Atmospheric Pressure

There is an envelope of air around us, which is commonly called as atmosphere. Atmospheric pressure is the force exerted by the atmosphere, per unit area, on the surface of earth.
The tremendous pressure exerted on our body due to air is counter balanced as our blood contains dissolved oxygen, which is at pressure more than the pressure of the atmosphere.

A simple barometer is used to measure the magnitude of atmospheric pressure. Torricellian Vacuum is an empty space which is created in the tube of the simple barometer by the flow of mercury in the downward direction.

Barometric pressure depends on factors like altitude, season, temperature etc.
Mercury is used as barometric liquid in  a barometer due to the reason that it is the densest liquid at room temperature, thus, a short column of mercury can counterbalance normal atmospheric pressure that is impossible in case of other liquids like water or alcohol. 
The two other important types of barometer are aneroid barometer and Fortin’s barometer. Aneroid barometer does not use any liquid and it is compact and portable. Fortin’s barometer is an improvement of simple mercury barometer in which defects of mercury barometer are removed.

Lift pump is a device that works on the principle that air exerts pressure. A lift pump has the capacity to support 10.34 meter vertical length of water in a closed tube.

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