248 PHYSICAL SCIENCE BK. vi 



\Vhen the winds struggle and the tempests roar, 



Can restrain them by its sway and rein them by bonds and prison. 



Doubtless the poets wished the place in which 

 the winds lay pent up underground to be con- 

 sidered a prison. But they did not perceive either 

 that what was shut up is no longer wind, or 

 that what is wind can no longer be shut up. 

 What is shut up is at rest, and the atmosphere 

 is at a standstill ; whereas all wind is in flight. 

 4 Besides these arguments, there is a considera- 

 tion by which it becomes manifest that motion 

 is brought about by air, namely, that our bodies 

 never tremble except when some cause produces 

 disturbance of the internal air, 1 which is contracted 

 by fear, grows sluggish in old age, languishes when 

 the veins are numbed, is checked with cold, or 

 after some attack of fever is quite driven from its 

 wonted course. As long as it flows unimpeded, and 

 moves in its wonted fashion, there is no quivering 

 of the body. When anything intervenes to prevent 

 its functioning, then being no longer able to main- 

 tain what it upheld by its vigour, it fails, causing a 

 collapse of everything that it had sustained when 

 unimpaired. 



XIX 



1 WE must now hear what Metrodorus of Chios 

 desires to urge by way of opinion. I do not allow 

 myself the liberty of passing over unnoticed even 

 opinions that I disapprove ; it is better to have the 

 largest possible variety of views, and to condemn 



2 rather than omit what we do not approve. Well, 

 then, what has Metrodorus to say ? He compares the 



1 Or spirit : there is almost a play upon the ambiguous meaning of the 



