152 THE PHYSICAL GEOGRAPHY OF THE SEA. 



Isthmus, and the colder and Salter, and therefore heavier water, 

 must either run out as an under current, or it must deposit its sur- 

 plus salt in the shape of crystals, and thus gradually make the 

 bottom of the Red Sea a salt-bed, or it must abstract all the salt 

 from the ocean to make the Red Sea brine — and we know that 

 neither the one process nor the other is going on. Hence we in- 

 fer that there is from the Red Sea an under or outer current, as 

 there is from the Mediterranean through the Straits of Gibraltar, 

 and that the surface waters near Suez are Salter than those near 

 the mouth of the Red Sea. 



414. And, to show why there should be an outer and under 

 current from each of these two seas, let us suppose the case of a 

 long trough, opening into a vat of oil, with a partition to keep the 

 oil from running into the trough. Now suppose the trough to be 

 filled up with wine on one side of the partition to the level of the 

 oil on the other. The oil is introduced to represent the lighter 

 water as it enters either of these seas from the ocean, and the wine 

 the same water after it has lost some of its freshness by evapora- 

 tion, and therefore has become Salter and heavier. Now suppose 

 the partition to be raised, what would take place ? Why, the oil 

 would run in as an upper current, overflowing the wine, and the 

 wine would run out as an under current. 



415. The rivers which discharge in the Mediterranean are not 

 sufficient to supply the waste of evaporation, and it is by a pro- 

 cess similar to this that the salt which is carried in from the ocean 

 is returned to the ocean again ; were it not so, the bed of that sea 

 would be a mass of solid salt. The equilibrium of the seas is 

 preserved, beyond a doubt, by a system of compensation as exqui- 

 sitely adjusted as are those by which the "music of the spheres" 

 is maintained. 



416. It is difficult to form an adequate conception of the im- 

 mense quantities of solid matter, in solution, which the current from 

 the Atlantic carries into the Mediterranean. In the abstract log 

 for March 8th, 1855, Mr. William Grenville Temple, master of 

 the United States ship Levant, homeward bound, has described 

 the indraught there : 



" Weather fine ; made 1^ pt. lee-way. At noon, stood in to Al- 



