376 WHY SHOULD LAND BE DRAINED? 



from becoming, for any considerable depth below the sur- 

 face, too dry for the purposes of vegetation. In the first 

 place, the water of heavy spring rains, instead of lying 

 soaking in the soil until the rapid drying of summer bakes 

 it into coherent lumps, settles away and leaves the clay, 

 within a few hours after the rain ceases, and before rapid 

 evaporation commences, too much dried to crack into 

 lumps. 



The other direct effect of under-draining is to remove 

 from below, water which, if not so removed, would be eva- 

 porated from the surface. 



The formation of a crust on the surface of the ground 

 is in direct proportion to the quantity of water that is 

 removed by evaporation, and the crust constitutes a barrier 

 against the admission of air. Consequently the larger the 

 quantity of water that is removed by the drains, the smaller 

 is the obstacle offered to the entrance of air. The more 

 constantly the lower parts of the soil are relieved from ex- 

 cess of water and supplied with air, the more deeply will 

 roots descend ; and the more frequently will the air in the 

 lower soil be changed, the easier its communication with 

 the atmosphere. 



On these two principles depends the immunity from 

 drought which under-draining helps us to secure. In dry 

 weather the soil gets its moisture from the deposit of dew, 

 on the surface during the night, and on the surfaces of the 

 particles of the lower soil constantly, day and night. 



