ABOUT VOLCANOS AND EARTHQUAKES. 45 



occurrence. This does not prevent, however, certain 

 conjunctures of atmospheric or other circumstances from 

 exercising a determining influence on the times of their 

 occurrence. According to the view we have taken of 

 their origin (viz., the displacement of pressure, resulting 

 in a state of strain in the strata at certain points, gradu- 

 ally increasing to the maximum they can bear without 

 disruption), it is the last ounce which breaks the camel's 

 back. Great barometrical fluctuation, accumulating at- 

 mospheric pressure for a time over the sea, and reliev- 

 ing it over the land ; an unusually high tide, aided by 

 long-continued and powerful winds, heaping up the 

 water ; nay, even the tidal action of the sun and moon 

 on the so/id portion of the earth's crust, all these 

 causes, for the moment combining, may very well suffice 

 to determine the instant of fracture, when the balance 

 between the opposing forces is on the eve of subversion. 

 The last-mentioned cause may need a few words of ex- 

 planation. The action of the sun and moon, though It 

 cannot produce a tide in the solid crust of the earth, 

 tejids to do so, and, were it fluid, would produce it. It 

 therefore, in point of fact, does bring tlie solid portions 

 of the earth's surface into a state alternately of strain and 

 compression. The effective part of their force, in the 

 present case, is not that which aids to ///? or lo press the 

 superficial matter (for that^ acting alike on the continents 

 and on the bed of the sea, would have no influence), but 

 that which tends to produce lateral displacement; or 

 what geometers call the tcnigeiitial force. This of neces- 

 sity brings the whole ring of the earth's surface, which 



