REPORT ON OCEANIC CIRCULATION. 
29 
trade winds into the Gulf of Mexico, its level is thereby raised fully three feet above the 
general level of the ocean. The following peculiarities distinguish this remarkable current : 
at its starting place in the Gulf of Mexico its waters have a higher specific gravity than 
those of any other ocean current at its starting ; it starts with a three-feet head of water ; 
and along its whole course, from the Straits of Florida to the Arctic Ocean, the prevail¬ 
ing winds at all seasons are substantially south-westerly or in the same line of movement. 
Hence the volume and speed of this great ocean current as it issues from the tropical 
region, and the enormous influence it has on the western climates of Eurasia. 
The Mediterranean Sea. 
The mean annual surface isothermal of 65° from the Atlantic passes eastwards through 
the Strait of Gibraltar, curves round by the north of Sardinia, and round by the Strait 
of Messina to Cephalonia; and the isothermal of 70° passes from Libya in a north¬ 
easterly direction to Tarsus ; and the isothermal of 60° just appears in the extreme north 
of the Gulf of Lyons. 
To the west of the Strait of Gibraltar the specific gravity of the surface is a little 
higher than L0270, but it quickly increases in advancing eastwards till, about long. 
10° E., it rises to L0280, and over the eastern basin of the sea it is L0290, showing thus 
a specific gravity higher than ever occurs in the open sea. Hence the water of the 
Mediterranean, even on the surface, is much salter than that of the Atlantic. This is 
due to the dry climate of the region, so that the loss from evaporation is not nearly com¬ 
pensated for by the fresh-water additions from the rainfall and by the rivers which 
empty themselves into its basin. 
The observations and discussions of Dr Carpenter on this sea and the Atlantic 
adjoining in 1870-71 must be regarded as a classical contribution to the question of 
oceanic circulation. The results of these and other observations and discussions estab¬ 
lished the important fact that in the Strait of Gibraltar there are, after allowing for 
tidal influence, two currents, one superimposed on the other. The upper current is an 
inflowing one carrying with it the surface water of the Atlantic, and the undercurrent 
is an outflowing one, carrying out with it into the Atlantic the warmer and denser 
underlying water of the Mediterranean. 
The submarine ridge at the Strait separating the deep water of the Atlantic from 
that of the Mediterranean is not quite 200 fathoms from the surface, and hence the 
direct communication between the seas is confined within that depth. A little way to 
the east of the Strait the temperature of the Mediterranean at 100 fathoms is about 
55° ; it increases eastward to 58° to the east of Malta, and rises a little above 60° in 
the Levant. The specific gravities to the east of the Strait are about 1 ‘0284, whereas in the 
Atlantic to the west specific gravity is but L0267. Hence the outflowing undercurrent 
