THE SOIL AND WATERCOURSES 39 



penetrating the minute spaces between the soil 

 particles. There is then an upward movement of 

 water, from the subsoil, due to the capillarity, and 

 a downward movement of the level from which 

 evaporation is taking place in the soil. This down- 

 ward movement is slower the farther from the sur- 

 face it is taking place : at a certain point the 

 upward and downward movements balance one 

 another, and at this level the moisture in the desert 

 soil remains constant, unless equilibrium is upset 

 by fresh rainfall or increased evaporative power of 

 the air, or some other disturbing factor. These 

 facts have a practical appUcation. Cultivation of 

 arid land is sometimes rendered possible by the 

 employment of a machine which reduces the sur- 

 face of the ground to fine powder (" dust-mulch "). 

 This layer of fine powder, penetrated by corre- 

 spondingly minute air-spaces, decreases the upward 

 diffusion of the air which has saturated itself with 

 moisture in the soil. The decreased upward diffu- 

 sion results in decreased evaporation, and equili- 

 brium is disturbed, to be re-established nearer the 

 surface, at a point where the upward movement 

 of the moisture once again balances the lessened 

 evaporation. But as the moisture in the soil now 

 lies at a higher level it is possible to plant and raise 

 crops which could not have come to maturity 

 without the aid of the " dust-mulch " (Livingstone). 

 Bearing it in mind, then, that terms such as 

 " sand desert," " gravel desert," are not capable 

 of exact definition, or of exact geographical de- 

 limitation, let us turn our attention to some of the 



