234 M. C. Martins on the Nature and Origin of 



in Westphalia may obscure the atmosphere at Bale, Paris, 

 Brest in the south, and Copenhagen in the north. Yet, not- 

 withstanding the extent, thickness, and density which must 

 be conceded to these clouds of smoke, we cannot admit that 

 they could cover a portion of Europe like certain general dry 

 fogs, such as those of 1764 and 1783, whose history has been 

 preserved to us by writers of the last century. These fogs 

 form a second class which I shall endeavour to characterize. 



II. General dry fogs produced by volcanic eruptions. 

 Trochner Nebel (Germ.) ; Dry Fog (Eng.) ; Sonnenrauch, 

 Kastner. 



To give an idea of this kind of fog, I think I cannot do 

 better than describe the most celebrated of all, that of 1783. 

 This task is the more easy, since the favour meteorology then 

 enjoyed has raised a crowd of observes who have transmitted 

 the most valuable documents respecting this curious phe- 

 nomenon in the Ephemerides de la Societe Meteorologique de 

 Manheim. 



I shall first trace the progress of this fog, that is to say, 

 determine the period of its appearance and disappearance at 

 the places where it has been observed ; I shall then treat of 

 its nature, and the phenomena which accompanied it. 



Dry fog of 1783. — Speaking in a general way, this fog 

 extended from Norway to Syria, that is to say, over a space 

 of 25 degrees of latitude ; and from England to Altai, that is, 

 over 120 degrees of longitude. It was observed, more or less, 

 during the whole period of time between the 24th May, the 

 day of its first appearance at Copenhagen, and the 8th 

 October, when Lamanon saw it for the last time in the val- 

 ley of Servieres in Dauphiny. 



Height of it. — When on Mount Ventoux, 1910 metres 

 above the sea, Lamanon still saw much of it above him. He 

 satisfied himself, by going from the sea- shore to the highest 

 mountains, that the lowest part was thickest and dryest. 

 Among the French Alps, the shepherds assured him that it 

 covered the highest peaks, which implied a thickness of 4000 

 metres. At Geneva, Senebier ascertained that it exceeded 

 the height of the great Saleve, which is 1484 metres above 

 the sea. De Saussure himself observed this fog at the hos- 



