156 THE PHYSICAL GEOGRAPHY OF THE SEA. 



425. In 1828, Dr. Wollaston, in a paper before the Philosopli' 

 ical Society, stated that he found the specific gravity of a specimen 

 of sea water, from a depth of six hundred and seventy fathoms, 

 fifty miles within the Straits, to have a "density exceeding that 

 of distilled water by more than four -times the usual excess, and 

 accordingly leaves, upon evaporation, more than four times the 

 usual quantity of saline residuum. Hence it is clear that an un- 

 der current outward of such denser water, if of equal breadth and 

 depth with the current inward near the surface, would carry out 

 as much salt below as is brought in above, although it moved 

 with less than one fourth part of the velocity, and would thus pre- 

 vent a perpetual increase of saltness in the Mediterranean Sea 

 beyond that existing in the Atlantic." 



426. The doctor obtained this specimen of sea water from Cap- 

 tain, now Admiral Smyth, of the English Navy, who had collected 

 it for, Dr. Marcet. Dr. Marcet died before receiving it, and it had 

 remained in the admiral's hands some time before it came into 

 those of Wollaston. 



427. It may, therefore, have lost something by evaporation; 

 for it is difficult to conceive that all the river water, and three 

 fourths of the sea water which runs into the Mediterranean, is 

 evaporated from it, leaving a brine for the under current having 

 four times as much salt as the water at the surface of the sea 

 usually contains. Very recently, M. Coupvent des Bois is said 

 to have shown, by actual observation, the existence of an outer 

 and under current from the Mediterranean. 



428. However that may be, these facts, and the statements of the 

 Secretary of the Geographical Society of Bombay (§ 421), seem to 

 leave no room to' doubt as to the existence of an under current 

 both from the Red Sea and ]\Iediterranean, and as to the cause of 

 the surfoce current which flows into them. I think it a matter of 

 demonstration. It is accounted for (§ 413) by the salts of the sea. 



429. Writers whose opinions are entitled to great respect differ 

 with me as to the conclusiveness of this demonstration. Among 

 these writers are Admiral Smyth, of the British Navy, and Sir 

 Charles Lyell, who also differ with each other. In 1820, Dr. Mar- 

 cet, being then engaged in studying the chemical composition of 



