70 



on the properties of the fluid. It is by no means certain 

 that this formula really applies to the movement of 

 air in soils hut whatever the actual expression, it 

 will probable contain a high power of the diameter of 

 the passage. It will be noted that the Poiseuille formu- 

 la contains the fourth. This means that the rate of 

 flow will decrease very rapidly as the passages become 



smaller and this conclusion is in general agreement with 



p 

 the results of experiments on the subject. It follows 



that ventilation by convectional circulation or as the 

 result of water movements will toe very much more diffi- 

 cult in clay soils than in sands. 



1. See, for instance, Leather,- Jour. Agr. Sci» 

 4: 303-304 (1912). 



2. On the flow of air through soil under exter- 

 nal head see: Ammon,- Porsch. Geb. Agr. Physik. 3: 209 

 (1883); von Welitschkowsky,- Arch. Ryg. 2: 483 ( ); 

 Hannan,- Forsch. Geb. Agr. Physik. 15: 6-25 (1891); 

 Wollny,- Forsch. Geb. Agr. Phys. 15: 193-222 (1893); 

 Heinrich,- Ber. landw. Vers. Stat. Rostock 2: 40-72 (1894); 

 Deherain and Demoussy,- G. R. 122 ; 109-112 (1896); King,- 

 U. S. Geol. Survey, Ann. Rep. 19, II; 59-294 (1897-1898), 

 Wise. Agr. Sxp. Sta., Report 1898 : 123-133, Science 22_: 

 495-499 (1905); Renk,- Zeits. Biol. 15: 205 ( ); 

 Green and Ampt,- Jour. Agr. Sci. 4: 1-24 (1911), 5_: 



1-26 (1912). 



