Far from being monotonous, the dunes present an infinite variety of form, their sweep of line and beauty of contour being in singular harmony with the wild, free life of the desert. I have remarked before that they can best be likened to a sea, for the wind which blows almost incessantly across these wastes has left the sand in waves and combers, arrested in the very act of breaking and stricken into immobility and deathly silence. But when the wind rises to a gale this paralyzed sea suddenly comes to life, for the dunes begin to move, or, as the Arabs term it, to “walk.” The word is highly descriptive, for the whispering of the sand as it tumbles along, and the rapid changes which take place in the shape of the drifts, their crests melting and smoking in the wind, give the impression that the whole landscape is in motion. When the dunes walk in good earnest, as in the case of a real sand-storm, nothing can arrest them. Whipped by a roaring wind, the sand rises in suffocating clouds; the sun is obscured; the hot blue sky changes to a murky red, then to an angry purple; the landscape is completely blotted out; it is impossible to see objects a hundred yards away; a sullen twilight descends upon the land, enshrouding it in gloom; the dunes heave and crumple; the surface of the earth flits past dizzily, as in a motion-picture taken from a moving train. It is one of the most curious, and at the same time one of the most terrifying, spectacles that Africa has to offer. The second night out from Tozeur we experienced a sand-storm. I have seen far more violent ones in the Sudan and in Northern Arabia, but it was exciting enough while it lasted. During the late afternoon menacing purple clouds had been piling up along the southern horizon. As darkness fell the gentle breeze which had been blowing throughout the day suddenly died down, to be succeeded by an oppressive hush, like that which in Mexico generally precedes an earthquake. On the desert a great silence fell, deathly and oppressive. Then, without the slightest warning, came the wind, a veritable hurricane, driving the sand before it as chaff is scattered by the propeller of an airplane. The flying particles cut our faces like driven sleet. Our skins felt as though they had been rubbed with emery-paper. The stars were obscured; the purple velvet sky deepened to the blackness of ink. The tent-pegs had been too firmly anchored to be torn out, but the canvas billowed like the sails of a racing-yacht; the guy-ropes hummed like bowstrings. The air was dense with driven sand; though we wrapped our heads in blankets, the dust filled our eyes, our ears, our nostrils, producing a sensation not far removed from suffocation. The palm-trees of the little oasis where we were encamped bent before the blast until their plumed tops swept the ground. From the darkness came eery shrieks and wails—the cries, so say the superstitious Arabs, of the malignant _afrits_ who let loose the sand-storms. Earth