different kinds of Dry Fogs. 235 



pice of Grimsel (1880 metres above the sea), on the 10th> 

 11th, and 12th July. It was little observable on the two 

 last days, but, according to the account of the people at the 

 hospice, it was as dense in the end of June on the Grimsel 

 as in the plain. 



At Narbonne, on the contrary, it never rose, according to 

 Marcorelle, above 780 metres ; at a greater elevation the 

 sky remained always clear. From Neufchatel the peaks of 

 the Alps were seen above the fog. But Saussure, who was 

 in the neighbourhood of Rolle on the 3d July, could not dis- 

 tinguish, between five o'clock in the morning and noon, the 

 peaks of the Jura, about three leagues distant. At Padua, 

 and even at Rome, the fog appeared suspended in the air and 

 not to touch the horizon. 



From what has been said it may be concluded that the fog 

 was variable in thickness ; it was so even according to the 

 hour of the day, for Lamanon being, on the 21st of June, on 

 the top of Ventoux before the rising of the sun, remarked 

 that the fog ascended as that luminary rose above the hori- 

 zon. 



Physical properties of this Fog — Its appearance. — 

 With the exception of Maret of Dijon, all observers were 

 struck with the extraordinary appearance of this fog. " It 

 was," says Senebier, " a bluish vapour, sometimes reddish, 

 never gray like ordinary fogs ; it coloured objects blue. 

 During the days on which it was dense, houses and trees 

 disappeared at the distance of a third of a league." Toaldo 

 at Padua, Marcorelle at Narbonne, Cotte at Laon, Praeus at 

 Sagan, Father Onuphre on St Gothard, and Saussure on the 

 Grimsel, compare it to a smoke, and even a dust totally 

 different from ordinary fogs. These testimonies are cor- 

 roborated by the examination of the other properties of this 

 vapour. 



Its hygrometrical state. — The very title of this memoir 

 imposes upon me the obligation of shewing that the fog of 

 1783 was completely dry, and had no effect on hygrometrical 

 instruments, nor on hygrometrical bodies. In order to prove 

 this, I have only to refer to the statements of the natural 

 philosophers who observed it. At Geneva, Senebier found 



