Move the hemispheres to the middle of the apparatus. Close the valve on the tube leading to the pump. Ask someone else to press both hemispheres against each other. Move the pump piston a couple of times. Check if the hemispheres can be easily separated. If not, they no longer need to be pressed. Work the piston a dozen more times to pump the air from the hemispheres inside out. Try to separate the hemispheres by pulling the ropes with your partner. Having conducted the experiment, open the valve.
Air molecules exert equal pressure on both inner and outer sides of the hemispheres. However, if we pump some air out, the pressure from the outside will increase; consequently, the hemispheres are kept pressed against each other. After approximately 50 piston movements the pressure inside hemispheres is about half of the atmospheric pressure, after 80 – 1/3, after 120 – 1/5.
The experiment with the metal hemispheres of the 42 cm diameter was conducted by Otto von Guericke in Magdeburg, in 1654. In order to bring those hemispheres apart, 16 horses were employed.