Marine chronometer by Eardley Norton

Description — British Museum

Marine chronometer; hexagonal; twenty-four hour movement; two-arm balance; Earnshaw detent escapement; circular white enamel dial; mahogany case.

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 Eardley Norton, c. 1790 Marine Chronometer Signature: On the dial 'E NORTON LONDON'. On the back plate '2322'. Box: The movement contained in a brass drum rigidly fixed inside an octagonal mahogany base. A brass bezel overlapping the base fits on the outside of the drum and is secured bayonet fashion, it has a bevel-edged flat glass. It is now impossible to remove the brass drum from the base, but tapped holes coming through the side of the latter may have been for the pivoting points of a gimbal ring. There are also two brass securing screws in the base. The dial plate fits flush in a recess in the top of the drum, with a positioning peg. Box 148 mm across the flats, h. 78 mm overall. Dial and hands: Slightly domed enamel regulator dial attached to the dial plate by the two small screws on the edge. There is no sign of scribed divisions under the enamel of the minute and seconds markings. Gold hands, hour hand not original. Movement: Dial plate diam. 139 mm; front plate diam. 108.4 mm; back plate diam. 108.4 mm; frame h. 30.1 mm. Frame: Full plate construction with four baluster pillars, the back plate retained by pins. The dial plate, with pierced centre to clear the motion and under dial work, attached to the front plate by four pillars and pins. It is of much larger diameter than the front plate and appears once to have been fitted with a case catch and hinge, various holes remaining. The balance staff passes through the frame and is pivoted on the front plate and in a pierced bridge on the back plate. There are a number of unused holes, including a corresponding pair in the plates near their edge, beside the fourth wheel, with a cock on the front plate, possibly for a train or balance stop device, and one in a pillar which is threaded. Fusee: Reversed fusee fitted with the usual stop-work and maintaining power. The barrel is of unusually large diameter and made from a piece of strip bent and soldered and the bottom soldered on. The setting-up-work is planted on the back plate with a very short extended arbor square on that end for setting up. The mainspring is grained white and not blued, it is not signed but punched with the figure 3 at the end, and hooked with hook and eye to both the solid unsnailed arbor and the barrel wall. The fusee has a single winding click and a brass maintaining wheel, the detent and the stop arm are fitted with brass return springs. Going train: The wheels riveted to the steel pinions; these, their arbors and the pivots polished. Jewelling: Only the pivots of the escape pinion arbor and balance staff in pierced jewels with flat endstones crudely set in strips of brass. The pierced jewels curved towards the endstones and with a sink on both sides. Escapement: Spring detent escapement of Earnshaw type but of massive proportions. The brass escape wheel sunk out on one side with only the tips of the teeth left the full thickness. The grained steel impulse roller set with a jewel leaning towards a radial to the tip. The discharge roller of brass, with an adjusting flat and a jewel set in a hole breaking into the edge. The detent straight except for the foot set sideways and screwed to a brass arm on the underside of the back plate. There was once an adjusting screw set in a slot in the edge of the back plate acting on the brass arm to move the detent towards or away from the balance axis. This screw has broken off and the arm is now fixed with two screws. The passing spring is steel, held under a screw head and the detent slotted for a flat locking stone. Banking for the detent is provided against a screw head set in a brass bracket screwed to the underside of the back plate. Balance: Bimetallic two-armed balance similar to the Arnold 'z' type with the rim sections screwed to integral blocks on the ends of the cross bar. There are steel timing screws at the ends of the arms and similar compensation screws set tangentially in brass blocks screwed to the free ends of the rims. The balance is riveted to a brass collet on the staff. Diam. of rim 39.9 mm, h. 4.3 mm. Balance spring: Blued steel spiral spring of about 5½ turns, without an overcoil or terminal curve, free-sprung to a brass stud on the back plate under the balance. Pinning to the stud is by a square pin in the square hole and to the collet by a wedge-shaped pin in a slot in the edge of the ring-shaped brass collet. Going-train counts: Great wheel (fusee) 48 teeth Centre pinion 12 leaves, wheel 64 teeth, 4 arms Third pinion 8 leaves, wheel 60 teeth, 4 arms Fourth pinion 8 leaves, wheel 56 teeth, 4 arms Escape pinion 7 leaves, wheel 15 teeth, 4 arms Beats per hour: 14,400 Motion work: Cannon pinion 14 leaves (held by a pin through the arbor), minute pinion 14 leaves Hour wheel 56 teeth (held by a pin through an arbor pivoted in the frame), minute wheel 42 teeth Provenance: Ilbert collection; Ilbert states that he acquired this chronometer from Clowes in 1937. Presented by Mr Gilbert Edgar C.B.E. in 1958. Exhibited: British Clockmakers Heritage Exhibition, Science Museum, London, May-September 1952; British Watches and Clocks, City of Birmingham Museum of Science and Industry, October 1953-February 1954. Bibliography: Bertele 1981, p. 150.