The Magdeburg hemispheres were one of Otto von Guericke's most interesting experiments, designed to illustrate the principle of air pressure. They were two hollow hemispheres a little over a foot in diameter which fitted together so well that the air could be pumped out from between them. The pressure of the surrounding atmosphere then held them together firmly. 1657, in a test before the Reichstag and the Emperor in Regensburg, it required sixteen horses, four pairs on each hemisphere to pull them apart.