90 IRRIGATION. 



SECTION II. OF IRRIGATION. 



This method of improvement is of course only ap- 

 plicable in particular situations, such as where a head 

 and flow of water can be obtained, and where also the 

 ground to be flowed is in grass or growing grain. All 

 water, except rain water, even that from the purest 

 springs, has mineral substances and organic substances 

 in solution. As it flows over the surface among living 

 plants, and in sinking through the soil comes in con- 

 tact there with their roots, it yields up these substances 

 for food. Beside such solid bodies, it contains in 

 solution carbonic acid and oxygen, both of which the 

 plant also receives with avidity. 



The surface of a field to be irrigated must of course 

 be somewhat sloping, and the water is brought on by a 

 main ditch at the head of the slope. In this main ditch, 

 at proper distances, are gates to regulate the flow of 

 water into smaller ditches, from the sides and ends of 

 which again run small cuts; these are so arranged that 

 every part of the field shall be flowed oyer by a thin 

 but regular sheet of water. At the foot of the slope 

 is another ditch, for the purpose of conveying away 

 such of the water as may not sink into the earth. 

 Where water is scarce, and the slope long, it is oc- 

 casionally used several times in succession. When 

 the flow has been continued for ten days or a fortnight 

 at a time, the supply gates are shut down, and the field 

 allowed to dry. The operation is often repeated once 

 or twice in a season. 



The effect of water in this case, is not like that 

 alluded to before in treating of swamps and wet land. 

 Here there is no stagnation; the water is always run- 

 ning and fresh. Land that is intended to be irrigated 

 should have a porous subsoil, or, if not, should be 

 underdrained; in either case the water sinks away as 

 soon as the flow is stopped, the soil dries, and the 



