1873.] 



of the Dardanelles and Bosphorus. 



389 



On all occasions when it has been practicable the specific gravity and 

 temperature of the water have been taken at different depths, at the same 

 times and places as the current observations. 



The general result of the undercurrent observations in the Dardanelles 

 is, that an undercuvvQJit exists whenever a surface one does, and invari- 

 ably in an opposite direction. 



Even with the imperfect means used, out of 12 days' observation 10 

 gave this result. 



The speed at which the under countercurrent travels seems to depend 

 upon that of the surface. 



The highest rate actually observed in the Dardanelles was y 8 ^ of a knot 

 an hour, when the surface-stream in the opposite direction was also the 

 fastest experienced, 3 knots. 



When the surface-current was slack so was the undercurrent. 



There was little or no deviation from this rule. 



The depth of the surface-current appears to be about 10 to 15 fathoms. 



Below this, the whole of the water to the bottom is generally on the 

 move in a contrary direction. 



At about 10 or 15 fathoms depth also the specific gravity of the water 

 generally alters rapidly, approximating to the Mediterranean density 

 below, and to that of the Black Sea above. 



The temperature also changes about this point. 



This shows that it is not superfluous surface-water that is running back 

 underneath, but that it is the Mediterranean supplying the place of the 

 lighter Marmara water. 



There appears to be little or no slack water between the two currents, 

 favourable observations having given them in opposite directions in a very 

 few fathoms different depth. 



Details of Undercurrent Observations. — Dardanelles. 



On July 29th an under countercurrent was obtained in the centre of 

 Gallipoli Strait at 30 fathoms. At the commencement of this observa- 

 tion it was calm, and the surface ran but a tenth of a knot, with a 

 counter undercurrent of two tenths ; later in the day a breeze sprang 

 up, when the surface ran six tenths, and the undercurrent was increased 

 to three tenths, showing an unmistakable connexion between wind and 

 current. 



Next day gave almost identical results. The current of seven tenths 

 of a knot at the surface decreased in strength with the depth, changed 

 its direction at about 15 fathoms, and increased in rate to the bottom, 

 where it showed three tenths. 



On August 13th five boats, equidistantly arranged across the strait, 

 took observations with various results ; but generally the surface-current 

 ran to N.E., and the under to the S.W., reversing the usual order ; all 

 rates very slow. 



VOL. xxi. 2 I 



