xiv EFFECTS OF AIR 243 



not delay for a time its attack upon some parts, 

 but with uniform regularity spreads over them all. 



Perhaps you had better assume, therefore, that 4 

 air from the surrounding atmosphere enters the earth. 

 As long as it has free egress, it glides through it 

 without doing harm ; but if it meet some obstacle 

 to block its way, then it is, to begin with, weighted 

 with the atmosphere that pours in on the rear ; 

 by and by it escapes with difficulty through some 

 chink, and makes its way with the greater violence 

 the narrower the opening is. That cannot take 

 place without a struggle, and a struggle involves 

 shaking of the earth. But if the confined air 5 

 cannot find even a chink by which to issue, it is 

 massed and becomes furious, and is driven round in 

 this direction and in that, overthrowing or bursting 

 one thing after another. It is excessively subtle, 

 and at the same time exceedingly powerful ; it can 

 worm its way into obstructions however great, 

 splitting and scattering whatever it enters. When 

 this occurs, then there is a regular tossing of the 

 earth. For the earth either opens to give room 

 to the wind, or, after giving room, is deprived 

 of its foundation and subsides into the very cavern 

 from which it allowed the wind to issue. 



XV 



SOME entertain the following opinion : The earth 

 is porous at many points, possessing not merely 

 those first shafts which it received as ventilators 

 at its creation, but many subsequently opened up 

 by various changes. In some places water has 

 washed away the soil that was on the surface; 



