502 DEPOSITS OF THE NILE. 



eive; its banks have been occupied by man probably twice as long. 

 But its geographical character has not been much changed in the 

 whole period of recorded history, and, though its outlets have 

 somewhat fluctuated in number and position, its historically 

 known encroachments upon the sea are trifling compared with 

 those of the Po and the neighboring streams. The deposits 



charge of the Mississippi is estimated at one-fourth of the precipitation in its 

 basin — certainly a very large proportion, when we consider the rapidity of 

 evaporation in many parts of the basin, and the probable loss by infiltration. 

 — Humphreys and Abbot's Report, p. 93. 



The basin of the Mississippi has an area forty-six times as large as that of 

 the Po, with a mean annual precipitation of thirty inches, while that of the 

 Po, at least according to official statistics, has a precipitation of forty inches, 

 Hence the down-fall in the former is one-fourth less than in the latter. Be- 

 sides this, the Mississippi loses little or nothing by the diversion of its waters 

 for irrigation. Consequently the measured discharge of the Mississippi is 

 proportionally much less than that of the Po, and we are authorized to con- 

 clude that the difference is partly due to the escape of water from the bed, or 

 at least the basin, of the Mississippi, by subterranean channels. 



These comparisons are interesting in reference to the supply received by 

 the sea directly from great rivers, but they fail to give a true idea of the real 

 volume of the latter. To take the case of the Nile and the Po : we have rea- 

 son to suppose that comparatively little water is diverted from the tributaries 

 ■of the former for irrigation, but enormous quantities are drawn from its 

 main trunk for that purpose, below the point where it receives its last affluent. 

 This quantity is now increasing in so rapid a proportion, that Elisee Reclus 

 foresees the day when the entire low-water current will be absorbed by new 

 arrangements to meet the needs of extended and improved agriculture. On 

 the other hand, while the affluents of the Po send off a great quantity of water 

 into canals of irrigation, the main trunk loses little or nothing in that way 

 except at Chivasso. Trustworthy data are wanting to enable us to estimate 

 how far these different modes of utilizing the water balance each other in the 

 case under consideration. Perhaps the Canal Cavour, and other irrigating 

 canals now proposed, may one day intercept as large a proportion of the sup- 

 ply of the lower Po as Egyptian dikes, canals, shadoofs and steam-pimips do 

 of that of the Nile. 



Another circumstance Is important to be considered in comparing the char- 

 acter of these three rivers. The Po runs nearly east and west, and it and its 

 tributaries are exposed to no other difference of meteorological conditions 

 than those which always subsist between the mountains and the plains. The 

 course of the Nile and of the Mississippi is mainly north and south. The 

 sources of the Nile are in a very humid region, its lower course for many 

 hundred miles in almost rainless latitudes with enormous evaporating power, 

 while the precipitation is large throughout the Mississippi system, except in 

 fthe basins of some of its western affluents. — See Article Po, in Johnson' t 

 •Cyclox>(Bdia. 



