AQUEOUS CAUSES OF CHANGE. 221 



CHAPTER V. 



Currents. 



** Thy shores are empires, changed in all save thee 

 Assyria, Greece, Rome, Carthage, what are they ? 

 Thy waters wasted them while they were free, 

 And many a tyrant since ; their shores obey 

 The stranger, slave, or savage ; their decay 

 Has dried up realms to deserts : not so thou, 

 Unchangeable, save to thy wild wave's play 

 Time writes no wrinkle on thine azure brow 

 Such as Creation's dawn beheld, thou rollestnow !'* 



Byron. 



WE are now to consider the remaining aqueous causes o-f 

 change, currents and tides. The joint action of these produce 

 mutations of great geological interest. The tides, or the great 

 tidal waves which flow over the surface of the ocean at stated in- 

 tervals, are mainly caused by the attraction of the moon, and 

 hence we may show, and by no very extended chain of causation, 

 that the effect of the moon in altering and keeping in a state of 

 perpetual mutation the face of the earth, is by no means incon- 

 siderable. A more remote cause, the rotation of the earth upon 

 its axis, produces in part at least, great currents which constantly 

 flow in vast circuits in the Atlantic, Pacific, and Indian oceans. 

 In addition to the circular currents which thus flow through th& 

 oceans which we have named, there are immense bodies of cold 

 water continually moving from the polar regions towards the 

 central portions of the earth, and, as these currents are exhibited 

 superficially, or on the surface, bearing down immense fields of 

 ice, and since the storms and fogs of those regions are not suffi- 

 cient to supply this waste of the waters, we may infer that an un- 

 der current of warmer water passes continually from the equa- 

 torial to the polar regions. The polar current of the southern re- 

 gions seems to be more powerful than that of the northern. lee 



