250 PHYSICAL SCIENCE BK. vi 



must be repeated in regard to water. When it 

 is accumulated at one place, which becomes too 

 small to contain it, it inclines in some particular 

 direction, and opens up a passage for itself, at 

 first by its mere weight, afterwards by the gathering 

 force of its current. Being long shut up it cannot 

 escape except down an incline, and it cannot drop 

 straight down with any gentleness, or without violent 

 shaking of the parts through which and on which it 



3 falls. Now, if after it has begun its rapid downward 

 movement it is checked at any point, and the force 

 of the current is thrown back upon itself, it is driven 

 back on the earth which encounters it, and attacks 

 the earth at the point where it is most insecure. 

 Moreover, the ground is sometimes so saturated 

 with the moisture it has received into its heart that 

 it subsides to a lower level and its very foundation 

 is destroyed. The pressure is then exerted on the 

 part toward which the weight of the descending 

 waters most inclines. Air, too, sometimes urges 

 the water. If it presses with some degree of 

 violence, it naturally moves the part of the earth 

 toward which it has urged the gathering of the 



4 waters. Sometimes, again, the air is driven into 

 passages through the earth, and in its search 

 for a way of escape causes a general movement. 

 The earth, as we know, is pervious to wind ; air 

 is too subtle to be excluded, too violent to be resisted 

 when excited to rapid movement. 



Turning from Democritus to Epicurus, we find 

 the latter to assert that all the foregoing may be 

 causes of earthquake, but he tries to introduce 

 some additional ones. He criticises other authors 

 for affirming too positively that some particular 

 one of the causes is responsible, as it is difficult 



