THE GULF STREAM. 23 



Carolina coasts. The salt-makers are in the habit of jiidgin"- 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 inter- 

 tropical 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 alwaj^s puzzled philosophers. Many are 

 the theories and numerous the speculations that have been 

 advanced with regard to it. Modern investigations and examina- 

 tions 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 tempe- 

 rature and saltness in the specific gravity of water in certain 

 parts of the ocean. Such difference of specific gravity is incon- 

 sistent with aqueous equilibrium, and to maintain this equi- 

 librium 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 per 

 petual work. 



73. Agencies concerned. — These agents, however, are not the 

 sole cause of currents. The winds help to make currents by press- 

 ing 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 superincum- 

 bent 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 sta- 

 tionary, 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 ageuts which 

 affect the specific gravity of sea-water and derange its equili- 

 brium, are as ceaseless in their operations as the sun in his 



