LAND DRAINAGE 645 



is wide in proportion to the length of the main drains, 

 the subdrains may branch out laterally at a right angle 

 or less. If the laterals on either side of the main drain 

 join at the same acute angle, the " herring-bone " system 



Fig. 65. — Sectional view of soil and rock formation, showing the under- 

 ground movement of water and the position of resulting wet areas 

 on the -urface. In addition to the springy places, the soil is kept 

 wet by the seepage of water along the top of the compact subsoil. 

 This figure also illustrates the reason for locating a cross drain 

 above the springy area in order to effect drainage. This method 

 cuts off the water supply. 



is formed. If the main drain is situated in the wettest 

 part of the field, this system has some advantage. If the 

 field is long and very narrow, the main drain may be along 

 the short side of the field, with long laterals leading up the 

 slope. If the land is of about equal wetness on a slope, 

 the drains should extend up and down rather than across 

 the slope. 



547. Grade of tile drains. — Where the land is rela- 

 tively flat or uneven, a survey should be made in order 

 to determine the distribution and extent of the grades. 

 This is necessary in arranging the system. Where the 

 grades are simple, the arrangement may be determined 

 by the eye, if the man in charge is experienced. 



