SOIL IMPROVEMENT BY DRAINAGE. 69 



No drain can do good work without a free outlet. The 

 main drains should follow the lowest land, the natural 

 water courses, unless this line be very crooked. Side 

 drains should, usually, run up and down the slope. 

 Where the surplus water mainly comes from higher 

 land, it is often possible to cut it off by a line of tile run- 

 ning across the slope. The tile for the main drains should 

 be selected with reference to the water they may be 

 ultimately required to carry, and not simply with 

 reference to present needs. Digging the ditch and 

 laying the tile should commence at the outlet. A 

 leveling instrument is advisable, but good results can 

 be obtained by noticing the flow of water in the bot- 

 tom of the ditch. The tile should be placed as close to- 

 gether as possible. The side drains should enter the 

 main at an acute angle, and a little above the bottom. 



The plow may often be profitably used for opening 

 the ditch. A common mistake in digging is in mak- 

 ing the ditch wider than is necessary. Another is in 

 determining the depth by measuring from the sur- 

 face. Many machines for digging ditches for the tile 

 drains have been invented. Some do good work un- 

 der favorable conditions. The great mass of the work 

 is still done by hand labor. 



In exceptional localities ponds and swamps may be 

 drained or good outlets secured for tile drains by 

 openings through the compact subsoil into beds of 

 sand or other porous subsoil underlying the impervi- 

 ous strata. 



Where there are large areas of level, wet lands, large 

 open ditches often have to be made as outlets for the 

 farm systems of tile drains. In some States great 

 canals many miles in length and often thirty feet 

 wide have been dug for this purpose. 



