Institute and Museum of History of Science, Florence, ITALY

      "At the bottom of an ocean of air"        
Why do aeroplanes fly? Weighing the air with mercury Forecasting the weather

Torricellian Experiment.
Gaspar Schott, Technica curiosa, sive, Mirabilia artis, Würzburg 1664



The barometer is the instrument which lets us "weigh" air. It works in much the same way as a set of scales: the column of mercury is counter-balanced by the pressure of air on a bowl full of mercury.
Torricelli discovered in 1644 that the level of mercury contained in a tube closed at one end, tipped upside down and lowered into a bowl of mercury, went down only partially because it was counter-balanced by the pressure of air on the mercury in the bowl. He also discovered that the height of the column of mercury varied, at the same altitude, with variations in temperature. Thanks to this discovery, it was possible to perfect the mercury thermometer.
At sea level, at 0°C and at a latitude of 45, the level of mercury in the little column stays at around 76 cm, whereas on top of a 2,500 m mountain, it settles at only 57 cm.




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