Pascal said that if it is the pressure of the air which raises the mercury under the vacuum, then the mercury in a barometer must stand lower on the top of a mountain than it does when the barometer is on the ground near sea-level. He asked a brother-in-law who lived in the south of France to take a barometer up a high mountain. It was found that the mercury column fell three inches, since less air remained above to exert any pressure. “This,” said the observers, “ravished us with admiration and astonishment.” Pascal also took a balloon half-full of air up a mountain. He found that it gradually filled out until it was quite tight, and that it collapsed again on descending. This result showed that the pressure which was not sufficient to fill out a balloon on the lower ground was quite sufficient to counterbalance the reduced pressure of the air upon the mountain. There were some who would not accept the view that the pressure of the air was sufficient to uphold a column of mercury against gravitation. They could not believe that the enormous pressure required could be furnished by such a rare substance as air. They weighed the pressure exerted by mercury on the supporting surface, and found that a column of mercury thirty inches long presses with a force of fifteen pounds upon every square inch of the supporting surface. They would not acknowledge that the air could exert such an enormous pressure, and showed that this pressure, exerted by the air on the surface of the human body, must amount to several tons. The inventors of the barometer replied that that enormous pressure on the human body was counterbalanced by the pressure of the air which is contained within the body itself, so that no inconvenience is felt. One philosopher claimed to have found that the mercury hangs by invisible threads from the upper end of the tube, and that he could feel those threads when he stopped the upper end of the tube with his finger. But that air is easily able to exert great pressure was proved afterwards by an Irishman called Robert Boyle, who showed that when air is compressed to half its volume, it exerts a pressure of quite thirty pounds to the square inch.