CUBRENTS OF THE SEA.' 179 



Tliese observations agree with tlie 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 Bed Sea. 



382. In the same paper, the temperatm-e of the an- between 

 Evaporation from. Suoz and Aden ofteu 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°, 

 and 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 Eed Sea to be no greater than that of Aden, a sheet of water 

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

 be carried off annually in vapour; 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, were no water to enter from the ocean, in one hundred 

 years. The waters of the Eed Sea, throughout, contain some fom^ 

 per cent, of salt by weight — or, as salt is a liaK heavier than water, 

 some 2.7 per cent, in bulk — or, in round numbers, say three per 

 cent. In the com^se of three thousand years, on the assumptions 

 just made, the Eed Sea ought to have been one mass of sohd salt, 

 if there were no cmTent running out." 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 cm-rents bring the 

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



383. Meditereanean Currents. — With regard to an mider 

 The mediterrakean current from the Mediterranean, we may begin by 

 CuERENTs. remarking that we know that there is a current 

 always setting in at the smface from the Atlantic, and that this is 

 a salt-water cmTent, which carries an immense amomit 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 liave 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. Smitb alludes to several hypotheses 

 which had been invented to solve these phenomena, such as subterraneous vents, 

 cavities, exhalation by the sun's beams, etc., and then offers his conjecture, which, 

 in his own words, is, " that there is an imder current, by which as great a quan- 



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