Magdeburg hemisphere

Etymology
They were designed by Otto von Guericke, a German scientist and mayor of Magdeburg, in 1656.

Noun

 * 1)  Either of a pair of large copper hemispheres, with mating rims, that were used to demonstrate the power of atmospheric pressure. When the rims were sealed with grease and the air was pumped out, the sphere contained a vacuum and could not be pulled apart even by teams of horses.