Amongst the local currents the existence of which has been most clearly established are those which convey the waters of the Sea of Azof into the Black Sea, and those of the latter into the Archipelago. The Don more than makes up for the loss by evaporation in the Sea of Azof, and its surplus waters find an exit through the Strait of Kerch into the Black Sea. Similarly the waters of the Dniester, the Dnieper, the Rion, and of the rivers of Asia Minor, and, above all, of the Danube, which by itself conveys a larger volume of water into the Black Sea than all the others combined, are discharged through the Bosphorus and the Dardanelles into the Archipelago. On the other hand, the Archipelago returns to the Black Sea, by means of a submarine counter-current and of lateral surface currents, a certain quantity of salt water for the fresh water which it receives in excess. This exchange accounts for the salineness of the waters of the Black Sea. The volume of fresh water discharged into it by the Danube and other rivers is so large that in the course of a thousand years its waters would become perfectly fresh, if there did not exist these compensatory highly saline counter-currents. Analogous phenomena take place at the other extremity of the Mediterranean. Evaporation there is excessive, owing to the neighbourhood of the burning sands of the deserts, the winds from which blow freely over the sea, absorbing the vapours and dispersing the clouds. The loss by evaporation amounts to at least seven feet in the course of a year, and as the annual rainfall is estimated to amount to twenty inches only, and the volume of water discharged annually by all the tributary rivers of the Mediterranean, if uniformly spread over its surface, would hardly exceed ten inches in depth, there exists thus an excess of evaporation amounting annually to more than four feet; and this excess has to be made good by an inflow of the waters of the Atlantic, which takes place through the Strait of Gibraltar, whose volume far exceeds that of the Amazon in a state of flood. This inflow of the waters of the Atlantic is felt, as a current, as far as the coasts of Sicily, and, like all other currents, it is bounded by lateral currents flowing in a direction contrary to that of the main current. During ebb the insetting Atlantic current takes up the whole of the strait, but when the tide rises the Mediterranean resists more successfully the pressure of the ocean, and this struggle gives birth to {28} two counter-currents, one of which skirts the coast of Europe, the other that of Africa between Ceuta and Cape Spartel; the latter is the larger and more powerful of the two. In addition to these, there exists a submarine current, which conveys the highly saline and heavier waters of the Mediterranean out into the Atlantic.