150 THE PHYSICAL GEOGRAPHY OF THE SEA. 



the stream of the oceanic or parent wave." " They are clearly to 

 be accounted for," says he, " by the streams acting obliquely upon 

 each other." 



403. It is not necessary to associate with oceanic currents the 

 idea that they must of necessity, as on land, run from a higher to 

 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 (§ 9). 



404. Tlie currents which run from the Atlantic into the Medi- 

 terranean, and from the Indian Ocean into tlie Red Sea, are the 

 reverse of this. Here the bottom of the current is probably a wa- 

 ter-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. 



405. 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 diiference 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. 



406. But from May to October is also the hot season ; it 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, we may suppose the daily evapora- 

 tion 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, v/hen taking the capacity of his feeders into calculation, 

 has to consider. With him it is an important clement ; 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 



* Johnston's Physical Atlas. 



