Question: what are magdeburg hemispheres ? 

Answer: The Magdeburg hemispheres were one of Otto von Guericke's most interesting experiments, designed to demonstrate the air pump he had invented. They were two hollow copper hemispheres a little over a foot in diameter which fitted together so well that they could hold a vacuum. The air was pumped out from between them, and they were then held firmly together by the air pressure of the surrounding atmosphere.
This question from the Yahoo Q&A forum is categorized in the topic: Computers & Internet