58 PHYSICAL SCIENCE BK. n 



opposite to them. This the Stoics call displace- 

 ment, in Greek it is peristasis, 1 which takes place 

 in air just as it does in water. For it literally stands 

 round every body by which it is pressed. There is 

 no need to assume an admixture of vacuum with 

 the element. But more of this another time. 



VIII 



1 FROM what has been said it must be inferred that 

 in nature there exists a principle of activity of 

 enormous force. For there is nothing that does not 

 become more active through tension ; and it is no 

 less true, nothing will be found capable of tension 

 from another body unless it have in itself capacity of 

 tension. 2 In the same way we say that nothing 

 could be moved by another body without possessing 

 the quality of mobility in itself. But what element 

 can be conceived more likely to possess tension in 



2 itself than air ? Will any one deny that it can be 

 subject to that force after seeing how it tosses 

 about the earth with its mountains, houses, and 

 walls and towers, and great cities with their in- 

 habitants, seas, and whole coast-lines ? The tension 

 of air is proved, too, by its velocity and expansion. 

 Illustrations of these properties are common : in an 

 instant the eye extends its sight over many miles; 



1 ireplffTa.ffis = a standing around. The Latin equivalent in the text is 

 circumstantia, rendered "displacement." 



2 The reading at several points is so uncertain that one cannot be at all 

 sure of the meaning. Probably the whole passage is very corrupt. So far 

 as the main theme is concerned, the argument seems to be, As mobility is a 

 presupposition of motion, so tensibility is a necessary condition of actual 

 tension produced in a body by another body. One is tempted to employ 

 " elasticity," but the term contains implications with which the author was 

 apparently unfamiliar. 



