Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/853

Rh MEDITERRANEAN 821 Cape Sta Maria cli Leuca and the island of Corfu to the Venetian shore in the Gulf of Trieste. Its average width is about 100 miles. A ridge with little over 400 fathoms appears to run across its entrance. Inside this the water reaches a depth of 765 fathoms, but shoals again rapidly towards Pelagosa Island, from which to the northward, including quite two-thirds of the sea, the depth is under 100 fathoms; indeed no part of the sea within 150 miles of its northern extremity is over 50 fathoms deep. There is authentic historical evidence of the encroachment of the Italian shores on the Adriatic, causing thereby a diminution of its area. As a consequence many towns which were ouce thriving seaports are now many miles inland ; thus Adria, which was a station of the Roman fleet, is now 15 miles inland, and there are many similar examples. The large rivers Po and Adige, which bring the drainage of the southern slopes of the Alps to the sea, deliver large quantities of sediment in the course of the year. The dis tribution of this mud is affected, not only by its own weight t-j tiling to make it sink to the bottom, but also by the set of the currents, which, running up the eastern coast, turn tj tha westward and southward at the upper end of the sea, and so tend to distribute the river mud along the bottom in the neighbourhood of the Italian coasts. The fact that towns which were formerly seaports are now 1 ihnd doss not therefore necessitate the assumption of a general rise of the land, it is merely a reclamation by nitural agencies of land from the sea at the expense of the inland mountainous country. Precisely similar pheno mena are observed in the neighbourhood of the mouths of the Rhone and of the Nile. Specific Gravity, Currents, dr. On the specific gravity Dr Carpenter reports many and interesting observations. In round numbers, that of the surface-water of the Atlantic off the Straits of Gibraltar is 1 0260 to 1 0270, that of the western basin of the Mediterranean 1 0280 to 1 - 0290, and that of the eastern basin 1 0290 to 1 0300, while that of the Black Sea is 1-0120 to 1-0140. It will thus be seen that the water of the Mediterranean proper is very much salter than either the Atlantic on the west or the Black Sea on the east, and this great density of the water affords a use ful means of recognizing it when investigating the inter change of waters which takes place at the two extremities of the sea. Both the temperature and the specific gravity of the water are evidences of the local climate. The great concentration of the water shows how dry the atmosphere at ths surface must be, and how insignificant the contribu tions of fresh water. With regard to the balance existing between the two factors, evaporation and precipitation, it would be impossible to give figures with any claim to accuracy, but a rough estimate may be formed by taking such data as Fischer has given. He puts the rainfall over the whole Mediterranean drainage area at 759 - 4 milli metres, or almost exactly 30 inches. If we remember that the average rainfall of the eastern slopes of Great Britain is less than 30 inches, and that therefore this may be taken as the maximum yearly supply to the North Sea, we may be sure that the Mediterranean does not receive more than 30 inches of fresh water in the year. With regard to the rate of evaporation over the area of the Mediterranean there is but very meagre information, but wherever it has been observed it has been found to exceed the rainfall, even as much as three times. Thus at Madrid it is 65 inches, or more than four times the rainfall, at Rome 105 inches, and at Cairo 92 inches. It may therefore without exaggeration be assumed that the evaporation is at least twice as great as the precipitation. Putting the latter at 30 inches, we should have 60 inches for the yearly evaporation, and a balance of 30 inches evaporation over precipitation. Were there no provision for making good this deficiency, the level of the Mediterranean would sink until its surface was so far contracted as to lose no tiiore by evaporation than would be supplied by rain. This condition would probably not be fulfilled before all the ^Egean and Adriatic and the whole of the western basin west of the island of Sardinia were laid dry, and what is now the Mediterranean would be reduced to two &quot;Dead Seas,&quot; one between Sardinia and Naples and the other between Africa and the mouth of the Adriatic. That the level and the salinity of the Mediterranean remain constant is due to the supply of water which enters at the Straits of Gibraltar. The currents in this passage have frequently engaged attention both from their scientific and their nautical interest. The most detailed investigation was that carried out by Captain Nares and Dr Carpenter in H.M.S. &quot; Shearwater &quot; in the year 1871. l From these investiga tions it appears that there are usually two currents in the Straits at the same time, one superposed on the other. Both are affected by tidal influence, but, after allowing for it, there is still a balance of inflow in the upper and of outflow in the under current. The waters of the two currents are sharply distinguished from each other by their&quot; salinity. Further, the upper current appears to affect by preference the middle of the channel and the African coast, while the under current appears to crop out at the surface on the Spanish coast. This distribution, however, is much modified by the state of the tide, and it must be remembered that in such places the surface separating the upper and under currents is rarely, if ever, a horizontal plane. That there is a balance of outflow over inflow at the bottom was well shown by the result of soundings as much as 200 miles north-west of the entrance of the Straits, where, in a depth of 1560 fathoms, water of decided Mediterranean origin was got from the bottom. There can be no doubt that this outflow of warm and dense Mediter ranean water is largely instrumental in causing the com paratively very high bottom temperature in the eastern basin of the North Atlantic. We have assumed that the balance of water removed by evaporation is 30 inches, or 2 5 feet. If we take the area of the Mediterranean to be 1,000,000 square miles, we have the volume of water removed t&amp;gt; = 2 5 x 36 x 10 12 = 90 x 10 12 cul&amp;gt;ic feet. This quantity of water has to be supplied from the Atlantic without raising the total quantity of salt in the sea. We have seen that the only provision for the removal of the surplus salt is the outward under current in the Straits. Hence the inward upper current must be sufficient to replace the water lost both by evaporation and by the outflow of the under current. We may take the Atlantic water to contain 3 -6 per cent, and the Mediterranean to contain 3 9 per cent, of salt. In order that the under current may remove exactly as much salt as is brought in by the upper one, their volumes must be in the inverse ratio of their saline contents, or the volume of the upper current must be to that of the under one in the ratio 39 : 36 or 1000 : 923 ; so that only 7 7 per cent, of the inflow goes to replace the water removed by evapora tion, while the remaining 92 3 per cent, replaces the water of the under current. We have then for the total volume of the inward current per annum V = --r-ll70xl0 ls cubic feet. 77 The width of the Straits from Tarifa to Point Cires is 8 miles, or 48,000 feet, and the average depth of the stream may fairly be taken as 100 fathoms; hence the sectional area is in round numbers 29,000,000 square feet. 1 Proc. Roy. Soc. (1872), xx. 97, 414.