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The barometer was invented by Evangelista Torricelli, a 17th century
physicist and mathematician. His colleague, Galileo, encouraged
him to use mercury, and today's barometers operate in much the same way
as the first barometer which Torricelli designed long ago in Italy.
Barometers are meteorological instruments which measure atmospheric pressure and can be used to forecast weather. Because atmospheric pressure changes with the elevation above sea level, a barometer can also function as an altimeter, to measure altitude. The two major types of barometer are the mercury barometer and the aneroid barometer. A mercury barometer indicates pressure by the height of a mercury column which rises or falls in response to atmospheric pressure changes. An aneroid barometer indicates pressure by the expansion and contraction of a metal disc covering a box containing a partial vacuum. Some barometers, called barographs, are designed to record changes in barometric pressure over the course of time. A barograph is made up of a partially evacuated metal cylinder. This cylinder is attached to a pen arm, and the pen moves vertically when pressure changes. The pen traces a record of pressure versus time on a chart. This chart is mounted on a drum, which rotates slowly by means a clockwork.
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