Swiss aluminium observatory chronometer deck watch Circa 1935

Aluminium deck watch in a mahogany box
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A fine and rare Swiss, chronometer deck watch, circa 1935. L’Heure de l’Observatoire by Longines, Swiss. With split-seconds chronograph, 36-hour power reserve indication and special device with revolving disc to set time to within a fraction of a second. Aluminium with reeded bezel, buttons on the band for stop/ start, the split seconds, and revolving the chronograph scale, mounted in a two-body mahogany box with hinged lid, the interior fitted with a plaque inscribed Le Chronometre Longines Donne Exactement L’Heure de L’Observatoire.
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Brushed silver dial with black Arabic numerals, outer minute track, outermost revolving chronograph ring with blue-painted second and fifths of a second division, Arabic five-second numerals, power reserve sector. Blued steel spade hands. A rhodium-plated, Geneva striping decoration, 26 jewels, straight-line lever escapement with gold pallet fork, monometallic balance, self-compensating blued steel Breguet balance spring with a special micrometric regulator, split-seconds chronograph with visible work. Dial, box and movement signed.
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Diameter: 70 mm
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Box: 160 x 140 x 60 mm
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Signed: Longines  Swiss
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The split-seconds hand has to be stopped at the hour signal; the revolving chronograph ring must be turned in order to set the division “60” facing the split-seconds hand, which is then released to join the other chronograph hand. The chronometer is therefore set to time within a fraction of a second. The aluminium case, monometallic balance and gold pallet fork make the watch antimagnetic. This type of chronometer is described and illustrated in L’Horlogerie – Une Tradition Helvétique, Samuel Guye, 1948, chapter: Les montres de bord pour automobiles et avions.