different kinds of Dry Fogs. 247 



instrument continued to advance to dry, that is to 47°'7 (84 

 deg. Sauss.). The temperature of the air was, during the time, 

 from 12 to 13 degrees. Although in the whalebone hygro- 

 meter the point of saturation in the air is not at 100 degrees, 

 but at 84°-5 (99 deg. Sauss.), this effect of a cloud on the 

 movement of the instrument appeared to me most extraordi- 

 nary. The fog continued sufficiently long to admit of the 

 fillet of whalebone, by its attraction for the molecules of 

 water, to elongate itself. Our clothes were not dampened. 

 A traveller, experienced in observations of this kind, recently 

 assured me that he saw on the naked mountain of Mar- 

 tinique, a similar effect of clouds on the hair hygrometer. It 

 is the duty of a natural philosopher to relate the phenomena 

 which nature presents, especially when he has neglected 

 nothing to avoid errors of observation." 



M. Rozet, an officer of the etat-major, has often observed, 

 during his investigations among the Pyrenees, a bed of hori- 

 zontal vapour at a height varying from 230 to 1150 metres 

 above the sea. But in order to determine whether these 

 vapours belong to the class of dry or humid fogs, it is neces- 

 sary that the observer should have been accustomed to hy- 

 grometrical experiments. 



In conclusion, it appears to me that the existence of true 

 dry fogs, as dense as ordinary humid fogs, has not been per- 

 fectly demonstrated. Saussure's bluish vapour is nothing 

 more than a disturbance of the transparency of the air, and 

 not a true fog. The insulated observation of M. de Hum- 

 boldt has been made with a defective instrument, and very 

 slow in its indications, — the whalebone hygrometer of Deluc. 

 On this subject, therefore, meteorology requires new re- 

 searches, undertaken with the new means of investigation for 

 which she is indebted to the progress of experimental physics. 

 It is unquestionable that the degree of humidity in fogs is 

 variable ; but it is not yet demonstrated that fogs exist so 

 dense as to veil objects at the distance of a kilometre, for 

 example, and so dry as in no degree to affect delicate psy- 

 chrometrical instruments ; at all events, that these fogs are 

 not the smoke arising from great combustions. This is a 



