78 



FARM DEVELOPMENT 



lower half of the mass. 



thrive so well as if it contained only 20 to 35 pounds 

 of water to the hundred pounds. If the rainfall 

 continues until no more soil with unsatisfied capillary 

 power exists below, the excess of water percolates to 

 the bottom of the pot, obeying the law of gravitation, 

 and there fills the larger interstices, 

 crowding out the air. This water is 

 called ground water. As the rain con- 

 tinues until the ground water has filled 

 all the openings in the soil, the sur- 

 face of this ground water gradually rises 



Figure 24. Pot of soil / 



ft fte un r tn n fife 8 f cap e niary amon g the soil particles in the bottom 

 ire^Mtisfled 811 and rfa t c hl ^ tne P ot > forcing the air out from 

 r gr a ounV aI Uef inui h among the soil particles. Thus in 

 Figure 24 the ground water is shown 

 to have filled the lower half of the pot, 

 while in the upper part the surfaces of 

 the particles are covered with what water the capillary 

 spaces and surfaces can hold. This excess of rain, upon 

 entering the soil, no longer retarded be- 

 cause all capillary force is satisfied, 

 simply obeys gravitation, percolates 

 vertically downward until it reaches the 

 surface of the ground water. In most of 

 our more open soils this excess water 

 sinks far downward until an impervious 

 layer is reached, where it forms the 

 ground water shown by the level of 

 water in the wells. With the continu- 

 ance of the rainfall, if the spouts A 

 and B in the pot be closed, the ground water will rise, 

 completely filling the interspaces within the soil. More 

 rain will cause the water to stand above the soil as surface 

 water, and to run over the rim of the pot, as in Figure 25. 

 If the spout A, Figure 26, be now opened, the surface 

 water and the ground water will gradually drain out to 



e u fJn o h f e w"Sr- pc ?S 



filling the open space 



in, the top of the pot 



and running off over 



1hc rim - 



