in.] PERCOLATION OF WATER 81 



tube (Fig. 4) as compared with the surface at b in the 

 wider tube. But the diminution of pressure caused by 

 a is greater than that caused by b, as shown by the 

 greater height to which water is raised in the tube; 

 hence in the same way the pressure inside the liquid 

 at a (Fig. 6) will be lower than that at b, and there 

 will be a flow of water from b to a, until the curva- 

 tures and corresponding surface tensions are equalised. 

 In a wet soil, then, surface tension is a force tending 

 on the one hand to retain a certain amount of water 

 round the particles, and on the other to equalise the 

 distribution of water, by causing movement towards any 

 point where the surface tension has been increased. 

 For example,(if the water in a soil is in equilibrium and 

 evaporation begins at the surface, the film there is made 

 thinner, and the curvature increased in the angles 

 between the soil particles : hence the pull exerted by 

 the film is increased, and water is lifted from below 

 against gravity. Per contra, jfrain fall on such a soil 

 the films round the upper particles are thickened, their 

 tension is lowered, and the pull of the film below now 

 acts with gravity in drawing the water down into the 

 soih_\ 



Percolation. 



The state of affairs illustrated by the model of balls 

 dipped in oil is seen in the case of a soil which has been 

 thoroughly saturated so that all the pore space is occu- 

 pied by water, and then allowed to drain until the remain- 

 ing water is held in the soil by surface tension only. 



In the upper layers the film will be stretched to the 

 utmost, or even broken by the pull of the water below ; 

 in the lower layers the film will be wholly engaged in 

 lolding the water immediately in contact with the 

 particles of the layer: these layers may be saturated, 



G 



