a1 University of Ottawa
Three hundred and forty-one years ago, on Saturday September 19, 1648, at eight a.m., Blaise Pascal's Great Experiment took place. Following Pascal's explicit instructions, his brother-in-law Florin Périer took two identical glass tubes and filled them with mercury. He inverted the tubes and submerged the open end in a bowl containing an inch of mercury. Thus he set up two Torricellian baroscopes in the garden of the Minim Monastery, at the base of the Puy-de-Dôme, one of the highest mountains in central France. The height of the mercury in each tube was carefully measured and marked on the glass. The measurements, which were identical, were recorded. One device was left in place, as a control baroscope, with an observer. The other was dismantled.