176 NATUEE AND PROPBETIES OF SOILS 



increases. The weight of the free water column may also 

 have some influence. Although King^ and Welitsehkowsky^ 

 have shown that definite relationships exist between the move- 

 ment of gravity water and both atmospheric pressure and 

 weight of water column, the practical field importance of these 

 factors are rather slight. 



A rise in temperature of the soil not only varies the relative 

 amounts of capillary and 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. On the other hand the swelling of hydrogels 

 which may be present tends to impede percolation to such an 

 extent that the movement of free water through a heavy soil 

 is often markedly checked by temperature rise. 



Of much more practical importance than either pressure 

 or temperature in the flow of gravity water is the texture and 

 the structure of the soil. In working with sands of varying 

 grades, Welitsehkowsky,^ Wollny,* and others have shown that 

 the flow of water varies with the size of particle, or texture. 

 King ^ has demonstrated that in general the rate of flow 

 through such is directly proportional to the square of the 

 diameter of the particles. By the use of the effective mean 



^ King, F, H., Principles and Conditions of the Movements of Ground 

 Water; U. S. Geol. Survey, 19th Ann. Dep'., Part II, pp. 67-206 j 1897- 

 1898. 



Kin^, F, H., The Soil, p. 180, New York, 1906. 



^Welitschkowsky, B. von., Experimentelle untersuchmigen uber die 

 JPermeabiUtat des Bodens fur Wasser; ArcMv. f. Hygiene, Band II, 

 Seite 499-512, 1884. 



Wollny, E., Untersuchungen u'ber die Fermeahihtat des Bodens fur 

 Wasser; Forsch. a. d. Gebiete d. Agr.-Physik, Band 14, Seite 1-28, 1891. 



® Welitschkowsky, D. von.^ Experimentelle untersuchungen u'ber d%e 

 jPermeabiUtat des Bodens fur Wasser; Arclaiv. f. Hygiene, Band II, 

 Seite 499-512, 1884. 



^Wollny, E., XJntersuchunger u'ber den Einfluss der Strulctur des 

 Bodens auf dessen Feuchtighetis — und Temperaturverhaltnisse ; Forsch. 

 a. d. Gebiete d. Agr.-Physik, Band 5, Seite 167, 1882. 



"King, F. H., Principles and Conditions of the Movements of Ground 

 Water; tJ. S. Geol. Survey, 19th Ann. Rep., Part II, pp. 222-224, 1897- 

 1898. 



