No. 17. 



Ivrigatinn. 



337 



ground very thoroughly of all subterraneous 

 water. Nor is this the only distinction be- 

 tween the two kinds of irrigation. In the 

 one, the water is generally allowed to stag- 

 nate until it shall have saturated the soil; in 

 the other, it is never allowed to stagnate, but 

 is maintained in a constant flow over the sur- 

 face. 



In one respect the two kinds of irrigatioti 

 may serve a common purpose, that is, by the 

 deposition of iriud or other fertilizing matter 

 upon the surface. The principal effect, we 

 may believe, which the Nile produces in its 

 periodical overflowings, is the supplying to 

 the soil of the mere element of water, with- 

 out which an arid soil, in a country where tiie 

 rain is scarcely known, would hardly produce 

 any thing. But this effect is plainly greatly 

 increased by the large quantity of mud which 

 the river deposites. In the case, too, of the 

 watered meadow, the water may, in like 



manner, deposite a fertilizing sediment ; but 

 this, though it alwa)'s adds greatly to the ef- 

 fect, is not essential to the producing of it; 

 and waters entirely free from all perceptible 

 sediment, are yet successfully employed in 

 the case of the watered meadow. 



In the watered meadow, a stream of water 

 is to be conducted to the surface, and caused 

 to flow over it in a constant manner; the 

 meadow to be watered, for the most part lying 

 upon the bank of the river from which the 

 water is conveyed, and forming a flat surface, 

 or rather a gently inclined plane. To the 

 highest part of this inclined surface, the wa- 

 ter is conveyed in what is termed the main 

 conductor, either by building a wear or dam 

 across the river where the water is to be ta- 

 ken off, or by bringing it from a higher source. 

 In the following diagram, A represents the 

 main conductor, and B the weat or dam. 



Fiff. 63. 



From the main conductor, and as near as 

 possible at right angles to it, are taken ofl^ 

 the various feeders, ccc, &c. These consist 

 of small trenches four or five inches in depth, 

 made widest, as a foot or sixteen inches, 

 where they issue from the main conductor, 

 and gradually lessening as they recede from 

 it. They may be formed at the distance from 

 each other of forty feet, or less, being nearer 



where the soil is stiff and retentive, and fur- 

 ther distant where it is loose and porous. 



The water is thus conveyed to the surface 

 of the meadow. But it is necessary that it 

 should maintain an equal flow over the 

 ground, and so be carried off as quickly ag 

 It is admitted. This is done by means of the 

 main drain dd, formed at the lower part of 

 the meadow, and the several smaller drains,. 



