Leonardo da Vinci designed a hygrometer to measure humidity in the air.
His proposed instrument used the changing weight of a moisture-absorbing material. As the material took up water from damp air, its weight changed; comparing that change could indicate the amount of moisture present. This was an early attempt to turn atmospheric humidity into a measurable physical quantity.
Humidity instruments later used hair, paper, salts, and other materials whose dimensions or mass changed with moisture. Modern hygrometers may use electronic sensors, but they still measure the same atmospheric property: water vapor content, often expressed as relative humidity.
Leonardo’s device should not be confused with a thermometer, which measures temperature, or a rain gauge, which measures liquid precipitation. The proposal illustrates his broader interest in instruments that connected careful observation with quantitative measurement.