88 LAND DRAINAGE 



tile used. " From the minimum size the tile will increase 

 in size according to the extent of the system. It is now 

 not uncommon for tile as large as two feet in diameter to 

 be used. Three-inch tile in lines not more than six 

 hundred feet long are usually best for lateral drains. 

 For drains up to fifteen hundred feet in length, four-inch 

 tile may be used, provided the grade is not less than four 

 inches per hundred feet. It is difficult to make an exact 

 statement concerning the proper size of main drains. In 

 general they should be capable of removing one-fourth of 

 an inch of water from the drainage area in twenty-four 

 hours." 



117. Grade or fall. Every line of tile should be so 

 laid that there is a gradual fall from the extreme end of 

 the drain to the outlet. This fall is usually spoken of 

 as the grade. It is desirable, when possible, to have a 

 fall of as much as 3 inches in every 100 feet. A carefully 

 constructed line of tile will work successfully on a much 

 less fall than this. Two inches is a common grade, and 

 in very flat areas a fall as slight as 1 inch to the hundred 

 feet is used ; and occasionally a fall of \ inch to the hun- 

 dred feet for tile as large as 8 inches. 



118. Relation of size of tile to the grade. The less 

 the fall, the greater must be the care exercised in laying 

 the tile, and the less will be its capacity to remove the 

 water and therefore the larger must be the tile. Elliott 

 says : " If we double the grade per hundred feet of the 

 drain we increase its carrying capacity about one-third." 

 If this be true, then if we lower the grade by half we 

 should decrease the carrying capacity by one-fourth. 



The following figures, from Fippin, give some idea of the 

 area of land drained by some common sizes of tile when 

 laid at different grades : 



