224 



SOILS. 



drostatic pressure. It may be presumed that at a certain dis 

 tance from the surface the downward movement becomes prac- 

 tically uniform, and independent of the pressure from above. 

 Summary. Two salient points are revealed by even a cur- 

 sory inspection of the preceding diagram, viz. : 



1. The downward percolation is most rapid in the same 

 soils in which the capillary ascent is quickest, that is, in the 

 coarse, sandy soil. 



2. The rapidity of percolation decreases materially as the 

 wetted soil column increases in length. 



The first point is readily foreseen and needs no comment. As re- 

 gards the second, it results from the fact that as the wetted column 

 lengthens the frictional resistance increasingly counteracts the effects 

 of the hydrostatic pressure from above, until the water's descent becomes 

 but little more rapid than would be its lateral diffusion, or its ascent at 

 the end of a similar column supplied by capillary rise from below. In 

 both cases the frictional resistance has so far counteracted the effect of 

 gravity that the capillary coefficients of the soil-material become the 

 controlling factors of the water movement. 



Influence of Variety of Grain-sizes. King (Physics of 

 Agriculture, pp. 159 ,160), compared the rapidity of the per- 

 colation of water through definitely graded pure sands on the 

 one hand, and a sandy loam and a clay soil on the other. The 

 materials were arranged in 8-foot columns fully saturated with 

 water at the outset, and then allowed to drain freely. The 

 following abridged table shows the tenor of his results : 



TABLE SHOWING RELATIVE RAPIDITY OF PERCOLATION IN PURE SANDS 

 AND SOILS, IN INCHES OF WATER DRAINED OFF. 



