574 



Dr. W. B. Carpenter on the 



[June 13, 



in the upper current, it is decidedly onwards in the wwcfer-currcnt 

 (§§ 53,56). 



II. The upper stratum here always consists of nearly unmixed Atlantic 

 water ; showing that the westerly flow which takes place during the ebb 

 is insufficient to bring Mediterranean water from the opposite end of the 

 Strait. The increase sometimes observable (§§ 51, 54) in the Sp. Gr. of 

 the surface-water of the Strait appears attributable rather to wind-drift 

 than to tidal movement. 



III. On the other hand, the wider stratum here (as at the opposite end 

 cf the Strait) either consists entirely of Mediterranean water, or shows a 

 large admixture of it ; and as this is the case alike during the mward and 

 the onward movement of the under stratum, it seems obvious that Medi- 

 terranean water must extend some distance to the westward, — since, if it 

 merely came up to the " ridge " without crossing it, the tidal inflow 

 would replace it by Atlantic water. And this inference is confirmed by 

 the fact that an admixture of Mediterranean water is distinctly traceable 

 far into the Atlantic basin (§ 49). 



IV. The stratum of Mediterranean water which is found at a depth 

 between 100 and 125 fathoms on the "ridge" (§ 52) must be derived 

 from the stratum which, in the deeper part of the Strait (extending west- 

 ward as far as Tarifa), lies at more than 200 fathoms from the surface. 

 Hence the lower or Mediterranean stratum must form an inclined plane 

 from Tarifa to the " ridge," the surface of which is about 100 fathoms 

 higher at its western than at its eastern end ; and as the distance between 



these two points is about 15 miles, the gradient will be about ^^^|j^ S 



or about 1 in 132. But it must be remembered that we have not here to 

 do with the absolute weight of this body of water, but only with the 

 difference in Sp. Gr. between water of (say) 1*027 and water of 1*029; 

 which is less than 1 -500th part of the absolute weight of the water which 

 thus runs up-hill. And with the evidence we have in the Florida Channel 

 (§§ 129-133) of the passage of the denser because colder water over the 

 shallowest part of the Narrows, there is no improbability in the like 

 flow of the denser because more saline water over the marine water- 

 shed which separates the Mediterranean from the Atlantic basin, if 

 there be a predominant westerly movement in the under- stratum. But 

 if there were no such movement, — the under-stratum being either at 

 rest, or its flux and reflux being equal, — there seems no reason why this 

 inclination of its surface should be maintained against the operation of 

 Gravity, which will be constantly tending to bring it to a level. 



59. I venture, therefore, still to maintain that the existence of a Tidal 

 flux and reflux in the Gibraltar currents does not prove that they are 

 entirely sustained by tidal action ; any more than the existence of an 

 alternate ebb and flow in a river proves that there is no down-flowing 

 stream. In every tidal river, the ebb is stronger on the whole than the 



