Very Rare Early Pocket Barometer by Negretti & Zambra c.1860

This is a rare and evolutionarily important transitional pocket aneroid barometer by Negretti & Zambra, London c1860. Antique pocket barometer having 2½” silvered and engraved dial, barometric scale calibrated in inches of mercury with a range from 23” – 31” divided down to 1/10th. Standard meteorological terms. Blued steel pointer. Heavy bevelled glass, with gilt brass index set to polished concave centre. Conventional early Vidie pattern movement driven from a single 2” capsule tensioned on a C spring. The whole contained within a drum form brass case, the dial sitting on internally pinned and threaded adjustable platform. Calibration port to verso. Presented in its original leather over timber, blue velvet and silk lined travel case snap closure on a button release. Condition: The subject of a full service, conservation, and calibration under laboratory conditions, see performance chart from dynamic test. The movement working exceptionally well showing hysteresis in descending pressure from 31” Hg (corrected). The dial with minor marks, some slight losses, oxidation at periphery. The brass case retaining a good proportion of original lacquer, some wear, oxidation. The travel case intact and structurally sound, small piece of leather replaced at lid rim; the internal fabric sound and vibrant, some discolouration and witness marks. Externally the leather worn, some losses and discolouration. Comments: This is a fascinating instrument on all levels, providing the link between the 4½” dialled instrument first produced by Vidie and the much more familiar ‘watch-size’ pocket barometers. Manufacture of this instrument is by the premier London maker Negretti & Zambra, as commissioned by Admiral Fitzroy, and of considerable quality. It conforms to the pattern of the earliest known pocket barometers originating in 1860 and only effectively produced in this size for a year or so before Negretti & Zambra successfully introduced the watch-size instrument, thus of considerable rarity (see Negretti & Zambra, A Treatise on Meteorological Instruments, Chapter V: Secondary Instruments, item 49 and accompanying Fig. 34). Two other similar instruments have been seen, one with the dial signed “Negretti & Zambra.” Aspects of this instrument would indicate a prototype or early production. Of most note is the dial platform enabling the precise height of the dial to be set - the ring is maintained on a fine internal thread and finally pinned when the right location achieved. Also noteworthy is the means by which the primary lever is driven from the capsule, connected to the C spring via a transverse steel rod.  In this condition and working as it does, very much a collector’s piece.