Institute and Museum of History of Science, Florence, ITALY

      "At the bottom of an ocean of air"        
Relative pressures 25 billion billion molecules Why do aeroplanes fly?

Experiment on the force of the vacuum.
Gaspar Schott, Technica curiosa, sive, Mirabilia artis, Würzburg 1664



Despite the fact that it seems untouchable, the air around us can hold up cargo-planes and blow down buildings. What gives it this substance, out of reach of our senses? Air contains an incredible number of molecules: around 25 billion billion per cm³ . In the space equivalent to a pin-head, there are millions of times as many molecules as there are people living on our planet.
Air molecules (which are 99% nitrogen and oxygen) move tirelessly at the speed of 450 metres/second (faster than the speed of sound) and each molecule hits other ones 5 billion times a second. The pressure of a gas comes from these collisions. This is why the billions of molecules which push, for example, inside a tube of a tyre, give a shove powerful enough to hold up a lorry. The movement of these molecules, and therefore also their pressure, goes up noticeably when the temperature goes up. And that's one of the basic principles of thermodynamics.



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