Leonardo da Vinci designed a hygrometer using a balance between absorbent and non-absorbent materials.
The instrument was intended to detect changes in humidity by comparing the weight or behavior of a moisture-sensitive material with a material that did not absorb water from the air. A change in atmospheric moisture would alter the balance and provide a visible indication.
Leonardo’s interest in humidity was part of his broader investigation of air, water, weather, and materials. His proposed hygrometer belongs to the early history of instruments that made atmospheric conditions measurable rather than merely observable.
A hygrometer is different from an anemometer, which measures wind, and a barometer, which measures air pressure. Leonardo’s design was an early humidity indicator, not a modern electronic sensor and not necessarily a device calibrated in today’s relative-humidity units.