2 4 o PHYSICAL SCIENCE BK. vi 



it happen that [at times] one of them agitates the 

 upper air, the other the nether ? l 



XIII 



1 IN this category you may rank Aristotle and his 

 disciple Theophrastus, a man of pleasant though not 

 of superhuman eloquence, as the Greeks considered 

 him, and of easy, polished style. Let me unfold in 

 more detail what they hold in common : There is 

 always evaporation of some kind going on from 

 the earth, which is at one time dry, at another has 

 an admixture of moisture. When this, rising from 

 the lowest parts of earth, has been raised to the 

 utmost extent, and has no place beyond into which 

 to issue, it is borne back and returns upon itself. 

 The struggle of the air in its ebb and flow tosses 

 to and fro all obstructions it meets, and, whether 

 its egress is stopped or whether it escapes through 

 the narrow openings, it causes movement of the 



2 earth and uproar. To the same school of opinion 

 belongs Strato, who made a special study of this 

 department of science, and was a diligent student 

 of natural philosophy. His verdict on the matter 

 is this : Cold and heat always move away from 

 one another in opposite directions, and cannot 

 remain in the same place. Cold flows into the 

 spot whence the influence of heat has departed ; 

 and, conversely, there is heat in the place whence 

 cold has been banished. The statement is beyond 

 doubt, but the contrariety of the two may become 



1 The argument seems to be : Two winds can blow simultaneously. One 

 may be beneath the earth (causing or during earthquake), one above. 

 Therefore, stillness of the upper atmosphere is not a necessary concomitant 

 of earthquake. The fact has at times been otherwise. 



