A rare gilt and lacquered brass miniature aneroid pocket barometer altimeter by William Callaghan, London, No 4811 c1875

Pocket barometer having silvered and engraved 1⅜” dial calibrated in inches of mercury with a range from 25 – 31, outer altitude scale marked from 0 – 6,000 feet and annotated “Airey’s Scale,” the upper dial with standard meteorological terms “Rain, Fair,” and signed by the maker “Callaghan,” the lower part with business address “23a New Bond Street, corner of Conduit street, London.” Blued steel pointer, bevelled glass set in a rotating bezel with point index. Early C-spring 1″ capsule driven movement, cast and machined primary lever, the cock struck “11,” the chassis verso struck “4811.” 

All maintained within a low barrel form case with engine turned detail, extension post, suspension ring, calibration port to verso. Case marked with serial no. “4811” and verso bearing hand-engraved stylised initials “MC.”

The whole contained within its original clam-shell leather over timber, blue silk and velvet lined case, the lid silk with maker’s logogram and Royal Warrant in gold block. Snap closure on a button release.

Condition: The subject of a full service, conservation, and calibration, see performance chart. Minor dial marks, slight losses to wax, the case with minor wear but retaining much original finish. The leather case structurally sound, the outer covering with good colour and much original gloss. The fabric interior bright and vibrant.

Comments: Miniature barometers might be considered quite unusual – this early one, by arguably London’s premier maker of these instruments, and marked with the Airey’s Scale, is of great interest. Makers of these instruments, assuming the response of their instruments to be linear, had been aware for a while that the generally accepted linear altitude scale could not be correct since the density of air diminishes with altitude – thus a change in altitude made from sea level to a thousand feet would be interpreted quite differently with a rise in altitude from five to six thousand feet. Airey’s Scale, which took this variation into account, was first employed by makers in 1867 and eagerly sort by those seeking accuracy in their instruments.