Otto von Guericke, a scientist and mayor…

Otto von Guericke, a scientist and mayor of Magdeburg, makes a presentation in front of the Imperial Diet and the Emperor Ferdinand III in Regensburg on May 8, 1654, and proves the existence of atmospheric pressure with his so-called Magdeburg hemispheres, a pair of large copper hemispheres with mating rims.

When the rims are sealed with grease and the air is pumped out, the sphere contains a vacuum and cannot be pulled apart by two teams of fifteen horses each.

The first artificial vacuum had been produced a few years earlier by Evangelista Torricelli, and had inspired von Guericke to design the world's first vacuum pump, which consists of a piston and cylinder with one-way flap valves.

Von Guericke uses the hemispheres these to demonstrate his air pump and the concept of atmospheric pressure.

One of these has a tube connection to attach the pump, with a valve to close it off.

When the air is sucked out from inside the hemispheres, and the valve is closed, the hose from the pump can be detached, and they are held firmly together by the air pressure of the surrounding atmosphere.

The force holding the hemispheres together is equal to the area bounded by the joint between the hemispheres, a circle with a diameter of fifty centimeters, multiplied by the difference in air pressure between the inside and the outside.

It is unclear how good a vacuum von Guericke's pump was able to achieve, but if it was able to evacuate all of the air from the inside, the hemispheres would have been held together with a force of around 20,000 N (4,500 lbf), equivalent to lifting a car or small elephant; a dramatic demonstration of the pressure of the atmosphere.