An unusual French hybrid brass open dial aneroid barometer no. 2, maker unknown c1858.

Early aneroid barometer having 4¾” zinc backed printed open centre card dial with weather prognostications to outer part, the barometric scale calibrated in cms Hg, marked “Baromètre” at 6 o’clock. The open centre with a hair spring tensioned rack and pinion drive assembled onto a lacquered brass sub-chassis. Blued steel pointer, bevelled glass with gilt index.

The unconventional movement based on Vidie’s basic arrangement, motive force delivered from a second generation 3” nickel alloy capsule tensioned on a coil spring, transmitting the signal via the bridge extension (primary lever) to a drop link and cross spindle with mounted 3rd lever captured in a semi-elliptical yoke set at 90° to a vertical spindle having end mounted rack.

All configured over a heavy circular iron chassis and contained within a spun drum / OG case, pendant and suspension ring, calibration port to verso.

Condition: The subject of a full service, conservation, and calibration under laboratory conditions, see performance chart from dynamic test. The movement with signs of oxidation but working with easily acceptable accuracy at less than 0.3 cm across the scale. 

The dial clean and crisp, the printed characters clear. The brass case worn with no obvious signs of original finish, remaining clean and dent free.

Comments: A very curious instrument which from a strictly mechanical view point makes little sense, other than the fact that it would probably have been far more robust than the standard Vidie design.

The designer has, at its simplest, transposed something of a Bourdon movement onto a Vidie design, the result being the elimination of the sometimes troublesome fine and delicate chain, essentially replacing it with a rack and pinion, characteristics of the Bourdon design.

Aneroid barometers are basically mechanical amplifiers with a very small movement at the pressure sensing unit amplified into a much larger movement at the reading point. Basic mechanics shows us that, along this amplification route, the energy available becomes substantially less and so friction on the bearing surfaces of the moving parts becomes a big issue. 

This design, in eliminating a potentially troublesome component, the fine chain, has increased the inefficiencies caused by static friction in adding at least a further two points of movement on bearing surfaces. The rack and pinion can be significantly less efficient than a fine chain – it will always be less efficient than a chain, despite the greatest care in manufacture, if executed badly or out of true. The only practical advantage that can be seen in this instrument is the rather more robust mechanism where fine chains are prone to oxidation and breaking at the anchor points.

As to the maker or originator, clearly French-made and most likely from a firm that has now reached near total obscurity – it is unlikely to be the work of Redier whose designs do have some similarities to the instrument, and Naudet can certainly be discounted.

Vidie marked all his early barometers “Baromètre Aneroide,” Naudet, “Baromètre Holosteric,” and Bourdon, “Baromètre Metallique.” The fact that this instrument is simply marked “Baromètre” suggest manufacture at or very near the expiration of the relevant patents, the maker not wishing to go too far in plagiaristic terms.

One thing is clear – the basic execution of the small parts and the design of the bridge do not immediately draw any conclusions as to origin. Although the making is acceptably good, the style, execution and finishing of the parts lacks any of the obvious cues found as hallmarks of other makers.

The instrument is quite aesthetically pleasing and that might have been partially responsible for this design above practical concern. The critical components of cast and machined brass carry serial no. “2” although the case is marked “XXI” (21). No 2 may therefore have been number 2 of a batch of say 12. However one views the serial numbers, this is pattern remains as yet unrecorded and because of its shortcomings was almost certainly a failure with very few produced.

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