Concept:A mercury barometer is an instrument used to measure atmospheric pressure.Explanation:It works by balancing the weight of a column of mercury against the pressure of the atmosphere.The mercury column is held in a glass tube with its lower end open to a reservoir of mercury.When atmospheric pressure increases, it pushes the mercury higher in the tube.When atmospheric pressure decreases, the mercury level in the tube falls.Thus, the height of the mercury column directly indicates the atmospheric pressure.Answer:B. atmospheric pressure.