SALINITY, GASES 59 



Generally speaking the salinity of ocean- 

 water beneath the surface diminishes from 

 the surface to a depth of 800 or 1000 fathoms 

 and then increases to the bottom. 



It has long been known that an interchange 

 of water takes place between the Mediterranean 

 and the North Atlantic at the Straits of 

 Gibraltar — an upper current flowing into 

 the Mediterranean from the Atlantic, and an 

 undercurrent flowing out of the Mediter- 

 ranean into the Atlantic — ^this interchange 

 being due to the lowering of the level of the 

 Mediterranean by excessive evaporation, and 

 to the extraordinary difference in the salini- 

 ties of the two seas. The submarine ridge 

 at the strait is covered by about 200 fathoms 

 of water, so that all direct communication 

 between the two seas is limited to that depth. 

 The outflowing warm dense saline water 

 gradually sinks on entering the North Atlan- 

 tic, and has a marked effect in raising the 

 temperature and salinity of the deeper waters 

 in that ocean. 



Again, between the Black Sea and the 

 Mediterranean there is a somewhat similar 

 arrangement of currents, in this case due to 

 the raising of the level of the Black Sea by 

 excessive inflow from riv^ers and to the differ- 

 ence in salinity of the two seas, the inter- 

 communication being complicated by the 



