25B 



EARTHQ,UAKES. 



perceived in some situations, it may be inferred, that this gas is hy- 

 drogen or sulphuretted hydrogen. In other instances, it mav be 

 stearn, which condensing again would produce a vacuum, and oc- 

 casion the external air to press downwards; this has been observed 

 in mines, immediately after the shock of an earthquake. 



The space over which the vibration of the dry ground is felt is very 

 great, but generally wider in one direction than another; and where 

 a succession of earthquakes has taken place in the same district, it 

 is observed that the noise and shock approach from the same quar- 

 ter. It has been before mentioned, that the earthquakes are most 

 frequent in volcanic districts ; but the shocks are not the most violent 

 in the immediate vicinity of volcanoes. Oii the contrary, they are 

 stronger in the more distant part of a volcanic country. The ground 

 is agitated with greater force, as the surface has a smaller number of 

 apertures communicating with the interior. " At Naples and Messina, 

 and at the foot of Colopaxi and Tunguraliua, earthquakes are dread- 

 ed only when vapours and flames do not issue from the craters." — 

 Hvmboldt. 



The connexion of earthquakes with volcanoes was noticed by an- 

 cient wiiters, and the latter were [)roperly regarded as the openings 

 through which the inclosed vapour and ignited matter, that occasion 

 earthquakes, found a passage. Strabo, in his Geography, states, that 

 *' the town of Regium, situated on the Italian side of the Straits of 

 Messina, was so called, according to jEschylus, from the circum- 

 stance that the island of Sicily was rent off from the continent by 

 earthquakes. Proofs of this arise out of the phenomena attending 

 ^tna, and other pans of Sicily and the Lipaia Islands, and even the 

 opposite continent. Now, indeed, when craters are opened, through 

 which fire and ignited matter and water are poured out, it is said that 

 the land near the Straits is seldom shaken by earthquakes: but for- 

 merly, when all the passages to the surface were obstructed, the fire 

 and vapour confined in the earth occasioned frequent earthquakes, 

 and the land, being rent, admitted the ocean. At the same time, 

 Prochyta and an adjacent island were also torn off from the continent, 

 while other islands rose from the ocean, as frequently happens at 

 this day." — (Strabo flourished in the reign of Augustus.) 



It is highly probable that every extensive eartliquake is- followed 

 by a volcanic eruption, more or less remote, unless (as not unfre- 

 quently happens) the elastic vapour immediately escapes from fissures 

 made at the time, in the countries that are the most violently convul- 

 sed. An earthquake was strongly felt in Geneva when I was there, ^ 

 February 19, 1822, and did considerable damage, in several towns 

 and villages in Savoy and France. A few weeks afterwards, I trav- 

 elled from Geneva to Lyons, and from thence to the ancient volca- 

 noes near Clermont. In the course of my route, I made frequent 

 inquiries respecting the effects of the earthquake : it appeared to 

 have been most strongly felt along the valley of the Rhone, and the 



