256 PHYSICAL SCIENCE BK. vi 



there in the immense caverns. Nay, says some 

 critic, but just as when we shiver from cold a 

 trembling follows, so, too, the earth is shaken by 

 air affecting it from without. This I deny can 

 by any possibility occur. Why, the earth must 

 get a chill in order to have the same happen to 

 it as to us, whom an external affection drives into 



4 a shuddering fit. I should quite allow that the 

 earth shows symptoms of much the same kind as 

 we do, but the cause is wholly different. An injury 

 of a deeper kind, more toward its centre, must affect 

 it, the very strongest proof of which may be found 

 in the fact that when through violent earthquake 

 the soil is laid open in wide destruction, the 

 chasm sometimes takes in and buries whole cities. 



5 Thucydides tells us that, about the time of the 

 Peloponnesian War, the island of Atalanta, either 

 wholly, or, at any rate, for the most part, was 

 swallowed up. You may take Posidonius for 

 witness that the same thing happened to Sidon. 

 But we do not require evidence of this. Within 

 our own memory the earth has been torn by 

 internal movement, adjoining places have been 

 rent asunder, whole plains have disappeared. I will 

 now explain how I suppose this sort of thing to 

 occur. 



XXV 



i WHEN air has completely filled a large vacant space 

 within the earth, and has begun to struggle and 

 meditate escape, it lashes again and again the sides 

 of the enclosure within which it lurks, and right over 

 which, as it happens, cities are sometimes situated. 

 The shaking is at times so violent that buildings 



