534 



Captain Spratt on the Undercurrent 



[June 15, 



Remarks and Experiments on the superficial and supposed under- 

 currents of the Mediterranean 



It can be easily understood that, if a superficial current of 1 knot is 

 observed to pass a float attached to a line which has a sinker or anchor at 

 the bottom, and also if the same amount of surface-current of 1 knot 

 passes a float which is attached to a suspended sinker or current-anchor 

 suspended halfway or at any depth, the sinker thus held suspended by the 

 surface-float is evidently as stationary as the one at the bottom, and there- 

 fore it must be in perfectly still water, whatever the depth may be ; con- 

 sequently the superficial current does not descend to that depth. 



Also, if another suspended sinker or current-anchor is lowered down a 

 few fathoms (say 10 fathoms) from the surface, and the float attached to 

 it has no current passing it, and consequently drifts away from the 

 stationary float attached to the bottom and near which it was lowered, it 

 is quite clear that the suspended sinker and its float are within the same 

 influence, in fact in the same amount of current. 



Again, if the suspended sinker be lowered to 20 fathoms by the side 

 of the stationary float, and a current of about half a knot be then observed 

 passing its float, although still drifting away from the stationary float, then, 

 as this latter float showed a current of 1 knot passing it, and the float of 

 the suspended sinker in 20 fathoms only showed a current of half a knot, 

 it is also clear that the current- anchor or suspended sinker was in a current 

 of only half the Speed of the superficial current, viz. of half a knot only. 



Also, if the suspended sinker be lowered to 50 fathoms, and the super- 

 ficial current passing its float be three-fourths of that passing the float at- 

 tached to the bottom, or running three-quarters of a knot, it is evident that 

 the current at the depth of 50 fathoms was three-fourths less than the sur- 

 face-current, or only running at the rate of one-fourth of a knot. 



In this manner, then, my experiments were carried out at different 

 depths, and at different times, in the Archipelago, Sea of Marmora, and 

 Dardanelles, as being favourable positions for testing the superficial currents, 

 and also of the existence of undercurrents, if any existed in these straits 

 and seas, as some have supposed. 



Thus, on the morning of December 19th, 1857, I hove to in H.M.S. 

 ' Medina,' between Bodosto and Marmora Island, near the eastern entrance 

 to the Dardanelles, and from a boat sounded with a shot and seine-twine in 

 350 fathoms ; I then attached to the twine a piece of light wood as a sta- 

 tionary float. The superficial current was then tested by the common log- 

 reel, run out from a boat kept stationary abreast of the stationary float, 

 when a current of 0'9 of a knot was observed to be running towards the 

 Dardanelles. 



Experiments for trying the rate o f the current at different depths were 

 then made in the following manner : — A flat piece of wood like a log-ship 



* Travels and Researches in Crete, vol. ii. p. 333. 



