262 Natural History of Volcanos and Earthquakes. 



the falling in of its cone at one period, appears to be balanced 

 by the accamulation of ashes at another. 



If a rent, reaching from the surface to the melted matters in 

 the interior, be of great length, but not open throughout its 

 whole extent, the first eruption will take place where there is 

 the least resistance. 



If this channel become obstructed, the volcanic fire will seek 

 another vent.* Violent concussions may open new fissuresf and 

 close old ones, by which frequent changes may be produced in 

 the channels of the lava and water. Fissures obstructed by lava 

 are closed so firmly as to be incapable of being re-opened ; new 

 ones, therefore, are formed. Thus it is, at least, if a volcano 

 produce eruptions from its sides. If it happen that a wide and 

 lasting vent be formed, all partial workings in the neighborhood 

 will cease. A similar combination, although on a somewhat 

 limited scale, is presented by groups of mineral springs, espe- 

 cially of hot springs. In such groups, new channels are seen to 

 open, new springs to rise, and old ones to close. The only dif- 

 ference is, that, as these changes are not accompanied with any 

 violent action, as is the case with volcanos, they require a greater 

 length of time for their accomplishment. 



We have, in the preceding inquiries, as yet only supposed the 

 admission of water from the sea. But this does not seem always 

 to be the case, even in volcanos situated near the sea. Accord- 

 ing to Hamilton,! the water of the springs and wells of Torre 

 del Greco diminished so much a few days before the great erup- 

 tion of Vesuvius, on the 15th June, 1794, that the corn mills at 

 the principal spring were nearly stopped, and it was daily neces- 

 sary to lengthen the ropes in the wells, in order to reach the 

 water. Some wells dried quite up, and on the morning of the 

 12th June, at Resina, a subterranean rumbling noise was heard 

 after a heavy rain. Monticelli and Covelli'§) relate that, before 



* Thus the interior of the crater of the Peak of Tenerijfe, shows it to be a vol- 

 cano, which for thousands of years has thrown out fire only from its sides. V. 

 Humboldt, Reise, t. i. p. 195. 



t According to the inhabitants of JVero Jindalusia, the soil in various districts in 

 their province has become more and more arid, in consequence of the frequent 

 earthquakes with which they are visited from time to time. — V. Humboldt, Reise, 

 t. ii. p. 21. t Phil. Trans, for 1795, p. 79. 



§ Loco cit. pp. 12 and 63. See also Monticelli, in Leonard's Taschenbuch fcir 

 die gesammte Mineralogie, vol. xiv, p. 87. 



