482 CONSTRUCTION, FARM ENGINEERING, MECHANICS 



These limits are based on perfectly laid tile, which is seldom 

 achieved. The lay of the land, the nature of the earth, the occur- 

 rence of rocks and trees and other obstructions, the necessity of 

 making detours, and other conditions, all influence the theoretical 

 limits of efficiency. 



Conditions that determine the size of the drains, particularly the 

 mains (Elliott) : 



1. 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. 



Number of acres from which H inch of water will be removed in 24 hours 

 by outlet tile-drains of different diameters and different lengths with dif- 

 ferent grades (adapted from Elliott) 



