30 AGRICULTURE FOR COMMON SCHOOLS 



same, but usually it should be 3 J to 4 feet. Such depth insures 

 the quick drainage of the surplus water from that part of the 

 soil in which the roots are mostly found. 



After the trench is dug the tiles are laid end to end in the 

 level surface of the bottom. Care must be taken that the 

 joints fit closely so that soil will not wash into the tiles and clog 

 them up. After the tiles are laid the covering of them must 

 be carefully done, so that they are not displaced. It is helpful 

 if an air vent is left at the upper end of the line of tiles. This 

 is made by setting a couple of tiles on end, or by filling up the 

 trench at this point with small boulders. These air vents 

 give a better "draw" to the system. A screen of some kind 

 over the outlet will prevent the entrance of rabbits or other 

 small animals which might get fast and die and clog up the 

 system. Draining costs from six dollars to thirty dollars an 

 acre and every care should be taken to keep the system in 

 good working order. > 



Irrigation. — Irrigation is artificially supplying water to 

 the land. It is practised in places where there is little or 

 no rainfall. It is also used to some extent in other regions 

 in dry times, particularly in gardens and truck fields where 

 the crop is very valuable. Irrigation is used quite extensively 

 in rice growing. 



Places where the rainfall is twenty inches or less during the 

 year are called arid; if the rainfall amounts to twenty to 

 thirty inches, the region is said to be semi-arid; where more 

 than thirty inches of rain falls in the year we have a humid 

 region. Parts of the following states need irrigation : Arizona, 

 California, Colorado, Idaho, Montana, Nevada, New Mex- 

 ico, Oregon, Utah, Washington, and Wyoming. Of these, 

 Nevada, Utah, Arizona, and Southern California are the dry- 



