1898-99.] Admiral Makaroff on Oceanographic Problems. 393 
draw your attention to the interesting phenomena of double 
currents in the Straits of Bosphorus, Gibraltar, Bab-el-Mandeb, 
Formosa, and La Perouse. 
The Strait of Bosphorus joins the Black Sea and the Marmora 
Sea. The Black Sea water has in it — roughly speaking — half 
the quantity of salt found in the water of the Mediterranean. The 
water of the lower strata of the Marmora Sea has the same com- 
position as the water of the Mediterranean. The upper strata, say 
from ten fathoms upward, contain water of intermediate salinity 
between the water of the Mediterranean and the water of the 
Black Sea (see Plate I., fig. 1). This difference in the salinity of 
the water is the chief reason of the enormous double current of the 
Bosphorus. Let us imagine that at a certain given moment the level 
of both seas is at the same height. The pressure of the column 
of water in the Marmora Sea will he greater than that in the Black 
Sea; the difference would increase with the depth and it would 
disappear at the surface. For this reason the water in the lower 
strata of the Marmora Sea rushes into the Black Sea, keeping close 
to the bottom. That rush of water after a certain time will raise 
the level of the Black Sea, producing a difference in the level of 
the two seas, which causes a superficial current to flow out of the 
Black Sea in the opposite direction to the under current. Here 
we see distinctly that the principal reason for the double current 
is the difference in the salinity of the water, and should that 
difference in salinity cease the double current would be discontinued. 
The fact is that in the Black Sea evaporation does not exceed the 
quantity of water supplied by rains and streams, and this excess 
of fresh water maintains the difference of salinity in the waters of 
the Black Sea and Mediterranean. 
The existence of double currents in the Bosphorus was known 
long ago, and Marsilli in 1681, in his letter to Queen Christina of 
Sweden, has described them. Later, they were somehow forgotten, 
and some interesting papers have been published, in which the 
authors try to prove that the double current was legendary. 
Rear-Admiral Sir W. J. L. Wharton, of your Navy (who is now at 
the head of the Hydrographic Office), was the first to show by direct 
observations that a double current existed in the Bosphorus. I was 
there a few years after him, commanding the stationary steamer 
VOL. XXII. 8/3/99 2 C 
