Marine chronometer by Brockbanks

Description — British Museum

Marine chronometer; two-day movement with Brockbanks affix balance and circular white enamelled dial; hinged bezel; rosewood box.

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 Brockbank, c. 1806 Marine Chronometer Signature: On the back plate 'Brockbank's LONDON No 632 TWO DAYS'. On the dust cover 'Brockbank's LONDON No 632'. On the dial 'BROCKBANKS LONDON'.(1) Box: The movement with a dust cover with bayonet fixing to the dial plate, contained in a brass drum, gimballed, in a fine brass-bound rosewood box of later date. The gimbal ring and movement drum can be locked, and the box is fitted with the usual lock, flush-fitting carrying handles, a glazed top with hinged lid and a brass inset disc in the front of the top. Set in the top edge of the bottom of the box where it meets the top is a thin tongue of wood fitting into a corresponding groove in the bottom edge of the top, to exclude dust from the interior of the box. A tipsy key in a corner bracket. The brass drum containing the movement has a hinged bezel with a domed glass which snaps shut and in the base a spring-loaded escutcheon plate with eccentric winding hole, standing proud of the bottom. Box 181.5 mm square, h. 188.5. Dial and hands: Flat enamel dial fixed by three short feet axially drilled and tapped for small blued steel screws, in holes in the dial plate. Dial plate scratched 637. The divisions on the dial are engine divided with scribed marks under the black enamel. There is no reserve of going indication. Flat gold hands, seconds hand screwed to its pipe. Movement: Dial plate diam. 101.4 mm; front plate diam. 78.6 mm; back plate diam. 77.8 mm; frame h. 22.2 mm. Frame: Full plate construction without a detachable barrel bridge, four turned pillars, the back plate retained by blued steel screws. The front plate fits into a turned recess in the dial plate and there are three equally spaced screws passing through the edge of the dial plate into the front plate. The balance potence has a moulded edge to the foot and there is a separate bridge on the front plate for the third, fourth and escape pinion arbor bearings. The front plate is not pierced for the third wheel. The balance potence with a figure 3 punched on the underside of its foot. Fusee: Fitted with the usual stop-work and maintaining power. The stop-piece return spring is formed from a piece of blued steel wire, the maintaining detent and its spring polished. A bell-shaped dust excluder is fitted tight on the squared part of the fusee arbor. The setting-up-work is fitted on the back plate, the solid unsnailed barrel arbor with an extended square at both ends. The mainspring appears to be original and is solid hooked at its outer end to a slot in the barrel wall. The mainspring is signed 'Geo Ansell & Son' on the outside and '1806' on the inside, also 'Wm Creghill Aberdeen Aug '72'.(2) Going train: Well made and finished, the wheels riveted to their pinions, with well-turned undercuts on both ends of each pinion, the leaves, arbors and pivots polished. Jewelling: The pivots of the escape pinion and balance staff in pierced jewels with endstones. A diamond endstone set in a blued steel ring, in the balance cock. The fourth arbor pivots are carried in bushes set like jewel settings, these may be made from a different alloy to that of the frame. Escapement: A development of Earnshaw's spring detent escapement with a steel escape wheel polished on both sides. The straight detent has a large triangular locking stone and a gold passing spring riveted on but held clear of the detent by a brass spacer. The foot of the detent is cylindrical and passes into a hole in a steel block screwed and steady pinned to the underside of the back plate. There is a lengthwise groove in the detent foot so that it can be clamped by a screw set in the block. Another groove, turned near the outer end for a flange-headed adjusting screw is also set in the block for adjustment towards or away from the balance. The steel block also has an extended overhanging portion with the detent banking screw set in the end. Beside the hole for the detent in the block is another similar, now empty. This was presumably for a separate passing spring which was never fitted or was removed when the present detent was fitted. There are also two empty tapped holes for clamping and adjusting screws for the absent passing spring. The impulse roller is remarkably thick (1.87 mm) and polished all over except on the top surface. It is jewelled, the impulse jewel leaning towards a radial to the tip, and there is a poising hole opposite the jewel. The polished leading edges of the escape teeth are severely cut near their tips. The discharge roller has no adjusting flat and is jewelled in a straight-sided slot. Balance: Bimetallic three-armed balance riveted to a brass collet. Limiting screws for the movement of the free ends of the bimetallic rims pass through clearance holes in the rims and into steel brackets integral with the arms. The outer sides of the rims are graduated with a series of inclined marks near their free ends. The brass compensation weights are fixed by a fitted plate, each with a screw and steady pin. Each weight has a brass screw set sideways on its outside, tangential to the balance; these are for fine adjustment of the compensation. Poising of the balance has been achieved by filing the undersides of the arms. Each steel timing screw set in the fixed ends of a rim has two holes in the end of the head instead of a screwdriver slot. Diam. of rim 29.7 mm, h. 2.17 mm. Balance spring: Unevenly blued steel helical spring of 7½ turns, with terminal curves, pinned in a square hole in poised brass collet. The overhanging adjustable brass collet is screwed and steady pinned with steel pins to the back plate. The clamp for the end of the spring is curved on the part touching the inside of the spring and flat on the other. Going-train counts: Great wheel (fusee) 90 teeth Centre pinion 12 leaves, wheel 90 teeth, no crossings Third pinion 12 leaves, wheel 80 teeth, 6 a.