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brass, gutta-percha
Before us is an original Portable Weather Forecaster, circa 1925, by one of the most renowned British makers of scientific instruments — Negretti & Zambra, London. The device is based on a system patented in 1915 (Patent No. 6276) and represents a characteristic example of the interwar period, when meteorological instruments combined engineering ingenuity with accessibility for a wider public.
The instrument is formed as a compact circular disc with a folding, retractable stand. The body is made of brass, with the reverse covered in gutta-percha, providing protection against corrosion. At the top of the case is a suspended brass ring element. This is a so-called split ring — a construction in which the metal forms a double coil, allowing the edge to be lifted and an object (typically a key) to be “threaded” into the enclosed loop. Such rings were invented in the 19th century by Samuel Harrison, and the design allowed objects to be easily added by advancing them along the spiral. It is noteworthy that Negretti & Zambra adopted this everyday engineering solution and integrated it into a scientific instrument.
The Weather Forecaster consists of a multi-layered rotating system of concentric discs, operating according to the principle of a mechanical “forecasting algorithm.” The construction is based on three functional zones:
The user first sets the outer ring (rear disc), aligning the current wind direction with the present state of the barometer: rising, falling, or steady. This forms the primary input parameter for the forecast.
The user then rotates the inner disc (front), setting the current barometric pressure in inches of mercury (previously corrected to sea level). In this way, the instrument receives the second key parameter — absolute pressure.
Once these parameters are set, the forecast automatically appears in a small aperture on the inner disc. If the pointer falls between two indications, the instructions recommend combining both results, interpreting the forecast as intermediate.
The instrument incorporates seasonal logic, making it not merely a mechanical calculator but an adaptive system for interpreting weather conditions:
Method of Use
Thus, the instrument functions as a mechanical model of synoptic logic, transforming observed parameters into a forecast.
The Weather Forecaster by Negretti & Zambra is not merely an auxiliary device, but an elegant example of an early “analogue computing” instrument designed to interpret meteorological data. The combination of thoughtful mechanics, compact form, and inventive construction makes it a vivid testament to a period when scientific instruments became part of everyday life while maintaining a high level of engineering sophistication.