360 THE POPULAR SCIENCE MONTHLY. 



turned around it, and every dune, however small, becomes a means of 

 so distributing the air-waves that their force and eroding power are 

 increased. From these and similar causes, the contour of the mobile 

 sand-hills is scarcely more permanent than that of the waves in whose 

 spray they lie. But it is the dry sand only that is put in motion by 

 the winds ; only a few inches below the surface it is uniformly moist, 

 and on that account somewhat adhesive. This moisture above where 

 the sand is saturated is capillary water, that is, water held by the 

 attraction of the sand-grains, and is about thirty per cent, of the mass 

 by weight. It rises through the sand to the surface as evaporation 

 goes on, and thus in this climate of rainfall the dunes are rendered 

 more permanent than on rainless deserts. 



The formation of a sand-dune seems a simple process, and it is 

 surprising how small an object may be the nucleus of one, and indi- 

 rectly of a series of them. A bush, or tuft of grass, or only a twig, 

 as we have seen, raised above a level surface, breaks the force of the 

 wind, and immediately the sand-grains, which are rolling along the 

 surface, are arrested, and form a minute hillock on the windward side 

 of the obstacle. This increases in size the sand-grains, as before ob- 

 served, are driven up its slope, and fall on the sheltered side. The 

 mound thus formed produces currents and eddies in the moving air, 

 and the form and position of other hillocks are determined by the 

 new conditions. By the means indicated, dunes are formed on our 

 narrow beaches thirty feet high ; but there are dunes on our coast 

 much higher than that, as will presently be noticed. Their size de- 

 pends mainly on the abundance and condition of the material, and ex- 

 posure to winds. On the coast of France they attain a height of 225 

 feet, and on the Atlantic border of the Sahara Desert are more than 

 twice that elevation. But the desert sands are exceedingly fine and 

 dust-like from attrition, and move in greater volume than is possible 

 for the coarser sands of our coasts. They are whirled and tossed in 

 the gale like dense smoke, but nowhere do they roll on as do waves 

 of the ocean, as is sometimes stated. The transition of a sand-dune 

 is by transfer and deposition of the individual particles of which it is 

 composed. 



A wonderfully vivid description of a sand-storm is given by Mr. 

 Southworth, in his " Four Thousand Miles of African Travel : " "I 

 was sitting at my table in the midst of the glorious sunshine of- Af- 

 rica. Slowly the southern horizon began to grow obscure. A huge 

 mountain of sand, growing grander and grander, advanced rapidly. 

 . . . The doom-palms and date-trees, frosted with clouds of white 

 birds, the spires and minarets slowly losing their outlines in the dense 

 obscurity. ... It came nearer and nearer. Its front was absolutely 

 perpendicular. To breathe was difficult and oppressive, and it was 

 darker than the darkest night I ever knew. Sand covered the ground 

 to the thickness of an inch." 



