CHAPTER II. PASCAL’S SCIENTIFIC DISCOVERIES.

“On the morning of Saturday, the 19th September, the day fixed for the interesting observation, the weather was unsettled; but about five o’clock the summit of the Puy page 34de Dôme began to appear through the clouds, and Périer resolved to proceed with the experiment.  The leading characters in Clermont, whether ecclesiastics or laymen, had taken a deep interest in the subject, and had requested Périer to give them notice of his plans.  He accordingly summoned his friends, and at eight in the morning there assembled in the garden of the Pères Minimes, about a league below the town, M. Bannier, of the Pères Minimes; M. Mosnier, canon of the cathedral church; along with MM. la Ville and Begon, counsellors of the Court of Aides, and M. la Porte, doctor and professor of medicine in Clermont.  These five individuals were not only distinguished in their respective professions, but also by their scientific acquirements; and M. Périer expresses his delight at having been on this occasion associated with them.  M. Périer began the experiment by pouring into a vessel 16 lb. of quicksilver, which he had rectified during the three preceding days.  He then took two 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 and at the height of 26 inches 3½ lines.  This experiment was repeated twice, with the same result.  One of these glass tubes, with the mercury standing in it, was left under the care of M. Chastin, one of the Religious of the House, who undertook to observe and mark any changes in it that might take place during the day; and the party already named set out with the other tube for the summit of the Puy de Dôme, about 500 toises (a toise is about six feet in length) above their first station.  Before arriving there, they found that the mercury stood at the height of 23 inches and 2 lines—no less than 3 inches and 1½ line lower than it stood at the Minimes.  The party were ‘struck with admiration and astonishment page 35at this result;’ and ‘so great was their surprise that they resolved to repeat the experiment under various forms.’  The glass tube, or the barometer, as we may call it, was placed in various positions on the summit of ‘the mountain’—sometimes in the small chapel which is there; sometimes in an exposed and sometimes in a sheltered position; sometimes when the wind blew, and sometimes when it was calm; sometimes in rain, and sometimes in a fog: and under all these various influences, which fortunately took place during the same day, the quicksilver stood at the same height of 23 inches 2 lines.  During their descent of the mountain they repeated the experiment at *Lafon-de-l’Arbre*, an intermediate station, nearer the Minimes than the summit of the Puy, ‘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, notwithstanding the inconstancy of the weather, which had been alternately clear, windy, rainy, and foggy.  M. Périer repeated the experiments with both the glass tubes, and found the height of the mercury to be still 26 inches 3½ lines.  On the following morning M. de la Marc, priest of the Oratory, to whom M. Périer had mentioned the preceding results, proposed to have the experiment repeated at the top and bottom of the towers of Notre Dame in Clermont.  He accordingly yielded to his request, and found the difference to be 2 lines.  Upon comparing these observations, M. Périer obtained the following results, showing the changes in the altitude of the mercurial column corresponding to certain differences of altitude of position:—