CURRENTS OF THE SEA. 131 



tilled water by more than four times the usual excess, and accord- 

 ingly leaves, upon evaporation, more than four times the usual 

 quantity of saline residuum. Hence it is clear that an under cur- 

 rent 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 w^ith less than 

 one fourth part of the velocity, and would thus prevent a perpet- 

 ual increase of saltness in the Mediterranean Sea beyond that ex- 

 isting in the Atlantic." 



The doctor obtained this specimen of sea water from Captain, 

 now Admiral Smyth, of the English navy, w^ho 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 w^ater, 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 con- 

 tains. 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. 



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

 Secretary of the Geographical Society of Bombay (§ 250), seem 

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

 both from the Red 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 accounted for (^ 245) by the salts of the 

 sea. 



253. Writers whose opinions are entitled to great respect differ 

 with me as to the proof 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. Marcet, be- 

 ing 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 (^ 252) taken fifty miles 

 within the Straits from the depth of six hundred and seventy 



