Students' ideas about Blaise Pascal experiment at the Puy de Dôme Mountain

International audience; In 1647, Blaise Pascal suggests to raise Torricelli's mercury barometer at the top of the Puy de Dome Mountain (France) in order to test the "weight of air" assumption which can be considered as the primitive form of the air pressure concept. This experiment, conducted in 1648, takes place in the backdrop of the controversy surrounding the existence of vacuum and is variously interpreted. Thus, the "weight of air" assumption as a cause of the variations observed during the ascent of the Puy de Dome is not unanimously approved among the scholars of the seventeenth century. We find a similar difficulty among students interviewed after instruction: they struggle in considering Torricelli's device as a measuring instrument associated with the changes of the air pressure.