Red Sea, an arm of the Indian Ocean, running NNW. from the Gulf of Aden, with which it communicates by the Strait of Bab-el-Mandeb, 13 1/2 miles across. Its length is about 1200 miles, and its greatest breadth 205 miles; it narrows towards the southern entrance, while in the north it is divided by the peninsula of Sinai into two gulfs, the Gulf of Suez, 170 miles long by 30 miles wide, and the Gulf of Akaba, 100 miles in length. The Arabian coasts of the Red Sea are usually narrow sandy plains backed by ranges of barren mountains; the African coasts towards the north are flat and sandy, but southwards high tablelands rise inland, culminating farther south in the lofty mountains of Abyssinia. A marked feature of the Red Sea is found in the partly upraised coral-reefs running parallel to both eastern and western shores; the Farisan Archipelago is in the eastern and larger reef, and the island of Dahlak, off Annesley Bay, in the western reef. There is also a volcanic group lying in 14° N. lat., the largest of which rises 2074 feet. The principal harbours are Mocha, Hodeida, Lokeyyah, Jiddah, and Yenbo' on the Arabian coast, and Massowah, Khor Nowarat, and Suakim on the African coast. A great trade route till the rounding of the Cape of Good Hope, the Red Sea has become a vastly more important route since the construction of the Suez Canal. The tides are very variable, depending largely on the direction and force of the winds. The mean temperature of the air generally ranges between 70° and 94° F. during the day, though readings of over 100° are often registered in the shade; but during the night the temperature may fall to the freezing-point, owing to radiation in the clear and cloudless atmosphere. The mean temperature of the surface water varies at the northern end between about 65° and 79° F., in the central regions between 75° and 86°, and at the southern end between 78° and 89°. The salinity of the water is almost constant at about 1.030 (ordinary ocean water is about l.026). No rivers flow into it, little rain falls, and evaporation is excessive. The greatest depth, near the centre, is about 1200 fathoms, and the mean depth about 375 fathoms. The marine fauna and flora are extensive. The origin of the name - Lat. Mare Rubrum and Gr. Erythra Thalassa - is much disputed.