184 THE PHYSICAL GEOGllAPHY OF THE SEA. 



have a " density exceeding that of distilled water by more than 

 four times the usual excess, and accordingly leaves, upon evapo- 

 ration, more than four times the usual quantity of saline residuum. 

 Hence it is clear that an under 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 prevent a perpetual increase of saltness 

 in the Mediteraanean Sea beyond that existing in the Atlantic." 

 The doctor obtained this specimen of sea water from Captain, 

 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. 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 Mediter- 

 ranean, is evaporated from it, leaving a brine for the under cur- 

 rent having /our times as much salt as the water at the surface of 

 the sea usually contains. Yery recently, M. Coupvent des Bois is 

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

 outer and under current from the Mediterranean. 



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

 The escape of salt the Secretary of the Geographical Society of Bom- 



and heavy water by - .^ cc^c\\ ^ i x J t,i. x 



undercurrents. bay (§ 382), sccm to Icavc no room to doubt as to 

 the existence of an under current both from the Ked Sea and 

 Mediterranean, and as to the cause of the surface current which 

 flows into them. I think it a matter of demonstration. It is ac- 

 counted for (§ 877) by the salts of the sea. Writers whose opin- 

 ions are entitled to great respect differ with me as to the conclu- 

 siveness of this demonstration. Among those writers are Admiral 

 Smyth, of the British Navy, and Sir Charles Lyell, who also differ 

 with each other. In 1820, Dr. Marcet, being then engaged in 

 studying the chemical composition of sea water, the admiral, with 

 his usual alacrity for doing " a kind turn," undertook to collect 

 for the doctor specimens of Mediterranean water from various 

 depths, especially in and about the Straits of Gibraltar. Among 

 these was the one (§ 885) taken fifty miles within the Straits from 

 the depth of six hundred and seventy fathoms (four thousand and 

 twenty feet), which, being four times Salter than common sea wa- 



