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