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 same 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 deptlx at an average — and this, most assuredly, is 
more than doubl«^ 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. Smitli appears to have been the first to conjecture this explanation, which he 
did in 1683 {vid^e Philosophical Transactions). This continual indraught into the 
I 
