THE GULF STREAM. 245 



stration of the fact that the waters of the polar regions pour 

 into the tropics along the bottom, just as the warmer equatorial 

 waters flow across the temperate zones near the surface, and 

 make their influence felt in the polar regions. 



The submarine ridges interrupt the flow of these cold polar 

 waters, and form the so-called closed basins, with a higher bot- 

 tom temperature than that of the adjoining oceanic basin. The 

 effect of such ridges upon the bottom temperature was first 

 traced by the soundings of the " Porcupine " in the North 

 Atlantic and in the Mediterranean. Subsequently, the " Chal- 

 lenger " discovered several such enclosed seas while sounding in 

 the East Indian Archipelago. 



The correctness of these results has been confirmed by the 

 Coast Survey, from soundings in the Caribbean and in the Gulf 

 of Mexico ; their bottom temperature (at a depth of over 2,000 

 fathoms) is exactly that (39i°) of the deejDest part of the ridge, 

 at about 800 fathoms, which separates them from the oceanic 

 Atlantic basin, with its temperature of 36° at the depth of 2,000 

 fathoms. 



The presence of thick layers of water having a higher bottom 

 temperatiu-e than that of adjoining areas would indicate the 

 presence of ridges isolating these warmer areas from the general 

 deep-sea oceanic circidation. A map of the Atlantic, made en- 

 tirely with reference to the temperatures, would correspond to a 

 remarkable degree with the topography of the bed of the ocean, 

 and show how and where the breaks in the continuity of the cir- 

 culation, both for the arctic and antarctic regions, occur in the 

 Atlantic. (See Figs. 140, 142.) 



It was not, however, until the Miller- Casella thermometer 

 came into general use for deep-sea investigations that a degree 

 of accuracy before unattainable in oceanic temperature became 

 possible. It soon was a well-recognized fact that as we go 

 deeper the temperature diminishes, and that at great depths the 

 temperature of the ocean is nearly that of freezing. In 1868— 

 69, in the Faeroes Channel, the " Porcupine " found a tempera- 

 ture of — 1.4° C. at a depth of 640 fathoms, and a temperature 

 of 0° C. at 300 fathoms, this being a southern extension, as was 

 subsequently found, of the deep basin of 1,800 fathoms lying 



