224 SOILS: PROPERTIES AND MANAGEMENT 



of movement. This indicates that the thickness of the 

 film covering the particles and connecting the interstices 

 containing the bulk of the capillary water is, within 

 certain limits, a dominant factor in rate of movement at 

 least. Let it be supposed that a withdrawal of water 

 occurs at A (see Fig. 37), the interstitial space between 

 two particles, the water surface being represented by the 

 dotted line aa . There is an immediate increase in the 



curvature of this surface, and 

 water tends to flow through 

 the capillary film spaces at c 

 and c', toward this area of 

 greater tension. If water con- 

 tinues to be withdrawn at A, 

 this adjustment continues 

 with considerable ease until 

 the film channel at c and c 

 becomes so thin as to cause 

 its surface (bb ') to approach the edge of the hygroscopic 

 film surrounding the particle. The viscosity of the 

 water gradually becomes a factor at this point, imped- 

 ing the capillary adjustment toward A. This point of 

 sluggish capillary movement has been designated by 

 Widtsoe l as the point of lento-capillarity. 



The amount of capillary water delivered at any one 

 point, therefore, will obviously be influenced by the 

 thickness of the film and may consequently be taken 

 as a measure of rate of rise. A short soil column 

 should deliver more water from a constant source 

 than a longer one, due to the thicker films at the sur- 



Fig. 37. — Diagram for the ex- 

 planation of the effect of 

 thickness of water fiJm about 

 soil particles upon ease of 

 capillary movement. 



1 Widtsoe, J. A., and McLaughlin, W. W. The Movement 

 of Water in Irrigated Soils. Utah Agr. Exp. Sta., Bui. 115, 

 pp. 223-231. 1912. 



