Wind Speed from a Cup Anemometer: Robinson's One-Third Law (1850)

The cups of a cup anemometer move at one third of the wind's speed. Count the turns in a minute and the notebook turns them into miles per hour and metres per second. Result: the wind speed, from Robinson's portable instrument (cups on a 5.6 in radius: turns per minute / 10 = mph). Source: T. R. Robinson, Transactions of the Royal Irish Academy vol. 22 (1850), pp. 158, 168-169. Photo: NOAA, public domain.