496 AGRICULTURE. 



becoming baked into lumps, or from being, for any considerable 

 depth below the surface, too dry for the purpose 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 clods, settles away and leaves the clay, within 

 a few hours after the rainfall ceases, and before rapid evapora- 

 tion commences, too much dried to crack into masses. Of 

 course, this is only the beginning of the operations of improve- 

 ment. 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 

 remove from below water which, if not so removed, would evap- 

 orate from the surface. The formation of a crust on the sur- 

 face 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, in direct proportion to its 

 thickness. 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 excess of water and supplied with air, 

 the more deeply will roots descend ; and the easier its communi- 

 cation with the atmosphere, the more frequently will the air in 

 the lower soil be changed. On these two principles depend the 

 immunity from drought which underdraining helps 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 driest weather gives up its 

 moisture (greater than at any other time), to the particles of the 

 cool, shaded lower soil with which it comes in contact. A box 

 of finely pulverized earth, two feet deep, previously dried in an 

 oven, placed in the sun and wind on the driest and hottest days 

 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 



