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. 



Novr, 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 w^ould, by 

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

 from its surface. 



242. 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 w^aters 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 w^ater 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 through the Straits of Gibraltar, 



