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Even though we can't feel it, air is constantly pressing down on us with a tremendous force -- 14.7 lbs. per square inch (100,000 newtons per square meters), to be exact! This was graphically demonstrated in 1654 when Otto von Gueicke, Burgmeister of the town of Magdeburg, Germany used a vacuum pump to remove almost all of the air from the space between two half-meter diameter hemispheres. The air pressure holding them together was so strong that two teams of horses couldn't pull them apart; when air was let back in, the hemispheres fell apart easily. Air pressure is created by the weight of the earth's atmosphere. Although we can't see air, the gas molecules still have mass, and gravity acts upon it. The air pressure changes daily due to the heating and cooling of the earth's surface. When air gets warm, it expands, becoming less dense, and therefore pushes with less pressure. We can measure changes in atmospheric pressure by using a barometer. Some barometers use long glass tubes filled with mercury inverted in a dish. Air pressing down on the surface of the dish forces the mercury up the tube. Normal air pressure can support a column of mercury about 760 mm high. When atmospheric pressure drops, the force of the air pushing on the dish isn't as great, so the column of liquid falls and we have a "falling barometer." When the atmospheric pressure increases, the mercury rises, thus a "rising barometer." We use air pressure all the time when we breathe. When our diaphragm moves down, air is pushed into our lungs from the outside, expanding the volume of the chest cavity. The diaphragm doesn't "pull" air in; it expands the volume of our lungs, and the air pressure fills the volume.