In 1657, in front of the Imperial Diet in Magdeburg, Otto von Guericke set up a famous and spectacular experiment witnessed by vast crowds of local residents.
Von Guericke used two perfectly matching hollow bronze half-spheres measuring about 50 centimeters in diameter. He fastened them together tightly to form a sphere. Using an air pump, he extracted the air from the sphere through a valve. When he had created the vacuum and removed the external connectors between the two hemispheres, von Guericke was able to show to the astonished spectators that two teams of eight horses pulling in opposite directions barely sufficed to separate the two emptied hemispheres.
This experiment demonstrated the force of atmospheric pressure when it is not offset by the internal pressure of the hemispheres. To separate them, all that was needed was to let the air back in. To open a compound sphere 50 centimeters in diameter similar to the Magdeburg sphere requires about 2,000 kilograms-force.