1872.] 



Currents in the Strait of Gibraltar. 



99 



the light line was incapable of doing. The line was made very fine, to eli- 

 minate as much as possible the error which must always creep in in conse- 

 quence of the suspending-line passing through currents running in different 

 directions and at different speeds. 



In practice it was found that the surface- and bottom-water were rarely 

 moving at the same speed, which was indicated by the buoy from which 

 the current-drag was suspended being pulled through the surface-water. 

 The movement of the current-drag itself floating in the lower stratum must 

 therefore have been affected to a certain extent corresponding to the force 

 of the upper current pressing against the buoy. In making a correction 

 for this movement, Captain Nares estimates that the current-drag was it- 

 self retarded to an extent equal to half the difference of their speed. 

 Although the pressure on the buoy carried it away from a position imme- 

 diately over the current-drag, yet the suspending-line was never more than 

 5° out of the vertical, showing that the current-drag did not vary its depth 

 of flotation to any appreciable extent. 



To obtain the surface-drift, a current-drag, lowered 2 feet below the sur- 

 face, was suspended from a cork sphere, 1 foot in diameter, which, floating 

 with the top just above water, presented as little surface as possible to be acted 

 upon by the wind, and kept the current-drag itself below the wash of the sea. 



As these observations were made in August, when the evaporation of the 

 Mediterranean is at its maximum, and the freshwater supply derived from 

 rain and rivers at a minimum, no great increase in the in-running superficial 

 current can be expected during the winter months when these conditions 

 are reversed. 



From these remarks we pass to the practical work of observation ; and 

 it may suffice to give in full detail the proceedings of one day only, which 

 will serve to show how the conclusions were arrived at. 



On August 22nd, in mid-channel 4 miles S.S.W. of Tarifa, wind east, 

 force 3, with a smooth sea, about one hour after high water, after sounding 

 in 325 fathoms, a current-drag was lowered 2 feet below the surface, and 

 allowed to drift, the ship keeping station close to it to enable its position 

 to be fixed occasionally by angles betw r een shore objects. 



It drifted in 28 minutes E.N.E. 0*8 mile = 1*7 mile an hour. By 

 observations immediately afterwards the temperature of the water at the 

 surface was found to be 63°, and at 200 fathoms below 57°. 



At 4| hours after high water, a boat was anchored in 330 fathoms. The 

 surface-current was then running towards the east at an estimated speed of 

 1*5 knot an hour. A current-drag was lowered 150 fathoms, but soon after 

 settling down, and before any measurement of the distance run could be 

 obtained, its suspending-line foaled the anchor-line and was carried away, 

 showing that a strong under-current was running to the eastward, which 

 subsequent observations proved was its natural course at this time of tide. 

 The anchor was then picked up to change its position rather to the west- 

 ward, where the strait was broader. 



