viii SURFACE AND UNDERGROUND WATER 119 



of water in the earth. On the outer surface are 

 huge marshes, great navigable lakes, and seas 

 covering immense tracts of earth and pouring over 

 its hollows. So in the interior of the earth there 

 is abundant store of fresh water, which overflows 

 great spaces no less than the Ocean and its gulfs 

 above ground ; in fact, still more extensively, as the 

 depth of the earth extends farther down than that 

 of the sea. From that supply in the deeps, there- 

 fore, those rivers of which we have spoken issue. 

 And why should one be surprised that the earth is 

 not sensible of their withdrawal since the sea is not 

 sensible of their addition ? 



IX 



SOME approve the following explanation : The i 

 earth contains, they assert, many hollow recesses 

 and a great quantity of air. This air, under pres- 

 sure of the gross darkness, of necessity freezes. 

 Then remaining sluggish and unmoved it ceases to 

 circulate and turns into water. Just as on earth a 

 change in the density of the atmosphere produces 

 rain, so beneath the earth the change of density 

 starts a river or a stream. In the former case the 

 air above our heads cannot long remain sluggish 

 and heavy ; for sometimes it is rarefied by the sun's 

 heat, sometimes expanded by the wind's force. 

 There are, therefore, long intervals between falls 2 

 of rain. But underground the forces, whatever they 

 are, that turn air into water, are constant perpetual 

 darkness, everlasting cold, inert density ; they can, 

 therefore, supply without a break the sources of 

 fountain or flood. We Stoics are satisfied that the 



