For more than 200 years, hygrometers have served as reliable instruments to measure relative humidity and thus to predict the weather. Mariners have long known that the likelihood of fog formation is directly correlated with the relative moistness of the air. But few people are aware that the first device to measure atmospheric moisture was invented in the 14th century to serve an entirely different purpose: namely, to settle the age-old quarrel between buyers and sellers of wool. Their bone of contention: the same amount of wool weighed more on humid days than on dry days, and was accordingly more costly when the air was moist. Cardinal de Cusa (1401 -1464) deserves credit for inventing a device that could measure the relative humidity and thus enable wool merchants to standardise the net price at wool markets.