422 NORTH-WEST AFRICA. of neutral zone, in which the normal succession of the aerial currents is suspended. One of the most dreaded winds is the sirocco, which blows from the south, often accompanied by whirlwinds of sand. These dense volumes of reddish dust sweep over the desert like sheets of flarae and with the velocity of tornadoes, now rising into the upper regions, now grazing the surface of the ground. The Caravan Routes. The moving sands, the long stony hamadas, the torrid heats followed by sudden returns of cold, the pestiferous winds, the dust-storms, the " cloudless skies and shadeless earth," the vast distances, the long intervals between the wells — at times dried up, at times held by hostile tribes — all tend to enhance the perils of the route, and to explain the solemn earnestness with which caravans equip themselves for the journey across the wilderness. The profession of guide, hereditary in certain . families, constitutes a sort of priesthood, for this person holds in his hands not only his own life, but that of all committing themselves to his guidance. At the departure of the caravan he meets with homage, mingled with entreaties ; on its safe arrival he is overwhelmed with thanks. In the most monotonous regions he is familiar with the slightest landmarks indicating the route to follow. The smallest plant, traces on the sands invisible to others, the atmospheric currents, all help to direct him along the right track or warn him of impending danger. He consults the fleecy cloudlet, and in the neighbourhood of the oases follows with the eye the long flight of the swallow and other birds of passage. Even unaided by sun and stars, he knows the right direction, and unerringly points to the precise quarter of the horizon leading to the wayside stations, and to the habitations of man on the verge of the desert. When he belongs to a race with complexion grey as the sands, or red as the soil of the hamada, he lays his garments aside and rolls naked on the ground, the better in this disguise to reconnoitre passing strangers and ascertain whether they be friend or foe. He knows the exact position of every oasis, of every pool or puddle, of every spring or well ; the path along the trackless desert is traced in his memory from station to station, and when compelled to fallow a new route experience must aid him in determining the right course. It happens at times that well-known highways become lost, either by the invading sands filling up the wells, or by hostile hordes seizing the stations along the route. Then the memory of the abandoned tract gradually fades into vague tradition, and in the popular imagination it becomes an earthly Eden. Thus was pictured the " Little Wau," before this long-lost oasis was rediscovered by the Arab explorer, Mahommed Tarhoni of Zella. The natives of Tibesti have lost the route followed by their forefathers across the Libyan desert to Egypt, and still speak of some delightful intervening oasis, where the sparkling waters murmur beneath the shade of the palm groves. On the other hand, the caravans of Wadai struck out fresh trade routes towards Tripoli tana in the years 1811 and 1813, although these highways have remained noglected for many decades. But even were the journey across the Sahara every-
