THE DRAINAGE OF FARM SOILS 211 



need draining are apt to be rather flat and there 

 may be considerable difficulty in deciding off-hand 

 where the lowest land is, and along what line be- 

 tween the outlet and the upper part of the field the 

 greatest fall may be secured. In doubtful cases 

 a level should settle the question. 



Having established the outlet and located the 

 main drain on the lowest land, next locate the lat- 

 erals, or collecting drains. The fall of the entire 

 system must now be considered. In general it 

 should be from 5 to 8 inches in 100 feet. If this 

 grade can be secured there should be no difficulty 

 in laying a system that will work perfectly. But 

 very often 2 or 3 inches in 100 feet, or even less, is 

 as much fall as can be had. Excellent drainage 

 systems are now in operation that have a fall of 

 2 inches in 100 feet, and there are occasional ex- 

 amples of farm drainage systems that work with a 

 fall of even ^-inch in 100 feet. But these can be 

 constructed only with the aid of a skilled engineer. 

 In farm drainage a fall of at least three inches 

 should be sought ; if one must content himself with 

 less grade he should employ a surveyor and give 

 greater attention to the laying of the tiles. 



On the other hand, too much fall in a drainage 

 system is equally undesirable. A fall of 12 inches 

 in 100 feet is considered about the limit of safety. 

 A greater grade would carry the water so fast that 

 there would be danger of loosening the tiles. This 

 is especially true on lighter soils, which frequently 

 have tile drains washed from them after a very 

 heavy rain. If any of the tiles are loosened suffi- 

 ciently to admit soil the whole system may be 

 ruined eventually. 



If possible it is best to have all the drains laid at 

 a uniform grade, from the upper end of the system 



