IRRIGATION. 



should have a corresponding channel [ 

 below to carry it off; for the water | 

 must never be allowed to stop and , 

 stagnate. When it has run 15 or 20 ! 

 feet, according to the declivity, over 

 the land situated below the feeder, or 

 the channel which brings the water, 

 it should be collected into a drain to 

 be carried off, unless it can be used 

 to irrigate lands which lie still lower. 

 Finally, it runs back into the river 

 from which it was taken at a lower 

 point of its course. 



"When there is a considerable fall 

 and a sufficient supply of water, a se- 

 ries of channels may be made, so sit- 

 uated below each other that the sec- 

 ond collects the water which the first 

 lias supplied, and in its turn becomes 

 a feeder to irrigate the lower parts 

 of the declivity ; a third channel re- 

 ceives the water and distributes it 

 lower down, until the last pours it 

 into the river : this is called catch- 

 work, because the water is caught 

 from one channel to another. This 

 method is only applicable where there 

 is a considerable fall of water and a 

 gentle declivity towards the river. 

 But it must be borne in mind that the 

 water is deteriorated for the purpose 

 of irrigation when it has passed over 

 the land, and that it is not advanta- 

 geous to let it flow over a great ex- 

 tent when a fresh supply can be ob- 

 tained ; but where only a small por- 

 tion of water can be commanded, 

 that must be made the most of; and 

 it will irrigate three or four portions 

 of land in succession without there 

 being any very marked difference in 

 the effect : beyond this it rapidly loses 

 its fertilizing qualities. 



" The general principle of irriga- 

 tion may be described as the supply- 

 ing of every portion of the surface 

 with an abundance of water, and ta- 

 king it off again rapidly. In many 

 situations, the great difficulty in irri- 

 gation arises from the want of a sup- 

 ply of water ; but even then a partial 

 irrigation may be effected, which, al- 

 though not perfect, will have its ad- 

 vantages. A small rill, which is oft- 

 en quite dry in summer, may still, i)y 

 judicious management, be made to 



O 2 



improve a considerable portion of 

 land ; its waters may be collected 

 and allowed to accumulate in a pond 

 or reservoir, and let out occasionally, 

 so that none be lost or run to waste. 

 If there is but a small quantity, it 

 must be husbanded and made to flow 

 over as great a surface as possible. 

 If there is water only at particular 

 seasons of the year, and at a time 

 when it would not be of much use to 

 the land, it may be kept in ponds, and 

 it will lose none of its qualities by 

 being exposed to the air. If animal 

 or vegetable matter, in a partial slate 

 of decomposition, is added to this wa- 

 ter, it will much improve its quality, 

 and by a judicious distribution of it 

 over the land, a great benefit may be 

 obtained. 



" If there is not a want of water, 

 there may be a want of declivity to 

 enable it to flow off, which, it should 

 always be remembered, is an essen- 

 tial part of irrigation : art may in this 

 case assist nature, by forming a pas- 

 sage for the water, either in its course 

 towards the land to be irrigated, or 

 from it after it has effected its pur- 

 pose. Where there is no natural ex- 

 it — and it might lead to too great an 

 expense to make an artificial one — 

 the water may sometimes be led into 

 shallow ponds, where a great part is 

 evaporated ; or porous strata may be 

 found by boring, into which it can be 

 made to run and be dispersed. Along 

 rivers where the fall is very imper- 

 ceptible, a channel, brought from a 

 considerable distance, may give such 

 a command as to throw the water 

 over a great extent of surface ; and 

 to carry it off, another channel may 

 be cut, emptying itself at some dis- 

 tance below ; so that lands which lie 

 along the banks of a river may be ir- 

 rigated, although they are actually be- 

 low the level of the river, and require 

 banks to protect them from inunda- 

 tion. 



" When the surface to be irrigated 

 is very flat and nearly level, it is ne- 

 cessary to form artificial slopes for 

 the water to run over. The whole 

 of the ground is laid in broad beds, 

 undulating like the waves of the sea. 



437 



