Breguet, Paris, c.1935 No.4379

1 day (‘Type 102’) Siderometre in mahogany box

For biographical details of the firm of Breguet of Paris, see pp.xxx

The Siderometre

Used for aerial navigation, this instrument is specially designed to indicate sidereal time both accurately and in a readily usable form, to enable rapid position fixing while flying. To make the necessary mathematical calculations easier, sidereal time is shown on this instrument in degrees, minutes and seconds of arc of the Earth’s rotation (as opposed to hours, minutes and seconds of time). The display has centre ‘seconds’ indication (which are actually arc-minutes), with split ‘seconds’ for accurate reading of the recorded instant. Degrees of sidereal time are shown digitally, with three apertures, the numbers running up to 360 and then reverting to 000. The high quality, gilt-brass movement has two separate spring driven parts. One is for timekeeping, the other for indexing the digital dials, at 4 minute (one degree) intervals. It does this in instantaneous jumps using a simple “escapement” controlled by a jewelled cam on the centre seconds wheel of the timekeeping train.

Box/Mounting

Two-tier, plain mahogany box measuring 80mm high, 133mm wide, and 110mm deep. The top of the box is screwed onto the sides with brass countersunk screws, flush with the top surface. The box sides are jointed with double mortices and the upper half is on a single piano-type, butt hinge, limiting the opening to just over 90°. There is a simple, two-nose, push-button catch with a brass button on the front of thye box. Inside the box are four wooden posts in the corners which support a gilt brass rectangular plate, onto which the movement is mounted. This plate is engraved: “BREGUET 4379” at the front, below the dial and there are four controls mounted around the dial. To the right of the dial, at the ‘3 o’clock’ position, is a knurled thimble for winding, marked on the plate: “REMOTAGE” with an arrow for clockwise direction of winding. Above this, is the control for hand setting, used in conjunction with the thimble. The lever, marked: “MISE / A / L’HEURE”, enables the thimble to be used for hand-setting instead of winding, while the lever is pushed across. At the ‘8 o’clock’ position is the stop/start control, marked: “MARCHE / ARRET” which stops the whole movement. A control at the ’11 o’clock’ position, marked “RATTRAPANTE” activates the split seconds indication. The movement itself is house in a gilt brass drum underneath the plate and there is a snap-on back covering the movement and a snap-on gilt bezel with low domed beveled glass over the dial. The underside of the box is covered with light green baize.

Dial and hands


The 58.0mm Ø white painted dial is mounted on a deep brass edge which is a friction fit on the main plate of the movement. The dial has a pair of blued steel centre sweep-seconds hands with ‘split-seconds’ facility, indicating 60 arc-minutes (one degree) in one complete rotation and has Arabic figures for tens of arc-minutes. The dial is divided down to 15-arc-second divisions. The dial is signed above the centre: “SIDEROMETRE / BREGUET TYPE 102 / BREVETE SGDG / 4379”. Degrees are indicated digitally with Arabic numerals appearing in three apertures, hundreds, tens and units, employing numbered “disc dials” under the dial plate. The back of the dial is stamped: “4379”.

The three, digital “disc-dials” are of brass, with French matt-silvering, and have the numerals printed on. They operate as follows:

The “Units” dial (marked 0-9) records units of degrees and is indexed round in one degree jumps by the movement. The dial simply jumps forward ten times in one rotation. It has a star wheel and jumper override for hand setting. On the periphery of this dial is an upper indexing finger and a lower “double” indexing finger. These advance the “Tens” dial as appropriate.

The “Tens” dial records tens of degrees and will be explained in the next section.

The “Hundreds” dial (marked 0-3 twice) records hundreds of degrees and is simply advanced to record successive hundreds of degrees. Once the third hundred is showing the “tens” dial advances the “hundreds” dial to 0 again after 60° have elapsed and the cycle repeats.

The “Tens” dial

The “Tens” dial, which is quite complex in operation, records tens of degrees, and is designed to indicate from 0-9 on the first three rotations, (i.e. up to 300°) and only up to 6 on the fourth (up to 360°), whereupon the dials all revert to 0. This is achieved on this "tens" dial by having a primary dial indicating 0-6 round the disc, with a secondary segmental dial marked 7-9, able to rotate independently about the same centre, above the primary dial, and covering about half of it.

Indicating 0-9 "Tens"

The primary dial is indexed from the finger on the “Units” dial, on notches in its periphery, but although there are seven figures on the primary dial (0-6), after the 6 has been indexed there is no notch – just a space- and the primary dial ceases to advance. However, the secondary, segmental dial, which was being carried round with the primary dial, has a notch in the correct position and this dial now advances three times independently of the primary, superimposing the 6 with 7, 8 and then 9. A further notch on the secondary segmental dial allows it to be advanced once more, but this time the secondary dial meets a projecting stud on the primary dial and both are now advanced so that the 0 on the primary dial is showing. Now there is a notch correctly positioned on the primary dial and it is now advanced again in the usual way. The secondary dial however is now held for three advances, until another stud projecting from the primary dial meets the back of the secondary dial and advances it with the primary. This cycle occurs three times, indicating 0-9 “tens” for one hundred, two hundred and then three hundred.

Indicating 0-6 "Tens"

However, on the forth rotation of the "Tens" dial something different happens.
Owing to the action of a system of levers, operated by a “Genera or Maltese cross” arrangement mounted under the primary dial, every fourth rotation the two studs projecting from the primary dial, (which control the secondary dial) are drawn in closer to the centre of this dial. The studs are now in a position where a groove turned in the underside of the secondary dial allows the primary dial to rotate without the secondary being carried forward, the studs passing right under the secondary dial.
In this way the 7-9 digits are excluded. However, once the 6 is displayed, without the secondary dial the primary could not be advanced. This problem is overcome using the same lever system as moved the studs in.
The system also moves an indexing piece out on the underside of the primary dial, and the lower “double” indexing finger on the “Units” dial advances the "Tens" dial, using this piece, exactly one unit after the 6 has appeared on the "Tens" dial. (The 6 on the "Tens" dial is only required for the duration of one degree- to read 360 – whereupon the dials must all be advanced to 000 again).
The secondary dial is attached to a seven tooth star wheel which is able to advance normally on 4 of the teeth, but is prevented from advancing on the other 3 by nibs on the tips of the teeth engaging on a nose on the jumper. This prevents the possibility of the secondary dial accidentally being carried forward on the primary dial when it should be waiting to be pushed on by the primary at the appropriate moment. The primary dial also has a seven point star wheel /jumper which can override the secondary dials “safety nibs” and allow the secondary to advance when appropriate.





Movement

The two-train, gilt-brass going-barrel, keyless movement with the main going train, having lever escapement, for timekeeping above the main plate and a separate short train for advancing the digital dials, mounted under the main plate. The general level of finish of the movement is high with most of the steel parts either highly polished or uniformly grained.

The main going barrel has Geneva-type ‘Maltese Cross’ stop work mounted on the cap. There is a five-wheel main train including the great wheel on the barrel, the third acting as a centre ‘seconds’ with split seconds work mounted over it on the front plate. In this train the wheels are all brass except the steel escape wheel and all are run under a single large bridge running across the movement. The third (centre ‘seconds’) wheel is run in a cock on its lower pivot.

The dial driving train consists of another, somewhat smaller going barrel, without stop-work, and with a solid steel second wheel meshing directly with an escape ‘vane’. This vane has two downward projections which interact like escape wheel teeth in a ‘claw’-shaped pallet frame. This pallet frame is swung from side to side, to enable the escaping, by an eccentric jeweled cam which runs in a rectangular hole in the body of the pallet frame, allowing the dial train to advance in steps.

There are four dots marked on all the wheels (except escape), on the inside of the barrels cap, on the outside wall of the barrels and under the male stop-work finger. There are four notches cut on dial driving barrel arbor and the dial train escape vane and on most parts of the split seconds work. All gilt-brass movement parts `are `stamped: “4”. The brass edge, the dial sub-plate and the main train bar are stamped: “4379”. The barrel bridge is stamped: “M”.

Mounted between the main plate and the dial is a sub-plate containing the split-‘seconds’ work.


Escapement, balance, spring and jewelling

Club-tooth, jewelled lever escapement with a compensation balance having 18 gold screws on the rims, and a flat spiral blued steel balance spring with overcoil and index on the balance cock engraved “R” and “A” for avance and retard. The escapement as a whole is mounted on a circular-shaped escapement platform screwed to the underside of the main plate.

The rubbed-in jewelling extends to the third wheel, including the eccentric jewel on its arbor for swinging the dial train pallet frame, and second wheel upper hole. The ‘split-seconds’ fly-back cam action includes a small red jewel acting on the heart-cam. The balance and escape wheel have endstones, the pallets have jeweled holes and pallet stones and the impulse pin is also jewelled, all stones a dark pink. None of the dial driving train is jewelled

Alterations/condition

The surface finish of the box is generally sound and clean.

The movement is in generally sound condition. The split-seconds jewel is broken in three pieces. One of the teeth on the castle wheel in the hand-setting is broken off. The movement was generally wiped clean and holes pegged perfunctorily, but the movement has not been fully cleaned.

Commentary, Provenance, etc

There is evidence that the movement has been altered during construction, with one or two steady pins being re-positioned and holes plugged, now just visible under the gilding. The calibre of this movement appears also to have been intended for another layout of escapement, with redundant holes present in the main plate which appear to have been for other balance and pallet centres

Main Plate: 56.0

Barrels:

Main

Inside barrel: 22.0 brace in barrel at 30° from hook

Arbor: 5.9 Steel, snailed.

Thickness: 0.20

Height: 4.4

Spring Signature: ? “Furysen 35” 24cms from the end.

Set-up: 2 ½ turns (1 ¼ turns as found). Stopwork gives 4 whole turns. (total output 9 turns)

Dial driving train

Inside barrel: 16.4 no brace in barrel

Arbor: 5.0 Steel, snailed.

Thickness: 0.15

Height: 3.0

Spring Signature: ? “Purysen” 36 cms from the outer end. (total output: 6 ¾ turns)



TRAIN COUNT


Wheel / Pinion (+ext dia) Comment: Crossings? Marks?Jewelled?

Great (going): 130 / 24.9

2nd: 100 / 18.8 + 12 / 2.4 6 tapered crossings. Light grained finish on top, rougher finish on underside. Pinion nicely finished

Third (centre ‘secs’): 96 / 15 + 8 / 1.6 “

Fourth: 60 / 10.5 + 12 / 2.0 “

Escape: 15 / 7.8 + 12 / 2.2 grey steel, tips polished

Balance Frequency: ?????

Great (dial): 96 / 19.4

2nd: 60 / 11.4 + 12 / 2.5 solid steel

Escape ‘vanes’: 8 / 1.6


Winding work

Winding bevel: 29 / 8.6 steel

Stem bevel: 25 / 7.0 “

Transmission bevel/dog clutch: 22 / 6.7 + 8 / 3.8 “

Dog clutch/castle wheel: 8 / 3.8 + 16 / 3.8 “

Double pinion: 24 / 5.8 + 16 / 4.6 “

2nd Transmission bevel: 30 / 8.7 “

Going trans. wheel: 30 / 8.5 “

Going trans. pinion: 18 / 5.7 “

Going ratchet: 65 / 18.8 “

Dial trans. wheel: 32 / 9.2 “

Ratchet (dials): 42 / 12.1 “

Balance Ø: 17.0 Balance Mass (incl. b/spring & stud): 0.9g

Balance spring Ø: 8.6 Material: Blued steel

Turns: 13 ½ incl overcoil.