RED SEA. The Red Sea runs north-north-west from the Gulf of Aden in the Indian Ocean for about 1200 u- miles, extending from 12° 40' to 30° N. lat. The Strait of Bab-el-Mandeb at the entrance to the Gulf of Aden is 13J miles across, and is divided by Perim Island into two channels, the north-eastern narrow and shallow, the south-western 10 miles wide, and deep. The sea widens rapidly to 230 miles in 16° N. lat., and more gradually to 250 miles off Kunfuda in 19° N. lat.; from this point it narrows to 130 miles in 24° N. lat., a breadth which is maintained up to 27° 45' N. lat., where the sea divides into two gulfs, those of Suez and 'Akaba. The Gulf of Suez continues in the north-north-west direction for 170 miles, with an average width of 30 miles; that of 'Akaba is narrower, and runs north-north-east for 97 miles. The Sinaitic peninsula between the two gulfs bounds the Bed Sea to the north; on the east the Arabian coast and on the west the coasts of Egypt, Nubia, and Abyssinia form the boundaries.

The Arabian coast (see ARABIA) is generally a narrow sandy plain backed by ranges of barren mountains abrupt in outline and of moderate height. Enormous coral reefs run along the coast in broken lines, parallel to the shore but not connected with it. They usually rise out of deep water to within a few feet of the surface; and a navigable channel of from 2 to 3 miles in width, in which the water is always calm, extends between them and the land. The Farisan Archipelago in 17* N. lat. is the largest and most important of the island groups of the eastern reef. It is entirely of organic formation. The most important har-bours of Arabia on the Bed Sea are Mokha in 13° 30' N. lat. (now nearly deserted for those of Aden and Hodeida, the port of San'a), Lokeyyah about 200 miles farther north, Jiddah in 21° 20' N. lat. (the only well protected harbour), and Yenbo' in 24° N. lat. The western coast is flat and desert in the north, but gives place farther south to high tablelands rising at some distance from the shore, and then to the lofty Abyssinian mountains (see ABYSSINIA, AFRICA, EGYPT). The parallel system of coral reefs- is not so extensive as on the east coast, and being nearer the land the inshore channel is narrower. The large and curiously shaped coral-rock island of Dahlak, lying off Annesley Bay, is the most important on the reef. There are seven or eight harbours, of which the best known are Massowah, a little to the north of Annesley Bay (the largest inlet on the sea), and the port of disembarkation of the British troops in the Abyssinian War of 1868, Khor Nowarat, which, though small, is the best bay in the Red Sea, and Sawakin (Suakim) in 19° 30' N. lat., the chief port of the Soudan trade.
The only islands of importance not already mentioned are those of the volcanic group in 14° N. lat., one of which, Jebel Zugur, 10 miles long and 7 wide, rises in a series of bare hills to the altitude of 2074 feet, and the islet Jebel Teir in 15° 30' N. lat., on which a volcano has only recently become inactive. A dangerous reef named the Daedalus in 24° 26' N. lat. lies right in the way of steamships traversing the sea; it is covered with a few feet of water or uncovered, according to the season, and, like most of the reefs and islands on the usual track of vessels, is furnished with a lighthouse.

The Red Sea area is in a state of gradual upheaval, the former seaport of Adulis on Annesley Bay is now 4 miles from the shore, and at Suez the former limits of the sea can be traced for several miles northwards; whereas the north coast of Egypt is undergoing gradual subsidence.

Tides.—The tides are imperceptible at many places on the Red Sea, and where observable they are extremely uncertain, varying both as to time and to amount of rise with the direction and force of the wind. At Suez, where they are most regular, the rise varies from 7 feet at spring to 4 feet at neap tides. The surface-currents of the sea are also variable and perplexing ; they are chiefly pro-duced by the wind, and change in velocity and direction accordingly.

Traffic.—From the decline of the old Indian trade with Egypt till the formation of the Peninsular and Oriental Steam Navigation Company and the overland route to India in 1840, traffic in the Red Sea was almost entirely confined to small native vessels trading with grain and fruit between Egypt and Arabia, and carrying pil-grims to Jiddah, the port of Mecca. Since 1840 passenger traffic, and since the opening of the Suez Canal in 1869 trade of all kinds in European vessels, have greatly increased. A telegraphic cable was laid from Bombay to Suez in 1859.

Meteorology.—The climate of the Red Sea region is one of the hottest in the world. The altitude of the sun, the almost con-tinually cloudless skies, the arid rainless character of the shores, and the complete absence of rivers combine to make the mean temperature high. That of the air usually ranges from 70° to 94° Fahr., though it has been frequently observed as high as 105° in the shade on board ship, and in the northern part of the sea the clearness of the nights promotes radiation, so that by morning the thermometer may fall to the freezing-point on shore. The atmo-sphere over the land is very dry ; the difference between the read-ings of the wet and dry bulb thermometers is frequently as much as 25°, and sometimes, during the prevalence of the desert wind, it rises to 40°. The evaporation from the Red Sea is naturally excess-ive ; the humidity of the air over the water is always great in summer, and when the wind blows off the sea the atmosphere is frequently saturated on shore. From the direction of the prevail-ing winds, precipitation takes place chiefly on the mountains of Abyssinia. North-north-west winds prevail on shore all the year round with very slight exception; but in the middle of the sea they are only universal from June to September, and are confined to the northern half from October to May. During the latter season south-south-east winds prevail in the southern part of the sea, while a belt of calms and light variable breezes occupies a changing position near the centre. The southerly winds are often accompanied with rain squalls, and in September there are frequently calms and hazy weather. Hurricanes and heavy storms seldom occur in the sea, but moderate gales are common and sandstorms not unusual. From the admiralty temperature charts it appears that the mean temper-ature of the surface-water at the four typical seasons of the year, taking all available data into account, is as follows :—

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The temperature of the water at the south end of the sea is usually in excess of that of the air, and it is on record that on four consecu-tive days the temperature of the surface-water was 100°, 106°, 100°, and 96°, while at the same time that of the air was 80°, 82°, 83°, and 82°. The surface-temperature varies from 70° to 90°, according to the position and the season. The winter mean of the northern part is about 71°, and this temperature continues to the bottom at that season. "When the temperature on the surface is higher than 71° it gradually falls as the depth increases, until at about 200 fathoms it becomes uniform in all parts of the sea as 71°, a temperature which is maintained from that depth to the bottom all the year round. This is in consonance with all observations made on enclosed seas, the water below the point to which the barrier reaches being of uniform temperature. According to some authori-ties this is the isocheimal or mean winter temperature of the sur-face ; but the researches of the " Challenger " seem to indicate that the temperature of the external ocean at the summit of the barrier is that which extends to the bottom of the enclosed sea.

Physical Conditions.—The greatest depth, which occurs in 21° N. lat., is about 1200 fathoms, and from this point the sea shoals to each end. The general conformation of the bottom is that of a series of gradually sloping rounded elevations with rounded basins between them. The water is shallow in the Gulf of Suez and also at the Strait of Bab-el-Mandeb, where the depth in the centre of the large channel is a little under 200 fathoms. The Red Sea basin is cut off from the general oceanic circulation by a barrier rising to within 200 fathoms of the surface in a channel that has a much smaller average depth and is only 13¿ miles wide. As no rivers discharge into it and little rain falls, it must be viewed as a purely evaporational area, and as such it is of extreme scientific interest. It reproduces and exaggerates all the special physical conditions of the Mediterranean ; but on account of the extremely trying nature of the climate it has not been so thoroughly investigated. The average amount of evaporation at Aden is variously estimated at from 0-25 to 0-75 inch per day, or from 8 to 23 feet per year ; in the Red Sea generally it must be at least equal to the smaller figure, and probably exceeds it. As the level of the Red Sea is not sub-ject to any permanent change it is evident that water must flow in through the Strait of Bab-el-Mandeb—the slight current through the Suez Canal need not be considered—to replace loss by evapora-tion. If there were no return current it is estimated that the Red Sea would become a mass of solid salt in one or two thousand years. Although the salinity of the water is higher than that of the water of the ocean it does not appear to be on the increase. The density of Red Sea water at 60° Fahr. is about 1'030, corre-sponding to 4 '0 per cent, of total salts, while that of average ocean water is l-026, which corresponds to 3'5 per cent, of salts. In order to account for the constancy of salinity in the Red Sea it is necessary to assume the existence of strong undercurrents of salt water passing out of the sea, beneath the opposite entering current of fresher water. These undercurrents have not yet been ob-served, but there are indirect proofs of their existence. During the hottest months (July to September), when there is most eva-poration, the prevalence of northerly winds drives the water out of the Red Sea as a rapid surface-drift; the south-west monsoon is blowing in the Indian Ocean at the same time, and the general level in the Red Sea is from 2 to 3 feet higher than during the cooler months, when evaporation is less, and when the north-east monsoon forces water into the funnel-shaped Gulf of Aden, and thence through the Strait of Bab-el-Mandeb. This is held by Dr W. B. Carpenter to be a proof of the existence of an undercurrent, for the north-east wind forms a head of water at the strait, which to equalize pressure over the area produces an under-return-current, and this greatly accelerates the flow of the regular undercurrent of the Red Sea, and so lowers the general level. In summer the outflow of dense salt water is slower, and this more than neutralizes the effect of the outward surface-drift, which to some extent reduces the volume of the entering fresher water at that season. In the Red Sea there is a constant and regular sub-surface circulation of water due solely to evaporation ; the surface-drifts caused by wind, although they form rapid currents and render navigation dangerous at times, are minor agents in the system and modify it only to a slight extent. The Rod Sea and the Persian Gulf serve as concen-tration areas for maintaining the salinity of the deep water in the Indian Ocean, in opposition to the currents of comparatively fresh water flowing northwards from the Antarctic Ocean.

Deposits.—In a sea so nearly landlocked and so narrow the deposits which cover the bottom are naturally of the order classed as terrigenous. The large quantity of sand blown into the sea, the immense abundance of corals and other calcareous organisms in the water, and the entire absence of rivers with their suspended sediments produce deposits more nearly resembling in some of their characteristics those of the open ocean than those of inland seas. But the sand and ooze from the bottom of the Red Sea have not yet been thoroughly examined.

Fauna.—Animal life in all its forms is extremely abundant in the Red Sea, which, however, cannot be said to have been any more completely surveyed from a biological than from a physical point of view, although several eminent zoologists have studied special types. Great numbers of new species have been discovered by each investigator, and it has been ascertained that the Red Sea fauna differs considerably from that of the Mediterranean, not more than twenty species being common, it is stated, to both, thus indicating that the separation of the two seas must have taken place at a remote epoch, which appears from geological evidence to be the Eocene period. It exhibits affinities with the fauna of the Pacific, particularly with that of the coast of Japan. Corals are more plentiful and more active in the Red Sea than in almost any other piece of water of its size, a result probably due equally to the high temperature, the great salinity of the water, and the abundance of food. (H. R. M.)