THE FORMS OF SOIL WATER 235 



159. Effect of temperature on the flow of gravity water. 

 A rise in temperature of the soil not only varies the 



amount of capillary water and thus increases the possible 

 free water present, but at the same time it increases the 

 fluidity and thus facilitates percolation. The expansion 

 of the soil air also tends to increase such movement. This 

 can be noticed in the operation of a tile drain in early 

 spring as compared with summer conditions. Calculated 

 effects of temperature change have been verified by con- 

 trolled experimental results. 



160. Effect of texture and structure on the flow of 

 gravity water. Of much more practical importance 

 than temperature, in the flow of gravitational water, 

 are the size and the arrangement of the soil particles. 

 In working with sands of varying grades, Welitschkowsky, 1 

 Wollny, 2 and others have shown that the flow of water 

 varies with the size of particle, or texture. King 3 has 

 demonstrated that in general with such materials the 

 rate of flow is directly proportional to the square of the 

 diameter of the particle. By the use of the effective 

 mean diameter 4 of a sand sample, he was able to calculate 

 a theoretical flow which compared very closely to observed 

 percolations. In sandy soils this law holds in a very 

 general way, but in clays it fails entirely. For instance, 



1 Welitschkowsky, D. von. Experimentelle Untersuchungen 

 iiber die Permeabilitat des Bodens fur Wasser. Archiv f. 

 Hygiene, Band II, Seite 499-512. 1884. 



2 Wollny, E. Untersuchungen iiber den Einfluss der 

 Struktur des Bodens auf dessen Feuchtigkeits- und Tempera- 

 turverhaltnisse. Forsch. a. d. Gebiete d. Agri.-Physik, Band 

 5, Seite 167. 1882. 



3 King, F. H. Principles and Conditions of the Movements 

 of Ground Water. U. S. Geol. Survey, 19th Ann. Kept., Part 

 II, pp. 222-224. 1897-98. 



4 This text, paragraph 87. 



