THE FOEMS OF SOIL WATFE 241 



At Cornell University ^ a system of cement tanks sunk 

 in the ground has been constructed. Each tank is about 

 four and a half feet square and four feet deep. A sloping 

 bottom is provided, with a drainage channel opening into 

 a tunnel beneath and at one side. As the tanks are ar- 

 ranged in two parallel rows, one tunnel suffices for both. 

 (See Fig. 39.) The sides of the tanks are treated with 

 asphaltum in order to prevent solution. The soil must 

 of course be placed in the tanks, this causing a disturb- 

 ance of its structural condition. As a consequence data 

 as to rate of flow and composition of the drainage water 

 are rather unreliable for the first few years. Such an 

 experiment must necessarily be one of long duration. 



166. Thermal movement of water. — Little has been 

 said as yet regarding this third mode of water movement, 

 the vapor flow, which is not peculiar to one form of soil 

 water but affects all alike. It is at once apparent that 

 the movement of water vapor can be of little importance 

 within the soil itself, since it depends so largely on the 

 diffusion and convection of the soU air. While the soU 

 air is no doubt practically always saturated with water 

 vapor, the loss of moisture by this means is slight. Buck- 

 ingham ^ has shown that, while sand allows such a move- 

 ment to the greatest degree, the loss occurring in a soil 

 with any appreciable depth of layer is almost negligible. 



The question of the thermal movement of water at the 

 soil surface, however, is vital in farming operations. At 

 this point the water films are exposed to sun and wind, 

 and drying goes on rapidly, the free, capillary, and a 



^ Lyon, T. L. Tanks for Soil Investigation at Cornell Uni- 

 versity. Science, N. Ser., Vol. 29, No. 746, pp. 621-623. 1909. 



^BucMngham, E. Studies on tlie Movement of Soil 

 Moisture. U. S. D. A., Bur. Soils., Bui. 38, pp. 9-18. 1907. 



