DRAINAGE. 9I 



provement. It is merely the foundation, but on heavy soils it is 

 the necessary foundation, of the processes (natural and artificial) 

 by which the improvement is effected and made permanent. The 

 only direct effects of draining are to prevent the soil from ever 

 being completely saturated for any considerable time, and to re- 

 move from below water, which if not so removed would be 

 evaporated 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 admis- 

 sion of ait in direct proportion to its thickness. Consequently, 

 the larger the quantity of the 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 excess 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 atmos- 

 phere. 



On these two principles depends the immunity from drought 

 which underdraining 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. 



The familiar example of the " sweating " of a cold pitcher that 

 stands in the sun and wind on a hot July day, illustrates the 

 manner in which the dew-laden air of our dryest weather gives 

 up its moisture (greater then than at any other time) to the parti- 

 cles of the cool-shaded lower soil with which it comes in con- 

 tact. A box of finely pulverized earth, two feet deep, previously 

 dried in an oven — placed in the sun and wind on the dryest and 

 hottest day of summer, would soon become sufficiently moist for 

 the growth of plants, by the deposit of dew among its lower and 

 cooler particles. Let the same earth be saturated with water and 

 closely compressed, and it would, under the same circum- 

 stances, be baked and dry throughout its whole depth. No 

 air could enter for the deposit of dew, and, from Its compact con- 



