- Museum number
SUBSIDIARY SECONDS POCKET-CHRONOMETER MOVEMENT WITH SPRING DETENT ESCAPEMENT AND BRIDGE-COCK.
Spring detent escapement.
- Production date
Diameter: 39.30 millimetres (back-plate)
Diameter: 44.60 millimetres (dial)
Thickness: 18.50 millimetres
- Curator's comments
- Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990)
Made by Thomas Earnshaw (1749-1829), c. 1783
Signature: On the back plate 'Robert Tomlin Watch Maker to the King London No 1400'.
Dial and hands: Flat enamel dial with subsidiary seconds, Roman hour numerals and Arabic five minute numerals. Minute hand not original, seconds hand missing.
Ebauche marks on the underside of the back plate (T.W over PT over 34.
Dial plate diam. 44.6 mm; front plate diam. 41.1 mm; back plate diam. 39.4 mm; frame h. 8.7 mm.
Frame: Full plate construction, turned pillars. No barrel bridge, unusual pierced and engraved balance bridge, balance potence with moulded edge. The brass parts gilded.
Fusee: With maintaining power having a brass ratchet wheel. Setting-up-work by worm and wheel under the barrel, an unusual feature in a chronometer.
Going train: Well made and finished, the brass wheels gilded and with a turned ring in the band of the wheel at the roots of the teeth on both sides. The pinions with deeply turned and polished undercuts, and the ends faced and polished. The pitch of the teeth rather coarse, especially the centre wheel and pinion.
Jewelling: Only the escapement pivots in pierced jewels with endstones. The jewels large and well made of pale ruby shot with darker colours, the endstones flat on both sides, the retaining screw heads and sinks conical.
Escapement: A very early form of Thomas Earnshaw's escapement. The detent mounting and adjustments similar to the illustration in Thomas Wright's patent. A screw in the edge of the back plate acts on the edge of the brass detent mounting, to move the detent towards or away from the balance staff. A tiny fine pitch screw on the back plate adjusts the height of the detent by acting on and bending the free end of the detent mounting. The tension of the detent spring towards the banking is also adjustable by a third screw set in the block on which the foot is mounted. By screwing in this screw and slightly releasing that holding the detent, the tension against the banking can be increased.
The escape wheel not sunk for lightness but of remarkably thin section, and gilded, mounted on a turned brass collet. Polished steel impulse roller, slightly countersunk at the centre, with a poising hole opposite the impulse notch. The impulse face leaning slightly towards a radial at the tip. The discharge jewel fitting into a dovetail in the roller, and with a depth adjustment by the head of a screw set in the roller. The roller also drilled for a brass pin also apparently for setting the depth but later abandoned. The detent somewhat deteriorated but probably original, with a gold passing spring pinned into a block, and a steel locking pin later filed and now having draw, forming part of the detent. A steel peg for banking against the inside of a steel screw head, with obvious signs of rust around the point of contact.
Balance: Plain steel three-armed balance, the crossings of triangular section, once polished but now deteriorated. Diam. 21.0 mm, h. 0.85 mm.
Balance spring: Polished and blued steel spiral balance spring of 7 open turns, pinned to an overhanging stud. The cock carries a well-polished index held by a brass ring with two screws. The spring projects through the stud for a quarter of a turn and so may not be original. There is no sign of there having been any thermal compensation.
Great wheel (fusee) 49 teeth
Centre pinion 12 leaves, wheel 60 teeth, 6 arms
Third pinion 8 leaves, wheel 64 teeth, 6 arms
Fourth pinion 8 leaves, wheel 70 teeth, 6 arms
Escape pinion 7 leaves, wheel 15 teeth, 4 arms
Beats per hour: 18,000
Cannon pinion 15 leaves, minute pinion 18 leaves
Hour wheel 60 teeth, minute wheel 54 teeth.
Provenance: Ilbert Collection, purchased by Ilbert from Goebel in 1932.
Note: Thomas Earnshaw's patent for his escapement and early bimetallic balance, No. 1354, was taken out for him by Thomas Wright in May 1783. In order to recover the costs of taking out this patent it was agreed between Wright and Earnshaw that the latter should make and sell watches to other watchmakers purchasing the right to use the escapement for £25. Each watch that Earnshaw made could be signed in the name of the purchaser but was required to bear a stamp for which a fee of one guinea was payable to Wright. This watch signed 'Robert Tomlin' is one of two examples known that is stamped in this way and made under this particular agreement. The other named 'Jessop Southampton' is no. 19 and is in the Time Museum, Rockford U.S.A. Eventually the agreement must have lapsed, and Earnshaw made a number of watches signed by other makers which do not carry the stamp or any other indication to connect them with Wright.
Bibliography: A. G. Randall, 'An Early Pocket Chronometer by Thomas Earnshaw signed Robert Tomlin', AH, June, 1984.
- Not on display
- Exhibition history
1998-1999 04 Dec-18 Apr, London, National Maritime Museum, Arnold & Earnshaw, Pioneers of the Chronometer
Latest: 3 (2017)
3 (Oct 1995) Seconds hand missing. Minute hand very badly repaired probably not original.
- Acquisition date
- Acquisition notes
- Following the successful acquisition of the celebrated Ilbert collection of clocks (1958,1006 collection), prints and other related materials made possible by the generous donation of funds by Gilbert Edgar CBE Ilbert's watches were then acquired using funds provided by Gilbert Edgar, public donations and government funds.
- Britain, Europe and Prehistory
- Registration number
- Additional IDs
Previous owner/ex-collection number: CAI.1752 (Ilbert Collection)
Previous owner/ex-collection number: M207 (Ilbert Ledger)