§ iVA, 43.-,. THE SPECIFIC GRAVITY OF THE SEA, ETC. 213 



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-wind regions, notwithstand- 

 ing the change of temperature. The temperature was a little 

 greater in the southeast 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 hjdrographers 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 right- 

 ly 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. According to Eodgers' observations, the average specific 

 Specific gravity of gravity of sca water, as it is taken from the sea on 

 average sea water. ^^^ parallels of 34° north and south, at a mean tem- 

 perature of 64°, is just what, according to thermal laws, it ought 

 to be ; but its specific gravity when taken from the equator, at a 

 mean temperature of 81°, is much greater than, according to the 

 same laws, it ought to be. The observed difference of its specific 

 gravity at 64° and 81° is .0015 ; whereas, according to thermal 

 laws, it ought to be .0025, or 67 per cent, greater than it actually 

 is. What makes this difference ? Let us inquire : 



435. The anomaly is in the trade-wind region, and is best de- 

 An anomaly, vclopcd {Fig. 2, page 214) in the North Atlantic, be- 

 tween the parallel of 40° and the equator. Though it is suffi- 

 ciently apparent both in the North and South Pacific {Fig. 1) — it 

 is marked by the Gulf Stream in the North Atlantic — commenc- 

 ing at the polar borders of these winds, the anomaly is develop- 

 ed as you approach the equator. The water grows warmer, but 



