Magdeburg hemispheres

Otto von Guericke, a German scientist and also the mayor of Magdeburg, invented a vacuum pump and studied the effects of vacuum and air pressure. In one of his most spectacular experiments, he fit two copper hemispheres together and used his pump to create a vacuum inside. This created a force holding the hemispheres together that was so great, teams of horses could not pull them apart until the valve was opened and air was let back inside. This experiment was illustrated here by Tim v.F. for the New Scientist LEGO contest.