Hygrometer 2

This instrument measures the moisture content of the air at the time the measurement is taken. A household name already in Leonardo's day, it consists of scales that contain a hygroscopic substance (sponge, cotton wool) in one pan and wax in the other (wax does not absorb water.) On dry weather conditions, the scales mark zero. However, as the moisture in the air increases - and the weight of the hygroscopic substance rises accordingly -, the scales will tip towards the pan where the hygroscopic substance is.
Codex Atlanticus, sheet 30 v.