Antique barometer having applied printed card dial, with dual concentric scales, the outer calibrated in inches of mercury with a range from 16 to 31, the  inner denoting altitude with a range from 0 to 18,000 feet, the upper part marked “Mountain Barometer,” the lower with Registration Mark diamond indicating the year 1862, and carrying the retailer’s details, Horne & Thornthwaite, London.

Blued steel pointer, the bevelled glass with centre knob and gilt index. 2” capsule driven early pattern movement tensioned on a C spring with lateral bracing bar, heavy cast primary lever, the transfer shaft with one fixed and one adjustable pivot mounted in a common sub chassis also supporting arbour and hair spring assembly. All maintained within a brass drum form case with adjustable platform determining dial height. Compensation port to verso.

Original oxblood leather over timber navy blue silk and velvet lined case, snap closure on a button release.

An unusual instrument manufactured for a brief period, probably no more than 18 months or so, as a transition from the larger 4” diameter instruments introduced by Vidi. The construction of the instrument is of interest, the movement having common attributes to a similar instrument attributed to CW Dixey, in particular the method of maintaining the height of the dial within the drum on an adjustable threaded platform ring, a characteristic that might suggest evolutionary production design. It is not possible to be sure of the maker’s identity, it seems likely though that most if not all instruments of this pattern were the work of Negretti & Zambra who made them available for preferred retailers and instrument makers alike. It is known that Negretti & Zambra were working to produce a watch sized version of the Vidi instruments now widely available. It is likely therefore that these instruments in various forms ranging from straightforward barometers to the much rarer and specifically designated Mountain Barometers aimed at alpine explorers were the work of Negretti & Zambra. In any event, it is a near certainty that they were all the work of one maker.

This instrument has been extensively tested, and the results show reasonable accuracy with reducing pressure. However as pressure is increased from 23”, a maximum error of 0.5” is evident at 30” – 31”  – see graph. The instrument is not faulty as such – the error can be explained by the phenomenom known as hysteresis, a shortcoming in response to pressure change caused by the memory effect of deforming metals. Essentially the pressure sensing capsule expands with declining pressure, and when the pressure is increased it does not return to the same dimensions as it started and so the reading obtained will show as low.

This shortcoming in the aneroid design became well known as manufacturers started to develop true altimeters such as the mountain barometer. In an effort to mitigate this effect, many different metals and amalgams were experimented with. Developments in the metallurgical structure of the capsule continued well into the 1950s.

Retains a good proportion of its original lacquered finish, some minor wear and discolouration, the movement performing well but within the restrictions of the original design, The case crisp with minor wear and marks, the internal fabrics clean with good colour and vibrancy.

This is one of the first dedicated altimeters and must be regarded as highly significant. Very rare, this wonderfully interesting instrument is in good working order and fine condition, having been the subject of detailed testing, service and conservation. Most definitely a collector’s lot.