- Museum number
SUBSIDIARY SECONDS POCKET-CHRONOMETER MOVEMENT WITH SPRING DETENT ESCAPEMENT AND SUGAR-TONGS TEMPERATURE COMPENSATION.
Altered spring detent escapement; sugar-tong compensation.
Diameter: 43.40 millimetres (back-plate)
Diameter: 48.50 millimetres (dial)
Thickness: 17.80 millimetres
- Curator's comments
- Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990)
Made by Robert Roskell, c. 1810
Signature: On the back plate 'Rt Roskell 5262'. On the barrel bridge 'Liverpool'.
Dial and hands: Flat enamel dial, secured to the dial plate by three feet and pins, marked on the reverse 'Cooper 72'. The circles on which the minute and second divisions are marked are scribed with a fine point.
The hands missing.
Dial plate diam. 48.5 mm; front plate diam. 44.6 mm; back plate diam. 43.6 mm; frame h. 7.7 mm.
Frame: Full plate construction with four turned pillars, the back plate retained by blued steel screws, detachable barrel bridge. The dial plate carrying the movement hinge, and attached to the front plate by three feet and pins. The case catch and spring mounted on the front plate under the dial. No dust cover or provision for one. All the brass parts gilded.
Fusee: Keywind fusee with the usual stop-work and maintaining power, all the steel parts blued. The maintaining ratchet wheel of brass, the steel detent set well in from the edge of the frame. A dust cup on the winding square. The setting-up-work on the front plate under the dial, the barrel arbor with an extended square on that end only.
Going train: A well made but quite standard train and layout. The brass wheels gilded, the steel pinions and arbors well finished and polished.
Jewelling: All the pivots of the going train and escapement arbors in pierced jewels, including the fusee, those of the escapement with endstones. A diamond endstone set in a blued steel ring on the balance cock.
Escapement: A spring detent escapement showing some similarity to Thomas Earnshaw's work. The three-armed escape wheel made of steel, and without the usual sunk out central portion, but with all surfaces black polished. The solid foot detent is presumably a later replacement, it has a steel passing spring pinned into a block in a similar way to Earnshaw's detents. There are three holes filled with brass plugs near the detent but not in the usual places for the adjustable brass foot of an Earnshaw detent, and there is no provision for the adjusting screw in the edge of the back plate (see registration nos 1958,1201.1733; 1958,1201.1731 and 1958,1201.1594). The locking stone is made from a half round piece of ruby, and the detent banking provided by the head of a steel screw set in a gilded brass piece with a foot screwed and pinned to the underside of the back plate.
The steel impulse roller polished on the bottom edge and crescent, and with a small rounded boss towards the balance, set with a jewel leaning heavily towards a radial to the tip. The leading edges of the escape wheel teeth only slightly marked. The usual jewelled discharge roller without an adjusting flat, polished on the bottom and edge.
Balance: Plain steel three-armed balance, the arms and both sides polished, mounted on a brass collet on the balance staff. Diam. 24.4 mm, h. 0.62 mm.
Balance spring: Open coiled spiral spring, poorly made, of 6½ turns, without an overcoil or terminal curve, pinned to a polished steel stud overhanging the balance, and round brass collet.
Thermal compensation: Sugar tongs compensation with two bimetallic arms screwed and pinned to a gilded brass plate. The brass plate has a pointer moving over a scale for mean time adjustment, and a central pierced boss concentric with the balance staff held in a hole in the back plate by two polished steel screws.
Great wheel (fusee) 60 teeth (worn)
Centre pinion 12 leaves, wheel 64 teeth, no crossings
Third pinion 8 leaves, wheel 60 teeth, 4 arms
Fourth pinion 8 leaves, wheel 70 teeth, 4 arms
Escape pinion 7 leaves, wheel 15 teeth, 3 arms
Beats per hour: 18,000
Cannon pinion 12 leaves, minute pinion 14 leaves
Hour wheel 42 teeth, minute wheel 48 teeth.
Provenance: Ilbert Collection, purchased by Ilbert from Bennet in 1942.
This movement shows remarkable similarity to those of a series made by Thomas Earnshaw (1958,1201.1733; 1958,1201.1731 and 1958,1201.1594). The layout of the parts, particularly on the back plate, also closely resembles these. The pillars were probably prepared to retain the back plate by pins, the marks are still visible on the back plate, suggesting that a standard ébauche was used.
- 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) Hour,minute and seconds hand missing.
- 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.1749 (Ilbert Collection)
Previous owner/ex-collection number: Q89 (Ilbert Ledger)