Marine chronometer by James Bullock

Description — British Museum

Marine chronometer; one-day Mudge-type lever escapement; light two-arm balance with trammel compensation; silvered-metal dial; regulator pattern indicating seconds, minutes and hours; mahogany case.

Curator's comments — British Museum

Comment from Richard Good, Catalogue of Watches in the British Museum. Vol. V (Unpublished manuscript) Made by James Bullock London, c.1795 A mahogany boxed one-day marine chronometer with an early lever escapement. Signature: 'James Bullock London' on the dial(1) Box: The movement is contained in a mahogany drum, brass bound at the top, with a hinged brass bezel and domed glass. The dial plate fits flush in a shallow rebate, and is positioned by brass pieces projecting into slots in the edge. A painted plaster weight used to level the drum is screwed to the underneath. The drum is gimballed in a plain green-baize lined mahogany box without a glazed panel in the top. There is an ivory escutcheon plate over the lock, an ivory disc inset in the front of the top and a brass one in the centre of the lid. The ivory disc in the front of the lid is inscribed 'BULLOCK'. The gimbal ring, numbered 217, can be locked by a brass rod passing through a bracket, a hole in the ring and into a hole in a brass block screwed to the wooden drum. This rod has a knurled knob and is threaded in the bracket. One of the retaining screws for this bracket is now missing. On the underside of the gimbal ring a length of large diameter brass rod has been crudely added for poising. A very small winding key is housed in a gusset in the box corner. The key is split and no longer fits the winding square. Strengthening quadrants have been added to the inside of the box at the bottom, each secured by three screws passing through the side from the outside inwards and through the base from the outside upwards. Carrying handles originally at the sides of the box have subsequently been removed and the screw holes filled. On the back of the case at the bottom there are further filled holes and marks showing the position of former brackets.(2) Dial & Hands: An engraved, silvered-brass dial of regulator type, with minutes around the outside and subsidiary dials for hours and seconds, secured by four feet and pins to the dial plate. There is a hole for winding just below the signature. Blued-steel hands, the hour and seconds with counterpoised tails, the minute without a tail. These hands are not original. Dial-Plate: The brass dial plate is held clear of the front plate and fixed by three pillars and pins. It is marked out for the dial on the underside and faintly scratched 1794. Movement Ebauche Marks: none. Frame: Full plate construction with four turned pillars. The wide balance cock is set in from the edge of the back plate, the balance staff passing through the frame to a cock on the outside of the front plate. There are two almost diametrically opposite brass blocks, screwed and pinned to the back plate. These may have been intended to protect the balance should the movement be placed balance cock down. However, their shape and position do not perform this function as the balance is too high and too far from them. There are also a number of plugged holes in the plates which appear to have been the result of alterations during manufacture. Fusee, Barrel & Mainspring: Fusee: a six-turn fusee with Harrisons maintaining power and with a brass maintaining ratchet wheel. The maintaining detent is held against the ratchet wheel by a piece of brass wire set in the back plate and pointing down. The block for the stop piece has a squared portion passing through the back plate and secured by a pin. The fusee is made as in clockwork with a single winding click and the great wheel is held by a brass plate and screw. Barrel: internal diameter 38.3 mm, height 17.2 mm. Mainspring: height 16.4 mm, thickness 0.45 mm, straight grained and not blued, and stamped with the number 3 at the end. Barrel Arbor: diameter 12.0 mm, not snailed. Hooking: Square. Going train: Well made but poorly finished, the fourth and escape wheels are mounted on collets. All the wheels have four crossings. Jewelling: There is no jewelling. The back plate bearings for both escape pinion and pallet arbors are in separate detachable plates with wedge-shaped steel end-pieces dovetailed in place. On the lower end of the balance staff is a small diameter pivot, not conical, running in a plain hole in the brass cock. The other pivot runs in a brass bush with the end resting on the polished end of a screw set in a steel plate which is held by two screws. Escapement: A straight-line layout ratchet tooth lever escapement without draw and with all the lift on the pallets. The massive steel lever and pallets have integral upstanding pads, screwed to a collet on an arbor pivoted between the back plate and a potence screwed and pinned to the front plate. The notch works with a roller on the balance staff pivoted between de-mountable brass shrouds. Safety action is provided by a long brass dart riveted to a collet on the pallet arbor, and a brass roller on the balance staff. Banking for the fork consists of a wide slot in a brass strip bent down at right angles and screwed to the back plate at the edge of the hole for the balance staff. No. of teeth embraced 42. Balance & Spring: Built up from two crossbars at about 30E to each other, one of steel, the other of brass. Two short brass rim sections are attached with pins in slots in the ends of the crossbars. The ends of the brass crossbars are attached at the ends of the rim sections; the ends of the steel crossbar nearly mid-way along the rim sections. At the free ends of the rim sections are steel screws with brass knurled heads. The steel cross-bar is riveted to a brass collet on the balance staff, the brass one is sandwiched between the steel crossbar and the brass balance spring collet. This collet is fixed with a clamping screw. Balance diameter 25.6 mm (rim), thickness 2.9 mm. A blued-steel conical spring of three turns, without terminal curves, pinned in a square hole in a brass collet on the edge of the balance cock. The spring is pinned in a similar square hole in the collet. Thermal compensation: The action of the temper.