Leonardo da Vinci designed a hygrometer to show changes in humidity with a balance.
A hygrometer measures moisture in the air. Leonardo’s proposed instrument used a moisture-sensitive material, traditionally described as wool, balanced against a fixed weight. As the material absorbed moisture, its weight changed and the balance indicated a change in atmospheric humidity.
The design reflects Leonardo’s broader interest in measuring natural phenomena mechanically. His notebooks include devices for observing air, water, wind, and weather, often translating an invisible physical change into movement that could be seen on a scale.
This device should not be confused with an anemometer, which measures wind speed, or a barometer, which measures air pressure. Leonardo’s hygrometer was an early concept rather than a standardized modern instrument, and the surviving descriptions do not establish that he produced a durable working model.