In 1643, more than two hundred years ago, an Italian, named Torricelli, filled a glass tube, 33 in. long and open at one end, with mercury. Putting his finger over the open end so as to keep the mercury from falling out, he turned it bottom upward into a bowl of mercury, and then removed his finger. As mercury is one of the heaviest things in the world, it would seem as if it should have run out of the tube into the bowl; yet it only fell a little way, and then remained standing in the tube. As mercury is about fourteen times heavier than water, Torricelli saw that the height of the mercury in the tube was about 1 /14 part of the 34-ft. column of water. He at once concluded that the mercury was held up by the pressure of air on the surface bowl. He afterward noticed that the mercury did not always stand at the same height, but that it rose and fell with the changes in the weather, the air pressure decreasing in damp, wet weather and increasing in dry, fine weather. This led to the making of the barometer, which is the same in principle as the tube used by Torricelli.