CURRENTS OF THE SEA. 151 



shores are burning sands ; the evaporation is ceaseless ; and none 

 of the vapors, which the scorching winds that blow over it carry 

 awaj, are returned to it again in the shape of rains. 



407. The Red Sea vapors are carried off and precipitated else- 

 where. The depression in the level of its head waters in the 

 summer time, therefore, it appears, is owing to the effect of evap- 

 oration as well as to that of the wind blowing the waters back. 



408. The evaporation in certain parts of the Indian Ocean 

 (§ 33) is from three fourths of an inch to an inch daily. Suppose 

 it for the Red Sea in the summer time to average only half an 

 inch a day. 



409. JSTow, if we suppose the velocity of the current which runs 

 into that sea to average, from mouth to head, twenty miles a day, 

 it would take the water fifty days to reach the head of it. If it 

 lose half an inch from its surface by evaporation daily, it would, 

 by the time it reaches the Isthmus of Suez, lose twenty-five inches 

 from its surface. 



410. Thus the waters of the Red Sea ought to be lower at 

 the Isthmus of Suez than they are at the Straits of Babelman- 

 deb. Independently of the waters forced out by the wind, they 

 ought to be lower from two other causes, viz., evaporation and 

 temperature, for the temperature of that sea is necessarily lower 

 at Suez, in latitude 30°, than it is at Babelmandeb, in latitude 13°. 



411. To make it quite clear that the surface of the Red Sea is 

 not a sea level, but is an inclined plane, suppose the channel of 

 the Red Sea to have a perfectly smooth and level floor, with no 

 water in it, and a wave ten feet high to enter the Straits of Babel- 

 mandeb, and to flow up the channel at the rate of twenty miles a 

 day for fifty days, losing daily, by evaporation, half an inch ; it is 

 easy to perceive that, at the end of the fiftieth day, this wave 

 would not be so high, by two feet (twenty-five inches), as it was 

 the first day it commenced to flow. 



412. The top of that sea, therefore, may be regarded as an in- 

 clined plane, made so by evaporation. 



413. But the salt water, which has lost so much of its freshness 

 by evaporation, becomes salter, and therefore heavier. The light- 

 er water at the Straits can not balance the heavier water at the 



