122 



LAND DRAINAGE AND IRRIGATION 



2. The Gridiron or Parallel System. — When an area requiring 

 several lines of tile slopes uniformly in one direction, or when it 

 slopes uniformly towards two adjacent sides, the ''gridiron" or 

 ^' parallel' system is the most economical (Fig. 65). This system 

 may also be used on level areas. 



3. The Herringbone System. — Sometimes land slopes towards 

 a middle and lower area. In such a case the ''herringbone" 

 system is most convenient (Fig. 56). This system may like- 

 wise be employed on level areas. 



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Fig. 56. — A combination of three systems of tile drainage A, natural system; B, parallel 



system, C, hernngbone system. 



4. Combination of Systems. — In actual practice it is usually 

 necessary and convenient to combine the different systems of 

 drainage, since conditions vary even in comparatively small areas. 

 Figure 56 illustrates this. 



Outlets of Mains Should be Protected. — ^Exposed outlets of 

 mains should be constructed to withstand frost, flooding or washing 

 and tramping of livestock. Moreover, they should be protected 

 by a screen to prevent their becoming clogged by anything that 

 might enter the mouth of the drains. Generally, it is best to use 

 glazed sewer pipe for about six to ten feet at the outlet, laid in 

 firm soil, or imbedded in concrete (Figs. 57 and 58). 



Tile Cheaper and Better Than Open Ditches.— In the long run 



