What kind of machine did Leonardo da Vinci design to measure humidity?

Leonardo da Vinci designed a hygrometer to measure humidity.

A hygrometer measures the amount of moisture in air. Leonardo’s design used materials whose weight changes when they absorb atmospheric moisture, allowing the effect to be shown on a balance. In commonly described versions, a moisture-sensitive substance such as cotton or wool is compared with a dry reference.

The instrument belongs to Leonardo’s broader investigations of weather, water, air, and physical measurement. It was not a modern electronic humidity sensor, and it did not provide a direct digital percentage. Instead, it translated an invisible atmospheric property into a measurable change in mass or balance.

Leonardo’s hygrometer is sometimes confused with his studies of weather instruments such as anemometers and barometers. Those devices measure wind and air pressure, respectively. Humidity concerns water vapour in the air, making the hygrometer a distinct instrument.