126 THE PHYSICAL GEOGRAPHY OF THE SEA. 



a lower level. So far from this being the case, some currents of 

 the sea actually run up hill, while others run on a level. 

 The Gulf Stream is of the first class (§ 10). 



238. The currents which run from the Atlantic into the Medi- 

 terranean, and from the Indian Ocean into the Red 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 Red 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 dis- 

 trict, 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 ex- 

 tends from latitude 13° to the parallel of 30° north. 



239. From May to October, the water in the upper part of this 

 sea is said to be two feet lowxr than it is near the mouth.* This 

 change or difference of level is ascribed to the effect of the wind, 

 which, prevailing from the north at that season, is supposed to 

 blow the water out. 



But from May to October is also the hot season ; it is the sea- 

 son 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,\ve may suppose the daily evap- 

 oration to be immense ; not less, certainly, than half an inch, and 

 probably twice that amount. We know that the waste from ca- 

 nals by evaporation, in the summer time, is an element which the 

 engineer, when taking the capacity of his feeders into calculation, 

 has to consider. 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 feed- 

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

 are burning sands ; the evaporation is ceaseless ; and none of the 

 vapors, which the scorching winds that blow over it carry away, 

 are returned to it again in the shape of rains. 



240. The Red Sea vapors are carried off and precipitated else- 

 where. The depression in the level of its head waters in the sum- 

 mer time, therefore, it appears, is owing quite as much to the effect 

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



* Johnston's Physical Atlas. 



