26 THE PHYSICAL GEOGRAPHY OF THE SEA. 



3. At the salt-works in France, and along the shores of the 

 Adriatic, where the ^^ salines" are carried on by the process of so- 

 lar evaporation, there is a series of vats or pools through which 

 the water is passed as it comes froni the sea, and is reduced to 

 the hrinj state. The longer it is exposed to evaporation, the Salter 

 it grows, and the deeper is the hue of its blue, until crystallization 

 is about to commence, when the now deep blue water puts on a 

 reddish tint. Now the waters of the Gulf Stream are Salter (§ 29) 

 than the waters of the sea through which they flow, and hence we 

 can account for the deep indigo blue w^hich all navigators observe 

 off the Carolina coasts. 



4. These salt-makers are in the habit of judging of the richness 

 of the sea- water in salt by its color — the greener the hue, the fresh- 

 er 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 the trade-wind re- 

 gions, and especially of the Indian Ocean, which poets have de- 

 scribed as the "black waters." 



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

 philosophers. Many are the theories and numerous the specula- 

 tions that have been advanced with regard to it. Modern inves- 

 tigations and examinations are beginning to throw some light upon 

 the subject, though all is not yet clear. 



Early v/riters maintained that the Mississippi River was the 

 father of the Gulf Stream. Its floods, they said, produce it ; for 

 its velocity, it was held, could be computed by the rate of the cur- 

 rent of the river. 



6. Captain Livingston overturned this hypothesis by showing 

 that the volume of water which the Mississippi E-iver empties into 

 the Gulf of Mexico is not equal to the three thousandth part of 

 that which escapes from it tlu'ough the Gulf Stream. 



Moreover, the water of the Gulf Stream is salt — that of the 

 Mississippi, fresh ; and those philosophers (§ 5) forgot that just as 

 much salt as escapes from the Gulf of Mexico through this stream, 

 must enter the Gulf through some other channel from the main 

 ocean ; for, if it did not, the Gulf of Mexico, in process of time, 



