Leonardo da Vinci designed a hygrometer using cotton and wax to measure atmospheric moisture.
A hygrometer measures humidity, meaning the amount of water vapor in the air. Leonardo’s design used materials whose weight or behavior could change with moisture. By comparing the response of cotton with a reference such as wax, the instrument could indicate whether the air was relatively damp or dry.
Leonardo’s hygrometer belongs to the early history of scientific instruments. It was not a modern digital humidity sensor, and its readings would not have had the precision of later calibrated instruments. Its significance is that it treated humidity as something that could be observed mechanically rather than described only by sensation.
Hygrometers are often confused with barometers. A barometer measures air pressure, while a hygrometer measures humidity. A rain gauge measures collected precipitation, which is related to weather but is not the same quantity as water vapor in the air. Leonardo’s design reflects his broader effort to quantify natural phenomena with simple materials and carefully observed changes.