1902 Encyclopedia > Mediterranean Sea

Mediterranean Sea




MEDITERRANEAN SEA. The southern shores of Europe are separated from the northern shores of Africa by the Mediterranean Sea. It extends in a generally east and west direction from longitude 5° 21' W. to 36° 10' E. Its length from Gibraltar to its eastern extremity in Syria is about 2100 miles. Its breadth is very various, being 400 miles from the mouth of the Rhone to the Algerian coast, 500 miles from the Gulf of Sidra to the entrance to the Adriatic, and 250 miles from the mouth of the Nile to the south coast of Asia Minor. From the very indented nature of its coasts, the general mass of the water is much cut up into separate seas, which have long borne distinctive names, as the Adriatic, the Aegean, the Sea of Marmora, the Black Sea, &c. The area of the whole system, including the Sea of Azoff, is given by Admiral Smythe as 1,149,287 square miles. If we deduct that of the Black Sea and Sea of Azoff, 172,506 square miles, we have for the area of the Mediterranean proper 976,781, or, roughly speaking, a million of square miles.

The Mediterranean is sharply divided into two great principal basins, the western and the eastern or Levant basin. The western possesses a comparatively smooth and unindented coast line. It is bounded on the south by the coast of Africa and the north coast of Sicily, and it is further enclosed by the coasts of Spain, France, and Italy, vhich form a roughly arc-shaped coast-lino. There are comparatively few small islands in this basin, though some of the more important large ones occur in it. The eastern basin is by far the larger of the two, and extends from Cape Bon to the Syrian coast, including as important branches the Adriatic and the Aegean. The latter is con-nected directly, through the Hellespont, the Sea of Marmora, and the Bosphorus, with the Black Sea. The entrance to the western basin and to the sea generally from the ocean is through the Straits of Gibraltar in 36° N. lat. If this parallel be drawn out through the sea it will be found that the western basin lies almost wholly to the northward, and the main body of the eastern one to the southward of it, the mean latitude of the western basin being about 39° 30', and that of the eastern basin 35°. They communicate with each other by the channels separ-ating Sicily from Italy and from Africa. The former is known as the Strait of Messina, and is of insignificant size, the latter is a wide channel apparently without any distinc-tive name, and generally shallow. The greatest depth on the shallowest rilge reaching from the African to the Sic lian coast is under 200 fathoms, and agrees very closely with the corresponding depth at the entrance to the Straits of Gibraltar.

Depth.—So far as is at present known, the maximum depth is pretty nearly alike in the two basins, being 2040 fathom in the western and 2150 fathoms in the eastern. Many lines of soundings have been run in the Mediterranean for telegraph purposes, and they afford a very good idea of the general configuration of the bottom. Between Marseilles and Algiers the depth ranges generally from 1200 to 1600 fathoms; between Naples and Sardinia from 1500 to 2000; between Alexandria and Rhodes from 1200 to 1600; and between Alexandria and Cyprus from 900 to 1100. The basin of the Mediterranean really begins about 50 miles to the westward of Gibraltar. It is here that the shallowest ridge stretches across from Africa to Spain ; the maximum depth on it is probably not more than 180, and certainly loss than 200 fathoms. From this ridge the bottom slopes quickly westward into the depths of the Atlantic, and gently eastward into the Mediterranean. The depth nowhere reaches 1000 fathoms until beyond Alboran Island, 120 miles east of Gibraltar. This is a small low island separated from the mainland on all sides by water of more than 400 fathoms; it must therefore be considered an oceanic as distinguished from a continental island.[820-1] Further to the north, and off the coast of Valencia, we have the Balearic Islands,—namely, Majorca, Minorca, Iviza, and Formentera. These also must be considered oceanic islands, and indeed two groups of oceanic islands. Iviza and Formentera are isolated both from the Spanish coast and from the other two islands by water of over 300 fathoms depth; Majorca and Minorca are connected by a bank with no more than 50 fathoms of water on it. Thirty miles east of Minorca there are more than 1400 fathoms ; beyond that there are no soundings between the Baleares and the large and important group of Corsica and Sardinia. These islands are continental, being connected with the Italian mainland by the bank on which Elba occurs, and which is covered by little over 50 fathoms of water. The Straits of Bonifacio, which separate Corsica from Sardinia, are also quite shallow, so that Corsica and Sardinia may be looked on as a secondary peninsula attached to the Tuscan shore of Italy by a shallow bank not more than 15 or 20 miles broad, the deep water coming close up all round it. Almost the same may be said of Sicily, including the Malta group, but excluding the Lipari group, which is purely volcanic. From Cape Passaro, in the south-east end of Sicily, a line can be drawn connecting it with the town of Tripoli, and without passing over water of more than 300 fathoms. As has already been said, the west end of Sicily is connected with the coast of Tunis by a ridge in no part covered by more than 200 fathoms of water. Between these two ridges lies a small but comparatively deep basin of 600 to 700 fathoms. At the western extremity of it lies the mountainous island of Pantellaria. The bank on which Malta is situated stretches for nearly 100 miles in a southerly direction from Cape Passaro in Sicily. Opposite, on the African shore, is a similar bank of much larger dimensions, on which are the small islands Lampion and Lampedusa belonging to Italy. In the deep channel between them and Malta is the small but lofty island Limosa. It is entirely volcanic, with an extinct crater on its north-eastern side, and three smaller ones to the south-ward. It resembles the Lipari group off the north coast of Sicily, which rise abruptly out of deep water, being connected by no bank either with the African or the Sicilian coasts. Some of the Lipari group are still active, Stromboli and Vulcano being of the number. Off the south coast of Sicily, and between it and the island of Pantellaria, occurs the famous Graham's shoal, the remains of what was for a tew weeks an island.[820-2]

The deepest water of the Mediterranean is found in its widest part between Malta and Crete, and the deep water comes close up to the Italian and Greek coasts, while on the African shore the water shoals more gradually. In the Strait of Messina, close to Beggio, there are depths of over 500 fathoms, and similar depths are found inside gulfs such as those of Taranto (nearly 1000 fathoms), of Corinth, Kalamata, and others. Also all through the Aegean in its many bights and channels very deep water is met with ; in the Sea of Marmora we have 500 fathoms, and in the Black Sea over 1000 fathoms. All along the south coast of Asia Minor the water is very deep, and the large islands of Cyprus and Crete are both separated by very deep water from the mainland. If we take the eastern basin, and run along its western and southern coasts from the mouth of the Po along the shore of Italy, Sicily, and Africa to the mouth of the Nile, and even further along the Syrian shore, we do not find a single off-lying island of any importance except the Malta group, while all along the eastern and northern coasts from Trieste to Asia Minor the coast is deeply indented, and the water broken up by many large and important islands. These islands are grouped along the west coast of Turkey and Greece, and irregularly throughout the Aegean. The east coast of the Adriatic, is studded with islands and inlets, and resembles in this respect the ^Egean; the west coast, on the othei hand, is low, and the water off it shallow, and there are lew harbours. The Adriatic stretches in a north-westerly direction for about 460 miles irom its entrance between Cape Sta Maria di Leuea 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 ['elagosa 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 once 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 distribution of this mud is affected, not only by its own weight tending to make it sink to the bottom, but also by the set of the currents, which, running up the eastern coast, turn to the 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 inland does not therefore necessitate the assumption of a general rise of the land, it is merely a reclamation by natural agencies of land from the sea at the expense of the inland mountainous country. Precisely similar phenomena are observed in the neighbourhood of the mouths of the Rhone and of the Nile.





Specific Gravity, Currents, &c.—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 P0260 to P0270, that of the western basin of the Mediterranean P0280 to 1-0290, and that of the eastern basin 1-0290 to 1-0300, while that of the Black Sea is P0120 to P0140. 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 the 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 more by evaporation than would be supplied by rain. This condition would probably not be fulfilled before all the iEgean and Adriatic and the wdiole of the western basin west of the island of Sardinia were laid dry, and what is now the Mediterranean would be reduced to two "Dead Seas," one between Sardinia aud Naples and the other between Africa and | the mouth of the Adriatic. That the level and the I 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. "Shearwater" in the year 1871.[821-1] From these investiga-tions it appears that there are usually two currents m 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 aud of outflow in the under current. The waters of the two currents are sharply distinguished from each other by their salinity. Further, the upper current appears to affect by preference the middle of the channel and the African coast, while the I 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 comparatively very high bottom temperature hi 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—

__________ cubic 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 tiie volume of the upper current must be to that of the under one in the ratio 39 : 36 or 1000: 923 ; so that ordy 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

_________ cubic feet.

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.

Dividing the volume by the area we have for the mean annual flow

== FORMULA ==

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 18'3 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.

== TABLE ==

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, E.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 (vithout 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 Ihe 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 3J 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 of 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 blows 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 clue 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 Sanssure extended to it his classical investigation into that of the Swiss lakes. In October 1780 he sank bis thermometer to a depth of 160 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 his famous expedition. There is some uncertainty about his observations in 1826 and 1829, and also about the later ones of Berard in 1831, as we are not informed whether the self-registering instruments used were protected from pressure or not. Mr Prestwich,[823-1] however, who has collected and critically discussed all the older deep-sea temperature observations, concludes, from a comparison of their results with those obtained by Aimé with protected instruments, that they were so protected, and admits their results into his tables without correction. In the deep water to the northward of the Balearic Islands D'Urville found in'April 1826 54°'5 F. in 270 fathoms, and in March 1829 54°'7 at the same depth, and the same temperature (54°'7) in 530 fathoms. Bérard, experimenting in the sea between the Balearic Islands and Algeria, found the temperature of the deep water nearly a degree higher, namely 55°'4 F., in depths of 500 to 1000 fathoms. Aimé[823-2] relates his own careful experiments on the temperature of both surface and deeper water in the neighbourhood of Algiers, and discusses them in connexion with those of other observers with very great ability. He concludes from his own observations and those of Bérard that the uniform temperature at great depths is 54°'86 F. From a consideration of the general climate of the Mediterranean, he comes to the conclusion that the temperature in the deeper layers of the sea ought to be lower than the annual mean of the surface, and that it ought to be not very different from the mean surface temperature in the winter months. From observations at Toulon and Algiers, he finds that at neither place does the surface temperature fall below 50° F., and that the mean surface temperatures in the months December, January, February, March, and April is at Toulon 53° '06 F. and at Algiers 56°'84 F. The mean of these two temperatures is 54°'9 F., which is almost exactly what he finds to be the mean annual temperature of the deepest water of the western basin. During the forty years which have elapsed since Aimé made his experiments and speculations, further observations have only tended to confirm his theory. It is true that the temperatures observed in the many soundings which have been made of late years have not shown absolute identity of temperature, and it is probable that the greater the refinement in the instruments used the more decided will the local differences appear. Especially it will be apparent that the bottom temperature varies with the climate of the preceding winter, and the distribution of temperature varies much with the prevalence of the winds. At the few stations where the temperature of the sea-water and that of the air are regularly examined, it appears that the water is generally for the greater part of the year warmer than the air, and in winter considerably so. The existing observations, however, are too few to justify any very definite statement on the subject. At Palermo the sea is warmer than tire air throughout the whole year with the exception of the months May and June. In Algiers Aimé found but little difference ; in autumn and winter the water was slightly warmer, in spring and summer slightly colder, than the air. In the eastern basin we have first Admiral Spratt's ob-servations in July 1845 in iEgina Gulf. In all his experiments made previous to the year 1860 he determined the temperature of the bottom water by taking that of the mud brought up in the dredge. This is a very excellent method ; in fact it is probably the best of all methods if a sufficient quantity of mud be obtained. From 1860 he used self-registering unprotected thermometers, which gave results necessarily too high, and it is impossible to apply any reliable correction to them without experimentally determining it on each thermometer which, was used. By the first method Admiral Spratt found 55°'5 F. at depths between 100 and 200 fathoms.

From these observations it seemed reasonable to conclude, as Aimé had done, that all over the Mediterranean a practically uni-form temperature is found at all depths greater than 100 or 200 fathoms, and that this temperature is 54° to 56° F. In order thoroughly to investigate this matter, as well as the biological con-ditions of the deep water of the Mediterranean, H.M.S. "Porcupine," Captain Calver, with Messrs Carpenter and Gwyn Jeffreys, visited the western basin of the Mediterranean in the autumn of 1870. A large number of temperature observations were made in the western basin near its southern coasts, and one sounding with temperature observation in the eastern basin a short distance from the Sicilian coast, the result of which was to confirm the conclusion arrived at from earlier observations, that, however high the temperature of the surface may be (and it may reach 90° F. ), the water becomes rapidly cooler as we go below the surface until we reach a depth of about 100 fathoms, where a temperature of 54° to 56° F. is found, and persists without sensible variation to the greatest depths. The average of all the bottom temperatures in the western basin was 54°'88 F. Three soundings were made in the intermediate basin to the eastward of Pantellaria in depths of 266, 390, and 445 fathoms, and in each case the bottom temperature was found to be 56°'5 F., or about a degree and a half warmer than in the deeper western basin. This is precisely what might have been expected from what we know of inland seas divided into several basins. In summer the shallower basin has usually a higher temperature at the bottom than is found at the same depth in the deeper one. Only one observation was made in the eastern basin, namely off Cape Passaio, in 1743 fathoms, with a bottom temperature of 56°'0 F. That the temperature in this basin should be lower than in the Pantellaria basin is due to its greater depth, and that it should be higher than is found in the western basin is due to its lower latitude. These researches were further prosecuted in the autumn of 1871 in the "Shearwater," Captain Nares, accompanied by Dr Carpenter. At two stations in the eastern busin " serial temperatures " were taken. At the first, 35° 54' N. hit., 16° 23' E. long., depth 1650 fathoms, the bottom temperature was 56°, or the same as had been observed the year before in 1743 fathoms ; at the second, 32° 17J' N. lat., 26° 44' E. long., depth 1970 fathoms, the bottom temperature was 56°'7, and the temperature at all intermediate depths was much higher than at the first station. The mean temperature of the water from the surface to a depth of 200 fathoms was, at the first station, 63°'75 F., and * the second 66°'78 F., or three degrees higher. At the first station all the temperatures down to 100 fathoms are higher than were observed in 1870 in the western basin, but it must be remembered that temperature observations made in different years cannot with justice be closely compared, as the climates of the two years are sure to differ considerably, and in the present case the difference in climate between the summers of 1870 and 1871 appears "to have been very considerable.

In the autumn of 1881 a very interesting series of observations were made by Captain Magnaghi, hydrographer of the Italian navy, and Professor Giglioli, on board the Italian surveying ship "Washington," in that part of the western basin which is enclosed between the islands Corsica and Sardinia on the one side and the Italian coasts on the other. It is here that the deepest water of the western basin was found; and, apart from the great interest attaching to the physical results obtained, the collections made with the dredge in the comparatively lifeless waters were of the very highest importance, showing, as they did, a practical identity in the abyssal fauna with that of the open ocean. This is the more remarkable as we have hitherto been accustomed to consider the similarity in the fauna of portions of the ocean remotely distant from each other as being due to the likeness of their temperatures. In the Mediterranean, however, the bottom temperature is quite 20° F. higher than is found in great depths anywhere in the open ocean.

For determining the temperature of the deep water Captain Magnaghi used the half-turn reversing thermometer of Negretti and Zambra, which in itself is a very beautiful instrument. The mechanical arrangement, however, for reversing, even as improved by Magnaghi, was not so satisfactory, and from certain irregularities in the temperature observations reported the writer is inclined to think that some of the remarkable results obtained, for instance on the 11th August, are due to this instrumental imperfection. On that day the water at 70 metres was found to have a temperature of 25°'l O, while that at 50 metres was 20°'l C, and that at 90 metres was 16°'7 C. The results obtained in the deep water are no doubt quite reliable, for the temperature is so uniform that a few fathoms more or less in. the depth at which it turned would make no difference in the temperature registered. In the more northern parts of this portion of the western basin, off the coast of Corsica, we find a practically uniform temperature from 250 metres down to the bottom in 2800 metres, the mean bottom temperature being 55°'96 F. Further to the south the temperature of the abyssal water appears to be distinctly higher. Tims between the south end of Sardinia and the Bay of Naples, in the deepest water, the practical uniformity of temperature is not reached until a depth of 1000 metres has been passed, and it is there 56°'7 F. It is un-fortunate that we do not know what the bottom temperature in other parts of the Mediterranean was. In this summer of 1881 it was quite one degree higher than that observed by Dr Carpenter in 1870.

The great value of such a volume of water as an equalizer of tem-perature on its shores must be apparent, though in this respect it is inferior to the Atlantic Ocean in its immediate neighbourhood. Places on the west coast of Spain and Portugal have a much higher winter temperature and lower summer temperature than places in the same latitude in Italy. The reason of this is simple: on the Atlantic coast the principal winds in winter are from the south-west, and have a warming effect, while in summer the source of the north-east trade wind is pushed back into the Bay of Biscay, causing in this season constant northerly winds along the coast of Portugal. The winds of the Mediterranean have no seas of remote latitudes to draw on either for heating or cooling purposes, though the sandy deserts of Africa which bound its southern coasts have at certain seasons a very decided influence on the climate. The tempering action of the sea does not extend very far inland, as is evident from the climate of inland towns in Italy. As the Mediterranean shores have so much importance as health-resorts, the data pre-sented in the following table are of interest They are taken chiefly from Theobald Fischer's Studien über das Klima der Mittelmeer-länder.

Table of mean January temperature (J.), of mean temperature of three winter months, December, January, and February (W.), also Rainfall (R.) in the same three months, for places on the Mediterranean, with those for some others for comparison.

== TABLE ==

Nature of the Bottom.—In the western basin the bottom consists chiefly of clay of a grey to brownish colour. Without doubt, when freshly collected, the surface layer is reddish-brown and the lower ones dark grey. There is always some carbonate of lime, chiefly due to Fm-aminifera. The mud very much resembles that obtained from similar depths in those parts of the open ocean whose bottom waters are shut off from free communication by ridges which may not approach within 2000 or 1500 fathoms of the surface, and with the exception of the Foraminifera it much resembles the mud from enclosed and comparatively shallow basins off the west coast of Scotland. In the following table the analyses are given of a few samples on the line of the submarine cable connecting Marseilles with Algiers.

== TABLE ==

To the student of the physical conditions of the sea the Mediter-ranean possesses a very high interest ; its size is such as to entitle it to rank among oceans, while it is so completely cut off from the remaining world of water that it presents us with a type which is purely local, and one might almost say provincial. (J. Y. B.)


Footnotes

820-1 Continental islands are those separated from the mainland by comparatively shallow seas, generally under 100 fathoms.

820-2 With regard to its appearance and disappearance Admiral Smythe (Mediterranean, p. Ill) says :—" It seems that, as early as the 28th of June 1831, Captain Swinburne, in passing nearly over the spot, felt several shocks of a sea-quake, proving that the cause was then in operation; but on the 19th of the following July the crater had accumulated to a few feet above the level of the sea, and was in great activity, emitting vast volumes of steam, ashes, and scoriae. From that time it gradually increased in all its dimensions till towards the end of August its circumference was about 3240 feet and its height 107 ; then from October various changes took place, and it entirely disappeared in December." Since that time it has changed considerably. In 1863 the least water on it was 15 feet. It has two heads close together, and at the distance of about 20 yards all round there are from 7 to 9 fathoms of water.

821-1 Proc. Roy. Soc. (1872), xx. 97, 414.

823-1 Phil. Trans., 1875, part ii. p. 601.

823-2 Ann. Chem. et Phys., 1845, xv. p. 5.



The above article was written by: J. Y. Buchanan, late of the "Challenger" expedition.



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