212 SOILS 



to the outlet. If it is necessary to change the grade 

 it should preferably be from a less fall to a greater, 

 say from 3 inches to 4 inches in 100 feet; if the 

 grade is reduced there is greater likelihood that the 

 sediment in the water will settle in the lower part of 

 the system. To avoid this, if a reduction in grade is 

 necessary, put a "silt basin" at the point where the 

 change is made. This is made by sinking an 

 8-inch, 10-inch, or 12-inch tile below the level of the 

 ditch, and notching it on one side for the drainage 

 water to flow in, with a lower notch on the opposite 

 side for the first tile of the new grade. The soil 

 dropped in here should be cleaned out occasionally. 

 Carry the silt basin to the surface with glazed 

 sewer tile ; or, if the line of tiles is large, dig a larger 

 basin and brick up the sides. Silt basins should 

 be covered all the time with iron, stone, or plank to 

 avoid accidents and freezing. Silt basins are really 

 small wells; they enable the farmer to see if his 

 drains are working properly, as well as collect silt. 

 They may be placed at the junction of the sub- 

 mains and the mains, even if there is no change in 

 grade, so as to give an opportunity to examine the 

 working of the drains. 



DEVICES FOR ESTABLISHING GRADE 



The most important part of under-drainage is 

 the grade. If tne grade is insufficient the water 

 stagnates and the tiles become filled with soil. If 

 anv part of the system, even a few feet of it, is 

 below grade the whole system suffers. Hence 

 the necessity of securing the services of a skilled 

 drainage engineer if a large area is to be drained. 

 The use of a surveyor's level is not indispensable 

 to good draining, out is extremely helpful. If a 



