A very unusual fine chart table marine aneroid barometer by Casella, Maker to the Admiralty and Ordnance, no. 4898 c1884

Aneroid chart table barometer having 4½” silvered, engine divided and engraved dial, the outer wheel driven time scale marked in Arabic numerals and divided to ¼ hours, the inner barometric scale calibrated in inches of mercury with a range from 28” – 31” and divided to 1/100”. The upper part signed by the maker “L. Casella, Maker to the Admiralty and Ordnance, London,” the lower annotated “Compensated” and serial no. “4898.” Dagger form blued steel pointer, bevelled glass. Bezel driven spine form setting index.

Conventional Vidie pattern movement driven from a single 2⅞” soldered capsule tensioned on a C spring, the whole contained within a drum form case, side mounted wheel and gear assembly, calibration port to verso with dust cover.

Presented in its original bespoke green baize lined teak sloping fronted case, inspection port with bevelled glass. Lock and key, silver plate affixed to front engraved “FELIXSTOWE, 2nd June 1884.”

Condition: The subject of a full service, conservation, and calibration under laboratory conditions, see performance chart from dynamic test. The movement working exceptionally well with a maximum logged error across the scale of 0.03”.

Essentially fine, the dial crisp and clean, the brass case retaining virtually all its original lacquered finish. The timber case with minor marks, light impressions, retaining much original lacquered finish. The lock working probably with original key, the tag marked “Aneroid”.

Comments: This is a very unusual, possibly unique instrument, having a setting time scale, the time of observation logged with the bezel set spine index – essentially the time of observation set at the head of the pointer, the bezel setting spine index aligned with both pointer head and time. It is then possible to determine the rate of pressure change as judged by the changed pressure value and elapsed time, an extremely useful observation at sea for determination of approaching squalls.

At time of writing this is the only such instrument we have known, though it is likely, given the obvious merits of such a system, that there may be others. It is unclear at this time if the system was patented or unique to Casella.

Further research continues into the provenance of this instrument.