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A325-26 · Aneroid
c.1870
Object Overview
This Casella Brass Drum Aneroid Barometer is an early English aneroid barometer manufactured around 1870 by the renowned London maker of scientific and meteorological instruments Louis Casella, an official supplier to the British Admiralty and Ordnance departments. The instrument bears serial number 1437, a relatively early number consistent with its proposed dating to the second half of the nineteenth century.
The barometer is a characteristic example of high-quality British instrument-making of the Victorian era, combining a traditional aneroid mechanism, a carefully crafted metal case, and an elegantly designed dial.
The barometer has a cylindrical, stepped and profiled case, characteristic of early aneroid instruments. Made entirely of brass, the case is relatively deep to accommodate the substantial mechanical assembly comprising the aneroid capsule, steel spring, and lever transmission. The front of the case features several concentric mouldings and smoothly contoured transitions, giving the substantial construction a distinctive yet restrained appearance.
aneroid capsule tensioned on a C-spring
brass, cast iron, glass, nickel silver, silvered brass, steel
A robust brass suspension ring is fitted at the top of the case, attached by a small articulated fitting that allows the instrument to hang vertically on a wall.
The unusual finish of the brass surface deserves particular attention.
The barometer's polished brass case displays an exceptionally beautiful, deep golden hue reminiscent of aged gilding. This colouration is most likely attributable to a protective lacquer coating, possibly based on natural shellac. Such transparent resin-based lacquers not only protected the metal against atmospheric corrosion but also imparted a characteristic warm golden tone, which could become even richer over time through the natural ageing and yellowing of the resin.
Unfortunately, it is impossible to establish whether the surviving coating is the original finish applied when the instrument was manufactured. It is more likely that the coating was renewed at a later date, possibly in the early 1920s. Much of the case still retains a bright metallic lustre, while numerous scratches, including initials deliberately engraved by a former owner, have acquired a dark brown or almost black appearance as a result of localised oxidation of the exposed brass.
Examination of the surface under ultraviolet light revealed a characteristic warm yellowish fluorescence, consistent with the presence of an old organic lacquer coating. Although the precise composition of the lacquer cannot be established without instrumental analysis, the combined observations suggest that the unusual golden colour of the case results from the preservation and natural ageing of a protective and decorative finish, rather than exclusively from the formation of a mineral patina.
The front of the case is enclosed by a thin brass bezel that fits closely around its outer circumference. The bezel retains a flat mineral glass crystal with a bevelled edge, protecting the dial, pointer mechanism, and mercury thermometer positioned in the lower section.
An additional pointer, used to indicate the trend in atmospheric pressure, is mounted at the centre of the glass. It is attached to a separate spindle passing through the crystal and operated by a small brass knob with a knurled edge. By turning this knob, the observer can align the auxiliary pointer with the current position of the main barometer hand, making it possible to assess the magnitude and direction of subsequent changes in atmospheric pressure.
The dial is made of silver-plated brass and is surrounded by a broad, polished reflector ring made of similarly finished metal. The combination of the matte silver surface, fine black engraving, brass components, and blued-steel barometer hand creates the characteristic contrast of materials and tones associated with high-quality English instruments of the nineteenth century.
The dial is divided into two principal functional sections.
The upper half carries a broad, curved barometric scale graduated in English inches of mercury (inHg). The measuring range extends from 28 to 31 inches of mercury, with each inch divided into fifty equal parts, corresponding to 0.02 inHg. Intermediate values are marked by small numerals from 1 to 9, representing tenths of an inch, while numerous fine graduations allow smaller variations in atmospheric pressure to be read.
Along the inner side of the scale are the traditional English weather indications:
A notable feature of the dial's design is the use of two different typefaces. The inscriptions RAIN, CHANGE, and FAIR are rendered in a decorative serif typeface, whereas the outermost indications, STORMY and VERY DRY, appear in a more restrained sans-serif style. All weather indications are written in uppercase letters. This combination of typefaces gives the scale a distinctive appearance and reflects the decorative preferences of Victorian instrument-making.
In the upper central section of the dial, immediately beneath the barometric scale, is an elegantly engraved maker's inscription:
L. Casella
Maker to the Admiralty & Ordnance.
LONDON
The inscription identifies the manufacturer's status as a supplier to British government departments responsible for naval affairs and military ordnance. Such a designation testified to the professional reputation of the firm and emphasised the precision and quality of its measuring instruments.
The main barometer hand is made of blued steel and has a characteristic elongated form. Its pointed indicating end moves along the upper curved scale, while the opposite end is shaped into a small decorative crescent, which also serves as a counterweight.
The lower half of the dial contains a mercury thermometer bent into a broad arc following the circular outline of the case. Its fine glass capillary tube is positioned against a separate temperature scale graduated in degrees Fahrenheit.
The temperature scale covers a range of approximately 10 to 130 °F, with numerical markings at ten-degree intervals. This arrangement makes efficient use of the lower portion of the dial without increasing the overall dimensions of the instrument, while preserving the visual symmetry of the composition.
The instrument operates by means of a conventional aneroid movement based on a large, circular, corrugated metal capsule from which the air has been evacuated. Changes in atmospheric pressure produce minute elastic deformations of the capsule walls, which are converted into angular movement of the indicating hand through a system of levers and a flexible chain transmission.
The capsule is mounted on a substantial circular metal plate finished in black, which also serves as the foundation of the entire measuring mechanism.
The upper movable wall of the capsule is mechanically connected to a powerful C-shaped steel spring. The lower end of the spring is secured to a substantial cast-iron crossbar positioned above the capsule, while its upper free end transmits the movements of the capsule's movable wall.
The spring performs the essential function of the principal force-bearing element of the aneroid mechanism. By opposing atmospheric pressure, it provides the necessary elastic load on the capsule, allowing its walls to move in response to changes in external pressure. The use of a separate steel spring gives the measuring system the required sensitivity and mechanical stability.
A long brass main lever extends from the free end of the spring. This rigid metal component transmits the minute movements of the spring to the subsequent elements of the mechanism.
The main lever is connected by a long coupling rod to a movable bell-crank lever mounted on pivot bearings. This bell-crank converts the transmitted linear displacement into a small rotational movement about its own axis. The ratio between the lengths of its arms provides additional mechanical amplification of the extremely small deformations of the pressure-sensitive element.
A fine, flexible fusee chain extends from the vertical arm of the bell-crank lever to the pointer spindle, forming the final element of the kinematic transmission.
A distinctive feature of this mechanism is the arrangement used to convert the linear movement of the chain into rotation of the indicating hand. A small cylindrical brass drum with a circumferential groove is mounted on the pointer spindle, and the fusee chain runs within this groove. As the bell-crank lever rotates, the chain moves, winding onto or unwinding from the drum and thereby producing a corresponding rotation of the pointer spindle.
Constant tension in the chain is maintained in the manner customary for such mechanisms, by means of a flat spiral spring mounted directly on the pointer spindle. This spring produces a small restoring torque, keeping the chain under tension and ensuring a stable mechanical connection between the bell-crank lever and the indicating hand when the direction of movement changes.
Overall, the mechanism represents a characteristic example of a mature Victorian aneroid design, incorporating virtually all the principal technical solutions developed during the first decades following Lucien Vidie's invention of the aneroid barometer in 1844.
The Casella Brass Drum Aneroid Barometer is a remarkable example of English scientific instrument-making from the early and middle Victorian period. Its historical significance lies not only in the distinguished reputation of its manufacturer and its relatively early serial number, but also in the combination of a traditional aneroid mechanism with fusee-chain transmission, an elegant silver-plated dial, and an unusual curved mercury thermometer.
The instrument owes much of its individual character to its substantial brass case, which retains an old lacquer coating whose deep golden hue contrasts strikingly with the silvered surface of the dial. Traces of prolonged use, darkened scratches, and occasional losses to the finish do not diminish its appeal; rather, they emphasise the authentic historical character of the object.
This barometer vividly illustrates a period when aneroid instruments had already achieved a high degree of technical sophistication while still retaining the distinctive craftsmanship and individual character of their makers. It is precisely this combination of mechanical ingenuity, meticulous finishing, and refined aesthetic quality that makes such instruments particularly valuable for the study of the history of meteorological instrumentation.