PHENOMENA OF OCEANIC CIRCULATION. 21 



accomplished every part of its beneficent mission on 

 land, the remainder pours down again in the form of 

 great rivers, to mingle with the ocean from whence it 

 originally came. 



Such is a brief outline of some of the principal 

 phenomena of the great circulation of water by the 

 atmospheric or gaseous ocean overhead, which we have 

 considered more minutely elsewhere in these pages.* 



Turning now to that of the liquid expanse of the 

 mighty deep we find that the evaporation of water 

 upon this gigantic scale, the alteration of its specific 

 gravity at the surface of the ocean, and its ex- 

 pansion by means of heat, produce there a vacuum, 

 which of course has to be filled by the heavier and colder 

 water from below: and in this way, as is generally 

 Relieved, a movement has gradually been communi- 

 cated to the whole body of water throughout the ocean ; 

 the dense cold water, as has been already pointed 

 out, creeping down along the bottom from both the 

 polar regions in the direction of the equator, thus 

 accounting for the icy chill of the water observed at 

 great depths ; and finally these opposite streams, meeting 

 in the equatorial region, cause an upcast stream of cold 

 water to be continually rising there, to supply the void 

 caused by the movement of the heated water above; 

 and so it is supposed the great ocean currents are set 

 in motion. 



Many learned works have- however been written 

 upon this complex and difficult subject, thus roughly 

 sketched in a few brief sentences; and a careful study 

 of the views held by the leading authorities shows 

 that this opinion that the cause of the great oceanic 



* See Chapter iv. on " Climates and Temperatures." 



