Saussure's hygrometer is an absorption hygrometer, created by the naturalist Horace Bénédict de Saussure (1740-1799) in 1775. It is based on the property of a well-degreased (for example with a solution of potassium or sodium carbonate, or sulphuric ether) human hair to absorb atmospheric water vapour in proportion to the amount of humidity in the air and to increase in length. The instrument shown here has a brass frame (width 10 cm, height 30 cm) and a wooden case with panes of glass and small side windows with netting; the perfectly degreased hair is fixed to the frame at the top by means of a small clamp the height of which can be adjusted with a screw; at the bottom it is wound around a pulley to which it is attached. A pointer, joined to the axis of the pulley, amplifies the miscellaneousations in the length of the hair on a scale with 100 divisions. ...continues