270 Bulletin 254. 



tile the same error in grade will cause very little harm. The size of the 

 tile to be used depends upon the length of the line. As a general rule, 

 there is a limit to the length of tile of any common size, which should 

 not be exceeded. These limits are also determined by the grade. The 

 relation of these factors is shown by the following table given by Elliott 

 in "Engineering for Land Drainage" (p. 142). 



Limit of si::c of tile to grade and leiiytli. 



Size of Minimum grade Limit of 



tile per 100 ft. length 



in inches in feet in feet 



2 10 600 



3 09 800 



4 05 1600 



5 05 2000 



6 05 2500 



7 05 2800 



8 05 3000 



9 05 3500 



10 04 4000 



II 04 4500 



12 04 5300 



These limits are based upon perfectly laid tile which is seldom 

 achieved. 



Elliott in Farmers' Bulletin 187 on the Drainage of Farm Lands, 

 gives the following summary of the conditions which determine the size 

 of the drains, particularly the mains. 



(0 The depth of water to be removed in twenty-four hours over the 

 area of the drainage system. 



(2) Rapidity with which the water is brought to the main, that is, 

 the number, size and fall of the laterals. 



(3) The- existence of emergency surface drainage. 



(4) The texture and physical condition of the soil, that is, whether 

 it is open and porous or dense and retentive. 



(5) The grade of the ditch. 



Ordinarily it is not possible to calculate the size of tile necessary 

 to be used with the same accuracy that a water conduit in a supply 

 system may be calculated. The character of the soil is the first factor 

 which renders such accuracy impossible. It has a great capacity to 

 retain moisture and in a clay loam soil the surface foot may retain, 

 two inches or more of water without becoming too wet. So that the 

 thoroughness of drainage is not determined directly by the rainfall. How- 

 ever, since the farmer must in some way gauge the size of tile used 

 to the area of land drained and the amount of water handled, the follow- 

 ing abridged table from Elliott, as quoted above, may be of use. 



