Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/855

Rh MEDITERRANEAN 823 we are not informed whether the self-registering instruments used were protected from pressure or not. Mr Prestwich, 1 however, who has collected and critically discussed all the older deep-sea tempera ture observations, concludes, from a comparison of their results with those obtained by Aime 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 547 at the same depth, and the same temperature (547) in 530 fathoms. Berard, 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. Aime 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 Berard that the uniform temperature at great depths is 54 S6_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 lie finds to be the mean annual temperature of the deepest water of the western basin. During the forty years which have elapsed since Aime 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 the air throughout the whole year with the exception of the months May and June. In Algiers Aime 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 1815 in /Egina 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 &quot;5 F. at depths between 100 and 200 fathoms. From these observations it seemed reasonable to conclude, as Aime 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. &quot; Porcupine,&quot; 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 1 Phil. Trans,, 1875, part ii. p. G01. 2 Ann. Chcm. et P 7 iys., 1845, xv. p. 5. 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 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. Those researches were further prosecuted in the autumn of 1871 in the &quot;Shearwater,&quot; Captain Nares, accompanied by Dr Carpenter. At two stations in the eastern basin &quot;serial temperatures&quot; 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 17| 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 6678 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 bo 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 &quot;Washington,&quot; 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 ab} r ssal 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 Negrctti 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 llth August, are due to this instrumental imperfection. On that day the water at 70 metres was found to have a temperature of 25 1 C., while that at 50 metres was 20 1 C., and that at 90 metres was 167 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. Thus 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 iibcr das Klima dcr Milicl nicer- lander.