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Magdeburg Hemispheres

Magdeburg hemispheres are two half-spheres of equal size. Placing them together traps air between them. This air is merely trapped, and not compressed, so the pressure inside is the same as the pressure of the atmosphere outside the spheres. That is to say, the air is pushing equally strongly inside as it is outside. The spheres thus pull apart with nearly no resistance. Now what if all the air were removed from the inside? Then no air remains to push on the inside of the sphere, but the air outside is pushing at atmospheric pressure. The push from the outside isn't balanced by another push on the inside, so the two hemispheres become very hard to separate.