UNDER-CURRENTS FROM THE POLES. 195 



the particles of powder, that currents are set up in 

 opposite directions through tlie water. Warm 

 water rises from the bottom, up through the 

 middle of the vessel, and spreads over the surface, 

 while the colder and therefore heavier liquid falls 

 down at the sides of the glass. Currents like 

 these must arise in all water-basins, and even in the 

 oceans, if different parts of their surface are un- 

 equally heated. 



The water that is cooled in the polar regions 

 sinks and travels from the poles towards the 

 equator, pushing away the warmer and lighter 

 liquid from the bottom of the sea, itself to give 

 way in turn, as it gets warm, to the colder water 

 that follows after it. This continual flow of the 

 water from the cold zones is replaced in a twofold 

 manner. The warm water of the tropical seas, 

 since it is the lightest, must spread itself north 

 and south over the surface of the ocean, and thus, 

 gradually losing its heat, be carried to the polar 

 regions. Between the tropics, too, evaporation 

 goes on most vigorously, and a great part of the 

 vapours formed fall again in rain and snow only in 

 higher latitudes. 



The direction of the surface-current of water 

 setting from the equator to the pole, and that of 

 the opposite movement in the depths of the ocean, 

 are somewhat altered by the turning of the world 

 round its axis. It is clear, that the only reason 



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