x The Hydrosphere 179 



warm water becomes banked up to a considerable depth, 

 displacing the cold lower water, which slips seaward as an 

 undercurrent (B, Fig. 35). During a prevailing sea-wind 

 the water along the shore assumes what may thus be called 

 an on-shore condition, just as by blowing steadily across 

 a milk dish one might drive the cream to one side, and 

 even blow it up on the shelving lip, completely displacing the 

 milk on that shallow coast. A wind from the land in like 

 manner drives the warm surface water seaward, and colder 

 water from a great depth wells up to take its place (A, Fig. 

 35), this being characteristic of an off-shore condition. 

 This enables us to understand how the permanent winds of 

 the Earth which blow steadily off shore (like the trade winds 

 from the west coasts of Africa and South America, 179) 

 cause cold water to well up from great depths. The up- 

 welling off the coast of south-western Africa and off the 

 coast of Morocco explains the exceptionally low sea surface 

 and air temperatures observed in these neighbourhoods, 

 and similar conditions are found on the west coasts of 

 Australia and South America. Where the prevailing winds 

 blow against the land, as on the north-east of South 

 America into the Caribbean Sea, and toward Western 

 Europe, the sea assumes a permanent on-shore condition, 

 the warm surface water from the tropics being piled up 

 against the land, while the colder deep water natural to 

 the locality slips away seaward. The effect of the pre- 

 vailing winds of the world is to set up a general skimming 

 of the ocean from the equator poleward, sweeping the 

 warm surface water away to one side t and allowing cold 

 water from the depths to rise up, completing the vertical 

 circulation. 



242. Wind and Ocean Currents. In a strong gale the 

 wind blows off the crests of the waves in spray or spindrift, 

 and even a moderate breeze sweeps forward a thin layer of 

 surface water over the ridged surface of the sea, giving rise 

 to what is called a surface drift. The currents of the Indian 

 Ocean and of the sea off the west coast of Central America 

 change twice a year with the changing of the monsoons, and 

 it is recognised that these currents are produced solely by the 



