520 DEPOSITS OF THE NILE. 
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 of the Nile are naturally greater in Upper than in 
Lower Egypt. They are found to have raised the soil at 
Thebes about seven feet within the last seventeen hundred 
years, and in the Delta the rise has been certainly more than 
half as great. 
We shall, therefore, probably not exceed the truth if we 
suppose the annually inundated surface of Egypt to have been 
elevated, upon an average, ten feet,* within the last 5,000 years, 
‘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 afiluents 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-pumps do 
of that of the Nile. 
Another circumstance is important to be considered in comparing the 
character of these three rivers. The Po runsnearly 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 the Mississippi is mainly north andsouth. 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 the basins of 
some of its western afiluents. 
* Fraas and Hyth maintain that we have no trustworthy data for calculating 
the annual or secular elevation of the soil of Egypt by the sediment of the 
Nile. The deposit, they say, is variable from irregularity of current, and 
especially from the interference of man with the operations of nature, to a 
degree which renders any probable computation of the amount quite impossible. 
—FrAAs, Aus dem Orient, pp. 212, 213. 
The sedimentary matter transported by the Nile might doubtless be esti- 
mated with approximate precision by careful observation of the proportion of 
suspended slime and water at different stations and seasons for a few succes- ° 
sive years. Figari Bey states that at low stages the water of the Nile contains 
little or no sediment, and that the greatest proportion occurs about the end of 
