42 ENGINEERING FOR LAND DRAINAGE 



Spring water when it proceeds from some distant and 

 constant source following a channel of its own until it 

 reaches a free outlet, where it shows its presence m a 

 definite and constant quantity. 



Systems of Tile Drains. 



The various arrangements of drains according to the 

 requirements of the surface and soil to be treated are 

 called systems. 



A Main Drain is one which is used to collect drain- 

 age water from smaller drains and conduct it to some 

 open ditch or natural stream. 



A Sub-Main is a drain which discharges into a main, 

 and is itself a receiving-drain for lines of smaller tile. 



A Lateral is the smallest drain in the system, and 

 discharges into a main or sub-main. 



The Natural System. — This is the kind of drainage 

 that is first practised, and consists in laying some lines 

 of tile in natural depressions which are particularly wet 

 and troublesome to the agriculturist, as represented in 

 Fig. 4. It deserves the name of system only because 

 in many kinds of soil and localities it is all that is re- 

 quired to make the drainage quite complete. It is an 

 aid to natural drainage, and in fact completes it where 

 the higher land naturally drains itself into adjoining 

 depressions. The occasional lines of tile then put in 

 must carry the drainage of the natural watershed, thus 

 compelling the drains to act as mains for a considerable 

 area. For this reason more complaint is made of the 

 incapacity of tile drains located in this way than where 

 more frequent drainage is practised. The Natural 



