Here's a classic experiment involving air pressure, one of the earliest demonstrations placed online by "Florencefst," the Fondazione Scienza e Tecnica's collection of scientific YouTube videos.
The introduction is Italian; in English, it says "Hemispheres of Magdeburg," and indicates that this demonstration apparatus dates from the first half of the nineteenth century.
The top and bottom brass hemispheres are placed together. Next the bottom hemisphere, which incorporates a valve, is connected through a hose to a vacuum pump (which is not visible) and a mercury-filled pressure gauge (the glass device on the right). As the pump is operated, we see the level of mercury change, indicating that the pump is removing most of the air from within the hemispheres.
The valve attached to the bottom hemisphere is closed. Now the vessel contains a vacuum. Well, it would be more precise to say that the air pressure within the hemispheres is much lower than the air pressure in the room outside, or that the hemispheres contain very little air.
In this state, it would take a very large force to separate the hemispheres. The total force caused by external air pressure is larger than the tiny force exerted by the small amount of air remaining within. Our demonstrator tugs on the apparatus, but is unable to pull the hemispheres apart.
But open the valve on the bottom hemisphere, and outside air rushes in. Now the force inside is the same as the force outside, and it has become quite easy to pull the two pieces apart.
For centuries, such hemispheres have been employed to impress students with the sizable forces exerted by the invisible air around us.
Why "Hemispheres of Magdeburg?" Because this experiment originated back in 1657, as researcher Otto von Guericke, of Magdeburg, Germany, fabricated a pair of copper hemispheres 50 centimeters in diameter. When von Guericke joined and pumped out these hemispheres, he showed dramatically that not even two powerful teams of horses could supply enough force to pull them apart. The demonstration was a spectacular sight to the onlookers in Magdeburg, and, in the centuries since, a vivid image to conjure in physics classrooms whenever the story is retold.
The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.