DEPOSITS OF THE NILE. 519 
sive, 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 
per second. Its smallest delivery is 186 cubic métres, or 6,569 cubic feet, 
its greatest 5,156 cubic métres, or 152,094 cubic feet. The average delivery 
of the Nile being 101,000 cubic feet per second, it follows that the Po con- 
tributes tothe Adriatic rather more than six-tenths as much water as the Nile 
to the Mediterranean—a result which will surprise most readers. 
It is worth remembering that the mean delivery of the Rhone is almost 
identical with that of the Po, and that of the Rhine is very nearly the same. 
Though the Po receives four-tenths of its water from lakes, in which the 
streams that empty into them let fall the solid material they bring down from 
the mountains, its deposits in the Adriatic are at least sixty or seventy per 
cent. greater than those transported to the Mediterranean by the Rhone, which 
derives most of its supply from mountain and torrential tributaries. Those 
tributaries lodge much sediment in the Lake of Geneva and: the Lac de Bour- 
get, but the total erosion of the Po and its aflluents must be considerably 
greater than that of the Rhone system. The Rhine conveys to the sea much 
less sediment than either of the other two rivers.—LOMBARDINI, Cargia- 
menti nella condiztone del Po, pp. 29, 39. 
The mean discharge of the Mississippi is 675,000 cubic feet per second, and, 
accordingly, that river contributes to the sea about eleven times as much water 
as the Po, and more than six and a half times as much as the Nile. The dis- 
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 ABsBorT’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. Besides 
this, the Mississippi loses little or nothing by the diversion of its waters for irriga- 
tion. Consequently the measured discharge of the Mississippiis proportionally 
much less than that of the Po, and we are authorized to conclude that the diifer- 
ence 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 thelatter. To take the case of the Nile and the Po: we have 
reason to suppose that comparatively little water is diverted from the tribu- 
taries 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 afiluent. 
This quantity is now increasing in so rapid a proportion, that Hlis¢e Reclus 
