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trated in figure 10, and is the phenomenon that makes ice skating 

 possible. Since the brine cells in the ice have a greater specific grav- 

 ity than the ice crystals, the ice in contact with the bottom of each 

 brine cell is under a slightly greater pressure than the rest of the 

 block, and the ice directly above each cell is under a slightly lower 

 pressure. Each cell therefore is slowly melting the ice directly under 

 it, while the ice is resolidifying over it. The brine therefore slowly 

 travels downward under the influence of gravity, leaving the lighter 

 ice behind. This process is greatly accelerated the following sum- 

 mer, when the temperature may not be low enough to refreeze the 

 melting above the brine cells. The result is that air holes remain in 

 the ice where the brine cells have been, and ice with a specific gravity 

 under 0.90 forms. 



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