Marine chronometer by Gibbons

Description — British Museum

Marine chronometer; Earnshaw spring detent escapement; movement in brass drum, outer brass canister-shaped bowl and brass-bound mahogany box; engraved silvered-brass dial, engraving filled with black wax, subsidiary seconds; blued steel hands.

Curator's comments — British Museum

Although referred to as Gibbons's Patent, this seems to relate to a patent Remontoire, the patent granted to George Prior 29th January 1818 - number 4214. Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990) Made by Gibbons (probably John), English, c. 1818 Marine Chronometer Signature: On the dial only 'Gibbons's Patent Remontoire'. Box: The movement protected by a brass drum fitting with a bayonet catch on a reduced diameter at the back of the dial plate. An outer brass canister-shaped bowl is pivoted in gimbals, it has a bezel with curved glass. There is a lock for the bowl and gimbal ring, the latter pivoting on the turned ends of brass screws in plates in the outside of the box. The winding key was fitted in a skew hole drilled in a corner of the box opposite the gimbal lock. The brass-bound mahogany box is fitted with handles, a glazed top with hinged lid and catch, a lock, and the top edge of the bottom is rebated on the inside for a strip of thick velvet to exclude dust. Box 164 mm square, h. 164 mm. Dial and hands: Engraved silvered brass dial, the engraving filled with black wax, subsidiary seconds dial. The dial is held in a recess in the dial plate by three screws on the inside of the ring of hour numerals. Blued steel hands. Movement: Dial plate diam. 89.1mm; front plate diam. 64.4mm; back plate diam. 64.4 mm; frame h. 23.9 mm. Frame: Full plate construction with a detachable barrel bridge, and four turned pillars, the back plate retained by large blued steel screws. The back plate is cut away for the escape wheel and impulse roller planted level with the plate. The front plate fits in a turned recess in the dial plate, there is a positioning peg and three eccentric-headed blued steel retaining screws. The centre of the dial plate is turned away to leave room for the under dial work. Fusee: Fitted with the usual stop-work and maintaining power, and with the setting-up-work on the barrel bridge. The maintaining detent is returned by a piece of blued steel wire pointing down from the back plate since the remontoire is planted in the space occupied by a normal return spring. A brass dust-excluding tube is carried on the winding square. The mainspring, apparently original but with new outer hooking, is signed 'Richd Smith, No. 5 Queen Street, Clerkenwell London, June 26th 1818'. Going train: The usual layout and construction, the brass wheels gilded, the steel pinions and arbors well finished and polished. Jewelling: The pivots of the fourth and all the escapement and remontoire arbors in pierced jewels, those of the escapement and remontoire where possible with endstones. A diamond endstone in a blued steel ring in the balance cock. Escapement: A development of Earnshaw's spring detent escapement. The brass escape wheel gilded and not sunk out. The steel impulse roller of comparatively large diameter, sunk out on its underside and not jewelled. The polished impulse face is radial, the edge of the roller is also polished. The straight detent, now damaged, is screwed to the bottom and follows the edge of a gilded plate screwed and pinned to the underside of the back plate. The locking stone is triangular, the passing spring gold, riveted to the side of the detent. Balance: Bimetallic two-armed balance of Earnshaw type, with small brass segment weights held by screws on the inside, and with brass timing nuts on the ends of the arms. The balance is held by two screws on a brass collet on the staff. Diam. of rim 27.1 mm, h. 2.9 mm. Balance spring: Blued steel helical spring of 9 turns, with terminal curves, pinned to a solid brass collet, and with the upper terminal clamped by a plate and screw on the edge of a steel stud on the balance cock. The upper terminal is very short and the spring is fixed almost at the circumference. Remontoire or constant force device: Such a device worked on the following principle: a helical spring colleted on the escape wheel arbor provides the torque to impulse the balance each time the escapement is unlocked. Towards the completion of impulse the escape wheel unlocks another similar coaxial wheel and causes the remontoire spring to be rewound by the train. The escapement is thus isolated from the train except during the moment of unlocking. The escape wheel arbor is pivoted at its upper end in a cock on the back plate. At its lower end it carries a pierced jewel in a hollow brass setting with the flat end of the arbor acting as an endplate. Below the escape wheel and arbor is the rewinding escape wheel, also of gilded brass, and similar to the upper one. It is mounted on the escape pinion and pivots in the frame below and a cock above. The upper pivot extends through the pierced jewel in the cock and passes into the jewel on the end of the escape wheel arbor. The two escape wheels are thus coaxial but can turn independently of each other. The blued steel helical remontoire spring, of some 13 turns, with terminal curves is pinned at its upper end to a brass collet immediately below the escape wheel. At its lower end it is clamped to the long extended upper pivot of the escape pinion. The lower escape wheel is locked on a spring detent. There was probably originally a locking stone but this has been replaced by a piece of round wire. Unlocking is achieved by a jewelled detent, the jewel acting on the extended end of the locking detent. An arbor pivoted in cocks in the frame carries the unlocking detent at its lower end and an arm in the path of the upper escape wheel teeth at its upper end. Safety device: It is important to prevent the lower escape wheel from being unlocked at the wrong moment and over-tensioning the remontoire spring. Alternatively, if the driving torque fails in the train, the remontoire spring could run right down. These two possibilities are prevented by an extra device. The steel frame clamped to the extended upper pivot of the lower escape arbor carries on an arm two pins s.