Dent, London c.1890/1970 No.1633

18 day marine chronometer, with Airy’s escapement remontoir, in glazed rosewood box.

Dent….

Box/Mounting

Single-tier, brass-bound box, cut down from a conventional three tier rosewood marine chronometer box, now measuring 126mm high, 168mm wide, and 199mm deep, the lid of the box glazed to view the dial. The front of the lower half has a brass, ‘scalloped star’ escutcheon and a circular ivory tablet inset, engraved: “DENT / 1633 / LONDON”, the area under the number showing signs of filing to remove an earlier number, parts of which are still just visible (possibly “0….7”). The back of the tablet is machine-engraved with a large “1633”. The box has an ebony beading running along the centre of the junction on the lower half, mating with a groove in the junction of the lid and forming a dust seal when closed. The lock is stamped: “SECURE LEVER” on its upper surface. The brass blocks, with brass screw-centres for the earlier gimbal pivots, are still on the sides of the lower half, as are the holes where the original drop handles and gimbal lock would have been mounted (the much darker finish of the rosewood in these areas revealing the current finish of the box to be very faded)

The movement, in its bowl, is securely screwed into the lower half of the box from beneath with two brass screws and is retained in a blue velvet-covered mount fitted inside the box. The wide bezel, with unsilvered sight-ring (2 ¼ turns to remove), has a simple, modern, knurling round the edge, and is fitted with a flat, beveled glass over the dial. The underside of the box has evidence of having had a green baize covering, now all missing.

Dial and hands

The 132mm Ø, engraved and wax-filled, silvered dial is attached to the brass edge on the front with three screws. The movement is attached to the brass edge inside with three large steel screws. The dial has roman hour numerals with a large seconds dial at VI o’clock having Arabic ten-second figures with straight batons at alternate five-second intervals. The dial is signed across the centre: “DENT 82 Strand / LONDON No.1633”. Two concentric ‘state of wind’ hands, one longer than the other, are placed at approx ‘1 o’clock’ on the main dial, but without any dial of their own to be read by. The longer hand advances during winding, the shorter during running, though as the running hand runs at three times the rate of the winding hand, it is unclear how this indication was intended to be read. The back of the dial is scratched: “DENT 1633”. Blued steel spade and poker-hands with a fine, blued steel pointer seconds hand with a counter-poised tail and simple blued steel subsidiary pointers.

Movement

Unusual, double-frame calibre movement with four, pinned, straight-sided, parallel pillars, and with three going barrels (one with stopwork), two engaging with one intermediate wheel and the other with a second intermediate, both intermediates meshing with a single centre wheel. This in turn meshes with a fourth wheel pivoted in the escapement platform, meshing with a fifth (seconds) wheel meshing with the escapement remontoir. The general level of finish of the movement is reasonably high with all movement parts flat-finished with some hand-curling or graining.

With the escapement at the front, the barrels/winding wheels are marked with dots: 1, 2 & 3, running anti-clockwise round the frame. Wheel 2 is also scratched “3” on its upper surface, and wheel 3 is marked “No.2” on its upper surface. Barrel 1 is scratched “No.1” on its end and barrel 2 has “3” scratched on its end. Intermediate wheel 1 (meshing with barrel 1) is scratched: “S” and intermediate wheel 2 (meshing with barrels 2 & 3) is scratched:”K” (?or L). The upper surface of the pillar plate is stamped: “1633” near the escapement sub-frame.

The escapement sub-frame is stamped “1633” on the underside of the base plate, on the underside of the balance cock foot, on the foot of the train cock and on the foot of the potence. Baseplate, upperplate, potence and train cock are all punched with three triangular dots. The upper plate has its underside stamped: “1” and there is a series of one large and three smaller plugged holes close to the present detent block fixing. These may have been the site of an earlier detent fixing. The fourth and fifth wheels on the escapement sub-frame have been repositioned, the earlier holes very neatly plugged but the openings in the baseplate for the earlier positions are still present.

Winding and State of Wind Indication

On the upper surface of the main frame is a central winding square with a steel pinion underneath its bridge, attached to a brass ratchet wheel which is held by two steel clicks, acting ½ a tooth space a part. There is a ‘releasing bar’ arrangement on top of the bridge, enabling both clicks to be released from engagement at once, to let the power down on the movement. The bridge, which has three feet, is marked with 2 dots on its upper surface.

One of the barrel arbors terminates under the dial in a pinion driving (16:1) a wheel with the ‘winding’ hand attached. A brass transmission wheel, driven off one of the great wheels, terminates under the dial in a pinion which meshes with another pinion driving (combined ratio:48:1) a wheel with the ‘running’ hand attached. Presumably the separation of these hands is supposed to represent the period run, in some way, but the exact means of reading these hands in unclear as the ‘running hand’ will revolve once in 16 days, whereas the winding hand revolves at three times the rate, needing to go round three full times during the winding in order to wind the springs up fully after that 16 days of running. The hands would however coincide again once winding is complete and this may basically be the intended means of showing ‘fully wound’.

Escapement, balance, spring and jewelling

The Earnshaw-type spring detent escapement is associated with an with Airy-type constant force remontoir, with an 8 ½ turn blued steel helical spring under the escape wheel, actuated by a lever released by the balance unlocking a lever-type escape wheel with’D’ section polished steel pallets, for locking the remontoir spring. The lower escape wheel pivot runs in a jewelled setting, with hole and endstone, mounted within a steel collar, side-screwed to the extended pivot of the escapement remontoir wheel. The cylindrical-foot detent is mounted on a brass block on the escapement sub-frame. The detent has a screwed-on, gold passing spring running alongside the detent blade, and with a red jewelled locking stone. The impulse roller has a radially set, red impulse jewel and the discharge roller has a clear sapphire-type stone inset. Below these is a ‘savage-type’ two-pin roller for unlocking the escapement remontoir.

The oversprung, four-arm, brass ‘horizontal staple’ balance has segmental brass weights at the ends of the two bimetal ‘staple’ compensation pieces. Three unused holes on the sides of the compensation weights suggests they have been positioned in a different place or have had other attachments to them in the past. Two brass mean-time screws are mounted at the ends of two of the arms, and two brass limiting weights, able to control the inward movement of the bimetal pieces, are mounted at the ends of the other arms.

The polished steel helical balance spring has terminals on both ends. The main jewelling, which is mounted in brass settings (upper balance diamond endstone is in a polished steel setting), extends to the balance and escape wheel with endstones and the escapement parts as mentioned. The remontoir holes (both locking pallet pivots and the upper locking wheel pivot) are all light pink rubbed-in jewels.

Alterations/condition

The ‘winding hand’ wheel has had a number of replacement teeth quite neatly dovetailed into the rim. There is some fingerprint staining on barrels and edge of plates, otherwise the surface finish is sound. The lower balance pivot has almost certainly been re-pivoted as the pivot is soft (was slightly bent and straightened easily) and staff and impulse roller are blue (from lowering temper of staff to drill for re-pivoting). The main driving movement is in almost unused condition, the wheelwork crisp and fresh with no signs of wear whatever and all very nicely finished. The pivots for the arbor of barrel 3 have been plugged/bushed, as have the pivots for the transmission arbor for the ‘going hand’ and the lower centre wheel. The movement has only been very lightly cleaned during inspection.

The surface of the box is sound with a few general slight knocks and marks and the brasswork of the box and bowl is lightly tarnished but is generally sound.

Commentary, Provenance, etc

The chronometer would appear to be a composite instrument, probably only completed in its present form in the 1970s. The main ‘motor frame’ movement, including the centre wheel, a design based on Dents patent No, xxx of 184x, appears to be of French origin and probably dates from the second half of the 19th century. The sub-frame incorporating the rest of the train, the escapement remontoir and the balance, also dates from this period, but has been adapted to fit this movement, having evidently been in another movement before. It is possible the sub-frame assembly had at that time a simple chronometer escapement and that the current escapement remontoir was fitted to it at the same time as the adaptation to this movement, probably in the late 19th century. However, the work on it then appears to have halted, only to be started again in modern times (perhaps near the closure of the company in the 1970s) when the movement was given the present bowl, bezel and glass, complete dial and hands and remains of a mid-19th century English chronometer box, affixed with re-numbered ivory tablet, were adapted to put it into a ‘complete’ and saleable form. The wheel for the ‘running hand’ was probably fitted at this time also.


Upper Plate Ø: 112.0

Pillar Plate Ø: 112.0

Plate distance: 37.6


Barrels:

Barrel 1 (nearest to balance cock foot and all parts marked with one spot)

Inside barrel: 38.6

Arbor: 11.6 steel, with steel hook, not snailed.

Thickness: 0.30 not noticeably tapered

Height: 21.5

Spring Signature: “Buel Jne Paris” on outside of spring, 50cms from end

Barrel 2 (between the other two and all parts marked with two spots)

Inside barrel: 38.6

Arbor: 12.1 steel, with steel hook, not snailed.

Thickness: 0.30 not noticeably tapered

Height: 21.5

Spring Signature: “Buel Jne Paris” on outside of spring, 50cms from end

Barrel 3 (furthest round anti-clockwise and all parts marked with three spots)

Inside barrel: 38.6

Arbor: 12.4 steel, with steel hook, not snailed.

Thickness: 0.30 not noticeably tapered

Height: 21.5

Spring Signature: “Buel Jne Paris” on outside of spring, 50cms from end

TRAIN COUNT

Wheel / Pinion (+ext dia) Comments

Great 1: 90 / 44.95
Winding: 40 / 44.2 Brass

Great 2: 90 / 44.8
Winding: 40 / 44.2 Brass

Great 3: 90 / 44.8
Winding: 40 / 44.2 Brass
Stopwork driver: Single steel finger
Stopwork driven: Brass ‘half wheel’; 2 double thickness teeth at 180 from each other, with four teeth in between (16.0Ø). 8 ¼ turns in total, 6 turns of barrel admitted by stopwork.

Inter 1 (barrel 1): 96 / 38.8 + 12 / 6.3 4 curved crossings. fine grained finish
Inter 2 (barrels 2 & 3): 96 / 38.6 + 12 / 6.3 4 curved crossings. fine grained finish


Centre/third: 90 / 31.5 + 10 / 4.35 5 tapered crossings. fine grained finish

Fourth: 80 / 21.6 + 12 / 4.4 “

Fifth: 80 / 18.4 + 10 / 2.8 “

Remontoir: 15 / appr.11.6 + 10 / 2.5 3 curved crossings

Escape: 15 / appr.12 “

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

Hour: 54 / 18.1

Minute Wheel: 56 / 19.3

Minute Pinion: 18 / 6.6 (Brass)

Cannon: 14 / 5.4 (Polished steel)

Winding pinion: 20 / 23.6 nicely grained steel

Ratchet wheel: 36 / 35.3 nicely finished brass

‘Running hand’:

Transmission wheel (meshing with barrel 3): 30 / 14.9 + 8 / 3.2

Reverser pinion: 8 / 3.2

Wheel: 128 / 41.7 4 curved crossings

‘Winding hand’

Driving pinion: 8 / 3.2

Wheel: 128 / 41.6 4 curved crossings

Escapement

Impulse pallet tip Ø: 5.6

Discharge pallet tip radius: 1.0

Balance Ø: 36.7 Balance Mass (incl. b/spring & stud): 8.5g

Balance spring Ø: 11.0 Material: bright steel

Turns incl. terminals: 10 (ac/w down)