COBALT BAROMETER — "The Gentleman with an Umbrella"
A weather card in the best English tradition: a wet High Street, a gas lamp, the Crown Inn, and a gentleman in a bowler hat who plainly knows something about the sky that we don't.
The umbrella is the working part. Its canopy is treated with cobalt salts — cobalt chloride — which bind and release water from the air and change colour as they do. Dry air keeps the crystals blue: fine weather. As humidity rises the tint drifts through violet — change. In damp air it turns pink, and the gentleman's umbrella is about to earn its keep.
Strictly speaking the card reads humidity rather than pressure, so "barometer" is a traditional name rather than a technical one. On a British street it still gets the forecast right more often than one might expect. No batteries, no calibration, no opinions of its own.
Best hung away from direct sunlight and well clear of kitchen steam. The colour is fully reversible: as the air dries, the umbrella comes back to blue.