"Golden Age," and Captain Toynbee, of the English East Indiaman the "Gloriana," have each returned to me valuable observations with this instrument. Rodgers, 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 use the hydrometer systematically 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 discovery 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 Rodgers, on board the U. S. ship "Vincennes," have shown that the specific gravity of sea water varies but little in the trade-wind regions, notwithstanding the change of temperature. The temperature was a little greater in the south-east trade-wind region of the Pacific; 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, yet the specific gravity of its waters at the two places in the Atlantic differs 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 Conference had in view when the specific gravity column was introduced 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 gravity 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 saltness, or both.
434. Specific gravity of average sea water.—According to Rodgers' 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 to saline and thermal laws, it ought to be; but its specific gravity when taken
