Air is so light that you need the special instrument called a barometer to measure air pressure. The barometer was first thought of by an Italian scientist named Torricelli, in 1644. A simple experiment showed him that air had pressure, and that this pressure could be measured. You can do this same experiment. All you need is a glass tube about 3 feet long, closed at one end; some mercury (a metal sometimes called quicksilver); and a small dish. Fill the tube and the dish with mercury.
Hold your finger over the open end of the tube and quickly turn the tube upside down in the dish. You now have a mercury barometer. The mercury in the tube will drop until it is about 30 inches above the dish. Fifteen pounds of air pressure, which is normal, will usually hold up about 30 inches of mercury. If the air pressure is lower, the height of the column of mercury will be less than 30 inches. If the air pressure is greater than 15 pounds, the column of mercury will be higher than 30 inches. You can use a mercury barometer to help predict the weather. When the column of mercury begins to fall rapidly, it usually means that bad weather is ahead, because windy rainy weather usually follows a drop in air pressure.
After a storm is over, the mercury will climb back to 30 inches You may wonder why water cannot be used instead of mercury. The reason is that water is so much lighter than mercury that it would take more than a 34-foot tube of water to measure air pressure. Three hundred years ago, a German named von Guericke made a water barometer that was taller than his house. He had to cut a hole in the roof so that it would fit. In order to see the top of the water column, he put a little wooden doll in the tube. The doll floated on top of the water. When the pressure was high, the doll would bob up above the house top. When the pressure was low, it would disappear below the roof. Von Guericke's neighbors did not know what made the doll appear and disappear. They did not know it was air pressure. They thought that Von Guericke was a magician. When they threatened to kill him, he took down his barometer.
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