Leonardo da Vinci’s wool-based humidity-measuring device was a hygrometer.
The principle was simple: wool absorbs moisture from the surrounding air, so its weight changes as humidity changes. By comparing the wool’s weight with a dry reference, an observer could estimate the amount of moisture in the atmosphere. Leonardo’s proposal turned a visible material change into a measurement.
This is an early example of a hygrometric method. Modern hygrometers may use electronic sensors, chilled mirrors, hair, or other materials, but they all seek information about atmospheric moisture. Leonardo’s wool design was not a thermometer, because temperature was not its measured quantity.
The device is also distinct from his anemometer, which concerns wind speed. Leonardo’s instruments show that his scientific interests extended beyond machines and weapons to weather variables that cannot be seen directly.