It is a scientific instrument used to measure ATMOSPHERIC PRESSURE.Because the atmospheric pressure varies with sea levels.It can also be used to measure ALTITUDE.
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A Barometer is an instrument to measure atmospheric pressure. It consists of a long tube of with mercury at the bottom.  Column of Mercury weight equals the pressure on the mercury container at the bottom of barometer.  Barometer uses the principle (Pascal's) that pressure is same at the same level in a liquid.

The meteorological department measures the pressure to predict weather.  Often people keep barometers mounted on walls in their houses.

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