In correspondence to division 0 and division 100 we see the words: Secheresse Extreme and Humidité Extreme.
The fixed points 100 and 0 were determined by Horace Bénédict de Saussure by placing the instrument under a bell in which the atmosphere was first saturated with steam and then dried with the introduction of melted potassium.
Since the tension of steam depends on temperature, the measurements performed were reliable only at temperatures close to the one at the time of calibration. For this reason, and because hairs deteriorate with the passing of time, Saussure's hygrometer was soon abandoned.