Marine chronometer by Barraud family

Description — British Museum

Marine chronometer; Earnshaw spring detent escapement; movement in brass bowl gimballed in brass-bound mahogany box; key for lock; silvered engraved brass dial, subsidiary seconds; blued steel hands.

Curator's comments — British Museum

Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990) Made by Barrauds, c. 1815 Chronometer movement Signature: On the back plate 'Barrauds' Cornhill LONDON 2/ 670'.(1) On the dial 'BARRAUDS' LONDON 2/570'.(1) Box: The movement contained in a brass bowl, gimballed, in a brass-bound mahogany box. The box contains a canvas tag marked c 793 and a crown with GR beneath. The brass bowl with a spring-loaded escutcheon plate in the bottom, with an eccentric winding hole, and a threaded bezel with a flat glass. A lock is provided for the gimbal ring and the brass bowl. The box has carrying handles, a lock with brass escutcheon plate, and a key to the lock. The top is glazed and provided with a hinged lid; it once had an ivory disc let into the front but this is now missing. The hinges are all stamped WT and S IMPROVED. A brass plate let into the lid matches the escutcheon plate over the lock. The lock stamped PATENT. A narrow strip of wood let into the top of the bottom part of the box fits into a groove in the bottom of the top when the box is closed, thus making a seal to help prevent dust entering the box. The box is recessed at the back to provide clearance for the gimbal ring and is 171.5 mm square, h. 182.5 mm. Dial and hands: Silvered, engraved brass dial graduated for 24-hour indication, the engraving filled with black wax. The subsidiary seconds dial in the upper half, reserve of going shown by a disc through an aperture in the lower half. The dial plate is recessed to take the dial flush with the edge, where it is held by three countersunk screws. Blued steel hands with bevelled top surface. Movement: Dial plate diam. 91.0 mm; front plate diam. 77.8 mm; back plate diam. 77.5 mm; frame h. 19.7 mm. Frame: Full plate construction with a high detachable barrel bridge, the back plate retained by blued steel screws. The dial plate is deeply recessed under the dial. The movement fitted into a deep recess in the dial plate from the movement side where it is held by three large brass screws passing into the front plate from under the dial. Accurate positioning is achieved by a small brass screw set in the outside of the front plate near the edge passing into a hole in the dial plate. There is a separate bridge for the third and fourth arbor lower bearings and the escapement is mounted in a separate frame screwed in a shallow recess in the back plate. This frame is built up on a brass disc with a large potence for the balance staff and escape arbor lower bearings and a cock for the balance staff upper bearing. A large curved chain guard extending across the frame is fixed to the underside of the back plate. The dial plate scratched 2/570 and FH 1117 on the edge. Fusee: Reversed fusee fitted with Harrison's maintaining power and English stop-work. The maintaining detent is returned by a tapered piece of blued steel wire set in a screw and pointing down from the back plate. The maintaining wheel of brass. The turned brass block for the stop-work arm is held by a large blued steel screw, and the return spring for the arm follows the edge of the back plate and acts in a slot in the outer end of the arm (cf. Barrauds' no. 982, cat. no. 82). Setting-up-work of blued steel is mounted on the barrel bridge, the barrel being considerably higher than the frame. The blued steel mainspring is signed but the signature is illegible, although it could be ' Catte 9" 514'. The spring is hooked at its inner end to the solid unsnailed barrel arbor, and at its outer end to a steel hook in the barrel wall. Going train: arranged with an intermediate wheel and pinion planted between barrel and fusee to go for eight days. A very well-made train with polished pinions and arbors, the fourth wheel mounted on a brass collet, the others riveted to their pinions. Jewelling: Only the pivots of the escape pinion arbor and balance staff in pierced jewels with endstones. A diamond endstone set in a blued steel ring in the balance cock. Escapement: A development of Earnshaw's spring detent escapement with a very light brass escape wheel and polished steel impulse roller, unjewelled, and with a convex impulse surface. The jewelled discharge roller, with an adjusting flat, fitted between the impulse roller and the brass collet on which the balance is mounted. The detent with D-shaped locking stone and steel passing spring is arranged in a slot in the escapement platform similar to that for an Arnold escapement, and banked on a brass plate secured by a single screw and with a slot for adjusting. The steady pin in the detent foot passes into a hole in a polished steel plate in the edge of the escapement platform, having a single screw to retain it and an adjusting slot so that the detent can be moved slightly, if required, towards or away from the balance centre. The style and finish of the platform and escapement suggests the work of Robert Pennington. Balance: Bimetallic two-armed balance very carefully made, the steel parts blued, with brass compensation and quarter screws. Screwed to the rims at the fixed ends are short sections of brass extending behind the free end of the opposite rim. A quarter screw passes through a hole in the free end of each rim section and into the brass piece behind. This arrangement may be intended to provide banking for the rims in extremes of temperature or simply to protect them during handling. The other two quarter screws are carried in brass tubes with a thinned slotted portion and riveted to the inside of the rims. Diam. of rim 22.0 mm, h. 2.4 mm. Balance spring: Blued steel helical spring of 9 open coils, with terminal curves, pinned to a split brass collet with concave upper surface, and to a brass diamond-shaped stud held with a single screw on the balance cock. A brass pin is positioned in the balance cock to act as a banking for the tail of the stud and fix its position. Going-train counts: Great wheel (fusee) 70 teeth Intermediate pinio.