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 indirectly 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 observed, 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 depends mainly on the abundance and condition of the material, and exposure 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-Africa. 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."
