236 PHYSICAL SCIENCE BK. vi 



of introduction to the neighbouring kings, and 

 so they had penetrated into the heart of Africa 

 and accomplished a long journey. "We came 

 indeed," I give their own words, "to huge marshes, 

 the limit of which even the natives did not know, 

 and no one else could hope to know ; so completely 

 was the river entangled with vegetable growth, 1 so 

 impassable the waters by foot, or even by boat, since 

 the muddy overgrown marsh would bear only a 

 small boat containing one person. There," my in- 

 formants went on, "we saw with our eyes two rocks 

 from which an immense quantity of water issued." 

 Now whether that is the real source or only an 

 addition to the river; whether it rises there or 

 merely returns to the surface after its previous 

 course underground ; don't you think that, whatever 

 it is, that water comes up from a great lake in the 

 earth ? The earth must contain moisture scattered 

 in numerous places and collected at depth in order 

 to be able to belch it out with such violence. 



IX 



1 FIRE is the cause assigned by some for earthquakes, 

 but they are not agreed as to its method of action. 

 First among them is Anaxagoras, who is of opinion 

 that pretty much the same cause produces concus- 

 sion in the earth as in the atmosphere. In the nether 

 parts of earth, air (gas) causes explosions of thick 

 atmosphere massed in clouds with the same violence 

 as on earth clouds are wont to be burst. Fire is 

 struck out by this collision of clouds and by the 



2 rush of the atmosphere that is forced out. This fire 



1 The so-called "sudd." 



