This physics tutorial explains how a barometer works to measure atmospheric pressure using a column of mercury.
The mercury liquid needs a resultant force to push it upwards and stop it from falling down into the bath. This force comes from the air pressure of the atmosphere acting through the liquid mercury.
However this pressure is not strong enough to keep the column in place and so it falls a few cm, creating a vacuum at the top.
This means that there is now less mercury in the column and therefore there is less weight pulling it down, so the atmospheric pressure upward force is then strong enough to keep the column at this 'equilibrium' position.
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