182 THE PHYSICAL GEOGRAPHY OF THE SEA. 



These observations agree "with the theoretical deductions just an- 

 nounced, and show that the surface waters at the head are heavier 

 and Salter than the surface waters at the mouth of the Ked Sea. 



382. In the same paper, the temperature of the air between 

 Evaporation from. Sucz and Aden oftcu rises, it is said, to 90°, "and 



probably averages little less than 75° day and night all the year 

 round. The surface of this sea varies in heat from 65° to 85°, 

 «nd the difference between the wet and dry bulb thermometers 

 often amounts to 25° — in the kamsin, or desert winds, to from 30° 

 to 40° ; the average evaporation at Aden is about eight feet for 

 the year." " Now assuming," says Dr. Buist, " the evaporation of 

 the Ked Sea to be no greater than that of Aden, a sheet of water 

 eight feet thick, equal in area to the whole expanse of that sea, will 

 be carried off annually in vapor ; or, assuming the Eed Sea to be 

 eight hundred feet in depth at an average — and this, most assur- 

 edly, is more than double the fact — the whole of it would be dried 

 up, were no water to enter from the ocean, in one hundred years. 

 The waters of the Ked Sea, throughout, contain some four per cent, 

 of salt by weight — or, as salt is a half heavier than water, some 2.7 

 per cent, in bulk — or, in round numbers, say three per cent. In 

 the course of three thousand years, on the assumptions just made, 

 the Ked Sea ought to have been one mass of solid salt, if there 

 were no current running out." Now we know the Ked Sea is 

 more than three thousand years old, and that it is not filled with 

 salt ; and the reason is, that as fast as the upper currents bring the 

 salt in at the top, the under currents carry it out at the bottom. 



383. Mediterranean Currents. — With regard to an under 

 TnEMKDiTEBitANEAiT currcut from the Mediterranean, we may begin by 

 (juEKE^Ts. remarking that we know that there is a current al- 

 ways setting in at the surface from the Atlantic, and that this is a 

 salt-water current, which carries an immense amount of salt into 

 that sea. We know, moreover, that that sea is not salting up ; and 

 therefore, independently of the postulate (§ 374) and of observa- 

 tions, we might infer the existence of an under current, through 

 which this salt finds its way out into the broad ocean again.* 



* Dr. Smith appears to have been the first to conjecture this explanation, which 

 he did in 1673 (vide Philosophical Transactions). This continual indraught into 

 the Mediterranean appears to have been a vexed question among the navigators and 

 philosophers even of those times. Dr. Smith alludes to several hypotheses which 



