546 



Captain Spratt on the Undercurrent 



[June 15, 



The foregoing quotations, and recapitulations of arguments and reasons 

 from ' Researches in Crete/ which, in my humble judgment and experi- 

 ence, seemed to be sound, in opposition to the undercurrent theory as a 

 grand circulation, and general and appreciable as a fact, I again offer in 

 concluding these remarks, but with all due deference and diffidence, al- 

 though I am strongly of opinion still, from my practical experience and 

 investigations regarding surface- and deep-water currents, that differences 

 of density and of level are more generally rectified by superficial and 

 littoral movements, than by undercurrents running up hill or burrowing 

 in mid-deeps. But if recognizable or measurable as a physical fact any- 

 where, it is only local and not universal, and is merely an atomic inter- 

 change of insensible amount in general, in the greater depths of the ocean 

 or of inland seas. 



On the Gibraltar Undercurrent. 



There are also strong reasons for inducing me to dissent from Dr. Car- 

 penter's conclusions as proofs of the undercurrent he asserts to having 

 found indisputable proofs of in the Gibraltar Straits ; for to my mind, on 

 carefully considering the observations, as well as the means employed, and 

 circumstances at the one trial (Station 64) which was accepted as an 

 undoubted evidence of such an undercurrent, against the four others that 

 showed no such result, there does not appear to be just grounds for assert- 

 ing that it really exists, as a positive result of the trials ; for in such a 

 question of science the fact should be free of any ground of doubt. 



In such a Strait as that of Gibraltar, however, where there are tidal in- 

 fluences combined with the general inset from the Atlantic, an under- 

 current at certain times is a possibility ; but, with all due deference to Dr. 

 Carpenter, I cannot agree with him in inferring from the single and, to 

 my experience, unsatisfactory result obtained at Station 64, " that a strong 

 presumption may be fairly raised for the constant existence of such a 

 return-current, though its force and amount are liable to variation," when 

 the results of his four other trials, viz. two at and near Station 39, and 

 one at 65 & 66, showed no undercurrent, the former being in the nar- 

 rowest part of the Strait, and the latter over the shallow ridge that unites 

 Europe with Africa, the average depth of which does not exceed appa- 

 rently more than about 130 or 140 fathoms, although there are depths of 

 160 near to the African side. It extends across, between Cape Trafalgar 

 and Cape Spartel, the two western capes of the Strait. 



The width of the Strait between these two capes is 22 J miles, and the 

 width in its narrowest part near Tarifa is only 7 J miles. 



There is therefore a great convergence of the confining coasts and de- 

 scending slopes to this part, and a necessarily convergence of the Atlantic 

 tidal wave, as well as general inset of the Atlantic current. 



In this constricted part of the strait also the greater portion of the 

 depth (fully 5 miles of the 7i across) is more than twice as great as on 



