570 



Dr. W. B. Carpenter on the 



[June 13, 



ging it fast through the surface-water, which was itself setting in the same 

 direction, against a light west wind ; and its rate, at 2\ hours after low 

 water, was estimated at 1*2 mile per hour, that of the surface being esti- 

 mated at 0*5 mile per hour. — Thus it appeared that in this situation both 

 the upper (Atlantic) current and the lower (Mediterranean) current fol- 

 lowed the direction of the Tidal movement, and that their rates were 

 greatly affected by it ; but a comparison of the relative amounts of the 

 flow in each case showed a decided excess in the movement of the upper 

 current to the eastward, and a no less decided excess in the movement of 

 the under- current to the westward. For taking 2\ hours after each turn of 

 the tide as the time of greatest velocity of each current, the comparison of 

 estimated rates (§ 60) stands thus : — 



2| hours after L. W. 2| hours after H. W. 



Surface-current W.-«s — 0*5 mile per hour. — *-E. 1*25 mile per hour. 

 Under-current W.^ — 1*2 mile per hour. — s-E. 0*88 mile per hour. 



Thus the excess of inflow of the upper current being at the rate of 0*75 

 mile per hour, that of onflow of the under- current was 0*32 mile per hour. 



54. Having then proceeded to Gibraltar, our first day's work (Aug. 26) 

 at the Mediterranean end of the Strait was chiefly devoted to experiments 

 with Mr. Siemens's Photometric apparatus ; the results of which, as they 

 were not altogether satisfactory, I think it better not to record. But the 

 opportunity was taken to obtain a series of Specific Gravities at different 

 depths in mid-channel between Europa Point and the African coast. A 

 strong easterly wind was blowing through the day, raising a considerable 

 swell ; and this had an obvious effect on the Sp. Gr. of the upper stratum, 

 which showed a decided admixture of Mediterranean water with the At- 

 lantic water properly forming it ; for instead of the 1*0271, which we had 

 found last year in this situation when a moderate westerly wind was 

 blowing, the Sp. Gr. of the surface-water proved to be 1*0277; at 100 

 fathoms it had increased to 1*0279 ; and the same Sp. Gr. was found 

 at 200 fathoms. At 250 fathoms, however, as in the two observations 

 made in the former year at the same part of the Strait, we came upon un- 

 mistakable Mediterranean water, its Sp. Gr. being 1029*5 ; this showed 

 itself again at 300 fathoms, and again in water taken from the bottom at 

 480 fathoms. Notwithstanding, therefore, the unusually high Sp. Gr. of 

 the upper stratum, the slightness of the increase down to 200 fathoms, 

 and the marked increase encountered between 200 and 250 fathoms, clearly 

 showed that its character was essentially Atlantic ; and thus we had di- 

 stinct evidence that the Mediterranean stratum here lies at least 200 fathoms 

 from the surface. It was further interesting to remark that the whole of 

 the lower stratum was composed of densest Mediterranean water ; whereas 

 in two observations made last year at an interval of six weeks, the water at 

 the bottom was found to be of less Sp. Gr. than the water at 250 fathoms. 

 It thus appears that in this peculiar channel considerable changes may 



