Sound needs a medium such as air or water to propagate. So, in vacuum, no sound is possible. This was discovered by the German physicist Otto von Guericke in 1654. To show that sound cannot be transmitted in vacuum, a bell was placed under a glass bell-jar. When the air pressure under the bell-jar is equal to the atmospheric pressure, the bell can be heard clearly. Then the bell-jar is evacuated slowly. The thinner the air, the softer the sound, and eventually the sound fades away completely. Von Guericke was also the inventor of an air pump, which he frequently used in his experiments. Most famous is his experiment with the so-called Magdeburg hemispheres [See elsewhere in this cabinet, 135], which showed the power of air pressure.