246 PHYSICAL SCIENCE BK. vi 



flows quietly along. When it is opposed and held 

 back it becomes furious, bursting all obstacles just 

 like that 



Araxes that ever spurned a bridge. 



2 As long as the river has a free easy channel it 

 rolls down its waters in due and regular succession. 

 But if through chance or by human agency rocks are 

 placed in its way to check its course, then it gathers 

 fresh strength from the barrier, and the more 

 numerous the obstacles opposed to it, the greater 

 the force that it musters to overcome them. For 

 all the water that accumulates behind, constantly 

 increases, and being at last unable to bear its own 

 weight manifests its violence through the havoc it 

 works in its descent, and escapes headlong down its 

 channel, bearing the very obstacles that blocked its 



3 path. The same thing occurs with air, only that, in 

 proportion to its greater strength and mobility, 

 it is the more rapidly carried onward, and bursts 

 the more violently all that encloses it. From this, 

 of course, there is a disturbance in the part of the 

 ground under which the struggle has occurred. The 

 truth of this assertion may be proved from the con- 

 sideration that often when an earthquake has taken 

 place, involving a breach of only some part of the 

 earth, wind has issued from it for several days. 



4 This is recorded to have taken place in the earth- 

 quake in which Chalcis suffered, as you will find 

 in Asclepiodotus, Posidonius' pupil, in his discus- 

 sion of my own topic of Physical Inquiries. In 

 other authors, too, you will find it stated* that after 

 a chasm had opened up at one spot, in no long time 

 wind issued from it, having no doubt made for itself 

 the way along which it travelled. 



