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 systém is to be laid out, an en- 
gineer should be consulted. 
