Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/33

Rh parallels of 20 and 30 S., and the meridians of and 25 W., over which there are no regular currents ; and to this the name Sargasso Sea is sometimes applied by analogy, although its surface has no covering of sea-weed. (See Plate I.)

Temperature of the Atlantic. The distribution of surface temperature over the area of the Atlantic has now been made out with considerable accuracy ; and it corresponds closely with what has been already stated as the course of the surface currents. There is, of course, a seasonal change, alike in its northern and in its southern division, this change being more and more marked as we recede from the equator. Following the course of the mean annual isotherms, however, we find that they cross the South Atlantic at nearly regular intervals, in an east and west direction, the principal departure from that direction being shown at their western end in the bend they take towards the south under the influence of the warm Brazil Current, and at their eastern in the still stronger bend they take towards the north under the influence of the cold South African Current, which reduces to about 75 the temperature of the southern equatorial that flows alongside the Guinea Current, whose temperature is 82. In the North Atlantic, however, the influence of the movement of oceanic water on the surface-temperature is very much more marked. The annual isotherms, which cross the Sargasso Sea with nearly regular parallelism, and on the African side tend somewhat to the south, where they meet the colder water of the North African Current, show a strong northward bend on the American side, along the early course of the Gulf Stream ; but as its excess of temperature above that of the Atlantic generally diminishes as we trace it towards the Banks of Newfoundland, this northward deflection progres sively becomes less. The marked contrast in temperature which is often there exhibited between two contiguous bands of water, a thermometer hanging from a ship s bow show ing a temperature of 70, whilst another hanging from the stern shows only 40, is due not so much to the elevation produced by the Gulf Stream as to the depression produced by the Arctic Current. This depression manifests itself in the southward bend given, on the American side, alike to the summer and the winter isotherms (see Plate), beyond the summer isotherm of 70 and the winter isotherm of 60, which may be considered as having nearly their normal position; whilst the northward tendency of these same isotherms on the European side not less conspicuously indicates a flow of warm water towards the western coasts of the British Isles, Norway, and even Iceland and Spitzbergen. It has been customary to regard this flow as an extension of the Gulf Stream ; but if that term be limited (as it ought) to the current that issues from the Gulf of Mexico through the Florida Channel, the hypothesis is found to be untenable so soon as the thermal phenomena of that current are carefully examined. For, in the first place, the popular idea that the Gulf Stream retains its high temperature with little diminution during its passage first northwards and then eastwards is clearly disproved by observation, as is shown by the following table of average temperatures taken at different seasons in the warmest of its bands : 3 &quot;a _&quot; ti t/T Summer Florida Channel 25 N. 77 7S 83 82 Off Charlestown 33 N. 75 77 82 81 Off Cape Hatteras 35 N. 72 73 80 76 S.E. of Nantucket Shoals S. of Nova Scotia 35 X. 35 N. 67 62 68 67 80 78 72 69 From this it appeal s that, while the high surface-tempera ture with which the Gulf Stream leaves the Florida Channel is retained in summer with only 5 reduction as far as Nova Scotia, there is a reduction of 5 in winter during its north ward passage to Cape Hatteras, and a further reduction of no less than 10 during its eastward passage from Cape Hatteras to Nova Scotia, making a total reduction of 15. In spring, again, there is a total reduction of 11, and in autumn of 13; and in both cases the reduction during the eastward flow under the parallel of 35 N. is greater than the reduction in the northward flow from 25 N. to 35 N. The explanation of this is plainly to be found in the fact that in the early part of the course of the Gulf Stream its superheated stratum is a thick one, so that when its superficial film is cooled down by a superincumbent atmosphere of lower temperature, it is replaced by the uprising of a deeper stratum having nearly its original temperature. But as the stream spreads out superficially, its superheated stratum becomes proportionally thinner, and will consequently be more and more rapidly cooled down by the superincumbent atmosphere. Even supposing, therefore, that it were not subjected to any special cooling influence, it appears certain that, as the rate of the current slackens and its depth diminishes, the cooling process must continue at an increased rate, so as to bring down the surface-temperature of the stream to the normal isotherm of the locality, long before it could reach the shores of Europe. But it has been shown that when it passes Newfoundland the Gulf Stream is subjected to a special cooling influence that of the Labrador Current with its fleet of icebergs, which melt away when borne into it ; and this produces such an immediate reduction of its surface- temperature, that it thenceforth shows very little excess, although its sub-surface stratum still appears to be warmer than that of the ocean through which it flows. But, further, the Gulf Stream, where it is last recog nisable as a current, is flowing due east, and its southern portion turns first south-east and then south, whilst, on the other hand, the course of the isothermal lines (see Plate) clearly shows that the flow of warm water which carries them northward spreads across the whole breadth of the Atlantic, from the British Isles to Labrador, even extending up to the west of north into Baffin s Bay. When we contrast this immense body of north-moving water with the thinned-out film of what is by comparison a mere rivulet, it becomes obvious (1) that its northward flow cannot be attributable to the vis a tergo of the Florida Current, whilst (2) its convection of heat to the Arctic Sea cannot be accounted for by any amount of excess of temperature that is limited to a small depth, since the temperature of such a stratum, moving north-east at a rate of (at most) 4 or 5 miles per day, must soon be brought down to that of the atmosphere above it. Influenced by these considerations, several eminent hydrographers, both British and American, have been disposed to deny, not only that the temperature of the North Atlantic is modified in any considerable degree by the true Gulf Stream, but that any other agency than that of warm S.W. winds is concerned in producing the climatic amelioration popularly attributed to it. They maintained, in fact, that the surface-temperature of the North Atlantic and Arctic Seas follows that of the superincumbent air, the atmospheric temperature not being in any degree raised by that of warmer water beneath. This doctrine, however, is found to be inconsistent with the results of careful com parisons recently instituted between marine and atmospheric temperatures along the western coasts of Scotland, the Orkney, Shetland, and Faroe Islands, and especially with those obtained along the western coast of Norway. For it is found that during the winter months there is a constant excess of sea-temperature above that of the air, averaging 