Where Pascal proved air has weight in 1648 using a mercury barometer at the summit versus the base.
Carsten Steger, CC BY-SA 4.0, via Wikimedia Commons
The weight of the world
On September 19, 1648, an important experiment took place on the slopes of the Puy de Dôme, a dormant volcano in central France. Blaise Pascal, the mathematician and physicist, sought to prove a revolutionary idea for the time: that the air in our atmosphere has weight. Unable to make the journey himself, he entrusted his brother-in-law, Florin Périer, to perform the measurements.
Périer began by setting up a Torricellian barometer in the town of Clermont-Ferrand, at the base of the mountain. The instrument consisted of a glass tube filled with mercury, inverted in a dish of mercury. The height of the mercury column was determined by the pressure of the surrounding air. Périer then ascended the Puy de Dôme, which rises to 1,465 meters (4,806 feet), and took another reading at the summit. Just as Pascal had predicted, the column of mercury at the top was shorter than the one at the base—specifically by about 85 millimeters.. This difference demonstrated that the air at higher altitudes exerted less pressure, proving that air indeed has mass. This experiment confirmed Evangelista Torricelli's theory about atmospheric pressure and forever changed our understanding of the Earth's atmosphere.
A modern mountain observatory
The scientific legacy of Pascal's experiment continues today at the Puy de Dôme Observatory. The first permanent mountain meteorology laboratory in France was established here in July 1876. Now operated by the Observatoire de Physique du Globe de Clermont-Ferrand (OPGC), the modern research station is a site for studying atmospheric composition..
The observatory sits within a protected natural park, and its location at 1,465 meters above sea level often places it in the free troposphere, above the local pollution layer. This makes it an ideal spot for collecting data representative of the regional background atmosphere. Researchers at the station conduct long-term measurements of greenhouse gases like carbon dioxide (CO2) and methane (CH4), as well as aerosols, cloud microphysics, and various reactive gases. The station is part of several international atmospheric monitoring networks, including the Integrated Carbon Observation System (ICOS) and the Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS). Data from Puy de Dôme contributes to a global understanding of climate change and air quality. The facility is equipped with a wind tunnel for studying cloud properties in natural icing conditions during the winter.
💡Fun Facts
Before becoming a scientific site, the summit was a spiritual center, housing a Roman temple dedicated to the god Mercury.
In 1911, aviator Eugène Renaux won a prize of 100,000 gold francs for being the first to fly from Paris and land on the summit.
The volcano is geologically young, having formed from an eruption around 10,700 years ago.
The Puy de Dôme is a UNESCO World Heritage site as part of the Chaîne des Puys volcanic alignment.
The site is accessible year-round. The Panoramique des Dômes train runs frequently, with schedules varying by season (every 20 minutes in very high season, to hourly in the off-season).
Admission
Access to the summit is free, but the rack railway, Panoramique des Dômes, has a fee. 2025 round-trip adult tickets are listed at €17.20. Prices vary by season.
Accessibility
The summit is accessible via a rack railway, the Panoramique des Dômes, which takes about 15 minutes and is accessible to people with reduced mobility. Visitors can also hike to the top via two main trails: the Chemin des Muletiers (a former Roman road) or the Chemin des Chèvres. Driving a private car to the summit is not permitted.
