PARADOXICAL CONTRACTION. 



4, instead of having any tendency to sink, 

 floats because it is lighter than the rest. It 

 therefore remains at the top, getting colder 

 and lighter still. The convection-currents cease 

 when once all the water has been cooled to 4. 



The top layer of water thus being un- 

 changed, and so being unceasingly acted on by 

 the cold wind and sky, is more rapidly cooled 

 down the few remaining degrees to freezing 

 point, and then begins to freeze, while the bulk 

 of the water beneath it is not cooled below 4. 

 The water having in the act of freezing ex- 

 panded much more than in cooling down to 

 freezing point, the ice so formed is lighter in a 

 still greater degree, and floats very strongly. 

 So we get from the paradoxical way in which 

 water and ice expand with cold the limitation 

 of ice to the upper layers, and the freedom 

 from severe cold of the water beneath it. 



Now, what would happen if water, like other 

 things, went on contracting indefinitely with 

 increase of cold ? In the first place, the con- 

 vection-currents would not be checked, and 

 would promote the cooling of the water till it 

 was cooled down to freezing point right through 

 to the bottom. Secondly, the formation of ice 

 being accompanied by still greater contraction, 

 the ice would be heavier than the water, and 

 therefore as fast as it was formed it would sink 

 to the bottom. The ice formed later would 

 sink in its turn and accumulate above that 



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