Institute and Museum of History of Science, Florence, ITALY

      "At the bottom of an ocean of air"        
The unbearable lightness of air Relative pressures 25 billion billion molecules

Experiments on the weight of air.
Daniello Bartoli, La tensione e la pressione disputanti qual di loro sostenga l'argento vivo ne' cannelli dopo fattone il vuoto, Roma 1677



Atmospheric pressure exerts itself in every direction. So, on different points of every surface there are vertical and horizontal pressures at work.
In the same way as happens for a body immersed in liquid, the result of the various pressures is a vertical push upwards equal to the weight of air displaced. Air-pressure, then, does not have the sole effect of crushing bodies, but can also lift them up, with a force which is usually weak, given that their mass is almost always greater than that of the air displaced. We see one use of this principle in hot-air balloons, which go up because their mass is less than that of an equal volume of air.
Moreover, atmospheric pressure does not squash our bodies, because they are counter-balanced by an equal internal pressure. A human being would explode in a vacuum, because his internal pressure would not be balanced by atmospheric pressure.




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