CURRENTS OF THE SEA. 175 



of his feeders into calculation, has to consider. With him it is an 

 important element : how much more so must the waste by evapora- 

 tion from this sea be when we consider the physical conditions 

 under which it is placed ! Its feeder, the Arabian. Sea, is a thou- 

 sand miles from its head ; its shores are bm-ning sands ; the eva- 

 poration is ceaseless ; it is a natm^al evaporating dish (§ 525) on a 

 grand scale ; none of the vapom^s which the scorching winds that 

 blow over it carry away are returned to it again in the shape of 

 rains. The Ked Sea vapom^s 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 

 evaporation, as well as to that of the wind blowing the waters back. 

 The evaporation in certain parts of the Indian Ocean is supposed 

 to be (§ 103) from three fomihs of an inch to an inch daily. 

 Whatever it be, it is doubtless gTeater in the Eed Sea. Let us 

 assume it, then, in the summer-time to average only half an inch a 

 day. Now, if we suppose the velocity of the cmrent 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 sm-face by evaporation daily, it would, by 

 the time it reaches the Isthmus of Suez, have lost twenty-five 

 inches from its surface. Thus the waters of the Eed Sea ought to 

 be lower at the Isthmus of Suez than they are at the Straits of 

 Babelmandeb. Independently of the forcing out by the wind, the 

 waters there ought to be lower from two other causes, viz., eva- 

 poration and temperatm^e ; for the temperature of that sea is 

 necessarily lower at Suez, in latitude 30°, than it is at Babel- 

 mandeb, in latitude 13°. 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 Eed 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 

 Babelmandeb, and to flow up the channel, like the present smface 

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

 cHned plane, made so by evaporation. 



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

 Fm?entrt hrLu*S ^^^^ ovaporation, becomes Salter, and therefore heavier. 

 straits explained. The lighter wator at the Straits cannot balance the 

 heavier water at the Isthmus, and the colder and Salter, and there- 



