114 THE PRINCIPLES OF 



is simply the artificial regulation of this natural deposition 

 of sediment. Tidal action is a necessary adjunct, as the rise 

 and fall of the tide, especially in spring tides, enables the 

 warper to alternately flood and free the area upon which he 

 is operating. The water from the river is allowed to enter 

 artificial canals at high tide by means of sluices, and is 

 conducted to the " compartment " which is to be covered 

 with warp. The water is then allowed to spread itself over 

 the area, the expansion being regulated in order to secure 

 a uniform deposit, by means of what are known as " call- 

 banks." At the fall of the tide the sluices are opened and 

 the water is allowed to flow back into the river channel. 

 Successive tides are employed for repeating the process, 

 until a regular deposit of soil has been formed to a depth of 

 from one to three feet. The soil thus made is of excep- 

 tionally rich character. As a case of natural warping the 

 valley of the Nile may be mentioned. The remarkable fer- 

 tility of Egypt has been produced and kept up for thousands 

 of years by the annual inundation of the Nile, and the 

 consequent deposition of mud brought from Central Africa 

 and the districts near and more remote which it washes in its 

 course towards the Delta. 



Another means of improving the physical character of soils 

 is good general cultivation. Thorough good ploughing and 

 seasonable cultivation of all sorts no doubt improves land ; 

 it accelerates the disintegration of mineral matter which is 

 constantly going on in soils. The soil is readily exhausted 

 of its available plant-food by over-cropping, but the natural 

 forces will continue to act, and the land will recoup itself 

 out of the vast store of insoluble materials which it contains. 

 This natural process is accelerated by cultivation ; the ground 

 is stirred, fresh surfaces are exposed, it is divided and re- 

 divided, and there is a more complete exposure to the forces 

 which convert the insoluble or dormant materials into an 



