86 



furnished me with observations which accorded 

 very well with each other. I have always pre- 

 ferred the old instrument with a single hair to 

 that of Richer, in which several hairs act at the 

 same time on the index, and with unequal ten- 

 sions. I can affirm also, that every thing Mr. 

 de Saussure has advanced, in his Essay on Hy- 

 grometry, of the long duration of his portable 

 hygrometers, is extremely exact*. I have pre- 

 served some without any alteration during three 

 years travels in the forests and mountains of 

 South America. Before my departure they were 

 compared by Mr. Pictet with the hygrometers 

 of the Observatory at Geneva ; and I have al- 

 most always found them at 99° or 100*5°, when 

 I have been able to expose them to a very thick 



fog- , 



As the fiftieth degree of the whalebone hygro- 

 meter corresponds to the eighty-sixth degree of 

 the hair hygrometer, I made use of the first at 

 sea and in the plains, while the second was ge- 

 nerally reserved for the dry air of the Cordilleras. 

 The hair below the sixty-fifth degree of Saus- 

 sure s instrument indicates, by great variations, 

 the smallest changes of dryness ; and has be- 

 sides the advantage of putting itself more rapidly 

 into a state of equilibrium with the ambient air. 

 Deluc's hygrometer acts, on the contrary, with 



* Ibid. §67. 



