519 



point of saturation in the air is not at 100°, but 

 at 84*5° (99° Sauss.), this effect of a cloud on 

 the instrument appeared to me very extraor- 

 dinary. The mist lasted long enough for the 

 whalebone to have increased it's length by it's 

 attraction of the particles of water. Our clothes 

 were not wet, A traveller, accustomed to ob- 

 servations of this kind, lately assured me, that 

 he had remarked on Mount P61ee in Martinico 

 a similar effect of clouds on the hair hygrome- 

 ter. It is the duty of the natural philosopher, 

 to relate all the phenomena that Nature dis- 

 plays to him, especially when he has neglected 

 nothing to avoid error in his observations. Mr. 

 de Saussure, in a violent shower, saw his hy- 

 grometer, which was not wetted by the rain, 

 keep up # (almost as at the Silla in the cloud) at 

 84*7° (48*6° Deluc) ; but it is easier to conceive, 

 that the air dispersed between the drops of rain is 

 not completely saturated, than to explain how 

 vesicular vapours, in immediate contact with 

 the hygroscopic body, do not impart to it great 

 er humidity. What can be the state of a va- 

 pour, which does not wet, and which is visible 

 to the eye ? We must, I believe, suppose that 

 a much drier air had been mixed with that 

 where the cloud was formed ; and that the vesi- 

 cles of vapour, the volume of which is much 

 ■ 



* See chap, iv, vol. ii ? p. 93. 



