48 THE BOTTOM OF THE SEA. 



the 16*th of south latitude, in the Atlantic Ocean. 

 Above these latitudes the saltness diminishes con- 

 tinually to the polar regions. 



On the other hand, we must not forget that there 

 exists near the equator a zone (the Equatorial Calm 

 Belt), where the precipitation of the vapour of water, 

 in the form of rain, is almost incessant. To the 

 north and south the trade-winds sweep the surface 

 of the sea, and the atmosphere is clear, or only- 

 shadowed by clouds floating towards the region of 

 rains. We also find, on approaching the poles, two 

 zones of variable winds, where frequent tempests 

 disturb the air, and cause abundant condensations. 

 The relation of evaporation and rain to the saltness 

 of the sea is thus made evident. 



In the polar regions the saltness of the sea is mo- 

 dified by another cause not less active than the 

 above — namely, the melting of the ice, accumulated 

 like two vast cowls over the extremities of the earth. 

 Every year, during the summer of each hemisphere, 

 torrents of fresh-w^ater are poured out towards the 

 temperate regions. These torrents gradually mingle 

 with the salt-water of the ocean, upon which they 

 first flow along as a river on its bed ; and as a con- 

 sequence of this, and the other active causes to which 

 we have alluded, the saltness of the sea grows less at 

 the surface in proportion as we approach the poles. 



