284 EARTHQUAKES. 



Captain Herschel, who found that the attractive force of 

 gravity in the neighbourhood of the Himalayas was not 

 so great as it ought to have been had these mountains 

 been solid. The Eev. 0. Fisher gives another explana- 

 tion of this phenomenon. Palmieri considers that the 

 terrible earthquake which devastated Casamicciola (1881) 

 was due to the hot springs having gradually eaten out 

 cavernous spaces beneath the town. The extremely local 

 character of this shock was certainly favourable to such a 

 view. 



The earthquake which, in 1840, caused Mount Cernans, 

 in the Jura, to fall, is also attributed to the solvent action 

 of waters in undermining its foundations. This under- 

 mining action was in great measure probably due to a 

 large spring, which, twenty-five years previously, had dis- 

 appeared, and which subsequently may possibly have 

 been slowly disintegrating the foundations of the moun- 

 tain. Earthquakes of this order would be principally 

 confined to districts where there are rocks which are more 

 or less soluble, as, for instance, rock salt, gypsum, and 

 limestone. 



Earthquakes and the attractive influences of the 

 heavenly bodies, — The most important attractions exer- 

 cised upon our planet are those due to the sun and moon. 

 To these influences we owe the tides in our ocean, and 

 possibly elastic tides in the earth's crust. Some theorists 

 would also insist upon liquid tides in the fluid interior of 

 our earth. The nature of the earth's interior is, however, 

 a question on which there is a diversity of opinion. 



One doctrine, which, until recent years, received much 

 support, was that the interior of the earth was a reservoir 

 of molten matter contained within a thin crust. Hop- 

 kins showed that the least possible thickness of such a 

 crust must be from 800 or 1,000 miles, otherwise the 



