Leonardo da Vinci designed a hygrometer to measure humidity by weighing an absorbent material.
A hygrometer measures moisture in the air. Leonardo's proposed instrument used a material that would gain weight as it absorbed atmospheric moisture, with a balance indicating the change. The idea belongs to his investigations of weather, air, water, and measurement.
This was an early conceptual approach to humidity measurement, long before electronic sensors. A weighing method can reveal relative changes in moisture, but its accuracy depends on the absorbent material, the balance, and careful calibration. Leonardo's sketch should not be confused with modern digital hygrometers or psychrometers.
The word hygrometer comes from Greek roots associated with wetness and measurement. Humidity instruments later used substances such as hair, which changes length as moisture changes, and thermometers combined in psychrometers. Leonardo's proposal is notable because it treats invisible atmospheric moisture as something that can be quantified mechanically.