21G rHTSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



" Golden Age," and (,^aptain Toynbcc, of the English East India- 

 man the " Gloriana," have each returned to me valuable observa- 

 tions with this instrument. Itodgers, however, has afforded the 

 most extended series. It embraces 128" of latitude, extending from 

 71" in one hemisphere to 57^ in the other. And here I beg to 

 remark that those navigators who u.se the hydrometer systemati- 

 cally and carefully at sea are quietly enlarging for us the 

 bounds of knowledge ; and they are gleaning in our field of 

 research. These observations have already led to the discoveiy 

 of new and beneficent relations in the workshops of the sea. In 

 the physical machinery of the universe there is no compensation 

 to be found that is more exquisite or beautiful than that which, 

 by means of this little instrument, has been discovered in the sea 

 between its salts, the air, and the sun. The observations made 

 with it by Captain Eodgers, on board the U. S. ship " Yincennes," 

 have shown that the specific gravity of sea water varies but little 

 in the trade-^vind regions, notwithstanding the change of tempe- 

 rature. The temperature was a little greater in the south-east 

 trade-wind region of the I'acific ; less in the Atlantic. But, 

 though the sea at the equatorial borders of the trade-wind belt is 

 some 20° or 25° warmer than it is on the polar edge, j-et the 

 specific gravity of its waters at the two places in the Atlantic 

 difters but little. Though the temperature of the water was 

 noted, his observations on its specific gravity have not been 

 corrected for temperature. The object which the Brussels Con- 

 ference had in view when the specific gravity column was intro- 

 duced into the sea-journal was, that hydrographers might find in 

 it data for computing the dynamical force which the sea derives 

 for its currents from the difference in the specific gravitj^ of its 

 waters in different climes. The Conference held, and rightly 

 held, that a given difference as to specific gravity between the 

 water in one part of the sea and the water in another would give 

 rise to certain currents, and that the set and strength of these 

 currents would be the same, whether such difference of specific 

 gravity arose from difference of temperature or difference of salt- 

 ness, or both. 



434. Specific gravity of average sea water. — According to Eodgers' 

 observations, the average specific gravity of sea water, as it is 

 taken from the sea on the parallels of 34° north and south, at a 

 mean temperature of 64°, is just what, according 1o saline and 

 thermal laws, it ought to be ; but its specific gravity when taken 



