The Atmosphere

Air and Air Pressure

Who showed that air pressure decreases as altitude increases?

French physicist and mathematician Blaise Pascal (1623–1662) was inspired by fellow physicist Evangelista Torricelli (1608–1647) to test the idea that the air in the atmosphere is much like seawater in an ocean. Since pressure in an ocean or lake increases as one descends into the depths, Pascal hypothesized that air pressure in a valley would be higher than on top of a mountain. To test the idea, in 1646 he asked his brother-in-law, Florin Perier (1605–1672), to use a barometer (a new invention at the time) and measure the pressure both at the top of the French volcanic peak Puy de Dôme and in the village of Clermont-Ferrand in Auvergne. The difference in elevation between the two is about 3,900 feet (1,200 meters). Taking along witnesses to verify his readings, Perier found that the air pressure at Clermont-Ferrand was 28 inches of mercury and at the top of Puy de Dôme it was only 24.6 inches. A monument to Pascal was later erected in Clermont-Ferrand to honor this achievement, and there is also a meteorological observatory atop Puy de Dôme.


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