228 



FLEXIBILITY OF THE EARTH'S CEUST. [Ch.XXXIII. 



or fall in the vent on tlie same principle as mercury in a 

 barometer; because the ebullition or expansive power of the 

 steam contained in the lava would be checked by every 

 increase* and augmented by every dimination of weight. 

 In like manner, if a bed of liquid lava be confined at an 

 immense depth below the surface, its expansive force may be 

 counteracted partly by the weight of the incumbent rocks, 

 and also in part by atmospheric pressure acting contem- 

 poraneously on a vast superficial area. In that case, if the 

 upheaving force increase gradually in energy, it will at 

 length be restrained by only the slightest degree of supe- 

 riority in the antagonist or repressive power, and then the 

 equilibrium may be suddenly .destroyed by any cause, such 

 as an ascending draught of air, which is capable of depressing 

 the barometer. In this manner we may account for 

 remarkable coincidence so frequently observed between tbe 

 state of the weather and subterranean. commotions, altlioiigli 

 it must be admitted that earthquakes and volcanic eruptions 

 react in their turn upon the atmosphere, so that disturbances 

 of the latter are generally the consequences rather than the 

 forerunners of volcanic disturbances.*^ 



From an elaborate catalogue of the earthquakes experienced 

 in Eurojpe and Syria during the last fifteen centuries, M. 

 Alexis Perrey concludes that the number which happen in 

 the winter season preponderates over those which occur in 

 any one of the other seasons of the year, there being, how- 



b 



the 



ever, some exceptions to this rule. 



as 



in the Pjnrenees. 



Curious and valuable as are these data, M. d'Archiac justly 

 remarks, in commenting upon them, that they are not as yet 

 sufficiently extensive or accordant in different regions, to 

 entitle us to deduce 



any general conclusions from them 

 subterranean movements throusrhout 



t 



M. Perrey has also, in a later report of earthquakes (1863), 

 inferred, as the result of 10,000 observations on the earth- 

 quakes of the first half of the present century, that they 

 occur more frequently and with more violence when the 



^ Scrope on Voleanos, pp. 58-CO. 



t Archiac, Hist, des Progres de la Geol. 1847, vol. i. pp. 605-610. 



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