Nicholas of Cusa’s Woollen Hygrometer

Exactly 400 years earlier, Nicholas of Cusa surmised that the weather could be predicted by measuring humidity. The German mathematician (and cardinal) designed a device that looked like a simple set of scales: a ball of untreated wool hung on one side, with stones on the other. As the wool soaked up water vapour in the air and became heavier, it would tip the scales towards it, moving a marker above that showed the increased humidity on a scale. Conversely, as the air became drier and the wool lighter, the marker would tip the other way. Clearly too busy with mathematical problems and high affairs of the Church, the cleric never got around to building his weather-divining gadget. It wasn’t until 1481 that Leonardo da Vinci made a prototype based on Nicholas’ drawing. Since then, reasonably accurate hygrometers have been created deploying all manner of water-absorbing materials including salt, string, paper, oats and whalebones, while a very popular 18th-century design used hair.