DrVEKSION OF EIVEES. 481 



to such a lieiglit as only partially to obstruct the surface drainage ; 

 or they may be provided witli sluices by means of which their 

 whole contents can be discharged in the dry season, and a summer 

 crop be grown upon the ground they cover at high water. The 

 expediency of employing them and the mode of construction 

 depend on local conditions, and no rules of universal apphcabihty 

 can be laid down on the subject.* 



It is remarkable that nations which we, in the inflated pride 

 of our modern civilization, so generally regard as little less than 

 barbarian, should have long preceded Christian Europe in the 

 systematic employment of great artificial basins for the various 

 purposes they are calculated to subserve. The ancient Peruvians 

 built strong walls, of excellent workmanship, across the channels 

 of the mountain sources of important streams, and the Arabs 

 executed immense works of similar description, both in the great 

 Arabian peninsula and in all the provinces of Spain which had 

 the good fortune to fall under their sway. The Spaniards of the 

 fifteenth and sixteenth centuries, who, in many points of true 

 civilization and culture, were far inferior to the races they sub- 

 dued, wantonly destroyed these noble monuments of social and 

 political wisdom, or suffered them to perish, because they were 

 too ignorant to appreciate their value, or too unskilful as practical 

 engineers to be able to maintain them, and some of their most 

 important territories were soon reduced to sterility and poverty 

 in consequence. 



Diversion of Rivers. 



Another method of preventing or diminishing the evils of in- 

 undation by torrents and mountain rivers, analogous to that 

 employed for the drainage of lakes, consists in the permanent 

 or occasional diversion of their surplus waters, or of their entire 

 currents, from their natural courses, by tunnels or open channels 

 cut through their banks. Natm*e, in many cases, resorts to a 

 similar process. Most great rivers divide themselves into several 



* The insufficiency of axtificial basins of reception as a means of averting 

 the evils resulting from the floods of great rivers has been conclusively shown, 

 in reference to a most important particular case — that of the Mississippi — ^by 

 Humphreys and Abbot, in their admirable monograph of that river. 

 21 



