234 SOILS: PROPERTIES AND MANAGEMENT 



column will accelerate the rate of flow. Under normal 

 conditions pressure may arise from two sources, baro- 

 metric pressure and the weight of the water column. 

 Changes in barometric pressure are communicated to 

 gravitational water through a movement of the soil air. 

 As the mercury column rises, more air is forced into the 

 soil and the pressure on the soil water increases. Such 

 a change has been observed by King 1 to produce as high as 

 a 15 per cent decrease in the flow of drains. King observed 

 also that the level of wells fluctuated from time to time for 

 the same cause. The expansion of the air of the soil due to 

 daily heatings was also observed to produce diurnal oscil- 

 lations in the level and the rate of flow of ground water. 

 Perhaps of greater import in the rate of percolation of 

 water is the pressure produced by the weight of the free- 

 water column. Working along this line, Welitschkowsky 2 

 has shown rather conclusively that with an ideal length of 

 column the flow varies directly with the pressure. His 

 ideal column with the sand with which he experimented 

 was 75 centimeters in length. With a longer column the 

 flow did not increase as fast as the pressure ; while with 

 a shorter column, doubling the pressure more than doubled 

 the flow. These results have been, verified by Wollny 3 

 and ably reviewed by King. 4 



1 King, F. H. The Soil, p. 180. New York. 1906. 



2 Welitschkowsky, D. von. Experimentelle Untersuchun- 

 gen iiber die Permeabilitat des Bodens fur Wasser. Archiv f. 

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



3 Wollny, E. Untersuehungen iiber die Permeabilitat des 

 Bodens fur Wasser. Forsch. a. d. Gebiete d. Agri.-Physik, 

 Band 14, Seite 1-28. 1891. 



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

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

 II, pp. 67-206. 1897-98. 



