As an artist and observer of the science of nature da Vinci was fascinated by the sky and sketched his observations of changes in the atmosphere.
His hygrometer is based on a simple balance scale. When dry the cotton and the weight opposite are in balance. As the cotton absorbs atmospheric moisture it grows heavier indicating an increase in humidity.
IMAGE: Hygrometer Photo: Vitaly Korovin / Shutterstock.com
Hygrometer
brass, Perspex, wadding and metal