90 ENGINEERING FOR LAND DRAINAGE. 



argued their favorite theories. It is one of those cases 

 in which theories do not always work out in practice, 

 the factor which prevents this being the variations in 

 the characteristics of the soil which is treated. In or- 

 der that any one theory may prove correct, it must 

 be assumed that a soil of a certain kind under certain 

 conditions is to be operated upon. This kind of soil, 

 however, is not always present, and the theory cannot 

 apply in full. 



In speaking of depth of drainage, 4 feet is called deep 

 drainage, 3 feet medium, and 2 to 2|- shallow drain- 

 age. If drains are laid deep the soil must be suscept- 

 ible to the ready percolation of water, and by this 

 process be converted into a soil of greater or less value 





hn^JT^^ 



4ft.> 



Fig. 19. — Effect of Depth of Drains on Open Soils. 



to plants. In Fig, 19 the difference in depth is shown 

 to the eye with its attendant advantage. Another ad- 

 vantage is that the soil has a greater reservoir capacity 

 for water which is valuable in times of excessive rain- 

 fall, and still another, the drains may be varied in dis- 

 tance apart upon the principle illustrated in Fig. 9. 

 Now, this is all true for deep, permeable, rich soils, 

 and with such there is no doubt as to the value of gen- 

 eral 4-foot drainage. 



On the other hand, many subsoils at a depth of 4 

 feet have no fertility in them. Though plant roots 



