CURRENTS OF THE SEA. 127 



241. 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. 



Now, if we suppose the velocity of the current which runs into 

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

 w^ould 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. 



242. Thus the w^aters 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 low- 

 er at Suez, in latitude 30°, than it is at Babelmandeb, in latitude 

 13°. 



243. 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. 



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

 clined plane, made so by evaporation. 



245. 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 

 Isthmus, and the colder and salter, and therefore heavier water, 

 must either run out as an under current, or it must deposit its sur- 

 plus salt in the shape of crystals, and thus gradually make the 

 bottom of the Red Sea a salt-bed, or it must abstract all the salt 

 from the ocean to make the Red Sea brine — and we know that 

 neither the one process nor the other is going on. Hence we in- 

 fer that there is from the Red Sea an under or outer current, as 

 there is from the Mediterranean throuffh the Straits of Gibraltar, 



