1872.] 



1 Shearwater ' Scientific Researches. 



573 



position occupied on the 28th ; with the view of experimenting in the 

 narrow part of the Strait for as long a period of the ^oo^-tide as possible, 

 so as to compare the results with those previously taken during the ebb. 

 The wind was here still from the East, with a force of from 3 to 4. A 

 surface-current- drag having been put over very soon after Low water, it 

 drifted at first nearly due South, or across the channel, but in the second 

 and third hours changed its course to the S.W., moving at the rate of 1*6 

 and 1*5 mile per hour. Having got into the in-shore current near the 

 African side, the drag was then taken up and started afresh in the middle 

 of the Strait, close to the under-current-drag ; and it then, between 4 and 

 5 hours after Low water, drifted W.S.W. at the rate of 0*55 mile per 

 hour. At High water, the surface-drag became stationary, and soon after 

 commenced drifting to the eastward. As the wind freshened at the same 

 time to a force of 6, the surface-drag was taken up ; but the drift of the 

 ship during the rest of the ebb-tide denoted a strong east-running surface- 

 current. The direction of the under-current, as indicated by a current- 

 drag at 250 fathoms depth, was very decidedly westward, while the sur- 

 face-buoy was moving nearly due south ; and its rate at 2|- hours from 

 the beginning of the ebb was estimated at 1*8 mile per hour. At from 

 3 to 4 hours after low water, the rate of the westward under-current was 

 1*6 mile; at from 4 to 5 hours it was still 1*25 mile; and between 

 5 hours and high water it was 0'8. The direction of the under-current 

 changed at High water; and at 1 hour afterwards it had acquired an eastward 

 rate of 1*0 mile, which increased to 1*6 at 2 hours after H. W., its mward 

 rate thus approaching to an equality with its outward rate during the flood, 

 which may not improbably have proceeded (as on the 28th) to an absolute 

 excess. — It may fairly be questioned, however, whether the strength of 

 this easterly Under-current during the ebb was not to be attributed (as 

 already suggested) to the continuance of the strong Easterly wind, which 

 by heaping up the surface-water, would augment the downward pressure 

 on the under-stratum. It is considered by Capt. Nares, whose judgment 

 on such a point is entitled to great weight, that the observations made at 

 the Mediterranean end of the Strait are not by any means so satisfactory 

 as those made on the "ridge" at the Atlantic end; since "the eddies 

 " which would naturally be expected at this part, in consequence of the 

 " funnel-shaped mouth of the Strait, complicate the movements and prevent 

 " such exact demonstrations as those found further to the westward, where 

 (t the current-stream runs more steadily." 



58. Giving our chief attention, then, to the phenomena presented at the 

 Western extremity of the Straits, we find these to be as follows : — 



L There is a reversal, with every ebb and flow of the Tide, in the direc- 

 tion alike of the upper and of the under-current : but while the wider- 

 current is the stronger during the flood or ow^going tide, the upper current 

 is the stronger during the ebb or Agoing tide ; and while the balance of 

 the onward and wward movement during each tide is decidedly /rewards 



