Although air is invisible, it still takes up space and has weight. In fact, it may surprise you to know that air weighs 14.7 pounds per column inch at sea level, or put another way, over 2,100 pounds per column cubic foot! Wow! The reason we don’t feel its weight is because air, like all fluids, doesn’t just push down. Instead, it pushes in all directions. Water has weight too, but you aren’t crushed when you swim to the bottom of a deep pool because water, like air, also pushes in all directions. But just try to lift all that water you’re swimming under. It weights over 62 pounds per cubic foot!
In 1656 Otto von Guericke performed a demonstration similar to the activity you just tried with the plungers. He used two metal hemispheres, which were 20 inches in diameter, and placed them together to form a single sphere. He had invented the world’s first vacuum pump in 1654. He used a vacuum pump to force air out of the sphere instead of into it. This created a vacuum which held the two hemispheres together tightly. No human could pull Otto’s sphere apart, so in front of Emperor Ferdinand III and other dignitaries, he attached two 15-horse teams to each end of his sphere. Despite a great effort, the horses could not pull his sphere apart. After all, they were trying to pull against a force of more than two tons!
When air is inside the sphere, it exerts the same amount of force as the air on the outside of the sphere. When you remove the air inside of the sphere, however, the air on the outside presses the two halves of the sphere together. If you peel the plungers apart slightly and let air back inside, the two sides will no longer stick together as the force on the inside and the outside will once again be the same.