Leonardo da Vinci described a hygrometer, an instrument for measuring humidity, using a balance-based arrangement.
The concept belongs to his broader study of atmospheric conditions. In a hygrometer, a material or object affected by moisture changes weight, length, or another measurable property. Leonardo's balance design used changes associated with dampness to compare atmospheric moisture with a reference condition.
This was an early attempt to turn humidity into a measurable quantity. Renaissance investigators lacked modern electronic sensors, calibrated polymers, and standardized humidity scales, so their instruments often depended on natural materials such as wool or other moisture-sensitive substances.
Leonardo's humidity instrument should not be confused with his anemometer. The anemometer concerns wind, while the hygrometer concerns moisture in the air. The modern relative-humidity percentage is a later scientific convention, not a reading that Leonardo's sketch could provide directly.