Another name which should be mentioned in connection with the barometer is that of Blaise Pascal, who in 1648 fully confirmed Torricelli’s results. Pascal saw that if the mercury column is really supported by the weight of the air, the height of that column must be less on the summit of a mountain than at the base, because there is less air over the top of the mountain than at the bottom, and therefore the weight of the air must be less at the summit. To prove this, he asked his brother-in-law Perrier, who lived at Clermont, in France, to carry the Torricellian tube up the Puy-de-Dôme, a mountain somewhat over 3500 feet high in Central France. This Perrier did on Sept. 19, 1648, and he found, as predicted by Pascal, that the mercury fell steadily in the tube as he went up the mountain, and that at the top of the mountain the column of mercury was over 3 inches shorter than at the base. The pressure of the atmosphere is a weather element which, unlike the other elements already considered, cannot be observed without an instrument. We cannot, under ordinary conditions at sea level, determine by any of our senses whether the pressure is rising or falling, or is stationary. The pressure on the upper floors of one of our high buildings is shown by a barometer to be considerably lower than it is at the level of the street below, and yet we notice no difference in our feelings at the two levels. It is only when we ascend far into the air, as in climbing a high mountain or in a balloon, that the much-diminished pressure at these great heights perceptibly influences the human body. Mountain climbers and aëronauts who reach altitudes of 15,000 to 20,000 feet or more, usually suffer from headache, nausea, and faintness, which have their cause in the reduced pressure encountered at these heights. [Illustration: FIG. 5.]