The Mediterranean - Hydroiogy
( Originally Published 1920 )
GREECE and its insular satellites prove sufficiently that the unstable I floods of the Mediterranean have exercised a greater influence upon the march of history than did the solid land upon which man trod. Western civilisation would never have seen the light had not the waters of the Mediterranean washed the shores of Egypt, Phoenicia, Asia Minor, Hellas, Italy, Spain, and Carthage. The western nations would have remained in their primitive barbarism if it had not been for the Mediterranean, which joined Europe, Asia, and Africa ; facilitated the intercourse between Aryans, Semites, and Berbers ; and rendered more equable the climate of the surrounding countries, thus facilitating access to them. For ages it appeared almost as if mankind could prosper only in the neighbourhood of this central sea, for beyond its basin only decayed nations were to be met with, or tribes not yet awakened to mental activity. " Like frogs around a swamp, so have we settled down on the shores of this sea," said Plato; and the sea he refers to is the Mediterranean. It is therefore deserving of description quite as much as the inhabited countries which surround it. Unfortunately many mysteries still remain hidden beneath its waves.
From an examination of the coasts, as well as from the traditions of the people inhabiting them, we learn that the Mediterranean has varied frequently in its contours and extent. The straits which connect its waters with those of the ocean have frequently changed their position. At a time when peninsulas like Greece, and even islands like Malta, formed part of continental masses—and that they did so in a comparatively recent geological epoch is proved by their fossil fauna—the waters of the Mediterranean covered large portions of Africa, of Southern Russia, and even of Asia. The researches of Sprat, Fuchs, and others have satisfactorily proved that towards the close of the miocene age a vast fresh-water lake stretched from the banks of the Aral, aeross Russia, the plains of the Danube and the Archipelago, as far as Syracuse in Sicily. Then came the briny waters of the ocean. There was a time when the Black Sea and the Caspian connected the Archipelago with the Gulf of the Obi. At another epoch the gulfs of the Syrtes penetrated far inland, and a large portion of what is now the Libyan and Saharan desert was then covered with water. The Strait of Gibraltar, which was torn asunder by Hercules according to the traditions of the ancients, is in reality but of recent origin, and has taken the place of a more ancient strait which joined the Mediterranean to the Red Sea and Indian Ocean: this strait has been restored by human hands, and is known now as the Suez Canal. The coast-lines of the Mediterranean are undergoing perpetual change, owing to the upheaval or subsidence of the countries surrounding it. The Nile, the Po, the Rhone, and other rivers incessantly enlarge the alluvial plains at their mouths, and still further encroach upon the sea. Actually the Mediterranean, with its subordinate seas from the Strait of Gibraltar to the Sea of Azof, covers an area about thirty t:mes that of the British Islands. This area is small if we compare it with the immense development of the coasts and the wealth in peninsulas, which impart an aspect of life and independence to at least one-third of the ancient world. The Mediterranean, though it takes precedence of all the oceans, in consequence of the part it has played in history, nevertheless only covers an area one-seventieth that of the Pacific. It is broken up, moreover, into several sep irate seas, some of them so small in extent that the navigator hardly. ever loses sight of the land. In the east we have the Black Sea, with its two dependencies, the Seas of Azof and of Marmara. The AEgean Sea, or Archipelago, with its numerous islands, extends between the deeply indented coasts of Greece, Asia Minor, and Crete. The Adriatic stretches towards the north-west, between the Balkan peninsula and Italy ; and the Mediterranean proper is divided into two separate basins, which might appropriately be called the Phoenician and Carthaginian Seas, or the Greek and Roman Mediterraneans. Each of these basins is again subdivided, the one by Crete, the other by the two islands of Sardinia and Corsica. These various subdivisions of the Mediterranean differ in area, and still more in depth. The Sea of Azof almost deserves the name of " Swamp," which was bestowed upon it by the aneients, for if a ship sinks in it the masts remain visible above the water. The Black Sea has a maximum depth of over 1,000 fathoms, but the narrow strait which joins it to the Sea of Marmara is shallower than many a European river. The cavity filled by the Sea of Marmara is far inferior to that of many an inland lake ; and the Dardanelles, like the Bosphorus, are hardly wider than a river. In the Archipelago and the eastern basin of the Mediterranean proper the depth corresponds with the protuberance of the land. Abyssal depths and " pits" of 260 and even of 540 fathoms are to be found in close proximity to the scarped mountain islands of the Cyclades, whilst on the low coasts of Egypt the water deepens only gradually, until in the centre of the Levantine Sea it attains a depth of 1,750 fathoms. The maximum depth—2,170 fathoms—is attained between Crete and Malta. If the whole of the waters of the Mediterranean were to be collected into an aqueous sphere, the latter would have a diameter of 90 miles; if it fell dow n upon the earth, it would not even wholly cov er a country like Switzerland.
The Ionian Sea is separated from the Adriatic by a submarine ridge rising in the Strait of Otranto, and bounded on the west by a shoal or submarine isthmus, already referred to by Strabo, which joins Sicily to Tunis. This isthmus forms the true geological boundary between the western and eastern basins of the Mediterranean, which are connected here by a narrow breach only, the depth of which hardly exceeds 100 fathoms. The western of these basins is the smaller and shallower of the two, but nevertheless it attains a depth of 1,100 fathoms in the Tyrrhenian, and of 1,360 fathoms and even 1,640 in the Balearic Sea, and is separated from the waters of the Atlantic by a submarine ridge lying outside the Strait of Gibraltar, and joining Europe to Africa.*
This subdivision of the Mediterranean into separate basins, divided from each other by shoals or submarine ridges, by islands and promontories, sufficiently explains the contrasts between the phenomena of the open ocean and those observed here. In the Mediterranean, it is well know n, the tides are almost everywhere irregular and uncertain. To the east of the Narrows of Gibraltar, in the sea extending between Andalusia and Morocco, the tides are hardly felt at all, and they are, moreover, interfered with to such an extent by currents that it is exceedingly difficult to determine their amplitude, or the establishment of the various ports. Nevertheless the rise and fall of the tidal wave are sufficiently marked to have attracted the attention of Greek and Italian navigators. On the coasts of Catalonia, France, Liguria. Naples, Asia Minor, Syria, and Egypt the oscillation is hardly perceptible, but on those of Eastern Sicily and of the Adriatic the tide sometimes rises three feet, and, if accompanied by storms, may even attain a height of ten feet in certain localities. The Straits of Messina and of Euripo (Euboea) have their regular tides, and in the Gulf of Gabes the waters rise and fall with the same regularity as in the open ocean. In the Black Sea, however, no tidal movements whatever have been discovered hitherto. It is nevertheless probable that more careful observations will lead to the discovery of a feeble tide, for it is believed that this phenomenon exists even on Lake Michigan, which has only one-fifth the area of the Black Sea.
The Mediterranean differs not only from the open ocean with respect to the feebleness and irregularity of its tides, but it is likewise without a great stream-current keeping in constant circulation the whole body of its waters. The currents which have been observed in various divisions of the Mediterranean can be ascribed only to local causes. An Italian geographer of the last century, Montanari, has advanced an hypothesis of a great circuit current which entered the Mediterranean through the Strait of Gibraltar, and, after having washed the shores of Africa as far as Egypt, returned to the west along those of Asia and Europe; but careful observers have vainly endeavoured to discover its existence. They have met only with local currents, produced by an indraught of the waters of the Atlantic, by winds, by the floods of rivers, or by an excess of evaporation. One of these currents sets along the coasts of Morocco and Algeria from west to east; another flows along the Italian coast of the Adriatic from north to south ; and a third from the mouth of the Rhone in the direction of Cette and Port Vendres. In fact, the configuration of the sea-bottom, and particularly the shoal between Sicily and Tunis, precludes the existence of any but surface currents in the Mediterranean.
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 Dmester, 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 pnenomena 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 Amason 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 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.
The quantity of salt held in solution in various parts of the Mediterranean differs widely, as the submarine ridges and shoals which divide it into separate basins do not permit its waters to mingle as freely as in the open ocean. Owing to the excess of evaporation, the quantity of salt is greater on the whole than in the Atlantic, and this is the case more particularly on the coast of Africa. But in the Black Sea it is far less, and near the mouths of some of the large rivers which enter that sea the water is almost fresh.*
The temperature of the Mediterranean is affected by the saine causes which produce its varying salineness, viz. the existence of sheals and banks, which separate it into distinct sub-basins. In the open ocean the currents convey to all latitudes large bodies of water, some of them heated by a tropical sun, others cooled by contact with the ice of the polar regions. But these layers of unequal density are regularly superimposed one upon the other, owing to the differences in their temperature : the warm water remains on the surface, whilst the cold water descends to the bottom. In the Mediterranean an analogous superimposition exists only to a depth of 110 fathoms, which is the depth of the Atlantic current, flowing into it through the Strait of Gibraltar. If a thermometer be lowered to a greater depth it will indicate no further decrease of temperature, and the immense body of water, remaining almost still at the bottom of the Mediterranean, has an equable temperature of about 56° F. Observations made at depths varying between 110 and 1,640 fathoms have always exhibited the same result. Professor Carpenter believes, however, that the abyssal waters of some of the volcanic regions have a somewhat higher temperature, which may be due to the presence of lava in a state of fusion.