396 THE PHYSICAL GEOGRAPHY OF THE SEA. 



742. The object of Plate IX., therefore, is to illustrate, as far as 

 Plate IX- the present state of my researches enable me to do, 

 the circulation of the ocean as influenced by heat and cold, and to 

 indicate, on one hand, the routes by which, the overheated waters 

 of the torrid zone escape to cooler regions, and to point out, on 

 the other, the great channel- ways through which the same waters, 

 after having been deprived of this heat in the extra-tropical or 

 polar regions, return again toward the equator ; it being assumed 

 that the drift or flow is from the poles when the temperature of 

 the surface water is helow, and from the equatorial regions when 

 it is above that due the latitude. Therefore, in a mere diagram, 

 as this plate is, the numerous eddies and local currents which are 

 found at sea are disregarded. Of all the currents in the sea, the 

 Gulf Stream is the best defined ; its limits, especially those of the 

 left bank, are always well marked, and, as a rule, those of the 

 right bank, as high as the parallel of the thirty-fifth degree of 

 latitude, are quite distinct, being often visible to the eye. The 

 Gulf Stream shifts its channel (§ 124), but nevertheless its banks 

 are often very distinct. Ships, in crossing the edges of it, can 

 sometimes know it by the color of the water ; at other times they 

 find, as they pass along, the temperature of the water to change 

 8° or 10° in the course of as many minutes; as an example of 

 this, I quote from the abstract log of the " Herculean," in which 

 Captain William M. Chamberlain, being in latitude 33° 39' north, 

 longitude 74° bQ' west (about one hundred and thirty miles east 

 of Cape Fear), remarks : " Moderate breezes, smooth sea, and fine 

 weather. At ten o'clock fifty minutes, entered into the southern 

 (right) edge of the Stream, and in eight minutes the water rose 

 six degrees ; the edge of the stream was visible, as far as the eye 

 could see, by the great rippling and large quantities of Gulf weed 

 — more ' weed' than I ever saw before, and I have been many 

 times along this route in the last twenty years." In this diagram, 

 therefore, I have thought it useless to attempt a delineation of 

 any of those currents, as the Eennell Current of the North Atlan- 

 tic, the "connecting current" of the South, "Mentor's Counter 

 Drift," " Eossel's Drift of the South Pacific," etc., which run now 

 this way, now that, and which are frequently not felt by naviga- 

 tors at all. In overhauling tlie log-books for data for this chart, 

 I have followed vessels with the water thermometer to and fro 



