UNDER-DRAINS 



251 



124, Covered or under-drains. Covered or under- 

 drains are any underground channels constructed for 

 the removal of water. Many kinds of material have 

 been used for this purpose. Some of the earlier materials 

 used were brush, stone, poles, boards, and brick. In 

 recent years these have been almost entirely supplanted 



^- -j?.. 











Fig. 81. Construction of a ditch for tile drains. 



by pipes made of clay or cement because of the greater 

 permanency and efficiency of the latter. 



The depth, frequency and size of drains depends on 

 the character of the soil and subsoil, the amount and 

 distribution of the rainfall, the topography of the sur- 

 face, the crop to be grown, the prevalence of under- 

 ground seepage, and the level of the ground water. 

 The system should always be arranged with reference 

 to these conditions. 



