262 Ground Water, Wells, and Farm Drainage 



quired to raise the surface of the ground water 1, 2, 3 and 

 4 feet in the sands of Fig. 94, after thorough drainage has 

 taken place. 



328. Law of Flow of Water Through Sands and Soils It 



has been generally claimed that the velocity of flow of 

 water through sands and soils is directly proportional to 

 the effective pressure and inversely proportional to the 

 length of the column through which the flow is taking 

 place. This means that to double the pressure will double 

 the rate of flow but to double the length through which 

 the water must flow will decrease the rate one half. A 

 law analogous is formulated for the flow of fluids through 

 capillary tubes and under certain conditions of pressure 

 and dimensions the law has been nearly fulfilled, both with 

 sands and capillary tubes. 



In practical measurements 1 of flow it is found that the 

 flow through some sands and some capillary tubes increases 

 faster ithan the pressure while in others it does not increase 

 so rapidly. 



The law of flow here referred to has been designated 

 <r Darcy's Law" and has been expressed by the formula 



_ p 



where 



V is the velocity, 



P is the difference in pressure at the ends of the column, 

 h is the length of the column. 



k is a constant depending upon the size of the soil grains, the 

 amount of pore space and the viscosity of the fluid. 



329. To Compute Flow of Water Through a Column of 

 Sand, Soil or Bock. Under the conditions where Darcy's 

 law may be fulfilled the amount of discharge may be com- 

 puted 'by means of the formula derived by Slichter 2 and 

 given below: 



1 Nineteenth Annual Report, U. S. Geol. Survey, Part II., p. 202. 

 Nineteenth Annual Report, TJ. S. Geol. Survey, Part II., pp. 301-321 



