438 Irrigation and Drainage 



136. That is, the water moves along the line of least 

 resistance, no matter how circuitous or how long that 

 may be. 



Where the cavities through which the water must 

 flow are those due to the diameter of the soil grains, 



Fig. 136. Movements of water toward tile drains where heavy clay 

 soils are underlaid with sand. 



the influence of size of grain on the rate of flow 

 is such that the amount of water passing a given 

 section under otherwise like conditions is somewhat 

 nearly proportional to the squares of the diameters. 

 This being true, if the effective diameter of the 

 grains in the clay is .004 m.m., while that of the 

 grains in the stratum of underlying sands is .07 

 m.m., then their squares will be .0049 and .000016 

 respectively, in which the ratio is nearly as 300 to 

 1, so that the water would flow through the same 

 length and section of sand about 300 times as rapidly 

 as it would through the clay. 



It is also true that the lengths of the soil pores 

 through which water flows decrease the rate in a ratio 

 nearly proportional to the lengths, so that the sand 

 column in the case cited, or, what is the same thing, 

 the distance between drains, could be 300 times as 

 great as with the clay and yet leave the rate of flow 

 just as rapid. It is plain, therefore, that the move- 



