CUIIUENTS OF THE SEA. 189 



sea. Writers whose opinions are entitled to great respect differ 

 with me as to the conclusiveness of this demonstration. Among 

 those writers are Admiral Smj'th, of the British Kavy, and Sir 

 Charles Lj^'ell, 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 nsual alacritj' 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 (§ 385) 

 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 water, left, as we have 

 just seen, no doubt in the mind of Dr. AVollaston as to the 

 existence of this under current of brine. But the indefatigable 

 admiral, in the course of his celebrated survey of the Mediterra- 

 nean, discovered that, while inside of the Straits the depth was 

 upwards of nine hundred fathoms, j^et in the Straits themselves 

 the depth across the shoalest section is not more than one 

 hundred and sixty* fathoms. "Such being the case, we can 

 now prove," exclaims Sir Charles Lyell, " that the vast amount 

 of salt brought into the Mediten-anean does not pass out again by 

 the Straits ; for it appears by Captain Smyth's soundings, which 

 Dr. Wollaston had not seen, that between the Capes of Trafalgar 

 and Spartel, which are twent^^-two miles apart, and where the 

 Straits are shallowest, the deepest part, which is on the side of 

 Cape Spartel, is only two hundred and twenty fathoms. \ It is 

 therefore evident, that if water sinks in certain parts of the 

 Mediterranean, in consequence of the increase of its specific 

 gravity, to greater depths than two hundred and twenty fathoms, 

 it can never flow out again into the Atlantic, since it must 

 be stopped by the submarine barrier which crosses the shallowest 

 part of the Straits of Gibraltar.!" 



387. Vertical circulation in the sea a physical necessity. — Accord- 

 ing to this reasoning, all the cavities, the hollows, and the 

 valleys at the bottom of the sea, especially in the trade-wind 

 region, where evaporation is so constant and great, ought to be 

 salting up or filling up with brine. Is it probable that such a 

 process is actually going on ? No. According to this reasoning, 



* "The Mediterranean." 

 t One hundred and sixty, Smyth. 

 J Lyell's Principles of Geology, p. 334-5, ninth edition. London, 1853. 



