CURRENTS OF THE SEA. 157 



sea water, the admiral, with his usual alacrity for doing "a kind 

 turn," undertook to collect for the doctor specimens of Mediterra- 

 nean water from various depths, especially in and about the Straits 

 of Gibraltar. Among these was the one (§ 425) 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 (§ 425), 

 no doubt in the mind of Dr. Wollaston as to the existence of this 

 under current of brine. 



430. But the indefatigable admiral, in the course of his cele- 

 brated survey of the Mediterranean, discovered that, while inside 

 of the Straits the depth was upward of nine hundred fathoms, yet 

 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 Mediterra- 

 nean 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 twenty- 

 two miles apart, and where the Straits are shallowest, the deep- 

 est part, which is on the side of Cape Spartel, is only two hundred 

 and twenty fathoms. t It is therefore evident, that if water sinks 

 in certain parts of the Mediterranean, in consequence of the in- 

 crease 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. "| 



431. According 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. 



432. According to this reasoning, the water at the bottom of 

 the great American lakes ought to be salt, for the rivers and the 

 rains, it is admitted, bring salts from the land continually and 



* " The Mediterranean." + One hundred and sixty, Smyth. 



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



