Leonardo da Vinci designed a hygrometer, a mechanical instrument intended to record changes in humidity.
His design used the changing weight of an absorbent material to indicate the amount of moisture in the air. As the material absorbed water, its weight changed; that change could be compared with a counterweight and read as an indication of atmospheric humidity.
The concept reflects Leonardo’s broader interest in turning natural observations into measurable quantities. Humidity was difficult to assess directly, so an instrument based on a moisture-sensitive material offered a practical way to compare damp and dry conditions.
A hygrometer measures moisture in the air, whereas a thermometer measures temperature and a barometer measures air pressure. Modern hygrometers may use electronic sensors, but Leonardo’s proposed instrument relied on a physical change in an absorbent substance.