38 ANIMAL LIFE IN DESERTS 



water, of allowing it to sink through them, and of 

 raising it from lower levels by capillarity. In 

 general we may say that evaporation of water 

 from the sm^face of the desert soil is due to the heat 

 of the sun, the movement and dryness of the air, 

 and the nakedness of the earth. Evaporation 

 (page 30) exceeds rainfall in deserts, except in 

 very short periods in the year : therefore the sur- 

 face of the soil is normally quite dry. But at 

 short distances beneath the surface one frequently 

 finds a proportion of moisture which is surprising, 

 and the proportion rapidly increases at sHghtly 

 greater depths : it is impossible in a general dis- 

 cussion to quote figures, and the depth varies with 

 such factors as type of soil, interval since last rain- 

 fall, and the depth of the permanent moisture in 

 the soil. The explanation of the dryness of the 

 surface and dampness of the deeper soU is that 

 after the soil has been saturated by a fall of rain 

 the surface rapidly dries : this drying process 

 penetrates to greater and greater depths, becoming 

 slower and slower, because the action of wind 

 is cut off, and the diffusion of the damped air is 

 retarded by the superficial layer of soil which is 

 already air-dry. There is continual upward mov^ 

 ment of moisture from the depths of the soil in 

 which water was accumulated when the rain fell, 

 and to which in certain cases water is brought 

 by seepage from springs, rivers, or water-bearing 

 strata. This upward movement depends on capil- 

 lary attraction, which enables the water to rise 

 above the level at which it would naturally lie, by 



