52 EGYPTIAN AGRICULTURE. 



respectively. The term percolation may include all those 

 movements. 



"Drainage," or the downward motion of water, is due 



to the action of gravity and therefore acts in a direction 



towards the centre of the earth. It is only natural that 



water should tend to fall, but this falling will only take 



place or continue when there are no other forces equal to or 



greater than gravity preventing the downward motion of 



the water. In clay soils the downward motion is difficult 



or impossible on account of the great resistance caused by 



the friction of the water passing down between the very 



fine particles of soil. Again, the surface attraction of 



the soil particles has power to overcome the force of 



gravity. This attraction however is of importance only 



when there is no excess of water and hence in ordinary 



loam and sandy soils after each irrigation there is a 



certain amount of drainage. The greater the head of 



water over the surface of the soil the greater is the 



pressure acting downwards and the greater is the amount 



of water lost by drainage. It follows that stiff clay 



soils should receive heavy waterings to overcome the 



resistance of the particles to the passage of water. When 



there is no drainage, the waste products of plant life 



in the soil, and salts, collect and tmd to render the land 



infertile. Hence, although by drainage soils lose a 



certain amount of plant food, this loss is far more than 



compensated for by the good effect produced by the 



washing away of undesirable bodies. The water table 



in the soil is raised by drainage water and is lowered by 



increased evaporation and by the water finding a lower 



