Page:Physical Geography of the Sea and its Meteorology.djvu/206

180 dependent for their well-being upon temperature, as are the fauna and the flora of the dry land. Were it not so we should find the fish and the algae, the marine insect and the coral, distributed equally and alike in all parts of the ocean. The arctic whale would delight in the torrid zone, and the habitat of the pearl oyster would be also under the iceberg, or in the frigid waters of polar seas.

372. Those of southern unlike those of northern seas.—Nevertheless, though the constituents of sea water be the same in kind, we must not infer that they are the same in degree throughout all parts of the ocean, for there is a peculiarity, perhaps of temperature, perhaps of transparency, which marks the inhabitants of trans-equatorial seas. MM. Peron and Le Sueur, who have turned their attention to the subject, assert that out of many thousand examples they did not find a single one in which the inhabitants of trans-equatorial were not distinguishable from, those of their species in cis-equatorial seas.

373. The capacity of water to convey heat.—Water, while its capacities for heat are scarcely exceeded by those of any other substance, is one of the most complete of non-conductors. Heat does not permeate water as it does iron, for instance, or other good conductors. Heat the top of an iron plate, and the bottom becomes warm; but heat the top of a sheet of water, as in a pool or basin, and that at the bottom remains cool. The heat passes through iron by conduction, but to get through water it requires to be conveyed by a motion, which in fluids we call currents. Therefore the study of the climates of the sea involves a knowledge of its currents, both cold and warm. They are the channels through which the waters circulate, and by means of which the harmonies of old ocean are preserved.

374. Currents of the sea to be considered in pairs.—Hence, in studying the system of oceanic circulation, we set out with the very simple assumption, viz., that from whatever part of the ocean a current is found to run, to the same part a current of equal volume is bound to return; for upon this principle is based the whole system of currents and counter-currents of the air as well as of the water. Hence, the advantage of considering them as the anatomist does the nerves of the human system—in pairs. Currents of water, like currents of air, meeting from various directions, create gyrations, which in some parts of the sea, as on the coast of Norway, assume the appearance of whirlpools, as