Marine chronometer by John Roger Arnold

Description — British Museum

Marine chronometer; pivoted detent escapement; movement in brass bowl gimballed in square mahogany box; engraved silvered brass dial; 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 John R. Arnold, 1813 Marine Chronometer 1-day Signature: On the back plate 'Jno R. Arnold London No 413 Invt et Fecit'. On the dial 'Arnold London 413'. Box: Square mahogany box with a glazed top missing its lid, fitted with handles, a lock and an ivory inset disc engraved with the British Government Property mark, 'Arnold' and '413'. The side sections are dovetailed together and around the edge of the top of the bottom part of the box is glued a strip of green baize. The movement contained in a brass bowl in gimbals in the box, with a bezel having a bayonet fixing on three pegs and with a domed glass. In a deep recess in the bottom of the brass drum is fitted a spring-loaded escutcheon plate with eccentric hole for the key. An ingenious lock is fitted to the gimbal ring consisting of a brass arm pivoted in the middle on a shoulder screw on the top of the wide gimbal ring. In the locked position one end fits into a slot in a piece screwed to the brass bowl and the other end under the head of a locking screw carried on a plate in the corner of the box. In the recessed position the arm pivots until it lies along the gimbal ring. 150 mm square, h. 141 mm, excl. missing lid. Dial and hands: Engraved, silvered brass dial, the engraving filled with black wax, attached to the front plate by three short pillars and pins. The pillars are turned with shoulders to leave a gap between the front plate and the underneath of the dial for the motion work. Blued steel hands, all of flat section, the seconds hand screwed to its pipe. Movement: Dial diam. 76.4mm; front plate diam. 64.4mm; back plate diam. 64.5 mm; frame h. 17.3 mm. Frame: Full plate construction with a detachable barrel bridge and four turned pillars, the back plate retained by pins. The balance staff lower pivot is planted in the front plate instead of in the usual potence. The front plate is not pierced for the third wheel although the third and fourth arbor lower pivots are carried in a separate bridge. Going barrel: Fitted with four-turn stop-work on the cover, the finger piece pivoting in the barrel bridge. The snap-on cover provided with a brass block fitting into any one of three slots in the top of the barrel to prevent the cover turning. The winding ratchet, click and spring fitted on the barrel bridge, and resembling J. R. Arnold's setting-up-work for a barrel with fusee. The inner end of the mainspring hooked to the solid snailed barrel arbor, the outer end with a solid block in a slot in the barrel wall, and signed 'R. Clark Decr 1813'. The barrel fixed to the great wheel in a turned sink by three recessed screws from the outside of the great wheel. Going train: The wheels riveted to their pinions, the latter and their arbors well finished and polished. The escape pinion is missing. Jewelling: The pivots of the fourth and escapement arbors in pierced jewels, those of the escapement with endstones, a diamond endstone in a brass ring in the balance cock. The jewelling is of the usual nineteenth-century English type. Escapement: An unusual form of pivoted detent, probably experimental. Unfortunately the escape wheel is missing but it must have worked as in an Earnshaw escapement with the detent in compression. The steel impulse roller carries a brass semicircular frame screwed to its upper surface in which is set a ruby pin to receive the impulse. It can be moved towards or away from the balance axis by a screw in the brass frame bearing on the balance staff. The ruby pin is carried between brass shrouds arranged in the gap in the steel roller normally occupied by the impulse jewel. The teeth of the escape wheel must have had nearly pointed teeth leaning slightly towards a radial to their tips. The light steel detent, mounted on a brass collet, is pivoted in the frame. It has a narrow flat locking stone projecting towards the back plate, indicating that it locked the teeth by their tips as in an Earnshaw escapement. The tail banks against a screw set in a brass piece screwed and pinned to the underside of the back plate. The gold passing spring is held by a single rivet. A spiral return spring for the detent is colleted to a brass block on the arbor and to a stud on a rotatable index on the inside of the front plate. The index consists of a split ring and boss, the latter held by two screws, and with a tail which can be moved to alter the set up of the spiral spring. Balance: Later two-armed bimetallic balance with brass screws and quarter nuts. From the height available under the overhanging part of the balance cock it seems likely that one of Arnold's flat bimetallic balances with upstanding rods carrying compensation nuts was originally fitted. Diam. of rim 22.7 mm, h. 3.0 mm. Balance spring: Blued steel open coiled helical spring of 8⅛ turns with terminal curves. A split brass collet on the balance staff and a brass diamond-shaped stud with lateral adjusting screw. Both collet and stud have round holes and D-shaped pins to clamp the spring. Going-train counts: Great wheel 80 teeth Centre pinion 8 leaves, wheel 75 teeth, no crossings Third pinion 10 leaves, wheel 64 teeth, 4 arms Fourth pinion 8 leaves, wheel 70 teeth, 4 arms The missing Escape pinion certainly 7 leaves and the wheel 12 teeth, as confirmed by the period of the balance and spring system Beats per hour: 14,400 Motion work: Cannon pinion 14 leaves, minute pinion 18 leaves Hour wheel 54 teeth, minute wheel 56 teeth Provenance: According to the Chronometer Ledgers preserved at the National Maritime Museum, Greenwich, No. 413's service with the Royal Navy was as follows: 7th Nov. 1823 Superb Captn. T. Hains 25th April 1826 Dryad Captn. Hon. R. Rodney 19th July 1830 Talbot Captn. R. Dickinson 12th Jan. 1837 Dido Captn. L. Davies 14th Nov. 1841 Serpent 30th Aug. 1847 H.M.S. Prometheus 25th Aug. 1852 H.M.S. Trident 5th Oct. 1857 Avon 15th May 1861 H.M.S.