246 Mr C. Martin on the Nature and Origin of 



Dry fogs properly so called. — It now remains for us to men- 

 tion certain fogs which are neither smokes produced by com- 

 bustions, nor callina, but vapours, among which the observer 

 finds himself, without experiencing the slightest sensation of 

 humidity, and hygrometrical instruments not indicating the 

 slightest trace of it. On this subject documents are still 

 rarer and more imperfect than in regard to the other three 

 species of fogs. Having never observed fogs of this nature, 

 which have been described by two great meteorologists, De 

 Saussure and Humboldt, I think I cannot do better than al- 

 low them to speak for themselves. 



" After many days of decidedly fine weather," says De 

 Saussure, " when the air is not perfectly transparent, we 

 perceive a bluish vapour floating in it which is not of an 

 aqueous nature, since it does not affect the hygrometer ; but 

 its nature is not yet known to us.'' 



It may be asked, in the first place, if this bluish vapour is 

 not the halo which accompanies the callina \ But how are 

 we to suppose that such an observer as Saussure should have 

 remarked the halo, and not attended to the horizon-smoke 

 which accompanied it ? It must be rare in Switzerland, for it 

 is never referred to in his Voyages dans les A Ipes ; and in 

 our ascents and prolonged stations among the mountains we 

 never saw it but once. 



The following notice from M. de Humboldt is a much bet- 

 ter characterized example of a true dry fog. On the sum- 

 mit of Silla, a mountain which rises near the town of Carac- 

 cas, to a height of 2630 metres above the sea, MM. de 

 Humboldt and Bonpland were much struck with the appa- 

 rent dryness of the air, which seemed to increase as the fog 

 formed. " When I took the hygrometer from its case," says 

 the illustrious traveller, "to subject it to experiment, it 

 marked 52 degrees (87 deg. Sauss.) The sky was clear, 

 yet tracks of vapour with distinct contours passed from time 

 to time amongst us, grazing the earth. Deluc's hygrometer 

 went back to 49 degrees (85 deg. Sauss.). Half an hour later, 

 a large cloud enveloped us ; we could no longer distinguish 

 the objects nearest us, and wc saw with surprise that the 



