84 THE BOTTOM OF THE SEA. 



miles an hour. The mean depth of the ocean be- 

 tween Japan and California is from 12,000 to 13,000 

 feet. 



Again, as the tides are movements produced in the 

 great mass of the waters, they are likely to cause 

 certain effects on the bottoms of seas which it would 

 be useful to ascertain. But that action takes place 

 near the surface, and is difficult to distinguish from 

 the effects of the weaves. 



All the causes of the agitation of the sea are ex- 

 terior to it. However far their influence extends, we 

 may yet conceive that a limit exists beyond which it 

 ceases to produce itself. The " waves of translation " 

 alone heave up the ocean from its lowest depths ; 

 but this phenomenon only takes place in connection 

 with earthquakes, and is, happily, of rare and but 

 momentary occurrence. 



Marine currents extend to a great depth; they 

 sweep through the ocean one above another, and it 

 has be©n proved, by the soundings made during the 

 lust few years, that their effects extend, we may al- 

 most say, to the lowest deeps. It has been ascer- 

 tained that in certain regions they are sensible at 

 from 10,000 to 12,000 feet from the surface. It is 

 certain, however, that their course is less violent in 

 proportion to their depth ; and in the deepest recesses 

 of the ocean, if any movement could be supposed to 



