178 THE PHYSICAL GEOGRAPHY OF THE SEA. 



it again in the shape of rains. The Red Sea vapors are carried 

 off and precipitated elsewhere. 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 fourths of an inch to 

 an inch daily. Whatever it be, it is doubtless greater in the Red 

 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 cur- 

 rent 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 evapo- 

 ration 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 Red 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., evaporation and temperature ; for the tempera- 

 ture of that sea is necessarily lower at Suez, in latitude 80°, than 

 it is at Babelmandeb, in latitude 13°. To make it quite clear that 

 the surface of the Red Sea is n®t 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 Babelmandeb, and to flow up the chan- 

 nel, like the present surface 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 inclined plane, made so by evaporation. 



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

 Upper and under cur- by cvaporatiou, bccomcs saltcr, and therefore heav- 

 expiainei"^ ^ ^^^ ^ lev. The lighter water at the Straits can not bal- 

 ance the heavier water at the Isthmus, and the colder and Salter, 

 and therefore heavier water, must either run out as an under cur- 

 rent, or it must deposit its surplus 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 



