126 



Physics of the Soil. 



It will be se.en from this table that when the grains are 

 so large that 10 of them will span a linear inch only 37 

 seconds are required for a pressure of .1 foot of water to 

 force 5,000 c. c., 5.3 quarts, of air through a column a foot 

 long and .01 of a square foot in cross section; but in the 

 finest clay soil, which makes the best grass land, where 

 5,125 grains must be set in line to measure a linear inch, 

 then the time required is 2,933,000 seconds for the same 

 amount of air under the same conditions to be forced 

 through, a ratio of 37 seconds to nearly 34 days. 



Table showing the differences in the rate of movement of air 

 through gravel, sand and soils of different types when the 

 columns are 1 foot long, .01 ft. in cross section and under a 

 pressure of .1 ft. of water. 



It should be understood that this slow rate of movement 

 of air through the finest clay soils was observed when the 

 air-dry soil had been pulverized in a mortar and made as 

 fine as practicable before packing into the aspirator. Un- 



