THE SOIL 



51 



Through A draw a horizontal line, BQ, It is evident now 

 that the downward pressure exerted by the forces acting upon 

 A are less if the surface be concave than if it were horizontal, 

 because there is, in the former case, the upward resultant of 

 the forces exerted by the particles in the shaded spaces. Let 

 this upward attraction be represented by jt?, and the downward 

 force, if the surface were plane, be represented by P. The 

 final resultant will be P for the plane surface and P -pfor the 

 concave, acting upon each particle on the surface. It is also 

 evident that the more concave the surface is, the greater will 

 be p and the less will he P-p, 



Hence when a tube which is wetted by a liquid, and which 

 therefore has within it a concave surface of the liquid, is im- 



FlG. 8. 



merged in a liquid the latter rises until the hydrostatic pressure 

 in the tube is capable of balancing the difference between the 

 pressure exerted by the liquid surface outside the tube (which 

 will be approximately P if the area of the vessel be large) and 

 that exerted by the surface in the tube — i.e., P-p. This is the 

 cause of capillarity, as the rise of liquid iti hair-like tubes is 

 called. 



Many writers on soils appear to think that in soils there are 

 such tubes, and they ascribe the rise of water to this action. 

 It is extremely improbable that this assumption is correct, for 

 the interstices of a soil are not filled with water, but, as is well 

 known, are largely occupied by air. True capillarity may per- 

 haps operate to a small extent, in some of the composite particles 

 of soil where the interstices among the cohering fragments 

 become entirely filled with water. But, in the great mass of 

 the sou, the motion of the water must be due to the action of 

 surface pressure exerted in the manner described. The rise of 



