460 ESTILTEATIOIT OF WATER. 



stored up in subterranean reservoii's in rainy seasons, is too ob- 

 vious to require to be dwelt upon. 



The readiness with wliicb water not obstructed by impermea- 

 able strata diffuses itself througli the earth in all directions — and, 

 consequently, the importance of keeping up the supply of sub- 

 terranean reservoirs — find a familiar illustration in the effect of 

 paving the ground about the stems of vines and trees. The 

 surface-earth around the trunk of a tree may be made almost im- 

 pervious to water, by flagstones and cement, for a distance as 

 great as the spread of the roots ; and yet the tree will not suffer 

 for want of moisture, except in droughts severe enough sensibly 

 to affect the supply in deep wells and springs. Both forest and 

 fruit trees attain a considerable age and size in cities where the 

 streets and courts are closely paved, and where even the lateral 

 ascess of water to the roots is more or less obstructed by deep 

 cellars and foundation walls. The deep-lying veins and sheets of 

 water, suppHed by infiltration from often comparatively distant 

 sources, send up moisture by capillary attraction, and the pave- 

 ment prevents the soil beneath it from losing its humidity by 

 evaporation. Hence, city-grown trees find moisture enough for 

 their roots, and though plagued with smoke and dust, often retain 

 their freshness, while those planted in the open fields, where sun 

 and wind dry up the soil faster than the subterranean fountains 

 can water it, are withering from drought.* Without the help 

 of artificial conduit or of water-carrier, the Thames and the 

 Seine refresh the ornamental trees that shade the thoroughfares 

 of London and of Paris, and beneath the hot and reeldng mould 

 of Egypt, the I^ile sends currents to the extremest border of its 

 valley.f 



* The roots of trees planted in towns do not depend exclusively on infiltra- 

 tion for their supply of water, for they receive a certain amount of both moist- 

 ure and air through the interstices between the paving-stones ; but where 

 wide surfaces of streets and courts are paved with air and water tight asphal- 

 tum, as in Paris, trees suffer from the diminished supply of these necessary 

 elements. 



f See the interesting observations of Keiegk on this subject, Schriften zur 

 allgemeinen Erdkunde, cap. iii., § 6, and especially the passages in Ritteb's 

 Erdkunde, vol. i., there referred to. 



The tenacity with which the parched soil of Egypt retains the supply of 

 moisture it receives from the Nile is well illustrated by observations of Girard 



