338 PHYSICAL SCIENCE 



disrupt all obstructions ; or when the pressure is less may cause 

 no more than a heaving of the surface. The idea that the shock 

 of an earthquake results from the removal of material underneath, 

 whereby the stability of the overlying portion is undermined, and 

 a collapse of the ground ensues, was held in various forms. 

 Some thought that this destruction arose from extensive combus- 

 tion within the earth. Anaximenes supposed that just as at the 

 surface, rocks and old buildings yield to the ravages of time and 

 fall down, so in the interior of the earth similar landslips may 

 occur and cause shocks to the districts above them (237). 1 



But the favourite opinion of antiquity regarded earthquakes 

 as primarily due to the violent commotion of air. Seneca com- 

 ments on the views of various philosophers, and more especially 

 Aristotle's, as to the way in which the air acts, and he then pro- 

 ceeds to deliver his own judgment. He has no doubt that, 

 though some of the other agencies may co-operate, the chief 

 motive force in earthquakes is air. By no part of nature, he 

 affirms, is such violent energy displayed as by air ; it kindles fire, 

 tosses the surface of the waters into waves, destroys large tracts 

 of the earth, uplifts new mountains, and raises in the midst of 

 the sea islands never seen before. Not only does air exist above 

 ground, but it also fills the hollows and interstices of the interior 

 of the earth, into which it freely enters from the surface. 

 Nothing in nature is so restless as air, and the earth cannot but 

 be affected by the movements of the air included in its inside. 

 The author agrees with the general opinion that when the air 

 begins to be agitated in a subterranean cavern which it has filled, 

 pressed by that which is still entering, it struggles to escape, and, 

 when it does so, emerges with a violence proportionate to the 

 narrowness of the passage for its exit. But if unable to make 

 its way out, it becomes furious, acts like a swollen impetuous 

 river, and that overthrows everything in its path. 2 



It is not difficult to realise how this explanation should have 

 been accepted in antiquity, and should have held its ground 

 down even into modern times. The violence of the commotions 

 of the atmosphere was a familiar feature on the surface of the 

 earth, although its physical causes, variously guessed at, were 

 utterly unknown. To minds that had no conception of the very 



1 The collapse of the roofs or sides of underground caverns may un- 

 doubtedly be in some instances the cause of local earthquakes. This 

 origin is enforced by Lucretius : 



terra superne tremit magnis concussa ruinis, 

 subter ubi ingentes speluncas subruit aetas. 



De Rer. Nat. vi. 544. 



2 Lucretius gives a picturesque recital of these views (De Rer. Nat. 

 vi. 535-607). 



