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:— Difference of altitude. Changes in the height of the mercury. Toises. Lines. 500 37½ 150 15½ 27 2½ 7 ½ When Pascal received these results, all the difficulties were removed; and perceiving from the two last observations in the preceding table that 20 toises, or about 120 feet, produce a change of 2 lines, and 7 toises, or 42 feet, a change of ½ a line, he made the observation at the top and bottom of the tower of St Jacques de la Boucherie, which was about 24 or 25 toises, or about 150 feet high, and he found a difference of more than 2 lines in the mercurial column; and in a private house 90 steps high he found a difference of ½ a line. . . . After this important experiment was made, Pascal intimated to M. Périer that different states of the weather would occasion differences in the barometer, according as it was cold, hot, dry, or moist; and in order to put this opinion to the test of experiment, M. Périer instituted a series of observations, which he continued from the beginning of 1649 till March 1651. Corresponding observations were made at the same time at Paris and at Stockholm by the French ambassador, M. Chanut, and Descartes; and from these it appeared that the mercury rises in weather which is cold, cloudy, and damp, and falls when the weather is hot and dry, and during rain and snow, but still with such irregularities that no general rule could be established. At Clermont the difference between the highest and the lowest state of the mercury was 1 inch 3½ lines; at Paris the same; and at Stockholm 2 inches 2½ lines.”