298 THREE CRUISES OF THE " BLAKE." 



quently ; while the carbonic acid, coming from the shores and 

 from the disintegration of the rocks, is held in solution. Were 

 it not for the constant movement of the sea, the stagnation of 

 life would soon become universal. 



The specific gravity of ocean waters, free from the effects of 

 local disturbing influences due to proximity of land, is found to 

 vary within very narrow limits, — between 1.024 and 1.028,^ 

 reduced to a standard temperature of 60° Fahrenheit, or 15.56'' 

 Centigrade. Mr. Buchanan found that the concentration of 

 the waters of the Atlantic was greater than that of either the 

 Pacific or the Southern Ocean, and greater in the North Atlantic 

 than in the South Atlantic. Below the surface the specific grav- 

 ity decreases gradually until a minimum is reached at about eight 

 hundred or one thousand fathoms,'^ and then increases slightly 

 towards the bottom, where in the South Atlantic and Pacific 

 quite a uniform specific gravity prevails.^ In the equatorial re- 

 gions the specific gra\dty first increases to a depth of from fifty 

 to one hundred fathoms, and then follows the same course as in 

 other parts of the ocean. 



In the North Atlantic, the bottom specific gravity is compar- 

 atively high. The principal causes for this concentration are 

 apparently the tradewinds, which, as has been suggested, in- 

 crease in their capacity for taking up moisture as they proceed 

 on their course from colder to warmer latitudes of the trade 

 regions in the Atlantic ; in higher temperate latitudes the oppo- 

 site effect is produced by prevaihng westerly ^vinds, which soon 

 become saturated with moisture in the warmer latitudes from 

 which they rise. A similar concentration is also brought about 

 by the formation of ice. The accumulation of salt in the 

 northern Atlantic is naturally removed by the slow flow from 

 north to south which probably takes place in the North Atlantic 

 below the depth of about one thousand fathoms. Similar condi- 

 tions undoubtedly exist in the Pacific, but it is more difficult to 

 trace them. 



^ The differences in specific gravity due sea-water is approximately compressed 



to differences of temperature are far in the ratio of 0.0009 for every hundred 



greater than those arising from the per- fathoms of depth, 

 centage of salt. 3 Qf 1.0257 to 1.02.59. 



- Buchanan's observations show that 



