Puy de Dôme

Puy de Dôme is perhaps the most well-known mountain in the Auvergne. For one thing it lies right outside a city of 150,000 inhabitants, Clermont-Ferrand, and you can even drive the 1100m/3300 ft. to the top on a road that spirals around the mountain, the Tour de France has been there several times too. The view is beautiful and very interesting with a large number of extinct volcanoes. Secondly, it was on Puy de Dôme that the existence of the atmospheric pressure was first proved by the famous French physicist and mathematician, Blaise Pascal. The Italian physicist Torricelli had in 1643 discovered that if you filled a glass tube about a meter (3. ft.) long and closed at one end with Mercury, put a finger over the open end and put it under the surface of a bowl filled with Mercury, the level of the mercury in the tube would drop to a height of 76 cm/29.92 in. and stay there, although moving up and down by a centimeter or two, especially in times of changing weather. This is what we now call a (primitive) barometer. Torricelli had proposed that it was the pressure of the air that carried the column of mercury, but had not been able to prove that. Pascal realized that if this explanation was true, then a barometer carried to a higher place should show a lower column of mercury. Enlisting his brother-in-law to help, he in 1648 had the set-up described above carried to the top of Puy de Dôme, and lo and behold, the column of mercury dropped by 8.4 centimeters/3.30 in., thus establishing the reality of the atmospheric pressure. This is the reason why pressure is now measured in Pascal (Pa).