20 THE PHYSICAL GEOGRAPHY OF THE SEA. 



Polar waters ; also of the dark blue of the trade-wind regions, and 

 especially of the Indian Ocean, which poets have described as the 

 ^ black waters." 



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

 Speculations con- philosophcrs. Many are the theories and numer- 

 stream, ' ous the spcculatious 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 

 entirely clear. But they seem to encourage the opinion that this 

 stream, as well as all the coiistant currents of the sea, is due main- 

 hj 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 equilib- 

 rium, 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 currents. 

 Agencies concerned. The wiuds lielj) to make currcnts 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 

 currents that were the consequence also 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 course, and in their 

 effects they are as endless. Philosophy points to them as the chief 

 cause of the Gulf Stream and of all the constant currents of the sea. 



74. Early writers, however, maintained that the Mississippi 

 Early writers. Rivcr was the father of the Gulf Stream. Its floods, 



they ^id, produce it ; for the velocity of this river in the sea 

 might, it was held, be computed by the rate of the current of the 

 river on the land. 



