SOIL WATER 83 



but an acute angle should be formed between the two streams 

 above the point of contact; otherwise the flow of water 

 will be impeded. For the same reason two laterals should 

 not enter a main drain opposite to each other. 



It is desirable to have as few main drain outlets as possible, 

 for the outlet is likely to be the weakest point in a drainage 

 system. If it becomes clogged, the entire system is put 

 out of action. It is more likely to be injured by freezing 

 than is the underground tile, and unless well protected it 

 affords an opening into which small animals may crawl and 

 clog the system. 



The quantity of water removed by tiles of various sizes, 

 and laid at certain distances and grades as well as other 

 operations that cannot be treated here, may be ascertained 

 from the books that deal exclusively with the subject of land 

 drainage. 



104. Digging ditches and laying tile. — The depth of 

 ditches for tile drainage varies from two to four feet. Three 

 feet is the usual depth. The closer together the laterals, 

 the shallower the drains may be laid. A compact soil, 

 through which water moves very slowly, will require the 

 use of shallow drains. A lighter soil underlaid by hardpan 

 will also require shallow drains. The shallower the drains 

 in any soil, the closer together they must be laid, the cus- 

 tomary range being from twenty to a hundred or more feet. 

 Surplus water enters the drains from the soil immediately 

 surrounding them. As the larger pore spaces become 

 partly empty, water enters them from surrounding soil, 

 and in this way drainage gradually extends. The soil mid- 

 way between the drains is the last to lose its surplus water, 

 and the water table is always higher between drains than 

 over them. 



The distance between drains must be small enough to 

 allow the water table to descend promptly to a point where 



