182 PHYSICAL GEOGKAPHY OF THE SEA, AND ITS METEOROLOGY. 



fathoms. " Such being the case, we can now prove," exclaims Sir 

 Charles Lyell, "that the vast amount of salt brought into the 

 Mediterranean 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 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 gTavity, 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. According to this reasoning, all the cavities, the hollows, 

 yertjcai circulation and tho vallevs at the bottom of the sea, especially 



in the sea a physical - n i -^ - i • i i-- 



necessity. m the trade- wmd region, where evaporation is so 



constant and great, ought to be salting up or fiUing up with brine. 

 Is it probable that such a process is actually going on ? No. 

 According to this reasoning, the water at the bottom of the great 

 American lakes ought to remain there for ever, for the bottom of 

 Erie is far below the barrier which separates this lake from the 

 Falls of Niagara, and so is the bottom of every one of the lakes 

 bellow the shallows in the straits or rivers that connect them as a 

 chain. We may presume that the water at the bottom of every 

 extensive and quiet sheet of water, whether salt or fresh, is at the 

 bottom by reason of specific gravity ; but that it does not remain 

 there for ever we have abundant proof. If so the Niagara Eiver 

 would be fed by Lake Erie only from that layer of water which is 

 above the level of the top of the rock at the Falls. Consequently, 

 wherever the breadth of that river is no gTeater than it is at the 

 Falls, we should have a cm-rent as rapid as it is at the moment of 

 passing the top of the rock to make the leap. To see that such is 

 not the way of Nature, we have but to look at any common miU- 

 pond when the water is running over the dam. The current in 

 the pond that feeds the overflow is scarcely perceptible, for " still 

 water runs deep." Moreover, we know it is not such a skimming 

 cmTent as the geologist would make, which runs fr'om one lake to 

 another ; for wherever above the Niagara Falls the water is 

 deep, there we are sure to find the cmTent sluggish, in compa- 



* One hundred and sixty, Smyth. 



t Lyell's Prmciples of Geology, p. 334-5, ninth edition. London, 1853. 



