”. The dial has “508” scratched and “F.V. 1853”, engraved on the back. Blued steel spade and poker-hands with a fine, blued steel pointer seconds hand with polished steel slotted centre and a counter-poised tail. The minute hand boss has a circular hole locating on a cylindrical shoulder on the cannon pinion, secured and orientated with a side screw locating on a flat on the side of the cylinder. Movement Full-plate fusee movement, without barrel bridge, with four pillars with flanges and fins at either end and one fin in the lower middle, pinned to the potence plate. The blued steel set up ratchet wheel is dot marked for position, and its blued steel spring is a later replacement and has had its position moved and the earlier holes plugged. There are two small drilled dots by the ratchet wheel to mark set-up positions against a nick on one edge of the square. The potence plate is engraved on its upper surface: “Jno. R Arnold London Inv.t. et. Fecit. No.508”. The outside of the pillar plate, and the inside of the potence plate are marked with a single engraved line, and the underside of the set-up ratchet wheel, the potence foot and balance cock have one dot drilled, The underside of the barrel is scratched: “508” and something indecipherable, but possibly: “Arnold”. The underside of the balance cock is engraved with: “F V” and scratched with “508”. There is a small hole in the pillar plate directly opposite the balance centre. The fusee cap is dot marked for the pin entry hole. The later signed blued steel mainspring has a steel square hooking in the barrel. There is a five-wheel train including great wheel, the fusee with Harrison’s maintaining power and with a straight-sided fusee pipe push-fitted to the square (dot marked for position). The train wheels are brass with the third, fourth and escape wheels run on a bar on the pillar plate. All train holes have been plugged and re-pitched from new, as well as the maintaining power detent. Escapement, balance, spring and jewelling Arnold-type spring detent escapement, the grey-finished detent with a highly polished foot. The detent banks against the usual eccentrically mounted banking disc (now in brass; a later replacement for gold?), recessed into a sink alongside on the potence plate. The rest of the escapement is probably the original, including the grey-finished impulse roller with its radial pink impulse jewel and the discharge roller, which also has a light pink stone inset. As currently adjusted, the escape wheel drops onto the pallet before the line of centres, and the tooth face meets the pallet half way down its curved impulse surface; the teeth bear the usual wear marks caused by the severe engaging friction. The Arnold-type bimetal oversprung balance, has very slightly tapered steel arms, the bimetal rim segments of Arnold’s type, with brass on the outside, in conjunction with steel. The rim segments extend just beyond 100° and have brass compensation nuts mounted on the curved threaded ends of the rims. There are brass meantime nuts attached to radial threads at the end of the arms and brass screws in the rims at 45°. There are also spare threaded holes in the rims at 90°. The later blued steel helical balance spring has terminals on b