214 COSMOS. 



The intimate connection of the phenomena which we han 

 considered is still hidden in obscurity. Elastic iluids are doubt 

 lessly the cause of the slight and perfectly harmless trembling 

 of the earth's surface, which has often continued several day* 

 (as in 1816, at Scaccia, in Sicily, before the volcanic eleva- 

 tion of the island of Julia), as well as of the terrific explosions 

 accompanied by loud noise. The focus of this destructive agent, 

 the seat of the moving force, lies far below the earth's surface ; 

 but we know as little of the extent of this depth as we know 

 of the chemical nature of these vapors that are so highly com- 

 pressed. At the edges of two craters, Vesuvius, and the tow 

 ering rock which projects beyond the great abyss of Pichin- 

 cha, near Quito, I have felt periodic and very regular shocks of 

 earthquakes, on each occasion from 20 to 30 seconds before 

 the burning scorise or gases were erupted. The intensity of 

 the shocks was increased in proportion to the time interven- 

 ing between them, and, consequently, to the length of time 

 in which the vapors were accumulating. This simple fact, 

 which has been attested by the evidence of so many travelers, 

 furnishes us -with a general solution of the phenomenon, in 

 showing that active volcanoes are to be considered as safety- 

 valves for the immediate neighborhood. The danger of earth- 

 quakes increases when the openings of the volcano are closed, 

 and deprived of free communication with the atmosphere ; but 

 the destruction of Lisbon, of Caraccas, of Lima, of Cashmir in 

 1554,* and of so many cities of Calabria, Syria, and Asia Mi- 

 nor, shows us, on the whole, that the force of the shock is not 

 the greatest in the neighborhood of active volcanoes. 



As the impeded activity of the volcano acts upon the shocks 

 of the earth's surface, so do the latter react on the volcanic 

 phenomena. Openings of fissures favor the rising of cones of 

 eruption, and the processes which take place in these cones, 

 by forming a free communication with the atmosphere. A 

 column of smoke, which had been observed to rise ibr months 

 together from the volcano of Pas to, in South America, sud- 

 denly disappeared, when, on the 4th of February, 1797, the 

 province of Quito, situated at a distance of 192 miles to the 

 south, suffered from the great earthquake of Riobamba. After 

 the earth had continued to tremble ibr some time through- 

 out the whole of Syria, in the Cyclades, and in Eubcea, the 

 shocks suddenly ceased on the eruption of a stream of hot mud 



* On the frequency of earthquakes in Cashmir, see Troyer's Germaii 

 translation of the ancient Radjataringini, vol. ii., p. 297, and Carl / 

 HOgel, Rei$cn., lid. ii.. s. 181. 



