Texture of Soils. 115 



amounts of water may be stored in them to drain away 

 gradually in the underflow. 



136. Subdivision of Pore Space Determines the Bate of Per- 

 colation and Drainage. If reference is again made in Km 

 34 it will be clear at a glance that water must flow through 

 spaces filled with these different sizes of spheres at very 

 different rates. Where the spheres are largest there are 

 16 passage-ways for the movement of air or of water ; but in 

 the middle section where the spheres have one-half the 

 diameter, the number of passages is 4 times as great, while 

 in the last section with spheres of one-quarter the size the 

 number of passages is 16 times as great. 



The aggregate area of the cross-sections of the pores is 

 exactly the same in the three cases, and from this it follows 

 that the areas of the cross-sections of single pores are to 

 each other as 16 : 4 : 1. 



The coarse spheres divide the column of water into 16 

 streams, the medium ones divide it into 64 streams, while 

 the smallest spheres divide the column into 256 streams, 

 each having only one-sixteenth the sectional area of the 

 first. But to subdivide the column into 256 streams in- 

 stead of 16 means that the friction must be much greater 

 in the aggregate on the smaller streams, and hence that the 

 flow must be slower. 



137. Method of Determining the Pore Space of Soil. The 



simplest method of determining the pore space of soil is to 

 pack the dry material into a cylindrical vessel containing 

 100 c. c. until it is even full, and then weigh and compute 

 the per cent, of pore space from the volume, weight and 

 specific gravity, using the formula 



Vd-W =p 



where V is tho volume of the vessel in c. c., d is the specific 

 gravity and W is the weight of the soil in grams. 



To determine the pore space in undisturbed field soil 



