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494

ZAA0052
Timekeeping

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Object details:

Object ID ZAA0052
Description John Roger Arnold, London c.1818 No.494 1 day marine chronometer in mahogany box For notes on John Roger Arnold, see pp.xxx Box/Mounting Three-tier plain mahogany box measuring 172mm high, 185mm wide, and 178mm deep. The later replacement, plain mahogany lid has plain brass hinges, opening right over to 180° to reveal a glazed panel retained with narrow wooden beading. The front of the upper half has a push button brass lid catch and a circular ivory tablet engraved and inked: “ARNOLD / 494 / ONE DAY”. The lock on the lower half has a narrow brass inset escutcheon. The box is of fine, concealed dovetailed construction at all four corners, with lapped mitres, and the upper half is now restricted to 90° opening by a brass strut on the right hand side in the box. There is a green baize lining inset in the upper edge of the lower half 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 square-section lacquered brass gimbal ring, which is stamped:”494” on its upper rear surface, supports a straight-sided brass bowl (tube with base soldered in), without poising weight inside, and which has a small circular brass winding shutter centred on the flat base, and a narrow brass bayonet-fitting bezel (2 prongs: 1 peg, 1 screw), with a thin convex glass over the dial. The bowl has a small hole in the base, function unknown, and two plugs on the lower side filling small casting faults. There is the faint trace of an inked inscription round the inside upper part of the bowl, reading: (?)“Fl 11 1837”. The interior fittings are otherwise of standard layout except that the rising ratchet winding key is mounted on a shelf at the rear left corner, and the 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 curved bar is stamped on its upper surface: “494”.The underside of the box has a green baize covering. Dial and hands The 78.3 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, and a later dog screw securing it in place at VI (3 holes for screw-fixing on the edge of the bowl at XII are also present).. 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. 494”. The dial has “X”, and a few indecipherable letters, near the centre, scratched on the back and one drilled dimple mark. 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 in a dimple on the side of the cylinder. Movement Full-plate fusee movement with four pillars with flanges and fins at either end and one fin in the lower middle, pinned to the potence plate. The pinned, polished steel set up ratchet wheel and its polished click would probably originally have been blued.. There are also one large and two small drilled dots by the ratchet wheel on the bridge, to mark set-up positions against two nicks on one edge of the square. The potence plate is engraved on its upper surface: “Jno. R Arnold London Inv.t. et. Fecit. No.494”. The outside of the pillar plate, and the inside of the potence plate, the potence foot, the train bar, the balance cock, the steel set-up ratchet wheel and the barrel bridge all have three punched dots. There is one (remaining?) dot near the turned centre of the third wheel. The minute wheel position has been laid out with a depthing tool scribed arc. The underside of the barrel is scratched: “Dent” “494”, “Rivolta”, and “R BLAKEborough. Bedale”. The hooking hole for the fusee chain has been repositioned and the old hole poorly plugged, and there has been a later, fixed round hook in the barrel wall, now poorly plugged and a poor square hooking fitted to the mainspring. The fusee cap is dot marked for the pin entry hole, and the fusee ratchet wheel and the underside of the maintaining power wheel have one drilled dot. The potence is scratched:”494” and the underside of the barrel bridge is scratched: “94”. 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 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 Earnshaw-type spring detent escapement with grey-finished, dovetail detent fitted to a brass block with integral banking piece under the potence plate and banking on a brass screw. The early-type, filed-finished detent has a gold passing spring riveted alongside the detent blade, and a pink jeweled, trapezoid section locking stone mounted in a slot on the detent. The escapement is probably the original, including the polished impulse roller with its raked pink impulse jewel and the discharge roller, which also has a light pink stone inset. The plain bimetal undersprung balance, has tapered steel arms, the bimetal rim segments extending fully round to the opposite arm. The rims have steel meantime screws attached at the end of the arms and at 90° from the arms, and brass compensation weights, just before the. 90° m/t screws and at the end of the bimetal rim segments. The blued steel helical balance spring has terminals on both ends, the lower terminal with a brass stud fixed with one screw and polished steel washer (originally two screws` in a slot) to a small brass plate screwed to the potence plate. The jewelling, which is all in clear or light pink stones mounted in brass settings, extends to the balance and escape wheel with endstones (faceted diamond in steel for the balance upper pivot) and the lower fourth wheel, and the escapement parts as mentioned. Alterations/condition The box is in sound and clean condition with a few small knocks and dents, but the later top is warped from the hinges to the front, and now has packing under the hinges to ensure it closes. There is a scratch on the left hand side of the upper half. The lacquering on the interior brasswork is mostly patchy, but still retain much of what is probably original lacquer. The ivory tablet on the front of the upper half is probably an early addition to the box. The brass strut in the box is a later addition, probably dating from when the balance was replaced, in the mid 1860s. The screw holding in the movement and the other, now missing, are later additions to the chronometer. The dial silvering is clean, but generally a little grey. The movement is in generally sound clean condition, though it was found to be thick with old, solidified oil. One of the train bar screws is a later replacement. The (replacement) mainspring has been re-hooked several times (rather badly) and is now too short. The square hooking itself is poor and too short for the barrel hole. The set-up work has been polished and left un-blued, at some later date. The balance cock has had its foot burred up at the edge to accommodate a new balance staff. The replacement balance was probably fitted in the 1860s after the chronometer was exchanged for new ones, with E.J.Dent. The movement has only been very lightly cleaned, and has been re-oiled, during inspection. Commentary, Provenance, etc The cylindrical cannon pinion boss prevents hand setting by use of a key. Potence Plate Ø: 64.8 Pillar Plate Ø: 65.5 Plate distance: 13.5 Inside barrel Ø: 28.8 Arbor Ø: 9.1 steel, snailed. Thickness: 0.23 - 0.33 Height: 11.35 (7 ½ full turns output from barrel – hooking could slip if forced) Set up: 20 teeth (25 teeth as found). Signature: “R Smills Octr 1864” (scratched on inside of spring, 20cms from end) TRAIN COUNT Wheel / Pinion (+ext dia) Comment: Fusee/Great: 70 / 33.9 No.of Turns: 6 (Chain 46cms, 120 links) Ratchet: 42 / 14.1 Brass, 2 steel clicks Maintaining Power: 120 / 32.1 Brass Centre/2nd: 75 / 26.9 + 14 / 7.2 Solid wheel. Finely finished pinion Third: 64 / 21.2 + 10 / 3.9 4 curved crossings “ Fourth: 70 / 17.7 + 8 / 3.0 “ Escape: 15 / app.12.5 + 7 / app.2.2 4 curved crossings Balance Frequency: 18,000 vbs/ hr (5 beats in 2 seconds) Hour: 60 / 15.6 Brass Minute Wheel: 64 / 16.8 “ Minute Pinion: 20 / 5.7 Highly polished steel Cannon: 16 / 4.5 Polished steel Set up ratchet: 20 / 10.0 Polished steel Impulse pallet tip Ø: 5.4 Discharge pallet tip radius: 1.5 Detent length: 23.0 Balance Ø: 24.1 Balance Mass (incl. b/spring & stud): 4.4g Balance spring Ø: 13.4 Material: Blued steel Turns incl. terminals: 8 ¾ (c/w down)
Date made circa 1818

Artist/Maker Arnold, John Roger
Place made London, England
Credit National Maritime Museum, Greenwich, London
Materials mahogany; steel; brass
Measurements Overall: 170 x 185 x 175 mm
Parts
  • 494 (ZAA0052)
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