This experiment is another way of showing
how strong the pressure of the atmosphere is. It was first done by the German scientist Otto
von Guericke in 1654 and uses two brass hemispheres, one with a tap.
The
hemispheres are put together and the air inside them pumped out using a vacuum pump.
The tap is then closed and the hemispheres disconnected from the pump. The difference in
pressure between the near vacuum inside the hemispheres and the normal air pressure
outside makes it very difficult to pull them apart. One early version of the experiment used
hemispheres about 30 cm in diameter and even two teams of horses couldn't separate
them!
