450 RESTORATION OF THE WATER. 



wholly diverted from their natural channels to supply the canals, 

 and their entire mass of water is completely absorbed or evapo- 

 rated, so that only snch proportion as is transmitted by infiltra- 

 tion reaches the river they originally fed. Irrigation, therefore, 

 diminishes great rivers in warm countries by cutting off their 

 sources of supply, as well as by direct abstraction of water from 

 their main channels. We have just seen that the system of irri- 

 gation, in Lombardy, deprives the Po of a quantity of water 

 equal to the total delivery of the Seine at ordinary flood, or, in 

 other words, of the equivalent of a tributary navigable for hun- 

 dreds of miles by vessels of considerable burden. The new canals, 

 executed and projected, will greatly increase the loss. The water 

 required for irrigation in Egypt is less than would be supposed 

 from the exceeding rapidity of evaporation in that arid climate ; 

 for the soil is thoroughly saturated during the inundation, and 

 infiltration from the ISTile continues to supply a considerable 

 amount of humidity in the driest season. Linant Bey computed 

 that, in the Delta, fifteen and one-third cubic yards per day 

 suflSced to irrigate an acre. If we suppose water to be apj)lied 

 for one hundred and fifty days during the season of growth, this 

 would be equivalent to a total precipitation of about seventeen 

 inches and one-thii'd. Taking the area of actually cultivated soil 

 in Egypt at the estimate of 4,500,000 acres, and the average 

 amount of water daily applied in both Upper and Lower Egypt 

 at twelve hundredths of an inch in depth, we have an abstraction 

 of about 74,000,000 cubic yards, which — the mean daily delivery 

 of the Nile being in round numbers 320,000,000 cubic yards — 

 is twenty-three per cent, of the average quantity of water con- 

 tributed to the Mediterranean by that river.* 



In estimating the effect of this abstraction of water upon the 

 volume of great rivers, especially in temperate climates and in 

 countries with a hilly surface, we must remember that all the 



* The proportion of the waters of the Nile withdrawn for irrigation is 

 greater than this calculation makes it. The quantity required, for an acre is 

 less in the Delta than in Upper Egypt, both because the soU of the Delta, to 

 which Linant Bey's estimate applies, lies little higher than the surface of the 

 river, and is partly saturated by infiltration, and because near the sea, in 

 N. L. 30°, evaporation is much less rapid than it is several degrees southwards 

 and in the vicinity of a parched desert. 



