Movements of Fluids Through Soils. 127 



der field conditions, as has been pointed out, a good clay 

 soil has its clusters of various sizes and there are passage- 

 ways of various sizes and forms which allow both air and 

 water to move much more freely than has been recorded 

 in the table and if it were not so plants could not thrive 

 in them. 



148. Permeability to Air of Undisturbed Field Soils. The 

 rate at which air may flow 



through soils in their natural 

 condition, in place in the field, 

 may be readily studied with an 

 apparatus such as is shown in 

 Fig. 40. When the soil tube 

 A is driven into the ground to 

 near the depth at which the 

 flow of air is to be measured it 

 is recovered, the core of soil re- 

 moved and the tube returned to 

 its place, when the aspirator is 

 connected as shown in the cut, 

 and the time required for a 



given volume of air to be drawn Flo . 4 o-showi P * apparatus for 

 through determined. In these 

 field studies it will be found 

 that the dryer the soils are the 

 more freely air passes through them but that when they 

 are saturated with water, as just after h^avy rains, little 

 or no air will pass through them even under a pressure of 

 12 inches of water. 



149. Weight of a Cubic Foot of Dry Soil. A cixbic foot of 

 undisturbed air-dry soil varies in weight between quite 

 wide limits, the humus soils being the lightest, and the 

 coarse sandy soils the heaviest. The writer bus found a dry 

 soil to have the weight per cubic foot given in the table be- 

 low: 



