Page:Popular Science Monthly Volume 43.djvu/55

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45
OUR KNOWLEDGE OF THE DEEP SEA.

shallower waters near coasts, but even in the greater depths of all oceans, animal life is exceedingly abundant. A trawling in a depth of over a mile yielded two hundred specimens of animals belonging to seventy-nine species and fifty-five genera. A trawling in a depth of about three miles yielded over fifty specimens belonging to twenty-seven species and twenty-five genera. Even in depths of four miles fishes and animals belonging to all the chief invertebrate groups have been procured, and in a sample of ooze from nearly five miles and a quarter there was evidence to the naturalists of the Challenger that living creatures could exist at that depth.

Recent oceanographic researches have also established beyond doubt that while in great depths the water is not subjected to the influence of superficial movements like waves, tides, and swift currents, there is an extremely slow movement, in striking contrast with the agitation of the surface water. Although the movement at the bottom is so slow that the ordinary means of measuring currents can not be applied accurately to them, the thermometer furnishes an indirect means of ascertaining their existence. Water is a very bad conductor of heat, and consequently a body of water at a given temperature passing into a region where the temperature conditions are different retains for a long time, and without much change, its original temperature. To illustrate: The bottom temperature near Fernando do Noronha, almost under the equator, is 0·2° C., or close upon the freezing point; it is obvious that this temperature was not acquired at the equator, where the mean annual temperature of the surface layer of the water is 21° C, and the mean normal temperature of the crust of the earth not lower than 8° C. The water must therefore have come from a place where the conditions were such as to give it a freezing temperature; and not only must it have come from such a place, but the supply must be continually renewed, however slowly, for otherwise its temperature would gradually rise by conduction and mixture. Across the whole of the North Atlantic the bottom temperature is considerably higher, so that the cold water can not be coming from that direction; on the other hand, we can trace a band of water at a like temperature at nearly the same depth continuously to the Antarctic Sea, where the conditions are normally such as to impart to it this low temperature. There seems, therefore, to be no doubt that there is a current from the antarctic to the equator along the bottom of the South Atlantic.

From the millions of reliable deep-sea soundings that have been made during the last forty years the more general features of the bathymetric chart of the world have been firmly established; and the ancient idea, derived chiefly from a supposed