It is supposed to extend to the height of _fifty_ miles. Do these fifty miles of air press upon the earth? They do press upon the earth. With how much force does the air press on the earth? With a force of _fifteen_ pounds on every square inch of surface. Then has the air weight? The air has weight. Why do we not feel the weight of the air? Because it presses with equal force, in every direction; up, down, and on every side. How can we show that the air presses upwards? Fill a tumbler with water and put a piece of writing-paper over the top. Hold the hand on the paper and quickly invert the tumbler, or turn it bottom upwards; then remove the hand, and the pressure of the air against the paper will keep the water in the tumbler. This is represented in Fig. 12. [Illustration: Fig. 12.] If a tumbler be filled with water in a bucket, and then raised bottom upwards until its edge is just below the surface of the water, it will remain full of water; why is this? The pressure of the air on the water in the bucket, keeps the water in the tumbler. If we use an iron pipe or a lead pipe, instead of a tumbler, will the pressure of the air keep the water in the pipe? It will, the same as it keeps it in the tumbler. How high will the air keep the water in the pipe? It will keep the water to the height of _thirty-four_ feet. Why does the air keep the water up thirty-four feet? Because the air presses down with a force of fifteen pounds on every square inch of surface, and a column of water thirty-four feet high presses down with a force of fifteen pounds on every square inch of surface. Does the weight of the air exactly balance the weight of the column of water? It does; they balance each other. Is water heavier than air? Water is heavier than air; because a column of water thirty-four feet high weighs as much as a column of air fifty miles high. Is quicksilver heavier than water? Yes; quicksilver is about thirteen times heavier than water. How high a column of quicksilver can the air keep up? The air can keep up a column of quicksilver only thirty inches high. Why is this? Because a column of quicksilver thirty inches high, weighs exactly the same as a column of air fifty miles high, or a column of water thirty-four feet high. How can this be shown? Take a glass tube about thirty-three inches long, cork up one end, and fill the tube with quicksilver; then put the open end of the tube beneath the surface of some quicksilver, in a basin, and the quicksilver in the tube will fall to the height of thirty inches, as is shown in Fig. 13. [Illustration: Fig. 13.] If the cork be removed from the glass tube, what will become of the quicksilver? It will immediately sink into the basin below. Why will the quicksilver sink into the basin? Because the air will then press upon the quicksilver in the tube, the same as it does on that in the basin; hence, all the quicksilver will fall to the same level.