22 PHYSICAL GEOGEAPHY OF THE SEA, AND ITS METEOBOLOGY. 



-vyIiicIi it flows, and hence we can account for tlie deep indigo 

 blue which all navigators observe in Gulf Stream water off the 

 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 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. What is the cause of the Gulf Stream has always puzzled 

 Speculations con- philoso j)hers . Many are the theories and nume- 

 s'iream. rous the speculatious that have been advanced with 

 regard to it. Modern investigations and examinations are begin- 

 ning to throw some light upon the subject, though all is not yet 

 entkely clear. But they seem to encom^age the opinion that this 

 stream, as well as all the constant cm^rents 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 equili- 

 brium, 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. These agents, however, are not the sole cause of cmTents. 

 Agencies concerned. The wiuds liclp to make currouts 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 at- 

 mosphere, by pressing with more or less superincumbent force 

 upon different parts of the ocean at the same moment, and as in- 

 dicated by the changes of the barometric column. But when the 

 winds and the rains cease, and the barometer is stationary, the 

 cmTents that were the consequence cease. The currents thus 

 created are therefore ephemeral. But the changes of temperatm-e 

 and of saltness, and the work of other agents Avhich affect the 

 specific gravity of sea-water and derange its equilibrium, are as 



