Barometers Realm

Mariner’s Aneroid Barometer, circa 1920, by Negretti & Zambra — a renowned British manufacturer of scientific and meteorological instruments, whose products were widely used in both maritime practice and scientific observation.

The barometer is housed in a compact cylindrical case made of heavy spun brass, providing durability and resistance to operational conditions. At the top of the case is a suspension ring, allowing the instrument to be conveniently mounted on a wall or within a ship’s cabin.

The front of the case is fitted with a thin tensioned bezel, retaining a thick flat mineral glass with a finely executed bevelled edge. Through a central aperture in the glass passes a knurled brass adjustment knob, which controls the setting pointer (trend indicator), enabling the user to record previous pressure readings.

The dial is made of silvered brass and features a high-precision scale produced by engine-division. It is calibrated in London inches of mercury, with each inch divided into 50 parts (lines), allowing for accurate readings. The barometric scale is supplemented by standard weather indications, executed in the characteristic style of Negretti & Zambra, derived from the traditional lettering of mercury barometer register plates.

The movement represents a refined aneroid construction. At its core is a corrugated chamber made of nickel-silver, from which the air has been evacuated. This sensitive element is mounted on a frame plate, above which is positioned a strong curved steel C-shaped spring, fixed at one end to a substantial support mounted between two screw pillars. This spring suspends the sensitive chamber, maintaining tension across its membranes.

A thick brass lever, attached to the upper end of the spring, transmits motion to the main arbor, which rotates in bearings. From there, the motion is conveyed via a fusee chain to the pointer arbor, driving the indicating hand. A spiral spring attached to the pointer arbor maintains constant tension in the chain.

The mechanism incorporates temperature compensation by means of a stable bi-metallic plate. This bi-metallic link system is formed by reducing part of the thickness of the main lever and replacing the brass layer with a steel plate. A similar principle was employed by John Earnshaw in the construction of compensation balances for marine chronometers.

This barometer represents a characteristic example of a high-quality ship’s aneroid from the early 20th century, combining robust construction with precise engineering and the established traditions of British instrument making. It reflects the level of development of maritime meteorology of its time and remains an important example in the evolution of practical aneroid barometers.