Marine chronometer by George Margetts

Description — British Museum

Marine chronometer; originally a remontoire; Earnshaw-type escapement steel pillars; white enamelled dial; Chinese rosewood stand carved with figures, buildings and rockwork.

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 George Margetts, c. 1790 Marine Chronometer (8-Day) Signature: On the dial only 'Margetts LONDON INVT NO 80'.(1) Box: The movement contained in a brass drum. The glazed bezel fits on the outside of the drum and is fixed by two screws passing into threaded holes in the dial plate. It has a domed glass. The bottom is removable but held by two screws in the side. It has a glazed panel over the balance and winding hole with a cover plate. There was once provision for winding at the side of the drum but this has been removed and the hole in the drum filled by a brass piece attached to the bracket on the back plate in which the winding wheel was pivoted. Drum diam. 130 mm, h. 88 mm overall. Dial and hands: Flat enamel dial, signed on the back in red in the enamel 'Weston', fitting flush in a recess in the dial plate and fixed by a central brass boss and a screwed fixing washer on the underside of the dial plate. The minute and seconds divisions do not appear to have scribed marks under the black enamel. Blued steel hands, the seconds held on its flanged tube by a screw in the end of the tube. Movement: Dial plate diam. 128.6mm; front plate diam. 123.5mm; back plate diam. 124.1 mm; frame h. 44.3 mm. Frame: Full plate construction with four lightly made steel baluster pillars. The front plate is secured by pins and the back plate by screws and washers in the pillars. The balance is planted inside the frame between a bearing in the front plate and another in a potence screwed to the top of the back plate and passing down at the edge of a hole in the plate. There are numerous empty holes in the plates and the movement appears to have undergone a good deal of modi-fication. Apart from the removal of the side-winding there was probably a revolving agate disc driven by the great wheel, on which the end of the balance staff rested in the dial up position, the disc moistened with oil from a piece of leather. There appears also to have been a remontoire on the fourth arbor between the pinion and the wheel. Fusee: Fitted with the usual stop-work and maintaining power, a toothed wheel is cut integrally with the larger end of the fusee to receive the drive from the side-winding wheel that was once fitted. The maintaining detent return spring is replaced by a piece of brass wire. A short squared arbor at the back for winding the fusee. The solid unsnailed barrel arbor with an extended square at that end only carrying the setting up ratchet. The winding ratchet works with two clicks and there are no marks or signature inside on the parts of the fusee assembly. The mainspring is a modern replacement punched A.M.F. and numbered 1322. There is no intermediate wheel, although the chronometer runs for eight days. Going train: Lightly made and crossed out, centre and third wheels riveted to their pinions, the latter, their arbors and pivots polished. Remains of the remontoire: The arbor of the fourth pinion is planted with one pivot in the back plate, the other extending through a hole in a potence screwed to the outside of the front plate. The end of the extended pivot is squared and carries a six-armed wheel with six square tooth-like extensions of the arms extending beyond the rim. The fourth wheel is mounted on a brass collet pivoted in the end of the squared arbor and in a cock on the front plate. The fourth wheel and pinion and the brass six-armed wheel are thus planted coaxially. A spiral spring is colleted on the fourth wheel arbor, and its outer end is pinned in a stud on the six-armed wheel. Between the spring collet and the fourth wheel is a thin brass pinion. At present the drive reaching the fourth pinion causes it to turn until two pins in the six-armed wheel passing between the spokes in the fourth wheel carry this wheel round by its spokes. The spiral spring is merely held in tension and neither this nor the brass pinion has any function. Jewelling: The pivots of the escape arbor and balance staff run in pierced jewels in brass settings, held by screws in suitable sinks in the frame. The lower escape arbor and balance pivots have endstones, that of the balance staff a diamond. The lower pivot of the fourth pinion arbor runs in a hole in a bush of silvery alloy and rests against an endstone. This assembly constructed as an ordinary jewel setting. With the exception of the fusee arbor, pivoted in the frame, the train pivots run in fixed bushes of different alloys. The centre arbor rear pivot bushing is a grey-white alloy. This has an ordinary brass replacement bush set in it, as has the third arbor rear pivot bushing which was originally a dull copper-red alloy. The other train pivots run in bushes of the same copper-red alloy. Escapement: A development of Earnshaw's spring detent escapement with unusual proportions. The brass escape wheel has twenty teeth, and the impulse roller is only slightly larger in radius than the radius to the tip of the discharge jewel. Both rollers are steel, dull polished and with their surfaces rounded. The impulse roller is jewelled radially and has a very large passing hollow. The detent has apparently been broken where the spring joined the original foot, then re-soldered to a replacement foot planted nearer the balance and to one side, thus putting the spring in greater tension. Locking is achieved against a steel face of the triangular-section block left integral with the detent. This face shows signs of wear. Banking is provided against the edge of the head of an eccentric screw. Both detent and banking are planted on the inside of the front plate. The gold passing spring is fixed under the head of a screw and lies parallel with but under the detent, the horn is formed with the end projecting down to meet it. Balance: Bimetallic three-armed balance with brass weights held by screws, and steel timing screws set in the rim at the ends of the arms. It.