Leonardo da Vinci designed a hygrometer to estimate humidity by comparing the weight of materials.
A hygrometer measures moisture in the air. Leonardo’s proposed instrument used a balance with materials that respond to humidity, such as wool or other absorbent substances, compared with a reference material. As the absorbent material took up moisture, its weight changed and shifted the balance. That movement provided an indication of how damp the air was.
The instrument was an early attempt to make atmospheric moisture measurable rather than merely observable. Leonardo’s work belongs to a long development of hygrometry; later instruments used hair, paper, salts, and other materials whose physical properties change with humidity. Modern electronic hygrometers use sensors rather than a simple balance.
A hygrometer is not a barometer. A barometer measures air pressure, while a hygrometer measures humidity. Leonardo’s design also differs from his anemometer, which concerned wind motion. These devices show how his notebooks connected natural observation with practical mechanical measurement.