Of all the branches of the Atlantic that enter deeply into the bosom of the land, the Mediterranean is the largest and most beautiful, covering with its dark blue waters more than 760,000 square miles. Situate in a comparatively low latitude, exposed to the heat of the African deserts on the south, and sheltered on the north by the Alps, the evaporation is excessive; on that account the water of the Mediterranean is salter than that of the ocean, and for the same reason the temperature at its surface is 3-1/2° of Fahrenheit higher than that of the Atlantic: it does not decrease so rapidly downwards as in tropical seas, and it becomes constant at depths of from 340 to 1000 fathoms, according to the situations. Although its own river domain is only 250,000 square miles, the constant current that sets in through the Dardanelles brings a great part of the drainage of the Black Sea, so that it is really fed by the melted snow and rivers from the Caucasus, Asia Minor, Abyssinia, the Atlas, and the Alps. Yet the quantity of water that flows into the Mediterranean from the Atlantic, by the central current in the Straits of Gibraltar, exceeds that which goes out by the lateral currents. Near Alexandria the surface of this sea is 26 feet 6 inches lower than the level of the Red Sea at Suez at low water, and about 30 feet lower at high water.[115] On the shore of Cephalonia there is a cavity in the rocks, into which the sea has been flowing for ages.[116] The Mediterranean is divided into two basins by a shallow that runs from Cape Bon on the African coast to the Strait of Messina, on each side of which the water is exceedingly deep, and said to be unfathomable in some parts. M. Bérard has sounded to the depth of more than 1000 fathoms in several places without reaching the bottom. At Nice, within a few yards of the shore, it is nearly 700 fathoms deep; and Captain Smyth, R. N., ascertained the depth to be 960 fathoms between Gibraltar and Ceuta. This sea is not absolutely without tides; in the Gulf of Venice they rise three feet, and at the Great Syrte to five feet at new and full moon, but in most other places they are scarcely perceptible. The surface is traversed by various currents, two of which, opposing one another, occasion the celebrated whirlpool of Charybdis, whose terrors were much diminished by the earthquake of 1783. Its bed is subject to violent volcanic paroxysms, and its surface is studded with islands of all sizes, from the magnificent kingdom of Sicily to mere barren rocks— some actively volcanic, others of volcanic formation, and many of the secondary geological period. Various parts of its coasts are in a state of great instability; in some places they have sunk down and risen again more than once within the historical period.