164 Physics of the Soil. 



The capillary pores in an ideal soil do not have a uni- 

 form diameter but are shaped like the cast shown in Fig. 



FIG. 55. Showing a cast of the pore space between spherical grains, 

 much enlarged. 



55, largest at one place and decreasing in either direc- 



tion to the area given by the equation above. The mean 



area of the section of the pore, is given by Slichter,* as 



mean area of section of pore = 0.2118 r 2 



which would make the largest or effective cross section 

 of the capillary pore not far from 



(.2118 X 2) .1613 = .2623 r 2 



From this the effective diameter of the capillary tubes 

 may be found, using the formula 



where r is the radius of the soil grain 



and D is the diameter of the capillary pore. 



* Theoretical Investigation of the Motion of Ground Waters, 19th 

 annual report of the Geological Survey, part II, p. 316. 



