Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/854

822 MEDITERRANEAN Dividing the volume by the area we have for the mean annual flow 1170 xio 12 R. 29 x 10 6 = 40xlO R feet, Reducing this to miles per day, we find that if the above data are correct the inflowing current at the Straits of Gibraltar ought to be equivalent to a current 8 miles wide, 100 fathoms deep, and running with the uniform velocity of 1S 3 miles in twenty-four hours. As the currents are reversed with the tides this is the balance of inflow over outflow in the upper current. It is worthy of remark that the flood tide runs to the westward at the surface and the ebb to the eastward. The following table of tides at places inside and outside the Straits will show that the mere differences of level due to the different tidal ranges at adjacent localities are sufficient to cause strong local currents. Places. High Water. Full and Change. Springs Rise. Neaps Kise. Neaps Range. Chipiona IT. M. 1 30 Ft. in. 12 5 Ft. in. 8 Ft. in. 3 6 Rota 1 24 12 6 8 3 6 Cadiz 1 23 12 9 8 2 Conil 1 18 12 7 5 3 3 Cape Plata 1 45 8 5 3 2 6 Tarifa 1 46 6 3 6 1 3 Al^osiras .. .... 1 49 3 9 2 6 1 3 Gibraltar 1 47 4 1 2 7 1 3 Ceuta 2 6 3 7 2 5 1 3 Tetuan 2 23 2 6 1 6 6 Tangier 1 42 8 3 5 1 2 Rabat 1 46 11 7 1 3 3 Mo^ador 1 18 12 4 8 3 6 A similar phenomenon is witnessed at the other end of the sea. Here the fresher waters of the Black Sea rush in through the narrow channel of the Dardanelles, causing a surface inflow of comparatively fresh water, while there is an outflow below of denser Mediterranean water. The dimensions of the Straits are too small to make the pheno menon of any importance for the supply of the Mediter ranean. The conditions both in the Dardanelles and in the Bosphorus were examined very carefully in the year 1872 by Captain Wharton, R.N., of H.M.S. "Shear water/ and his results are published in an interesting report to the admiralty, of that date. It is remarkable that the comparatively fresh water of the Black Sea persists without sensible mixture through the Sea of Marmora and into the Dardanelles, while there is constantly a current of Mediterranean water running underneath, and the depth in the two channels is only from 30 to 50 fathoms. There can be little doubt that the saltness of the Black Sea -is due wholly to the return current of Mediterranean water enter ing through the Bosphorus. Were the exit of the Black Sea a channel with sufficient fall to bring the surface of the Sea of Marmora below the level of the highest part of its bottom, so that no return current could take place, the waters of the Black Sea would be fresh. In the body of the sea the rise and fall are much less than at any of the places in the above table. At Algiers a self-recording tide gauge was set up by Aime", and from its records he deduced a rise and fall of 88 millim. (say 3^ in.) at springs and half that amount at neaps, a fluctuation which would escape ordinary observation, as it would be masked by the effects of atmospheric disturbances. At Venice and in the upper reaches of the Adriatic, the true lunisolar tide seems to be more accentuated than in other parts ; but here also its effects are subordinate to those of the wind. In summer the Mediterranean is within the northern limit of the north-east trade wind ; consequently, through out a great part of the year, the winds are tolerably constant in direction ; and, blowing as they do over large areas ol water, they are instrumental in moving large masses of it from one point to another, and so producing streams and currents. The effect of wind on a surface of water is twofold : it produces the rhythmic motion of waves and the motion of translation of currents. Besides the motion produced by the direct action of the wind on the surface-water, there are currents due in the first instance to the accumulation of water produced by a wind which has been blowing constantly in one direction. The phenomenon of an abnormally high tidal rise with a gale of wind blowing on shore is one with which inhabitants of the British Islands are familiar. It is also a matter of frequent observation that, for instance, a south-west gale which exaggerates the height of high water on the western coasts of Britain reduces it on the east coasts. It blovrs the water on the west coast and off the east coast, so that the difference in the high-water levels on the two coasts is very pronounced. Supposing free communication were quickly made between the two coasts, a current would be the result, and its violence would be much greater than would be due to the local action of the wind on its surface. In the Mediterranean the winds blow during a great portion of the year very constantly from one direction or another, and generally from north and east. The extent of the sea is so great that the slope produced by the transference of the surface water constantly in one direction might have a sine or arc capable of being measured in feet and inches when the radius is as much as 200 miles long. Thus at Port Mahon, in the island of Minorca, according to the Admiralty Sailing Directions, the water rises and falls according to the direction of the wind. With wind from south-east or south-west the water rises, but from north west or north-east it falls. When northerly or north westerly winds prevail, and this is the case for two-thirds of the year, a strong current sets to the south-west off Ayre Island, which is reversed in seasons when south-westerly winds prevail. This current is due to the water escaping round the end of Minorca having been driven southward so as to raise a head on the north coasts of the island. Similarly in the Faro or Strait of Messina the currents, of which the famous Scylla and Charybdis are swirls or eddies, are the evidence of a tendency towards equalizing the levels of the eastern extremity of the western basin and of the western extremity of the eastern basin. In addition to this peculiarity of position with reference to the two basins, it has been found that there is a very strong purely tidal influence at work which alone produces an alteration in the direction of the currents, and thus adds to the confusion of the waters. At Capo di Faro the rise is scarcely per ceptible, at Messina it may attain a maximum of 10 to 13 inches. In the Straits of Bonifacio, between Corsica and Sardinia, the currents follow entirely the direction of the prevailing winds, and are at times very rapid. In the channel between Sicily and the African coast the currents also follow the winds. In long periods of calm weather a steady easterly set is observed, no doubt a prolongation or reproduction of the Gibraltar current, Temperature. Nothing whatever was known of the temperature of the deep water of the Mediterranean until Saussnre extended to it his classical investigation into that of the Swiss lakes. In October 1780 he sank his thermometer to a depth of ICO fathoms off Genoa and of 320 fathoms off Nice, and at both depths he found the temperature of the water to be 55 8 F. These observa tions have a special value, for, owing to Saussure s method of ex perimenting, his results were not affected by the pressure obtaining at great depths in the sea. Fifty years elapsed before any similar experiments were made, when D Urville, in the "Astrolabe," made a few observations at the beginning and the end of Ids famous e-xpedition. There is some uncertainty about his observations in

1826 and 1829, and also about the later ones of Berard in 1831, as