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A classic mercury barometer consists of a glass tube with one end open and the other end sealed close. The tube is filled with mercury and then the open end is inverted and placed into an open reservoir of mercury. The resulting vacuum in the tube keeps the mercury in the tube from completely emptying into the reservoir.  The pressure inside the tube eventually equalizes with the atmospheric pressure on the outside of the tube.  When the atmospheric pressure lowers and the resulting pressure on the mercury reservoir lowers, the mercury level in the column will drop as well as a little more mercury empties from the tube.  When the atmospheric pressure increases, the pressure on the reservoir increases and forces a little more mercury into the column, thus raising the level in the tube.  This is how a basic mercury barometer works.

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