262 
NATURE 
[JuLy 13, 1899 
© The water of the lower strata or the Marmora Sea has the same 
“composition as the water of the Mediterranean. The upper 
Strata, say from ten fathoms upwards, contain water of inter- 
mediate salinity between the water of the Mediterranean and the 
water of the Black Sea. This difference in the salinity of the water 
is the chief reason of the enormous double current of the Bos- 
phorus. Let usimagine 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 be 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 (who is now at 
the head of the Hydrographic Office) was the first to show bydirect 
observations that a double current existed in the Bosphorus. I was 
there a few years after him, commanding the stationary steamer 
Zaman. 1 began to take observations of the specific gravity of 
the water at different depths, and I found out that the water 
forming the lower strata contained twice as much salt as the 
water of the upper strata ; after this, a double current was quite 
evident to me. 
I do not wish to detain you with an account of the different 
results referring to the velocity of both currents, and will only 
‘point out to you that the lower current is similar in many details 
to an ordinary river, while, on the contrary, the upper current 
differs much from an ordinary river, probably for the reason 
that, while the surface of it is falling gradually down, the 
bottom rises constantly. 
The difference of level of the Black Sea and :Marmora Sea, 
calculated from the difference in the specific gravity of the 
water, I found for the month of July 1882 to be 1°396 feet. 
In the Strait of Gibraltar I had only five stations, and made 
my observations one day only. I had no opportunity of 
measuring the velocity of the current, but the phenomenon is 
very similar to what I found in the Bosphorus. The water of 
the Atlantic rushes into the Mediterranean, the difference 
between the surface levels being, according to my calculations, 
0°54 foot. 
The evaporation of water from the Mediterranean is greater 
than the quantity supplied by rivers and rains. For this reason, 
the water becomes more dense, settles down, and goes back to 
the Atlantic by the under current. 
I wish to point out here that the temperature of the lower 
strata of the Mediterranean coincides with the mean winter tem- 
perature of the air in the eastern part of the sea. This is quite 
evident, because in winter the temperature of the water to a 
great depth corresponds to the temperature of the air. In 
summer, the surface water is much warmer, but this high tem- 
perature cannot penetrate to a great depth. Iam sorry that I 
have not time to discuss more fully this question, but in the 
Straits of Bab-el-Mandeb we have the same phenomena as in 
the Gibraltar Strait and Mediterranean. Here again—by my 
observations—the temperature of the lower water strata coincides 
with the winter temperature of the air at the place where the 
water settles down. 
In the three straits already mentioned we have a double 
current: superficial and bottom current. In the Straits of 
Formosa and La Pérouse there are also two currents, but both 
are superficial. 
{ ought to mention that the influence of the rotation of the 
earth on the direction and velocity of the currents cannot be 
over-estimated. I shall not discuss this question fully, but the 
fact that in every salt inland sea there is a circular rotation of 
NO. 1550, VOL. 60] 
the water in a direction opposite to the apparent movement of 
the sun, shows that the rotation of the earth has very much to 
do with the direction of the currents. In the vicinity of islands, 
for the same reason, the water follows a direction coinciding 
with the apparent movement of the sun. It is for this reason 
also that the water alongside the Chinese coast flows to the 
south during the north-easterly monsoon as well as during the 
south-westerly monsoon. The Kuro-Siwo current going to the 
north and north-east cannot touch the Chinese coast because 
there is brackish water flowing to the south-west. 
In the Strait of Formosa the specific gravity and temperature 
of the water at the Chinese coast are quite different from what 
is observed off the coast of Formosa. This difference in the 
temperature and specific gravity may give to a sailor a good 
guide for a fair passage through the Strait. The temperature of 
the water, say, in the month of February at the Chinese coast 
is 11° C., while at the coast of Formosa it is 20°. If the captain 
will try during the month of February to follow the line of the 
temperature of 15° he will pass at a good distance from the 
dangers of both coasts. Moreover, at the Chinese coast in 
winter it is possible to find water at less than 1I‘0240 
(si), while at the coast of Formosa it is seldom less than 
1'0265. 
Every sailor knows how difficult is the passage through 
the Strait of Formosa. During the north-easterly monsoon the 
weather is very thick, and the depth of the sea cannot in these 
places be regarded as giving a good means for determining the 
position of the ship. It may happen that after a ship leaves, 
say, Nagasaki the captain never knows his position until he 
runs on the Chinese coast and wrecks his ship. My opinion is 
that a regular temperature service should be arranged from 
Turnabout Lighthouse ; everyday a pilot boat should put to sea, 
taking temperatures both going out and returning, and the tem- 
perature of the water should be wired to all Chinese and Japanese 
ports for the information of the captains. By these means many 
ships would be saved from danger. | 
The currents in the Strait of La Pérouse are very complicated. 
There is a very narrow and long strip of cold water, which lies 
in the direction from N.W. to S.E. ; a vessel crossing that strip 
in July may have temperatures of 18° C., then 5°, and again 16° 
or 18°. It would take me too long to explain the source from 
whence the cold water comes, and why it is constantly there ; it 
is the cause of fogs which render navigation in that place very 
difficult. I may briefly say that the Kuro-Siwo current partly 
enters the Sea of Japan, and the excess of water escapes partly 
through the Strait of La Pérouse into the Okotsk Sea. Due to 
the rotation of the earth, the current turns to the south-east and 
flows alongside the Island of Yezo. This water is warm and 
dense, having much salt init. The water of the Okotsk Sea— 
particularly in the vicinity of the Island of Saghalien—is in 
summer also pretty warm, but it is much lighter than the water 
of the Kuro-Siwo, and thus while the denser water sinks down, 
the lighter water tries to rise on the top of it. The difference 
of level which is produced hereby brings to the surface the cold 
water of the lower strata. 
I studied this Strait in 1887 and 1888, and published the 
results of my study, but when I came to the Pacific again in 
1895, as the Admiral commanding the squadron, I was very 
anxious to go to the Strait of La Pérouse to re-investigate the 
currents, and now I am in possession of very valuable material 
on this subject, which is almost ready for publication. 
I do not propose to take up more of your time at present with 
particulars of these five straits. I only wish to remind you what 
important information the thermometer and hydrometer can give 
in the study of the different parts of our so little-known planet. 
You know better than I that studies in that direction ought to 
be continued, and no nation in the world has been so liberal as 
England, which found means to send out for four years the 
Challenger with a scientific staff to explore the deep sea. But 
it is not always possible to find such means, and it is advan- 
tageous to associate ordinary seamen with that kind of work. 
I should be very glad if oceanographers would come to 
certain definite opinions with regard to the mode of collecting 
the information about the temperature of the surface water. It 
would be a great advantage to knowledge to divide the study 
of the sea with regard to the temperature. Suppose Russia 
should take Okotsk Sea, Bering Sea, or Sea of Japan, Black 
Sea, White Sea, Kara Sea, and the Finnish Gulf, England takes 
| the Atlantic, United States takes Northern Pacific, Germany 
