different kinds of Dry Fogs. 239 



mony of Hemmer, Senebier, Toaldo, and Marcorelle, I may 

 add that of Van Swinden, who was astonished to see it con- 

 tinue in spite of rains, winds, and storms. 



Origin of the dry fog of 1783. — Every meteorologist who 

 has taken the trouble to read the preceding details, will be 

 persuaded, like myself, that the fog of 1783 was not composed 

 of aqueous vapour. The hygrometrical experiments of Se- 

 nebier, Van Swinden, and Lamanon, — its continuance for two 

 months of the summer, in all kinds of weather, and during all 

 kinds of winds,- — sufficiently prove this. 



This fog was smoke — Toaldo, Marcorelle, Cotte, and De 

 Saussure are all agreed on this point ; the latter supports his 

 opinion by that of the Bernese mountaineers who, he says, 

 are so well experienced in fogs. 



Its origin appears to us to be that already assigned to it 

 by some observers of the period, namely, the earthquakes 

 and volcanic eruptions which in the same year shook Iceland 

 and Calabria. We know as a fact that in these eruptions 

 the volcanoes threw up into the air masses of ashes, which 

 formed true clouds, which the winds carried to a distance. 

 In the neighbourhood of the volcano, the light of the sun 

 was completely obscured by them, as in the eruption of Ve- 

 suvius, in the year 70, when, according to Pliny the younger, 

 the obscurity was like that of a shut-up apartment. On the 

 22d and 23d October 1822, lanterns were used in the villages 

 near Vesuvius. M. de Humboldt, who bears testimony to 

 these facts, compares them to what so often takes place at 

 Quito during the eruptions of Pichincha. During the erup- 

 tion of Catopaxi, 4th April 1768, the shower of ashes at 

 Hambato and Tacunga was such that the inhabitants like- 

 wise went about in open day with lanterns. These pheno- 

 mena were also observed at great distances from the 

 ignivomous crater. During the eruption, in the month of 

 December 1760, the smoke of Vesuvius, carried by the wind, 

 darkened the sun for an entire day at Cuccaro and Cilento, 

 towns in the principality of Salerno, situate 92 kilometres 

 from the mountain. On the following day the grass was 

 covered with ashes. 



r2 



