Leonardo da Vinci’s moisture-measuring device was a hygrometer. His design used a balance and a hygroscopic material whose weight changed as it absorbed moisture from the air.
The concept reflects Leonardo’s interest in measuring physical conditions rather than merely describing them. A hygroscopic substance gains mass in damp air and loses mass in dry air, so a balanced scale can indicate relative changes in humidity. The design belongs to the early history of weather instruments.
A hygrometer measures moisture, while a barometer measures air pressure, a thermometer measures temperature, and an anemometer measures wind speed. Leonardo’s device should therefore not be called a general weather machine. Later hygrometers became more standardized, but the underlying principle of using moisture-sensitive materials remained important.