Unlike the ocean waves, in which only the form moves forward, while the water composing it moves up and down only, the sand dune and the material of which it is composed are both moving in the direction of the wind. A breeze even of five or six miles an hour will keep the lighter surface dust moving freely, while a twelve-mile wind will not only sweep along much larger particles but it also carries more of them. And just as the surface, or "skin," friction forms waves at the surface of water, it also piles the desert sand in wave-like dunes. The loose bits of rock waste are carried along, up the windward slope of the dune until they roll over its crest, where, no longer impelled by the wind, they come to rest. Thus, the crest, built forward by new material constantly added, is advancing. Valleys are filled; old stream channels are obliterated; and the inequalities of the surface are levelled off until the whole landscape is one of shifting, drifting sand. Notwithstanding all these drawbacks, the Sahara and the arid lands southward to the Sudan are by no means destitute of life and wealth. It is an almost universal custom to speak of the barren condition of the desert. The contrary is the truth; there is no soil elsewhere so fertile and productive. It is vastly superior even to the soil of the lands reclaimed from the bottom of the North Sea. Water is the magic wand that makes the sands of the Sahara bring forth crops that are marvellous both in quantity and quality. No fruit grown elsewhere in the world can compare with that grown on desert lands, and the French engineers are planning the means whereby water may be obtained. Surface water that is available to irrigate the wastes of the Sahara does not exist. The level of the Nile is so far below the surface on both sides of its own flood-plain that its waters cannot be used for the reclamation of any part of the Libyan Desert, and the same is practically true of the Niger, which barely more than touches the borders of the Sahara. The few wadys, or "dry washes," are destitute of water except when a cloud-burst may fill them; but this happens at intervals of years only. The engineer takes into his confidence a caravan driver--perhaps an Arab, possibly a Berber, but quite as likely a slave. And the long experience has taught the caravan man where to find the precious water. The engineer then brings his science into play and drives an artesian well. The well thus driven may be a "gusher," but for most of them pumps are required to raise the water to the surface. The best well, however, furnishes water enough to irrigate but a very small area. Indeed, all the lands of the Sahara together irrigated by artesian wells would make an area scarcely larger than the State of Delaware, and all the water thus obtained would not supply New York City!