John Roger Arnold, London c.1812 No.407

1 day marine chronometer in mahogany box

For notes on John Roger Arnold, see pp.xxx

Box/Mounting

The box and gimbal mounting for this chronometer is no longer present. The movement is retained in a straight-sided brass bowl (probably a cast tube with a second, seamed tube added over, and soldered onto a cast base) with a brass poising weight inside, and which has a circular brass winding shutter centred on the flat base. The bayonet-fitting bezel (2 prongs: 1 peg, 1 screws) is also no longer present.

Dial and hands

The 78.4 mm Ø, engraved and wax-filled, silvered-brass ‘regulator’ type dial has three small riveted feet which fix to the pillar plate with pins and the dial itself seats in the recess in the bowl, a notch at XII and a notch in the bowl orientating it correctly.. The main dial has arabic minute figures and a subsidiary hours dial above the centre, with roman hour numerals and there is a large seconds dial below the dial centre having arabic ten-second figures with straight batons at alternate five-second intervals. The dial is across the centre: “Arnold London”, and is engraved: “407” in the upper half of the seconds dial. The dial has scratched on the back: “407”, “R Good”, “13/1”, “D ADAMS / CRICKLEWOOD” and “Adams / 673”, and one drilled dimple mark. Blued steel spade and poker-hands with a fine, blued steel pointer seconds hand with polished steel slotted centre and a counter-poised tail. The later minute hand has a later blued steel washer retaining it with the cross pin.

Movement

Full-plate fusee movement with four pillars with flanges and fins at either end and one fin in the lower middle, pinned to the potence plate. The pinned, polished blued steel set up ratchet wheel is dot marked for correct position on the square which has a small notch on one corner, and there is a single dot on the barrel bridge for set-up position. The potence plate is engraved on its upper surface: “Jno. R Arnold London Inv.t. et. Fecit. No.407”. The underside of the pillar plate, and the inside of the potence plate, the potence foot, the train bar, the great wheel, the centre wheel, the balance cock and the barrel bridge all have two punched or drilled dots. The underside of the balance cock foot is scratched: “407”. There is one (remaining?) dot near the turned centre of the third wheel. The fusee cap is dot marked for the pin entry hole, and the fusee ratchet wheel has one drilled dot. The signed blued steel mainspring has a blued steel square hooking in the barrel. There is a five-wheel train including great wheel, the fusee with Harrison’s maintaining power and with a fusee pipe push-fitted to the square (dot marked for position, and originally drilled for a cross-pin, now plugged). The train wheels are brass with the third, fourth and escape wheels run on a bar on the pillar plate. All train holes have been plugged and re-pitched from new, as well as the maintaining power detent. One of the steady pins on the potence has had to be repositioned during manufacture.

Escapement, balance, spring and jewelling

Arnold-type spring detent escapement, the detent now missing except for the foot, which has a highly polished foot, and a short length of grey-finished detent spring. The detent banked against the usual eccentrically mounted gold banking disc, recessed into a sink alongside on the potence plate. The rest of the escapement is probably the original, including the grey-finished impulse roller with its radial clear impulse jewel and the discharge roller, which also has a clear stone inset. On impulse, the escape wheel drops on to the impulse pallet half way down the curved impulse tooth surface.

The Arnold-type bimetal undersprung balance, has very slightly tapered brass arms, the bimetal rim segments of Arnold’s type, with his special ‘platina’ alloy on the outside, in conjunction with steel. The rim segments extend just beyond 90° and have brass compensation nuts mounted on the curved threaded ends of the rims. There are steel meantime screws attached at the end of the arms and large brass nuts mounted on short threaded extensions to the rims on the other side of the arms. The flat-section, blued steel helical balance spring has terminals on both ends, the lower terminal with a brass stud fixed with two screws with polished steel washers to a small brass plate screwed to the potence plate. The small brass plate may have been first pinned on another movement and was used during the construction of this one, two earlier steady pin holes being plugged on the piece. The jewelling, which is all in clear stones mounted in brass settings, extends to the balance and escape wheel with endstones and the lower fourth wheel, and the escapement parts as mentioned.

Alterations/condition

The lacquering on the bowl brasswork is patchy and spotty, with a few large spots of tarnish through the lacquer, but it still retains much of what is probably original lacquer.The dial silvering is good, with light yellowing of the laquer.. Rather horrible 20th century blued steel clock hand and collet for minutes.

The movement has evidently had a fall and the shock was evidently taken on the barrel. The cap was bent and the lower pivot of the barrel arbor was sheared off. This has been re-fixed (Loctite 638) but is not a permanent solution AND THE CHRONOMETER MUST NOT BE WOUND. The upper balance pivot is broken, the lower balance jewel hole is cracked in two places and the centre wheel rivet on the pinion was knocked slightly askew (repositioned correctly). Slight wear on escape wheel teeth. The steel m/t screws on the balance are a little rusty (surface rust removed with fine wire wool). Little evidence of use. The movement is generally otherwise in sound clean condition, though it was found to be thick with old, solidified oil. The movement has only been very lightly cleaned, and has been re-oiled, during inspection.

Commentary, Provenance, etc

The collet over the cannon pinion boss prevents hand setting by use of a key. The movement condition is v well preserved; probably much of original finish survives and this represents a fine ‘type specimen’ example for Arnold’s chronometer finish at this period.


Potence Plate Ø: 64.8

Pillar Plate Ø: 65.4

Plate distance: 13.2

Inside barrel Ø: 28.6

Arbor Ø: 9.2 steel, snailed.

Thickness: 0.28 - 0.36

Height: 11.45

(6 full turns output from barrel)

Set up: teeth (3 teeth as found).

Signature: “Robt Clark June 1812” (scratched on inside of spring, 18cms from end)

TRAIN COUNT


Wheel / Pinion (+ext dia) Comment:

Fusee/Great: 70 / 34.1 No.of Turns: 6 (Chain 47cms, 145 links)

Ratchet: 34 / 15.8 Brass, 2 steel clicks

Maintaining Power: 120 / 32.3 Brass

Centre/2nd: 75 / 26.4 + 14 / 7.2 Solid wheel. Finely finished pinion

Third: 64 / 21.3 + 10 / 3.9 4 curved crossings “

Fourth: 70 / 17.6 + 8 / 3.0 “

Escape: 12 / 12.3 + 7 / app.2.3 4 curved crossings

Balance Frequency: 14,400 vbs/ hr (half seconds)

Hour: 60 / 16.0 Brass

Minute Wheel: 64 / 16.8 “

Minute Pinion: 20 / 5.6 Highly polished steel

Cannon: 16 / 4.8 Polished steel

Set up ratchet: 20 / 9.0 Blued steel

Impulse pallet tip Ø: 6.8

Discharge pallet tip radius: 1.4

Detent length: 22.5

Balance Ø: 29.2 Balance Mass (incl. b/spring & stud): 7.1g

Balance spring Ø: 15.5 Material: Blued steel

Turns incl. terminals: 11 ½ (c/w down)