Biographical Memoir of Horace Benedict de Saussure. 231 



teors ; the anemometer, for determining at once the direction, 

 velocity, and strength of the currents of the air ; lastly, it was 

 from these motives that he invented the cyanometer and diapha- 

 nometer, for comparing the degrees of colours and of transparency 

 of the air at different heights. It is unnecessary for us to say that 

 the measurements of heights by the barometer must also have 

 been a continual object of his investigations. Thus, in examining 

 the mountains as a natural philosopher, he explored the atmo- 

 sphere as a geometrician and a chemist ; and it is to him, in fact, 

 that we owe all the positive information which we possess re- 

 garding the composition and motions of the fluid by which we 

 are enveloped. 



These different applications of physics form so many interest- 

 ing digressions in the narrative of his journeys. We follow him 

 with delight in these delicate investigations ; we find him never 

 neglecting, in the most agreeable as in the most fatiguing situa- 

 tions, to impress upon his observations that scrupulous accuracy 

 which forms the seal and suretiship of correctness. He wrote, 

 however, a separate essay upon Hygrometry, which was the most 

 complicated and the most delicate of these measurements ; and 

 this work is one of the most beautiful with which natural philo- 

 sophy was enriched at the close of the eighteenth century. 



The question to be solved is, to ascertain how much water in 

 vapour is contained in a given volume of air. In order to solve 

 this problem, it would be necessary to separate the vapour from 

 the air, or, in other words, to dry the air completely. This 

 operation, however, is impossible, and the object can only be at- 

 tained by approximation, and at the expence of much tim&, by 

 employing substances that have a great affinity for moisture. 

 We therefore content ourselves with a body capable of putting 

 itself into a certain equilibrium of humidity with the surround- 

 ing air, and of indicating the moisture which it has taken up 

 by more or less apparent changes of weight or dimensions ; and 

 as the fibres of organized bodies are eminently endowed with 

 the property of being elongated by moisture, and contracted 

 from dryness, it is these substances, especially, that are employ- 

 ed for making hygrometers, or rather hygroscopes ; for, as we 

 have seen, they do not afford an exact measure, but only an 

 approximate indication. It is obvious that there must be great 

 differences of sensibility and exactness between the different 



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