Sahara (Arab. Sāh'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 the vicinity of the Niger and Lake Tsad. It is usual to regard the Libyan Desert, lying between Egypt, the central Soudan, and Tripoli, as a separate division. Both are, however, links in the chain of great deserts that girdle the Old World from the Atlantic coast across Africa, Arabia, Persia, Turkestan, and Mongolia to the Pacific. It was long customary to assert that the Sahara was the bed of an ancient inland sea, and that it consisted of a vast, uniform expanse of sand, swept up here and there into ridges by the wind. But this idea is utterly erroneous. Since the French became masters of Algeria, and more especially in recent years, they have completely revolutionised our knowledge of the Sahara, at all events of the country immediately to the south of Algeria and Tunis. 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. But, in spite of our knowledge of this part of the world having been so greatly increased of late, there are several extensive tracts as to which we have next to no information. The present article will therefore only touch upon the more outstanding features.
From the neighbourhood of Cape Blanco in the west a vast bow or semicircle of sand-dunes stretches right round the northern side of the Sahara to Fezzan, skirting the Atlas Mountains and the mountains of Algeria. This long belt of sand-hills varies in width from 50 to 300 miles, and is known by the names Igidi and Erg, both meaning 'sand-hills.' The hills rise to 300 feet (in one place, it is said, to more than 1000 feet), though the average elevation is about 70 feet. They are composed of pure quartz sand, reddish brown in colour; are stationary in character, though constantly changing their outward form and configuration; and lie as a rule in parallel chains, whose outward slopes are fairly gentle, but their inward slopes steep. Water is nearly always to be found below the surface in the hollows between the different chains of these sand-hills, and there a few dry plants struggle to maintain a miserable existence. South of Algeria, on the other side of the Erg, the country rises into the lofty plateau of Ahaggar (4000 feet), which fills all the middle parts of the Sahara. Its surface runs up into veritable mountains 6500 feet high, which, incredible as it may seem, are covered with snow for three months in the year. On the south it apparently falls again towards the basins of the Niger and Lake Tsad; nevertheless there are mountain-ranges along the eastern side reaching 8000 feet in Mount Tusidde in the Tibbu country, and a mountain-knot in the oasis of Air (or Asben) which reaches up to 6500 feet. Mountainous tracts occur also in the west, between Morocco and Timbuctoo, but of inferior elevation (2000 feet). These mountainous parts embrace many deep valleys, most of them seemed with the dry beds of ancient rivers, as the Igharghar and the Mya, both going some hundreds of miles northwards towards the 'shotts' (see below) of Algeria and Tunis. These valleys always yield an abundance of water, if not on the surface in the watercourses, then a short distance below it, and are mostly inhabited, and grazed by the cattle and sheep and camels of the natives. 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,'—the best known is the Hammada el-Homra, south-east of Ghadames and on the border of Tripoli—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, in all which localities water is wont to exist, there are oases—habitable, cultivable spots, islands of verdure in the midst of the ocean of desert. These oases occur in greatest number along the southern face of the Atlas and the Algerian mountains, on the northern side of the Ahaggar plateau, and along certain definite lines, the chief of which extend between Murzuk in Tripoli and Lake Tsad, the Igharghar and Sokoto by way of Air, the Igharghar and the bend of the Niger by way of Timissao, Morocco and Cairo by way of Taflet, Tuat (Aïn-salah), and Ghadames, and Morocco and Timbuctoo by way of Tenduf and Taudeni. These lines of oases mark the great caravan-routes between the central Soudan states and the Mediterranean.
A large portion of the Sahara, though not the whole, was undoubtedly under water at one time, probably in the Cretaceous period and earlier. Then the surface seems to have been in great part elevated, so that the water remained only in some lakes and in gulfs near the Mediterranean coast. The physical features that at present characterise the Sahara are undoubtedly due in their broad essentials to atmospheric, chemical, and even mechanical causes, and only in a very small degree to the action of water. Water has exercised scarcely any influence on a large scale here since the Tertiary period; and there can be no doubt that a process of desiccation, similar to that which is now going on in the Turkestan deserts, has been in operation throughout the whole of this region from the earliest historic time. The Romans had colonies or military posts a long way southwards, in what are now desert regions; and both Herodotus and Pliny tell us that the elephant, the rhinoceros, and the crocodile, all animals that only live near abundant supplies of water, were common throughout North Africa in their day. None of the Egyptian inscriptions or animal-sculptures represent the camel, nor do the Greek and Roman historians mention it either as being a denizen of North Africa. The camel is now the principal carrier across the Sahara, and must have been introduced since the beginning of the Christian era. The inference from these and other facts is that the process of desiccation has gone on more rapidly during the last 2000 years. The position of the sand-dunes is determined by the unchangeable configuration of the surface; the wind and chemical action do all the rest. The sand itself is simply the Saharan rocks (granite, gneiss, mica-schists, and cretaceous rocks) ground to dust. The great heat by day causes the rocks to expand; the great fall of the temperature at night, combined with the enormous evaporation that then takes place, makes them split and crack, and break into pieces; and the strong, often violent, winds use these fragments like files, or even sand-blasts, with which to grind to pieces other rocky fragments. The terrors of the desert sand-storm have been often described (see DESERT). Thick deposits of Saharan quartz sand-dust were discovered by the Challenger on the floor of the Atlantic a long way west of the African coast. The sand in the dunes is so dry that in several places the tread of a camel or a man will make the hill hum, or even thunder, as a vast quantity of it slips down to a lower level. The range of temperature is exceedingly great: often the thermometer falls from considerably more than 100° F. during the day to just below freezing-point at night. In the west of the Sahara the daily average is 85° in the shade in the month of May. Rain does fall in certain parts of the Sahara with more or less frequency; but in most districts on the average after 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. But the atmosphere is so dry and clear that objects can be seen and sounds heard at a vast distance. The Mirage (q.v.) is no uncommon feature. Owing to this extreme dryness of the air, the Sahara, especially where it is reached by the prevailing west and north-west winds, is very healthy.
The plant-life is very rich in the oases, the date-palm, which has its home in these regions, being the principal ornament as well as the most valuable possession of these fertile spots. But fruit trees, as oranges, lemons, peaches, figs, pomegranates, &c., are also grown, with cereals, rice, durra, millet, and such-like food crops. In the desert regions the plant-life is confined principally to tamarisks, prickly acacias and similar thorny shrubs and trees, salsolaceæ, and coarse grasses. The animals most commonly met with include the giraffe, two or three kinds of antelope, wild cattle, the wild ass, desert fox, jackal, hare, lion (only on the borders of the desert), ostrich, desert lark, crow, viper, python, locusts, flies. The people keep as domestic animals the camel, horse, ox, sheep, and goat.
The human inhabitants, who are estimated altogether at between 1,400,000 and 2,500,000, consist of Moors, Tuareg, Tibbu, Negroes, Arabs, and Jews. The Moors and Tuareg are both Berbers (q.v.); the former live between Morocco and Senegal, the latter in the middle, south of Algeria and Tunis. The Tuareg are great traders, and control the principal caravan-routes. The Tibbu, who number about 200,000, and are regarded as being ethnically intermediate between the Berbers and the Negroes, occupy the oases between Fezzan and Lake Tsad. The Arabs of pure stock are very few; they have become mixed with the Berbers and the Negroes. The most valuable products of the Sahara are dates and salt, the latter collected on the salt pans, and made from the rock-salt of Taudeni in the west, and of Kawar (Bilma) in the east; the remaining products are horses, soda, and a little saltpetre. But for many long years there has been a very active trade carried on by caravans, between the central Soudan and Niger countries and the Mediterranean states, the ivory, ostrich-feathers, guns, 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, &c.) of European countries. The French desire to get this trade into their own hands, and are proposing to construct a trans-Saharan railway, light and of narrow gage, from the coast to the shores of Lake Tsad and the Niger.
They also entertain the grandly ambitious idea of uniting their possessions on the Senegal and on the Niger with Algeria and Tunis. This union has, indeed, been theoretically accomplished already 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 Tsad), was acknowledged 'to be within the French sphere of influence.' The proposed trans-Saharan railway would make this union more practical, especially if the railway line were taken from Algeria to near Timbuctoo, a distance of 1750 miles, as one scheme proposes. Alternative routes are to connect the Algerian system with Kuka on Lake Tsad (2250 miles), to build a line from near Cape Nun on the Atlantic to Timbuctoo (1100 miles), and to connect the Senegambian coast by a line over Futa-Jallon with the upper Niger.
Within recent years scientific men have eagerly discussed the possibility of reclaiming the Sahara from the arid desolation to which such a vast proportion of its surface is now abandoned. That no amelioration can be effected in the great bulk of its area is pretty well agreed; and if the desiccation is principally due, as has been maintained, to continental changes of elevation, it is pretty certain that nothing can be done. But the destruction of forests on the northern mountain-slopes is believed to be a co-operating cause. If so—for the fact is doubtful—this could be remedied. Two other schemes have, however, been proposed, and one of them has been carried out with admirable success. Westward from the Gulf of Cabes stretches for 250 miles a chain of salt lakes (shotts) right along the south of Tunis and Algeria, to the meridian of Biskra. Into these Captain Roudaire proposed (1874) to let the waters of the Gulf of Cabes by cutting through a ridge, 13 miles wide and 150 feet high, and so making an inland sea of some 3100 sq. m. in area with an average depth of close upon 80 feet. The scheme is, in point of engineering, practicable; but it is questionable whether it would accomplish the desired effect of modifying the climate and soil of the surrounding regions any more than the Sea of Aral or the Caspian does. At all events the proposal has been allowed to drop. In 1877 Mr Donald Mackenzie propounded the idea of flooding the western Sahara, the district called El Jnf, by letting in the waters of the Atlantic; but the German traveller Lenz ascertained that El Jnf was not a vast depression, but only a small valley. The other measure is the boring of Artesian Wells (q.v.), and with the water so obtained irrigating the soil in the vicinity. This method of reclaiming the desert, which was apparently known to the ancients, has been prosecuted by the French with great energy since 1856. By 1890 they had made a string of these wells from the cultivated districts of Algeria as far as Tugurt, on the edge of the desert, south of Biskra. Water is generally found at depths varying from 10 to 300 feet, and in great abundance. Wherever these wells have been bored the date-palm groves and the orchards have increased greatly in extent, and the population has become much denser. In 1890 Cardinal Lavignerie, Archbishop of Carthage (Tunis), founded a missionary order called the Armed Brothers of the Sahara. 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 estimated at 2,000,000 square miles.
See reports of French explorers in the Proceedings of the Paris Geographical Society; Zittel, Die Sahara, ihre physische und geologische Beschaffenheit (Kassel, 1884); Nachtigal, Sahara und Sudan (3 vols. 1879-89); Barth, Travels in North and Central Africa (5 vols. 1857-58); Lenz, Timbuktu (1884); Rohlfis, Quer durch Afrika (1874), &c.; Duveyrier, Les Touaregs du Nord (1864); Tchihatchef, The Deserts of Africa and Asia (Brit. Assoc. Reports, 1882); Rolland, Géologie du Sahara (1891); French books of travel by Soleillet (1876), Choisy (1881), Largeau (1882), and Doulx (1888). For the railway schemes, see Comptes Rendus of Paris Geog. Soc. (1890); and Donald Mackenzie, Flooding of the Sahara (1877).