LAND-DRAINAGE 217 



to be carried off may enter where the sections join. 



' ' The effect of drainage is not merely to carry off 

 the superfluous water and thus promote the aeration 

 of the soil to the depth reached by the roots ; it also 

 keeps the soil cool and moist by the constant pres- 

 ence of water in the drainage ditches or pipes. 

 When a heap of sand is watered at its base, the mois- 

 ture is seen to mount higher and higher until it 

 reaches the top. In like manner the water collected 

 in our drainage ditches soaks into the upper soil in 

 a dry time and thus reaches the roots of plants grow- 

 ing there, so that water which is superfluous or even 

 harmful at certain periods is held in reserve and 

 gradually distributed at the right moment. 



"Another advantage of a drainage system is that 

 it prevents that cooling of the soil which would re- 

 sult from prolonged evaporation. In taking the 

 form of vapor water chills the objects that help to 

 promote the evaporation. For this reason we feel 

 a decided chill on emerging from a bath; the film 

 of moisture that covered us is passing off in vapor- 

 ous form. Similarly a constant evaporation at the 

 surface of a water-soaked tract of land chills the 

 ground and we have a cold soil. But if the water 

 is carried off by proper drainage, evaporation ceases 

 and there is no further chilling of the surface soil. 

 Now, a high temperature is always favorable to vege- 

 tation. 



Draining is so beneficial that it is not confined to 

 marshy ground, which without it would be quite un- 

 productive, but is applied also to ordinary arable 



