Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/35

Rh established : Of the water which fills the deep trough of the North Atlantic (fig. 1) between Teneriffe (lat. 28 u N.) and St Thomas (lat. 18 N.), divided by the &quot;Dolphin rise into an eastern and western basin, by far the larger

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mass has a temperature ranging from 40 downwards, in the eastern basin, to a bottom-temperature of 35^, whilst in the western basin apparently under the influence of the Antarctic underflow the bottom-temperature sinks to 34 4. A tolerably regular descent is shown in this sec tion, from a surface-temperature rising near St Thomas to 75, to the bathymetrical isotherm of 45, which lies be tween 400 and COO fathoms depth ; there is then a stratum between 45 and 40, of which the thickness varies from about 250 to 450 fathoms, the isotherm of 40 lying at between 750 and 1000 fathoms depth, while below this, down to the bottom at between 2000 and 3000 fathoms, the further reduction to 34 4 is very gradual. The same general condition prevails in the South Atlantic (fig. 2), between Abrolhos Island (lat. 18 S.) on the coast of Brazil, and the Cape of Good Hope (lat. 34-J- S.), this trough also being divided into two basins by the elevation of the bottom which culminates in the island of Tristan da Cunha. The temperature of the water that occupies it, however, is lower through its whole vertical range than that of the North Atlantic. The stratification is nearly uniform from the surface downwards to the isotherm of 40, which lies at from 300 to 450 fathoms depth, the isotherms of 39 and 38 also lying within about 500 fathoms; there is then a slower reduction down to the isotherm of 35, which lies between 1400 and 1800 fathoms; while the whole sea-bed is covered by a stratum of about 600 fathoms thickness, whose tempera ture ranges downwards from 35 to 33. The whole of this deepest stratum is colder than any water that is found in the corresponding portion of the North Atlantic, except near St Thomas.

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It is not a little remarkable that the sub-surface stratum of water, having a temperature above 40, is thinner under the equator than it is in any other part of the Atlantic from the Faroe Islands to the Cape of Good Hope. Not withstanding the rise of the surface-temperature to 76-80, the thermometer descends in the first 300 fathoms more rapidly than anywhere else; so that polar water is met with, as shown in fig. 3, at a much less depth than in the North Atlantic (fig. 1), and 100 fathoms nearer to the surface than even in the colder South Atlantic (fig. 2) ; whilst the temperature of the bottom is but little above 32. Thus the influence of the polar underflow is more pronounced under the equator than it is elsewhere ; as is distinctly seen in the section shown in fig. 4, which is taken in a north and south direction so as to exhibit the relation of the thermal stratification of the North to that 