§ 375, 37G. CURRENTS OF THE SEA. I77 



375. It is not necessary to associate with oceanic currents tlie 

 Marine currents do idea that thcv must, of neccssitv, as on land, run 



not, like those on -i • ^ i i irinr> i- 



land, run of neces- troni a higher to a lower level. 00 far from this 



sity from higher to . , - r»xi ^ n 



lower levels. bcmg the casc, some currents 01 the sea actually run 



up hill, while others run on a level. The Gulf Stream is of the 

 first class (§ 83). 



376. The currents which run from the Atlantic into the Med- 

 The Red Sea current, itcrraneau, and from the Indian Ocean into the Eed 

 Sea, are the reverse of this. Here the bottom of the current is 

 probably a water-level, and the top an inclined plane, running 

 down hill. Take the Eed Sea current as an illustration. That 

 sea lies, for the most part,- within a rainless and riverless district. 

 It may be compared to a long and narrow trough. Being in a 

 rainless district, the evaporation from it is immense ; none of the 

 water thus taken up is returned to it either by rivers or rains. It 

 is about one thousand miles long ; it lies nearly north and south, 

 and extends from latitude 13° to the parallel of 30° north. From 

 May to October, the water in the upper part of this sea is said to 

 be two feet lower than it is near the mouth.* This change or dif- 

 ference of level is ascribed to the effect of the wind, which, pre- 

 vailing from the north at that season, is supposed to blow the wa- 

 ter out. But from May to October is also the hot season ; is is the 

 season when evaporation is going on most rapidly ; and when we 

 consider how dry and how hot the winds are which blow upon this 

 sea at this season of the year ; that it is a narrow sea ; that they 

 blow across it, and are not saturated, we may suppose the daily 

 evaporation to be immense. The evaporation from this sea and 

 the Persian Gulf is probably greater than it is from any other 

 arms of the ocean. We know that the waste from canals by evap- 

 oration, in the summer time, is an element which the engineer, 

 when taking the capacity of his feeders into calculation, has to con- 

 sider. With him it is an important- element ; how much more so 

 must the waste by evaporation from this sea be when we consider 

 the physical conditions under which it is placed. Its feeder, the 

 Arabian Sea, is a thousand miles from its head ; its shores are 

 burning sands ; the evaporation is ceaseless ; it is a natural evap- 

 orating dish (§ 525) on a grand scale; none of the vapors which 

 the scorching winds that blow over it carry away are returned to 



* Johnston's Physical Atlas. 



u 



