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An instrument that measures air pressure is called a barometer. One of the first barometers was developed in the 1600s. The original instrument had mercury in the small basin, with an upside down glass tube placed in the mercury. As air pressure increased, the pressure would force more mercury in the tube. When pressure decreased, the level of mercury in the tube would go down. (We now know that mercury is a very dangerous substance, and people should never touch it or breathe in its vapors!) This type of barometer is known as a mercurial barometer. The height of mercury in the glass tube could be measured in inches. After measuring sea level air pressure many times, we now know that average sea level air pressure is 29.92 inches. Using the metric system, this equals 76 centimeters.
