CURRENTS OF THE SEA. 129 



249. These observations agree with the theoretical deductions 

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

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

 Red Sea. 



250. In the sailie paper, the temperature of the air between 

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

 ages little less than 75° day and night all the year round. The 

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

 ence 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 Red 

 Sea to be no greater than that of Aden, a sheet of water eight feet 

 thick, equal in area to the whole expanse of the sea, will be car- 

 ried off annually in vapor ; or, assuming the Red 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, 

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

 The waters of the Red 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 Red Sea ought to have been one mass of solid salt, 

 if there were no current running out." 



251. Now we know the Red 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. 



252. Mediterranean Currents. — 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 

 carries an immense amount of salt into that sea. We know, 

 moreover, that that sea is not salting up ; and therefore, inde- 

 pendently of the postulate (^ 236) and of observations (^ 253), 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 1683 {vide Philosophical Transactions). This contmual indraught into the 



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