I02 THE MOVEMENTS OF SOIL WATER [chap. 



point is only in evidence when the brush is partially wet. 

 Water or other liquids cling to solid bodies which they 

 " wet," because they enclose them within the elastic film 

 formed by their surface, and as the liquid inside the 

 film is free to move, the film must be equally stretched 

 at every point when equilibrium has been attained and 

 the liquid is at rest When the shape of the wetted 

 body is irregular, the film that encloses it will always 

 adjust itself to develop its minimum of surface. Coming 

 back to the beads in oil or wax, the film on the upper 

 surface is stretched to the thinnest extent that is 

 consistent with not breaking, because all the liquid 

 inside that does not form the film itself is free to 

 move, and sinks by gravity as low as possible. On the 

 lowest row the film is at its thickest, all the pull of the 

 elastic skin being employed in holding up the weight of 

 liquid inside the skin. In the angles between the beads 

 the layer is thicker, because the skin is, as it were, 

 stretched across a corner in order to reduce its area to 

 a minimum. With this idea of a stretched outside skin 

 in our minds, let us imagine what would happen if 

 liquid were added or taken away from any point. 

 Suppose liquid is added at the top, the skin becomes 

 at once more stretched, and tries to readjust matters by 

 pressing the liquid down and establishing an equal 

 stretching all over, and in this downward pressure it is 

 aided by gravity. Once the liquid has arrived at the 

 lower layer, the skin there has to carry an increased 

 weight, too much for its elasticity, so that the skin breaks 

 and a drop falls. It is in this way that water percolates 

 downwards in consolidated soil where there are no 

 actual channels through which to flow ; it is handed on 

 from particle to particle, until at last it reaches an open 

 space into which a drop can form and break off. This 

 open space may be a drain-pipe, which then begins to 



