462 LIFE OF HORACE BENEDICT DE SAUSSURE 



De Saussure used his hygrometer as frequently as he did 

 the barometer and the thermometer, and by its means he dis- 

 covered that the atmosphere was subject to great variations 

 of humidity by night as well as by day. These he was unable 

 to explain, but they were probably associated with Fohn (a 

 phenomenon which, so far as I have been able to ascertain from 

 studying the index to the Voyages dans les Alpes, de Saussure did 

 not investigate). One interesting point noted was that at night 

 the lowest humidities on the Col du Geant occurred when the 

 humidity at Chamonix was relatively high. Dealing with the 

 question of the rate of evaporation, he showed that on the Col 

 du Geant a given rise of temperature evaporated three times the 

 weight of water that it did at low levels, a well-known fact 

 associated with diminution of pressure now used commercially 

 in the vacuum pans for concentrating sugar solutions and the 

 like. 



De Saussure was unable to deal fully with the temperature 

 relations of evaporation, because, as he regretfully acknowledged, 

 there was no method of determining the absolute zero of tem- 

 perature. The advancement of science has found such a method, 

 and meteorologists are now beginning to reckon temperature 

 from the absolute zero on account of the facility it gives to thermo- 

 dynamic calculations. 



The innumerable observations on storms, mist, and haze 

 made by de Saussure in the course of his wanderings are full of 

 interest, and he often detected facts the full explanation of which 

 was beyond the reach of the science of his day. His observations 

 on the formation of cloud were particularly interesting, and their 

 value is not much affected by the assumption he made that the 

 hypothesis of the time was correct which attributed the main- 

 tenance of a cloud in the air to the hollow vesicles of which it was 

 believed to consist. He showed in a very convincing way that 

 the cloud banners which are often seen streaming from a mountain 

 are not at rest, but are continually forming where the moisture - 

 laden air encounters the chill of the peak and evaporating as it 

 passes beyond its influence. 



The many lakes of the Alps received the attention of de 

 Saussure less from the point of view of a geographer than 

 from that of a natural philosopher intent upon establishing 

 a theory of the earth. Thus he makes no suggestion of a 

 systematic survey of lake -basins, but contents himself in 

 most cases by ascertaining the depth of the part reputed 

 locally to be deepest. This found, he devoted much ingenuity 



