566 



Dr. W. B. Carpenter on the 



[June 13, 



the Royal Society by the Hydrographer to the Admiralty, and has been 

 published in its 'Proceedings' (Jan. 18, 1872), there will be no occasion 

 for me to do more than state the results of my own Sp. Gr. determinations, 

 and compare these with the results obtained by the use of the " current- 

 drag," in such a manner as to enable every reader to judge for himself 

 whether my own conclusions are or are not justified by the facts now adduced. 

 — That the complete elucidation of the question requires a much more pro- 

 longed and systematic study than it was in our power to make on this 

 occasion, is the opinion at which we have both arrived ; and since the know- 

 ledge to be gained by such an investigation would be highly serviceable 

 to the Navigator, whilst affording most valuable data for the Scientific 

 study of Tidal and Current movements generally, I shall include in this Sec- 

 tion of my Report some suggestions for its further prosecution. 



47. It will be remembered that between Capes Trafalgar and Spartel, 

 at the western or Atlantic entrance of the Strait, is a "ridge" which con- 

 stitutes a kind of marine " watershed," looking on one side towards the 

 Atlantic, and on the other towards the Mediterranean. The depth of the 

 northern half of the channel across this section scarcely anywhere exceeds 

 50 fathoms ; whilst in its southern half the depth does not seem anywhere 

 to reach 200, and may be considered to average 150 fathoms. From this 

 "ridge" the Atlantic slope deepens gradually westwards, until, at a dis- 

 tance of about 45 miles, a depth of from 500 to 600 fathoms is reached. 

 On the other hand, the Mediterranean slope deepens gradually eastwards 

 along the whole length of the Strait (about 35 miles), as far as its embou- 

 chure in the Mediterranean between Gibraltar and Ceuta, where the depth 

 of the deepest part of the channel exceeds 500 fathoms. Thus it appears 

 that the Strait is to be considered as a prolongation of the Mediterranean 

 basin, not of that of the Atlantic. If its bottom were to be elevated 200 

 fathoms, the "ridge" would become dry land, entirely cutting off the 

 Mediterranean from the Atlantic ; but though the channel between the 

 European and the African shores would be considerably narrowed, it would 

 still extend further west than Tangier. If thus completely cut ofT from 

 the Atlantic, the Strait would be in every sense a part of the Mediterranean, 

 and would be entirely filled with the denser water of that great Inland Sea. 

 But in virtue of its communication with the Ocean outside, and of the con- 

 tinual inflow (modified by tidal changes) of a surface-current from the 

 Atlantic, the whole upper stratum of the water of the Strait has a purely 

 Atlantic character, which is as distinctly recognizable by the Specific- 

 Gravity test at the Mediterranean as at the Atlantic end of the channel. 

 On the other hand, the lower stratum was last year found no less distinctly 

 to correspond in Specific Gravity with the denser water of the Mediter- 

 ranean ; so that its presence could be recognized by this character no less 

 certainly on the summit of the " ridge " than in the deepest portion of the 

 Mediterranean embouchure. (See Report for 1870, §§ G 1—69.) 



48. I made it, therefore, my first object to ascertain whether the pre- 



