Page:Physical Geography of the Sea and its Meteorology.djvu/61

 Rh marvel is, that they in their violence do not, by mingling the Gulf and littoral waters together (§ 70), sooner break up and obliterate all marks of a division between them. But the waters of the Gulf Stream differ from the inshore waters not only in colour, transparency, and temperature, but in specific gravity, in saltness (§ 102), and in other properties, I conjecture, also. Therefore they may have a peculiar viscosity, or molecular arrangement of their own, which further tends to prevent mixture, and so preserve their line of demarkation.

101. Action on copper.—Observations made for the purpose in the navy show that ships cruising in the West Indies suffer in their copper sheathing more than they do in any other seas. This would indicate that the waters of the Caribbean Sea and Gulf of Mexico, from which the Gulf Stream is fed, have some peculiar property or other which makes them so destructive upon the copper of cruisers.

102. Saltness of the Gulf Stream.—The story told by the copper and the blue colour (§ 71) indicates a higher point of saturation with salts than sea-water generally has; and the salometer confirms it. Dr. Thomassy, a French savant, who has been extensively engaged in the manufacture of salt by solar evaporation, informs me that on his passage to the United States he tried the saltness of the water with a most delicate instrument: he found it in the Bay of Biscay to contain 3½ per cent, of salt; in the trade-wind region 4 $$\scriptstyle\frac{4}{10}$$ per cent.; and in the Gulf Stream, off Charleston, 4 per cent., notwithstanding the Amazon and the Mississippi, with all the intermediate rivers, and the clouds of the West Indies, had lent their fresh water to dilute the saltness of this basin.

103. Agents concerned.—Now the question may be asked. What should make the waters of the Mexican Gulf and Caribbean Sea salter than the waters in those parts of the ocean through which the Gulf Stream flows? There are physical agents that are known to be at work in different parts of the ocean, the tendency of which is to make the waters in one part of the ocean Salter and heavier, and in another part lighter and less salt than the average of sea-water. These agents are those employed by sea-shells in secreting solid matter for their structures; they are also heat and radiation, evaporation and precipitation. In the