TENTH ANNUAL YEAR BOOK— PAGE X 567 



2. More or less work with a level may be required in order to obtain 

 the facts necessary for determining upon the best plan of work. The 

 slopes may be so slight or so deceptive to the eye and the lines of natural 

 drainage and best points of outlet so obscure that it will require an in- 

 strumental survey to determine them. 



After the lines have been located upon the ground, then their location 

 in the ground as to depth and grade must be done with the level, if any 

 degree of accuracy in the construction of drains is expected to be at- 

 tained. 



The grade of a drain is its rate of fall and is expressed in decimals of 

 a foot per hundred feet and is called so much per cent. Determining 

 the grades upon which the drains should be laid requires much skill 

 and knowledge of practical details of construction, together with the un- 

 derstanding of the requirements of the soil, capacity, cost and efficiency 

 of various kinds and sizes of drains. 



The minimum grade that may be used successfully for the tile drains 

 is a matter of great moment where level lands are treated, and will de- 

 pend much upon the accuracy with which the drains will be con- 

 structed. 



Grades as low as one-half inch per 100 feet are in successful operation, 

 giving good results on thousands of acres of land. Mains laid on a level 

 are sometimes used with success, the flow through such mains depend- 

 ing upon the head given by the free water in loose soils, and by lateral 

 drains having a grade greater than that of the main. The lack of fall must 

 be offset by increased size of drains and by the greater degree or accuracy 

 in their construction. 



When a sub-main or lateral enters another drain it is best to have an 

 outfall from the branch line into the main. This is commonly called a 

 "drop" and should be proportionate to the size of the tile used on 

 both lines. For example, branches into a 6-inch main should drop .20 

 feet; into an 8-inch main, .30 feet; 10-inch, .40 feet; 12-inch, .50 feet. The 

 depth with which drains should be laid is a matter which has received a 

 good deal of attention since the time that under-drainage began to be 

 practiced. Advocates of deep and shallow drains have very earnestly 

 argued their favorite theories. 



It is one of those cases in which theories do not work out in practice, 

 the factor which prevents this being the variations in the characteristics 

 of the soil to be treated. 



In speaking of depth of drainage, 4 feet is called deep drainage, 3 feet 

 medium, and 2 to 2i/^ feet shallow drainage. If drains are laid deep the 

 soil must be susceptible to the ready percolation of water, and by the 

 process be converted into a soil of greater or less value to plants. An- 

 other advantage is that the soil has a greater reservoir capacity for water, 

 which is valuable in times of excesive rainfall, and still another, the 

 drains may be laid further apart. 



This is true for deep, permeable, rich soils, and with such there is no 

 doubt as to the value of general four feet drainage. 



On the other hand, many subsoils at a depth of four feet have no fertil- 

 ity in them. Though plant roots often penetrate them, seeking for mois- 



