The summit of Mont Blanc – today measured at around 4805 m – was conquered for the first time 240 years ago, on 8 August 1786, by the doctor Michel Gabriel Paccard and by the crystal seeker Jacques Balmat. This is where the truth comes in deus ex machinof this story, de Saussure, who didn’t even climb Mont Blanc that day in 1786. In fact, air has weight and above our heads there is a column of air kilometers high that presses downwards: this is atmospheric pressure. Mont Blanc offered the opportunity to repeat that experiment at a much higher altitude and with instruments that had in the meantime become more manageable and precise. It was the second ascent of Mont Blanc.