122 OUR HERITAGE THE SEA ' 



Tide, on the other hand, resembles the course of 

 a body of water that is regulated mechanically, as by 

 pumping, and is regularly propelled in one direction 

 for a certain number of hours per day, and in the 

 opposite for another similar period, independent of 

 any considerations of weather, rainfall, etc. Here, 

 again, the simile is weak and halting, but it must 

 serve as giving a slight idea of the difference between 

 tide and current. 



Now, as regards the permanent character of oceanic 

 currents in speed and direction, there is very much 

 to be said. Indeed, it would appear as if, at the 

 outset, we must admit that no ocean current is per- 

 manent in the particular sense with which that word 

 is used. In a general sense, the great oceanic currents 

 are permanent, that is, they run continually in the 

 same general direction and at the same general rate, 

 only varying either under some great atmospheric or 

 submarine disturbance. Now, when we say the great 

 oceanic currents, we mean the Gulf Stream and 

 equatorial current in the Atlantic, the Kuro Siwo or 

 Japanese current and equatorial current in the Pacific, 

 these Atlantic and Pacific currents being curiously 

 alike, allowing for the different configuration of the 

 land and magnitude of the two oceans. The Mozam- 

 bique current in the Indian Ocean and Humboldt's 

 current in the South Pacific complete the list of the 

 great oceanic currents that have received specific 

 names. 



It has, however, been considered that all the great 

 currents of the ocean commence within the tropics, 

 and that their primary cause is the enormous amount 

 of evaporation that is continually going on under the 



