124 SOILS: PROPERTIES AND MANAGEMENT 



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from which the effective surface exposed can be deter- 

 mined. Thus, designating a soil as having an effective 

 mean diameter of particle of .0052 millimeter merely 

 indicates that this particular soil shows an air and 

 water movement the same as would be shown by a 

 homogeneous soil with spherical particles of that diameter. 



The apparatus x for the de- 

 termination consists of a cylin- 

 der in which is placed a sample 

 of air-dry soil, the pore space 

 being carefully determined by 

 weighing. The rate of air 

 movement is then determined 

 by connecting with an aspi- 

 rator, the temperature and 

 the pressure being constantly 

 under control. The reading is 

 usually calculated to a tem- 

 perature of 20° C. The fact 

 that the structural condition 

 of the soil is likely to be dis- 

 turbed in placing the sample 

 in the aspirator tube detracts 

 from the accuracy, especially 

 in fine soils. Nevertheless, 



Fig. 20. — King's aspirator for £ing f oun( J fa res ults fairlv 

 the determination of the rate j 1 1 1 . 



of air movement through accurate, and showed that 

 soils. (G), Pressure gauge; tne calculated and the ob- 



(<S), soil column ; (L), water; in* 1 1 



U), aspirator; (HO, weight. . served flow ot water through 



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

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

 II, pp. 222-224. 1899. A complete discussion is given of 

 King's ideas in this article, pp. 67-294. 



