HOW TO PREPARE THE SURFACE. 189 



CHAPTER XVII. 



PREPARING THE SURFACE FOR IRRIGATION. 



The method of farming by irrigation is very simple and 

 easy to learn. The principals upon which it is managed 

 are summed up in the general laws that water always runs 

 down hill, and that a certain quantity of it is needed for 

 the growth of a plant. In preparing the ground for irri 

 gation, then, it is only necessary to remember these facts 

 and conform the practice to them. The surface of a cul 

 tivated field should therefore be of slight slope, generally 

 in one direction, and of an even, smooth character, free 

 from irregularities or knolls. If, however, the character 

 of the surface is such that it is variously inclined with 

 irregular depressions, having a general course downward 

 from the level of the water supplv, the courses of the 

 distributing channels may be so laid out as to practicably 

 reduce the whole field to a regular slope and make it very 

 easily irrigated. In the first case, the water taken from 

 the canals of supply will be brought into the main dis 

 tributing channels, the course of which will be down the 

 slope ; directly, if the declivity is not too great, and 

 diagonally if not more than three feet in a hundred. 

 From these channels the water will be taken laterally in 

 to other channels, and from them spread over the ground. 

 This plan being suitable only where the soil presents a 

 plane surface, inclined from the canal downward, is ob 

 viously fitted for only a very few cases, for those in which 

 the land is altogether free from swells and variations from 

 a level, are very rare naturally and not very common arti 

 ficially. 



Where, however, it is possible so to prepare the land 

 that this even plane surface can be secured, it will 

 manifestly be the best and cheapest in the end, so to pre 

 pare it. The great majority of the river bottoms in those 



