Sahara (usually Sa-hay'ra, properly Sah'a-ra; Arab. Sah'ra), the vast desert region of North Africa, stretching from the Atlantic to the Nile, and from the southern confines of Morocco, Algeria, Tunis, and Tripoli southwards to near the Niger and Lake Chad. The Libyan Desert, lying between Egypt, the central Soudan, and Tripoli, is a separate tract. The surface, instead of being uniform and depressed below sea-level, is highly diversified, and attains in one place an altitude of fully 8000 feet. From Cape Blanco in the west, the Erg, a vast semicircle of sand-dunes (50 to 300 miles wide and 70 to 300 feet high), stretches right round the northern side of the Sahara to Fezzan. In the centre the country rises into the lofty plateau of Ahaggar (4000 feet), with veritable mountains 6500 feet high, and actually covered with snow for three months in the year. There are mountain-ranges in the east reaching 8000 feet. The mountainous parts embrace many deep valleys, most of them seamed with the dry beds of ancient rivers, which yield abundance of water, if not on the surface, then a short distance below it, and are inhabited, and grazed by cattle, sheep, and camels. Another characteristic type of Saharan landscape is a low plateau strewn with rough blocks of granite and other rocks, and perfectly barren. These elevated stone-fields, called 'hammada,' alternate with tracts of bare flat sand, with broad marshes, where water has stood and evaporated, leaving salt behind it, and with extensive tracts of small, polished, smoothly-rounded stones. In very many parts of the Sahara, especially in the valleys of the mountainous parts, in the recesses or bays at the foot of the hills, alongside the watercourses, and in the hollows of the sand-dunes, there are oases - habitable, cultivable, watered spots. Lines of oases mark the great caravan-routes between the Soudan states and the Mediterranean.

A large portion of the Sahara, though not the whole, was undoubtedly under water at one time; and a process of desiccation has been going on throughout the whole region from the earliest historic time; the Romans had colonies or military posts a long way to the south. The sand is simply the Saharan rocks (granite, gneiss, mica-schists, and cretaceous rocks) crumbled to dust by the alternations of heat and cold. The range of temperature is exceedingly great: often the thermometer falls from more than 100° F. during the day to just below freezing-point at night. Rain does fall in certain districts at intervals of two to five years. After a fall of rain it is not unusual to see the river-beds in the mountainous regions filled with foaming torrents. Owing to the extreme dryness of the air, the Sahara is very healthy. The plant-life is very rich in the oases, the date-palm, oranges, lemons, peaches, figs, pomegranates, etc, being grown, with cereals, rice, durrha, and millet. In the desert regions are found tamarisks, prickly acacias and similar thorny shrubs and trees, salsolaceAe, and coarse grasses. The animals include, besides the camel, horse, ox, sheep, and goat, the giraffe, antelopes, wild cattle, the wild ass, desert fox, jackal, hare, lion, ostrich, desert lark, crow, viper, python, locusts, flies. The inhabitants, estimated at between 1,400,000 and 2,500,000, consist of Moors, Tuareg, Tibbu, Negroes, Arabs, and Jews. The chief products are dates and salt, also horses, soda, and saltpetre. A very active trade is carried on by caravans, between the central Soudan and Niger countries and the Mediterranean states, the ivory, ostrich-feathers, gums, spices, musk, hides, gold-dust, indigo, cotton, palm-oil, shea-butter, kola-nuts, ground-nuts, silver, dates, salt, and alum of the interior lands being exchanged for the manufactured wares (textiles, weapons, gunpowder, etc.) of European countries. The French desire to get this trade into their own hands, and have proposed to construct a light trans-Saharan railway from the coast to the shores of Lake Chad and the Niger. They have done much to realise the ambitious idea of uniting their possessions on the Senegal and on the Niger with Algeria and Tunis - a union theoretically accomplished by the agreement of 1890 between Great Britain and France, by which the whole of the Sahara, except the west coast (which is claimed by Morocco and Spain and Great Britain) and the extreme east (beyond a line drawn from Murzuk in Fezzan to Lake Chad), was acknowledged to be within the French 'sphere of influence.' There have been schemes for flooding the low-lying 'shotts' south of Tunis, and much has been done towards improving certain areas by boring artesian wells and so irrigating the country around. By a series of conventions between Britain and France (1893-99), one of which recognised the right of France to all territory west of the Nile basin, practically the whole of the Sahara is now accounted French; and the area of the French Sahara is about 2,000,000 sq. m. See German works by Barth (1858), Nachtigal (1879-89), Rohlfs (1874), Zittel (1884), and Lenz (1884); French works by Rolland, Cat, Bissuel, Vuillot, Toutee, Schirmer, Bonnefon, and Foureau (1891-1902); and Somerville's Sands of Sahara (Phila. 1901).