and armies engulfed in the moving sands may be set down as imaginary, however, save, perhaps, in some instances in the Libyan Desert, where such tragic episodes are known to have occurred. The investigations of French geologists have completely disproved the theory that the Sahara represents the dried-up bed of what was once a vast inland sea and that its dunes are composed of sand left there at that period. It appears, on the contrary, that the sand, instead of having been deposited by the sea, comes from the stony plateaus of the _hammada_. The soft Cretaceous sandstone, heated by the sun until it is as hot as the top of a stove, is suddenly chilled at night by rapid radiation. The rock, fractured by the abrupt change from heat to cold, disintegrates; and the sand thus produced, sifted and drifted by the wind, takes the form of dunes. The slightest breeze is enough to make the desert smoke with dust, while a strong wind produces a weird singing of the sands, a peculiarly eery sound which indicates that the dunes themselves are shifting. The loose particles of sand which cover wide tracts of the Sahara are a cause of acute discomfort not only to the countries lying along the whole north coast of Africa but even to those on the opposite side of the Mediterranean, for the sand-laden wind from the Sahara, known in southern Europe as the sirocco and in Egypt as the _khamsin_ (because during the spring it blows at intervals for about fifty days) frequently affects regions a thousand miles away, covering them with fine red dust. Though to all intents and purposes waterless so far as rivers are concerned, the Sahara is thickly sprinkled with fertile spots, varying in area from a few acres to hundreds of square miles, where the water comes to the surface in the form of springs or is found in shallow wells. Thanks to the enterprise of the French government and the ability of French engineers, these oases have been greatly expanded during recent years, for experiments have proved that an unlimited supply of aërated water is obtainable from the great reservoirs which underlie the Sahara by the sinking of artesian wells, water generally being found at a maximum depth of two hundred feet. When one is in the midst of the desert, surrounded by blinding white sand-dunes which support no living thing, the very idea of water seems absurd and its existence impossibly remote, yet it is often only a few yards beneath one’s feet. In this fact of a secret water-supply lies the hope of the regeneration of parts at least of the Sahara, for, once it has been introduced to water, this seemingly hopeless region will blossom like the rose.