188 THE OCEAN. [Book VIH. 



actually exists is caused, rather than how a system of 

 circulation might be caused in the absence of any 

 more powerful cause. And, if the actual system of 

 oceanic circulation be in accordance with the theo- 

 retical action of vis-inertise, it must then be admitted 

 that this force is the great prime cause of the ocean 

 currents by which a constant interchange of equa- 

 torial and polar waters is effected ; so that, by its 

 action, all portions of the waters of the ocean are in 

 their turn alternately exposed to the heat of equa- 

 torial and the cold of polar regions without the 

 action of the agencies so interestingly described 

 by Captain Maury, tending to cause differences of 

 specific gravity, being brought into play. Regarding 

 the action of these agencies and that of the winds as 

 comparatively trivial forces, the theoretical action of 

 vis-inertise described in Book 11. serves to discover 

 and to explain the actual course of the currents by 

 which the circulation of the ocean is effected. 



Sir John Herschel is surprised how Captain 

 Maury can doubt that the winds are the great prime 

 cause of ocean currents ; and, indeed, theoretically 

 considered, it appears plausible enough to assume 

 that the winds must tend, to some extent, to cause 

 a system of currents, by driving the surface-water 

 before them ; which, wherever it accumulates against 

 obstructions, must tend to run off in streams. But, 

 practically considered — that is to say, considering 



