No. 4.J DRAINAGE. 379 



In the statements which have been made concerning the distance 

 between different Unes of drains reference has been made to the drain- 

 age of areas where soil and water conditions are uniform in character. 

 There are, of course, many fields with varying soil and water condi- 

 tions, in which uniform spacing of the lines of tiles is not advisable. 

 Under these conditions each field is a special problem in itself. 



The distance between drains which will give satisfactory results 

 will be influenced much by the depth of the drains. The deeper these 

 are the farther apart the lines may be. In compact clays a fairly safe 

 rule is to place the lines G or 7 feet apart for each foot of depth in the 

 drain, while in loams the distance may vary at double this rate. That 

 is, it may be from 12 to 14 feet for each foot of depth. 



The Proper Depth of Drains. 



The depth to which drains can be placed is in some instances de- 

 termined Ijy the level of the water in the open channel into which the 

 tiles nuist eventually discharge. Where this level is relatively close to 

 the surface, the question will sometimes arise with how little depth 

 may fairly satisfactory results in drainage be anticipated. In the 

 writer's judgment it will not be advisable to put in underdrains unless 

 the depth possible is equal to at least 2h feet from the surface. Here 

 and there for short distances the tile may be laid at less depth, but it 

 will not be safe, from the action of the frosts, at less than the depth 

 indicated. 



In all cases where conditions permit it seems best to place under- 

 drains at the average depth of from 3^ to 4J feet below the surface. 

 At this depth drainage in the case of compact clays and hardpans may 

 be relatively slow at first, but later the soil will gradually become more 

 open and porous under the influence of root action, the tunneling of 

 earth worms and the formation of cracks as the soil dries out, so that, 

 in the course of a relatively short time, the drainage of even clays is 

 sufficiently rapid with underdrains at the depth indicated. 



The Proper Grade. 

 In the case of fields which are relatively level it is necessary to 

 consider with how small a grade satisfactory drainage can be secured. 

 Experience indicates that with very careful work satisfactory drainage 

 is secured with grades not exceeding 3 inches in 100 feet; indeed, 

 engineers often put in drains with less grade than this. To put in 

 drains which will work satisfactorily with so flat a grade requires ex- 

 tremely careful work, and with the kind of labor usually available on 

 the farm, even under the careful oversight of the owner, who in most 

 cases will have little experience in this kind of work, a steeper grade 

 will be much safer. With ordinary skill and care excellent work can 

 be done with grades ranging from 3 to 5 or G inches to each 100 feet. 

 Whatever the grade, much care should be taken to make it as uniform 

 as possible. 



