In 1654, two teams of 15 horses could not pull apart two copper hemispheres held together only by a vacuum.

In May 1654, Otto von Guericke, the mayor of Magdeburg and a physicist, performed this famous experiment before Emperor Ferdinand III. He used two copper hemispheres about 50 centimeters in diameter, known today as the Magdeburg hemispheres. By using a vacuum pump he had recently invented, he removed the air from the sealed sphere.The experiment demonstrated that the atmosphere exerts a pressure of approximately 101,325 Pascals at sea level. Because there was no air inside to push back, the external air pressure pushed the hemispheres together with a force equivalent to lifting a weight of over 2,000 kilograms. This massive pressure effectively locked the copper bowls together through the weight of the air alone.Guericke used two teams of 15 horses, totaling 30 animals, to pull against each other in an attempt to separate the vacuum-sealed halves. Despite their immense power, the horses were unable to overcome the atmospheric force. This proved his theory that the Earth is surrounded by a heavy atmosphere that exerts constant pressure on everything.The experiment was a landmark in the history of physics, as it challenged the long-held Aristotelian belief that 'nature abhors a vacuum.' It provided the first clear evidence of the physical weight of air and the existence of a vacuum. Guericke's work laid the foundation for future developments in steam engines and thermodynamics.