Sahara, the largest desert region in the world, stretches E. and W. across Northern Africa, from the Atlantic to the valley of the Nile, a distance of 3000 m., and on the N. is limited by the slopes of the Atlas Mountains, and on the S. by the valleys of the Senegal and Niger Rivers. The surface is diversified by long sweeps of undulating sand-dunes, elevated plateaux, hill and mountain ranges (8000 ft. highest) furrowed by dried-up water-courses, and dotted with fertile oases which yield date-palms, oranges, lemons, figs, &c. The most sterile tract is in the W., stretching in a semicircle between Cape Blanco and Fezzan. Rain falls over the greater part at intervals of from two to five years. Temperature will vary from over 100°F. to below freezing-point in 24 hours. There are a number of definite caravan routes connecting Timbuctoo and the Central Soudan with the Niger and coast-lands. Dates and salt are the chief products; the giraffe, wild ass, lion, ostrich, python, &c., are found; it is chiefly inhabited by nomadic and often warlike Moors, Arabs, Berbers, and various negro races. The greater part is within the sphere of French influence. "When the winds waken, and lift and winnow the immensity of sand, the air itself is a dim sand-air, and dim looming through it, the wonderfullest uncertain colonnades of sand-pillars whirl from this side and from that, like so many spinning dervishes, of a hundred feet of stature, and dance their huge Desert waltz there."