OASES. Throughout the great belt of desert extending from the west coast of Africa to central Asia, various fertile tracts occur, clothed with vegetation and watered by springs, to which the name oases has been applied. Those which are best known are met with in the central and eastern portions of the Great Sahara and in the Libyan Desert. In that region they consist generally of deep depressions or valleys, locally termed "wadis," where the water comes to the surface in natural springs, or where it may be procured by sinking wells. Under the influence of these beneficent springs vegetation bursts forth and covers a more or less extensive area, which becomes a halting-place for travellers in the desert, and frequently supports a considerable population. Many of the oases are situated in the mountainous regions, where the ground is sufficiently elevated to precipitate the moisture in the atmosphere. The rain which falls, however, is rapidly alisorbed by the rocks or sandy soil, and much of it collects in depressions at no great depth from the surface. The Arabs have long i been in the habit of tapping these subterranean waters by sinking wells, a copious supply being usually obtained at depths varying down to 200 fathoms. Indeed, so rapidly does the water ascend when the aqueous strata are pierced in certain localities that the well-sinkers are sometimes suffocated ere they reach the surface. In the Algerian Sahara a large number of artesian wells have been sunk by the French, resulting in the formation of oases, which have to some extent affected the habits of the native tribes by inducing them to become cultivators of these fertile tracts. It is evident, therefore, that, notwithstanding the arid climate which prevails generally throughout the African deserts, a tolerably plentiful supply of water can be obtained by artificial means at various points. The springs, being essential to the very existence of the oases, are naturally guarded with care so as to prevent the sands encroaching on them. Should they cease to flow, the decay of the vegetation rapidly ensues and the oases disappear. Another characteristic feature of these fertile tracts is the palm-tree forests, which are admirably adapted to such unfavourable conditions. The date and the dam palm are the commonest species met with in the forests ; they are highly prized by reason of the produce which they yield to the cultivators of the soil and the shelter which they afford from the scorching sun. Indeed, so serviceable are they that in some cases the Arabs artificially create an oasis with the aid of a few palm trees by digging holes deep enough to allow the roots to pierce the aqueous strata.

From the accounts given by various travellers it would appear that, while the larger oases cover extensive tracts, the smaller ones are liable to be effaced by drifting sand. As an example of one of the largest, Air or Asben may be mentioned, which measures 180 miles from north to south. Such extensive fertile tracts are dotted over with villages, whose inhabitants carry on the cultivation of the soil and export various articles of produce. By means of irrigation different cereals are successfully cultivated, such as barley, rice, and millet.

In the western Sahara the chief oases are : - (1) Twit, about 1000 miles to the south-west of Tripoli, the principal town being lnsalah ('Ain Salah) ; (2) Taudeni, south-west of Tuat ; (3) 'Arawan, south of Taudeni and north of Timbuktu ; and (4) Wilata, southwest of 'Arawan. In the eastern portion of the Great Desert the important oases arc : - (1) Fezzan, the capital of which is Morzfik, lying to the south-south-east of Tripoli ; (2) Ghadames, north-west of Fezzin ; (3) Tibesti, south of Fezzan ; (4) Bilina,west-south-west of Tibesti ; and (5) Air or Asben, west from Bilma and north-west from Lake Tchad. The last is perhaps one of the most remarkable oases in the African desert, forming a tableland whose average elevation is 2000 feet, with peaks rising to a height of 5000 feet. Heavy tropical rains are precipitated by this lofty plateau,. and i hence the valleys are clothed with vegetation. This oasis is richly cultivated, producing barley, maize, and millet. The capital is Agades, a town with 7000 inhabitants, which is situated on the caravan route between Jlorzdk and Sokoto, and constitutes one of the important centres of trade in central Africa.

In the Libyan Desert the chief oases are : - (1) khargeh or Kharija (the outer oases), sometimes termed the oasis 9),tagia, about. 120 miles to the west of Thebes ; (2) Dakhel or Dakhda (the inner oases), situated to the west of Ehargeh ; (3) Farafreh (Farafira), north-west of Dakhel ; (4) Baliriya (northern), to the north of Farafreh ; (5) Siwa (the famous oasis of Jupiter Ammon), at the northern limits of the Libyan Desert ; and (6) the Knfra group, south-west of Siwa. Perhaps the most fertile of these tracts is Dakhel, which was first made known to Europeans in 1819 by Sir A. Edmonstone. The chief produce consists of dates, rice, olives, and apricots, but durra, barley, lemons, citrons, and figs also grow on the rich soil of this oasis. It contains eleven villages, the total population being estimated at 6000.

Similar fertile tracts, though of smaller extent, occur in Arabia and in that part of Persia lying to the west of the Salt Desert. lii central Asia the great desert of Gobi, ranging from Turkestan to Manchuria, is interspersed with a few oases, the chief one being Kand, which is characterized by a rich growth of vegetation.

OAT, Avena sativa, L., one of about forty species mostly dispersed through the temperate regions of the Old World. It belongs to the tribe Avenew of the order Graminea or Grasses. The spikelets form a loose panicle, familiar in the cultivated oat, the flowering glume having its dorsal rib prolonged into an awn, which is in some species twisted and bent near the base.