54 EGYPTIAN AGRICULTURE. 



In sandy soils which offer little resistance to the passage 

 of water; drainage may be very easily and successfully 

 accomplished, whereas in clay, which offers considerable 

 resistance, the water table midway between the drains may 

 be too high unless the drains are sufficiently near to each 

 other. In Egypt the object of drainage is more to keep 

 the soil from saltness than to lower the water table to the 

 level best suited for plant growth. To obtain the same 

 reduction of water table, deep drains may be made at a 

 greater distance apart than shallow ones. 



The horizontal motion of water in the soil takes place 

 under the action of gravity and external pressure. This 

 motion is closely allied to drainage as the weight of the 

 water is the moving factor. Infiltration may be regarded 

 as a special case of drainage in soils, where the sinking 

 water is unable to find a downward discharge and therefore 

 takes a more or less horizontal direction and filters into the 

 neighbouring soils. Infiltration includes upward motion 

 when this is caused by pressure of water from behind, and 

 it includes motions which are partly vertical up or down 

 and partly horizontal. Infiltration is usually caused by 

 water from some high level canal penetrating the sur- 

 rounding soil. At first, the water draining from the 

 canal drains downwards, but this gradually raises the water 

 table, and with the head of water behind, the water is 

 forced along in a horizontal direction. Naturally the 

 result of infiltration is worst when the water table in the 

 soil is already high, but even where the water table is low, 

 the decreasing permeability of the soil as greater depths 

 are reached offers even more resistance to the drainage of 



