NOTES 



327 



show any perceptible gain. He merely notices the opinion which 

 some philosophers had expressed that the sea does not get larger 

 because it restores to the earth as much water as it receives, 

 allowing its own saline water to sink through endless subterranean 

 winding passages wherein it is purged of its saltness and rises on 

 the land as pure fresh water. 1 Another view, that most of the water 

 supplied by rain eventually finds its way into the rivers, is approxi- 

 mately that at which modern research has arrived, but it meets 

 with our philosopher's strong opposition. His first objection is 

 derived from his own observation. He tells us that, as a diligent 

 digger among his vines, he can confidently affirm that even the 

 heaviest rain does not penetrate to a depth of more than ten feet 

 from the surface. What is not absorbed by the upper crust of 

 the ground runs at once into river channels, and thence into the 

 sea. He next asks how rain, which immediately flows off the 

 surface of naked rocks, can possibly be the source of the springs 

 and rivers that issue from bare crags, or how springs that appear 

 on the very summit of mountains can be due to rain. Though 

 he could not but be aware of the close connection everywhere 

 observable between evaporation, rainfall, and the volume of 

 springs and rivers, he does not seem to have reflected on its 

 meaning how in seasons of drought the surface waters fail first, 

 how by degrees the springs begin to lessen and even to cease, 

 how the rivers dwindle until in many cases their beds become 

 almost or quite dry, and yet how, when welcome rains set in, the 

 springs and rivers gradually resume the bulk they had before the 

 dry weather impoverished them. He had made no study of the 

 way in which rain percolates through the soil, subsoil, and 

 rocks underneath, though there are places, such as his vineyard 

 may have been, where,' from some impervious material, only a 

 feeble or inappreciable flow of moisture descends beyond a few 

 feet from the surface. Nor was he aware of the innumerable 

 lines of joint by which the most solid rocks are traversed, and 

 which serve as passages for the descent and ascent of water. 

 Had he climbed many mountains, he would have failed to find a 

 spring on the summit of any one of them, unless there had been 

 a sufficient area of higher ground at hand to serve for the supply 

 of the water. 



The origin of underground water is regarded by Seneca as 



1 This is the view expressed by Lucretius : 



. . . ut in mare de terris venit umor aquai, 

 in terras itidem manare ex aequore salso ; 

 percolatur enim virus, retroque remanat 

 materies umoris et ad caput amnibus omnis 

 confluit, inde super terras redit agmine dulci. 



De Rer. Nat. vi. 633. 



