176 The Realm of Nature CHAP. 



Red Sea would become a mass of solid salt in less than 

 2000 years. Since there is no. perceptible change in its 

 salinity, it is certain that a deep undercurrent of salt water 

 passes out through the Strait of Bab-el-Mandeb sufficient to 

 carry back to the Indian Ocean all the salt received from 

 it. The circulation of the Mediterranean is carried on in 

 the same way, as the rainfall it receives is only equal to 

 about one-quarter of the evaporation from its surface, and 

 its water, although of higher salinity than the neighbour- 

 ing Atlantic, is not growing salter. The outflowing current 

 through the Strait of Gibraltar underneath the inflowing 

 fresher current has been observed, and the deep water of 

 the Atlantic in that neighbourhood is perceptibly wanned 

 and increased in saltness by the outflow. 



238. Circulation of Seas by Dilution. The Black 

 Sea is a deep basin cut off from the Mediterranean by the 

 shallow Bosphorus, the Sea of Marmora, and the Dar- 

 danelles. This sea contains only about 2 per cent of salts, 

 its water being very much freshened by the Don, Danube, 

 and other great rivers which flow into it, supplying more 

 water than is removed by evaporation, and raising its level 

 about 2 feet higher than that of the Mediterranean. A 

 steady surface outflow of brackish water from the Black Sea 

 consequently sets through the Bosphorus ; but a slower 

 stream of very salt Mediterranean water forces its way 

 along the bottom into the Black Sea, so that the sea is not 

 permanently freshened. The cause of the undercurrent of 

 salt water between seas of different salinity is that in order 

 to produce equilibrium the pressure exerted by two adjacent 

 columns of a fluid must be the same. A column of salt 

 water exerts the same pressure as a column of fresh water 

 higher in proportion to the difference of salinity. But (38) 

 water cannot stand at a higher level beside water at a lower 

 level, and the fresher water pours over the surface of the 

 salter column, upon which the pressure is thereby increased 

 and the undercurrent is produced in order to equalise 

 matters. As long as the supply of fresh water is kept up 

 there can be no equality, and thus the circulation con- 

 tinues. The Baltic Sea has a somewhat similar circulation. 



