SEA WATER 



233 



uniform salinity cooling in its surface layers, when the den- 

 sity of this surface water increases and it sinks downwards. 

 Upwelling of water occurs when a deep current meets a 

 submerged bank or the shelving bottom of coastal regions : 

 it can also be brought about by persistent off-shore winds, 

 which blow the surface water outwards and this is replaced 

 by water from below (Fig. 48). The upwelling of water 

 is of the greatest importance, for the deep waters of the 

 sea abound in those nutrient salts, the phosphates and 

 nitrates, and when water from deeper levels rises to the 

 surface it brings with it 



this store of nutriment 

 to replenish the 

 impoverished surface 

 waters (see page 246). 

 This is why, in the 

 region of submerged 

 shelves and in coastal 

 areas, life is so abun- 

 dant. 



Tides 



On shore m'nd 



Fig. 48. — Diagrams to show the currents 

 set up by on- and off-shore winds. 



Apart from these 

 great circulatory cur- 

 rents in the ocean there are other periodic movements 

 brought about by the attraction of the sun and moon on 

 the waters. These are the tides. 



The mass of the moon exerts an attracting force on the 

 particles on the earth. This force is greater on those 

 particles which lie closest to the moon, but it is extremely 

 small, being only one ten-millionth of the earth's pull on 

 those same particles. Let us imagine the earth to be 

 stationary and covered uniformly all over with water. 

 The pull of the moon on the water immediately beneath 



