What is "magdeburg hemispheres"

Magdeburg \Mag"de*burg\, n. A city of Saxony.

Magdeburg centuries, Magdeburg hemispheres. See under Century, and Hemisphere.

Hemisphere \Hem"i*sphere\, n. [L. hemisphaerium, Gr. ?; ? half = ? sphere: cf. F. h['e]misph[`e]re. See Hemi-, and Sphere.]

1. A half sphere; one half of a sphere or globe, when divided by a plane passing through its center.

2. Half of the terrestrial globe, or a projection of the same in a map or picture.

3. The people who inhabit a hemisphere.

   He died . . . mourned by a hemisphere.
   --J. P. Peters.

   Cerebral hemispheres. (Anat.) See Brain.

   Magdeburg hemispheres (Physics), two hemispherical cups forming, when placed together, a cavity from which the air can be withdrawn by an air pump; -- used to illustrate the pressure of the air. So called because invented by Otto von Guericke at Magdeburg.

The Magdeburg hemispheres are a pair of large copper hemispheres, with mating rims. They 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 by teams of horses. The Magdeburg hemispheres were designed by a German scientist and mayor of Magdeburg, Otto von Guericke, to demonstrate the air pump that he had invented, and the concept of atmospheric pressure. The first artificial vacuum had been produced a few years earlier by Evangelista Torricelli, and had inspired Guericke to design the world's first vacuum pump, which consisted of a piston and cylinder with one-way flap valves. The hemispheres became popular in physics lectures as an illustration of the strength of air pressure, and are still used in education. A pair of the original hemispheres are preserved in the Deutsches Museum in Munich.