CUEEENTS OE THE MEDITEEEANEAN. 87 



one day, and during another day there is no sensible movement in 

 cither direction. Often, too, according to Forchhammer, the two 

 contrary currents glide one above the other ; the lighter on the 

 surface, coming from the Baltic, and the other from the Nortb 

 Sea, heavier by reason of the salt it contains, flowing beneath. 



At the other extremity of Europe similar phenomena occur in the 

 Bosphorus, at the outlet of the Black Sea. This strait, which receives 

 the superabundant waters of the Euxine, presents a mean breadth of 

 more than a mile, with a depth of 15 fathoms,* so that if the waters of 

 the sea flowed there in a continuous manner as in the bed of a river, and 

 the swiftness of the current were only about 1^ mile per hour, it would 

 not discharge less than nearly 36,000 cubic yards per second. But 

 it is probable that all the united affluents of the Black Sea and the 

 Sea of Azof supply hardly the half of this quantity ; and, besides, a 

 great part of the water brought by them is carried off again by evapora- 

 tion. The Bosphorus is therefore too large to serve as the bed of a 

 single current flowing from the Black Sea into the Sea of Marmora. 

 It has been observed that the waters ordinarily descend towards 

 the Mediterranean, with a speed of from 2 to 4 miles an hour ; but 

 the existence of tolerably rapid lateral counter-currents has also been 

 established ; and sometimes the winds blowing from the west cause 

 the principal current to flow back through the strait. A submarine 

 movement of the waters in the direction of the Black Sea also exists, 

 as already ascertained by Marsigli in the last century. 



At the western part of the Mediterranean, between Gibraltar and 

 Ceuta,, the normal current is that coming from the ocean. In fact, 

 the Mediterranean has not many considerable tributaries. It only 

 receives a single river having a really great mass of water, namely, 

 the Danube ; its other affluents of any importance, the Rhone^ the 

 Po, the Dniestr, the Dnieper, the Don, and the Nile, bring, on an 

 average, not more than 19,620 cubic yards of water per second.f 

 On the other hand, evaporation is very rapid in the basin of the 

 Mediterranean, especially on the coasts of Egypt and Tripoli. We 

 may admit that the quantity of water taken from this basin by 

 the solar rays, and not directly restored by rain, annually represents 

 a section of about 4| feet ; which is probably near enough to the 

 truth, as in the neighbourhood of Genoa, Beaucaire, Aries, and 

 Perpignan, on the northern shores of the sea, the evaporation exceeds 

 four-tenths of an inch per day in the great heat, and nearly 2 feet 



* Tchihatchef, Asie Mineure. 

 t See in Vol. I. the section entitled, Rivers, 



