Magdeburg Hemisphere
The two hemispheres are joined with an airtight seal and a vacuum pump draws air out of the sphere. Initially the air pressure inside and outside the sphere is the same (normal atmospheric pressure) so there is a balance of forces on the inside and outside surfaces of the sphere.
By removing some of the air from inside the sphere the internal pressure decreases. Inward force from the outside pressure now exceeds the outward force from the internal pressure and the two hemispheres are forced together by the pressure difference.
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