Leonardo da Vinci designed an early hygrometer to estimate humidity by comparing the changing weight of moisture-sensitive materials.
The instrument relied on the fact that some substances absorb water from damp air. Leonardo proposed placing such a material on one side of a balance and a dry reference material on the other. A change in balance could therefore reveal that the air had become more humid.
This was an early form of hygrometry, the measurement of moisture in air. It was not a modern electronic sensor and did not provide the precise relative-humidity percentage familiar today.
Leonardo’s interest in humidity connected natural observation with practical engineering. Moisture affects materials, weather, agriculture, and the performance of machines, so measuring it could support both scientific study and design.