98 



Capt. G. S. Nares on the 



[Jan. 18, 



Atlantic during one flooi-tide under the most favourable circumstances, 

 viz. strong continuous east wind 3 , was only two miles, while the easterly 

 set was found to be at hast ten miles during the ebb. 



This, however, is the case with the surface-water only; the bottom stratum 

 is unaffected by the in-running current from the Atlantic and sets east or 

 west according to the tides for equal periods ; and there is no reason to sup- 

 pose that it is interfered with by westerly winds as the upper water un- 

 doubtedly is. 



On the shallow ridge at the west entrance of the Strait the in-running 

 current from the Atlantic is insufficient to counteract the effect of the 

 west-running flood-tide, and a tidal influence was shown on both the surface- 

 and bottom-water. 



It was found that the surface-water moved towards the east during the 

 ebb-tide faster than the bottom-water ; with the flood it set towards the 

 west slower than the bottom-water ; and it ran towards the east faster than 

 it ran towards the west. The bottom-water ran to the westward with the 

 flood faster than it ran to the eastward with the ebb. The power exerting 

 itself to produce this excess of west-running water at the bottom is there- 

 fore sufficient to annihilate the in-running current which makes itself so 

 easily felt in the water above. 



The prevailing set of the surface-current running from the westward into 

 the Mediterranean being met and checked by an advancing tidal wave coming 

 from the eastward, the water is raised and causes the flood -tide (i. e. on 

 the shore) ; after the wave has passed the water falls, running to the east- 

 ward with the prevailing current ; thus the flood or rising tide by the shore 

 in the Strait is produced by the west- running current. The change of the 

 tidal stream in the bottom stratum corresponds with the time of high and 

 low water at Gibraltar. With east winds, the surface-current stops 

 running to the east from one to two hours after low water ; it turns to run 

 to the east again at high water. 



These observations may be of great practical use to the large number of 

 sailing-vessels engaged in the Mediterranean trade. A^essels intending to 

 beat out of the Strait to the westward should get under weigh, or sail out 

 from under shelter, at low water ; at half flood long tacks may be made 

 from shore to shore, but shelter must be gained before high water. 



The method and apparatus employed in observation of the currents were 

 similar to those with which we have become familiar through the reports 

 of recent deep-sea researches. The current-drags were made of canvas, 

 hanoing 4 feet below two light wooden crossed yards, each 4 feet long, 

 secured in the middle and weighted to 75 lbs., the weights being attached 

 to the bottom of the canvas. This was suspended at the requisite depth 

 by a fine line, 0*2 inch in diameter, hanging from one end of an iron 

 buoy, 5 J feet long, 1 foot in diameter in the middle, and pointed at each 

 end, each buoy being capable of floating 100 lbs. weight. Close to the 

 current-drag was a stouter line by which to lift it out of the water, which 



