What Leonardo da Vinci instrument estimated humidity using a balance?

Leonardo da Vinci’s balance-based instrument for estimating humidity was a hygrometer.

A hygrometer measures humidity, the amount of water vapor in the air. Leonardo described an early device in which a moisture-absorbing material was balanced against a dry material. As the absorbent material took up water from the air, its weight changed and the balance moved, indicating a change in atmospheric moisture.

The instrument belongs to Leonardo’s wider studies of weather, water, and physical measurement. It was an important conceptual step because humidity cannot be seen directly, so the device translated an invisible atmospheric condition into a measurable change in mass.

Leonardo’s hygrometer should not be confused with a barometer, which measures air pressure, or an anemometer, which measures wind speed. Nor was it equivalent to the electronic humidity sensors used today. Its significance is the use of hygroscopic material and a balance to compare dampness systematically.