154 THE PHYSICAL GEOGRAPHY OF THE SEA. 



just announced, and show that the surface waters at the head are 

 heavier and Salter than the surface waters at the mouth of the 



Eed Sea. 



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

 Suez and Aden often rises, it is said, to 90°, " and probably aver- 

 ao-es little less than 75° day and night all the year round. The 

 surface of this sea varies in heat from 65° to 85°, and the differ- 

 ence between the wet and dry bulb thermometers often amounts 

 to 25° — in tlie 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 Eed 

 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 car- 

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

 hundred feet in depth at an average — and this, most assuredly, is 

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

 w^ere no w^ater to enter from the ocean, in one hundred years. 

 The waters of the Eed Sea, throughout, contain gome 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 Red Sea ought to have been one mass of solid salt, 

 if there were no current running out." 



422. Now we know the Eed 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. 



423. Mediterranean Cueeents. — With regard to an under 

 current from the Mediterranean, we may begin by remarking that 

 we know that there is a current always setting in at the surface 

 from the Atlantic, and that this is a salt-water current, which car- 

 ries an immense amount of salt into that sea. We know, more- 

 over, that that sea is not salting up ; and therefore, independently 

 of the postulate (§ 401) and of observations, 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 



