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The first person to measure atmospheric pressure was a mathematician by the name of Evangelista Torricelli, in 1643. He did so by using a tube of mercury, closed at the top, held in a bowl of mercury.

He noticed that some days the mercury in the tube was at a higher level than other days. He found out that when air pushed down on the bowl of mercury, it was pushed up the tube. This is why you sometimes hear 'inches of mercury' as a measurement of air pressure!