109 



PERCOLATION. 



The permeation of water through soils of different qualities has 

 been studied by WelitschUowsky (AVoIhiy, 1888, X. p. 20:1.) He 

 maintained a layer of water at a constant height above the material 

 through Avhich it permeated ; therefore the pressure forcing the water 

 through was constant. He found that the quantity of flow increased 

 at first rapidly, then slowly for several days, depending on the thick- 

 ness of the stratum of soil and the pressure of the water, until the 

 permeation reached the maximum; then the rate of flow diminished 

 slightly for a day or two until it became constant. He found that 

 the quantity of water delivered in a unit of time has no simple rela- 

 tion to the pressure forcing it through the soil or to the thickness of 

 the layer of soil through which it flows, but the relation is more 

 nearly expressed as follows: If the pressure be increased by regular 

 additions the flow of water increases in an arithmetical progression 

 such that the quantity equals (A) plus a constant factor (D) times 

 the pressure (P) less unity; A-f-D (P — 1). The numerical values 

 of these terms can be deduced from his extensive tables of experi- 

 ments, of which the following table is an abstract: 



" The capacity for water is expressed as a percentage of the weight of the dry soil. 



The general laws of the flow of waters through soils of difi'erent 

 natures have been elaborately investigated by Milton AMiitney in a 

 series of pajjers published in Agricultural Science, Volume IV, to 

 which the reader must refer for the details. 



The percolation of water through the soil, whether it goes down- 

 ward as drainage or upward to be evaporated from the surface, 

 depends not merely upon the degree of comminution of the soil and 

 its compactness, but also, among other things, to a slight extent, upon 

 the barometric pressure of the atmosphere, so that a falling barometer 

 is, according to E. S. -Goff, generally accompanied by a corresponding 

 increase in the rate of drainage or of percolation downward. (Agr. 

 Sci., Vol. I, p. 173.) 



