The Mediterranean Sea 



between Africa and the mouth of the Adriatic. That the 

 level and the salinity of the Mediterranean remain constant is 

 due to the supply of water which enters at the Straits of 

 Gibraltar. The currents in this passage have frequently 

 engaged attention both from their scientific and their nautical 

 interest. The most detailed investigation was that carried out 

 by Captain Nares and Dr Carpenter in H.M.S. "Shearwater" 

 in the year 1871 l . From these investigations it appears that 

 there are usually two currents in the Straits at the same time, 

 one superposed on the other. Both are affected by tidal 

 influence, but, after allowing for it, there is still a balance of 

 inflow in the upper and of outflow in the under current. The 

 waters of the two currents are sharply distinguished from each 

 other by their salinity. Further, the upper current appears to 

 affect by preference the middle of the channel and the African 

 coast, while the under current appears to crop out at the 

 surface on the Spanish coast. This distribution, however, 

 is much modified by the state of the tide, and it must be 

 remembered that in such places the surface separating the 

 upper and under currents is rarely, if ever, a horizontal plane. 

 That there is a balance of outflow over inflow at the bottom was 

 well shown by the result of soundings as much as 200 miles north- 

 west of the entrance of the Straits, where, in a depth of 1560 

 fathoms, water of decided Mediterranean origin was got from 

 the bottom. There can be no doubt that this outflow of warm 

 and dense Mediterranean water is largely instrumental in 

 causing the comparatively very high bottom temperature in 

 the eastern basin of the North Atlantic. 



We have assumed that the balance of water removed by 

 evaporation is 30 inches, or 2-5 feet. If we take the area of the 

 Mediterranean to be 1,000,000 square miles, we have the volume 

 of water removed: 



v = 2'5 x 36 x io 12 = 90 x io 12 cubic feet. 



This quantity of water has to be supplied from the Atlantic 

 without raising the total quantity of salt in the sea. We have 

 seen that the only provision for the removal of the surplus 



1 Proc. Roy. Soc. (1872), xx. 97, 414. 



