PHYSICAL PARADOXES. 



Let Fig. 31 represent a section through a 

 pond. The water on the surface A, exposed to 

 cold east winds, or radiation into a clear sky, 

 is cooled. Cooling makes it contract and 

 become heavier than the rest, so that it sinks 

 and forces the lighter warm water to the top. 

 The cooled water becomes slightly warmed 

 by contact with the ground at the bottom, 

 BCD, while the water now at the top be- 



Flo. 31.-CONVECTION-CURRENTS IN WATER. ! ; 



comes as cold as that which was originally there, 

 and in time colder. It is therefore now the 

 heavier, and sinks in its turn, driving the other 

 up again. Thus are maintained the convec- 

 tion-currents which keep quietly circulating 

 until the water is all cooled down to 4 C. 



What happens then ? The top layer, which 

 gets cooled below 4, ceases, as we have seen, 

 to contract, and begins to expand again, as it 

 gets colder. But expansion makes it lighter. 

 The same quantity occupies more space. The 

 same space, therefore, contains a smaller quan- 

 tity or weight. Thus the water cooled below 



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