"To these objections Pascal replied in two letters, addressed to [Stephen] Noel; but though he had no difficulty in overturning the contemptible reasoning of his antagonist, he found it necessary to appeal to new and more direct experiments. The explanation of Toricelli had been communicated to him a short time after the publication of his work; and assuming that the mercury in the Toricellian tube was suspended by the weight or pressure of the air, he drew the conclusion that the mercury would stand at different heights in the tube, if the column of air was more or less high. These differences, however, were too small to be observed under ordinary circumstances; and he therefore conceived the idea of observing the mercury at Clermont, a town in Auvergne... and on the top of the Puy de Dome, a mountain 500 toises above Clermont The state of his own health did not permit him to undertake a journey... but in a letter dated the 15th November 1647, he requested his brother-in-law, M. Perier, to go... M. Perier began the experiment by pouring into a vessel sixteen pounds of quicksilver which he had rectified... He then took two [straight] glass tubes, four feet long, of the same bore, and hermetically sealed at one end, and open at the other; and making the ordinary experiment of a vacuum with both, he found that the mercury stood in each of them at the same level... This experiment was repeated twice with the same result. One of these... was left under the care of M. Chastin... who undertook to observe and mark any changes... and the party... set out, with the other tube, for the summit of the Puy de Dome... Upon arriving there, they found that the mercury stood at the height of 23 inches, and 2 lines—no less than 3 inches and 1½ lines lower... The party was "struck with admiration and astonishment at this result;" and "so great was their surprise, that they resolved to repeat the experiment under various forms." During their descent of the mountain, they repeated the experiment at Lafond de l'Arbre, an intermediate station... and they found the mercury to stand at the height of 25 inches, a result with which the party was greatly pleased, as indicating the relation between the height of the mercury and the height of the station. Upon reaching the Minimes, they found that the mercury had not changed its height..."