Barometers Realm

This refined brass desk weather forecaster (Patent No. 6276/1915, London) с.1925 designed originally by E. W. Kitchin and later improved and perfected by Negretti & Zambra, represents one of the most sophisticated mechanical forecasting devices available to the public in the early twentieth century. It is constructed from three concentric, finely engraved brass rings surrounding a central plate that bears detailed operating instructions.

The outermost ring is set first: the user aligns the engraved scale with the observed wind direction and notes whether the barometer is rising, falling, or steady. The central pointer is then rotated to indicate the current barometric pressure (corrected to sea level). Once these two settings are established, the forecast automatically appears in the small aperture on the central plate. If the pointer falls between two indications, the instructions advise the user to combine the readings. Seasonal rules are also provided: for rain read snow when temperatures are sufficiently low; winter rules apply from October to March, and summer rules from April to September.

Devices of this type were revolutionary. Although Admiral FitzRoy had earlier published detailed barometer-based weather rules for domestic use, the barometer itself remained merely a register of current pressure unless the observer possessed both discipline and meteorological knowledge. Negretti & Zambra’s patented forecaster solved this problem elegantly: with a single adjustment of the brass discs, an accurate short-term forecast could be read instantly, requiring no expertise beyond access to a barometer.

Promoted in contemporary catalogues as “invaluable to motorists, golfers, hunting men, and all outdoor sportsmen”, the instrument quickly became popular. Pocket versions were widely produced, yet these larger desk models were both more accurate and more visually impressive.

This forecaster was supplied either on its own or together with the so-called “Forecasting aneroid” which was particularly convenient, since the forecaster requires the input of barometric values reduced to sea level, whereas ordinary domestic barometers generally indicated relative pressure depending on their geographical location and altitude. The forecasting aneroid, by contrast, was equipped with a quick-adjustment mechanism for switching to sea-level pressure.

For enthusiasts of early meteorology, they remain remarkably functional even today—over a century after their invention—demonstrating the ingenuity of pre-computer weather prediction in the era before numerical modelling transformed the field.