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 (393°) of the deepest 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 
temperature than that of adjoining areas would indicate the 
presence of ridges isolating these warmer areas from the general 
deep-sea oceanic circulation. 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 Færöes Channel, the “ Porcupine” found a tempera- 
ture of — 1.4? C. at а 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 
