Irrigation and Drainage 147 



Installing the drainage system. 



The first step in draining land is to lay out the system. 

 Some kind of instrument for getting levels must be used 

 to determine the contours and to decide where to place 

 the drain lines. A level is also necessary to find the 

 proper depth for the trenches. After the system is laid 

 out, the ditches are dug either by hand or by machinery. 

 In early days they were practically always dug by hand, 

 but modern machinery now does the work much more 

 cheaply. Tile should probably not be placed nearer the 

 surface than two feet or deeper than six or eight feet ex- 

 cept in unusual cases. Usually five feet is a good depth. 



The bottom of the ditch should have a uniform grade ; 

 otherwise, the flow of drainage water will be uneven and 

 silt will be deposited in low places. In sections where 

 springs of fresh water occur, there is a tendency for roots 

 to clog the drains. They must then be placed deeper 

 than would otherwise be necessary. Care should be 

 taken to have the joints of the tile fit well together to 

 avoid filling with dirt. The work of covering can usually 

 be done with a team and scraper. The outlet should be 

 screened to keep out small water-loving animals, and 

 should be constructed so that it will not be clogged easily. 

 If an extensive drainage system is to be laid out, an en- 

 gineer should be consulted. 



