Page:Physical Geography of the Sea and its Meteorology.djvu/49

Rh Carolina coasts. The salt-makers are in the habit of judging of the richness of sea-water in salt by its colour—the greener the hue the fresher the water. We have in this, perhaps, an explanation of the contrasts which the waters of the Gulf Stream present with those of the Atlantic, as well as of the light green of the North Sea and other Polar waters; also of the dark blue of intertropical seas, and especially of the Indian Ocean, which poets have described as the "black waters." Seamen who visit the Falls of Niagara never fail to remark upon the beautiful green of the water in the river below, and to contrast it with the dark blue of the sea in the trade-wind regions.

72. Speculations concerning the Gulf Stream.—What is the cause of the Gulf Stream has always puzzled philosophers. Many are the theories and numerous the speculations that have been advanced with regard to it. Modern investigations and examinations are beginning to throw some light upon the subject, though all is not yet entirely clear. But they seem to encourage the opinion that this stream, as well as all the constant currents of the sea, is due mainly to the constant difference produced by temperature and saltness in the specific gravity of water in certain parts of the ocean. Such difference of specific gravity is inconsistent with aqueous equilibrium, and to maintain this equilibrium these great currents are set in motion. The agents which derange equilibrium in the waters of the sea, by altering specific gravity, reach from the equator to the poles, and in their operations they are as ceaseless as heat and cold; consequently they call for a system of perpetual currents to undo their perpetual work.

73. Agencies concerned.—These agents, however, are not the sole cause of currents. The winds help to make currents by pressing upon the waves and drifting before them the water of the sea; so do the rains, by raising its level here and there; and so does the atmosphere, by pressing with more or less superincumbent force upon different parts of the ocean at the same moment, and as indicated by the changes of the barometric column. But when the winds and the rains cease, and the barometer is stationary, the currents that were the consequence cease. The currents thus created are therefore ephemeral. But the changes of temperature and of saltness, and the work of other agents which affect the specific gravity of sea-water and derange its equilibrium, are as ceaseless in their operations as the sun in his