Magdeburg's hemispheres(cast iron) evacuated, can't be separated.

Magdeburg's hemispheres (cast iron) evacuated, can't be separated. When air is allowed into the cavity,the hemispheres can be separated easily. When cavity is evacuated with vacuum pump, hemispheres cannot be separated... FLUID STATICS.
UCB Index: 
C+30+5
PIRA Index: 
2B30.30
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