Answer
A barometer works by atmospheric pressure pushing down on the liquid and pushing the liquid up in the tube. Therefore, when there a large volume of mercury in the tube, a greater atmospheric pressure is pushing down on the barometer and vice versa.
A manometer works by comparing the pressure of the gas inside the container and the atmospheric pressure. If the pressure of the internal gas is greater, more mercury is pushed towards the atmospheric side of the manometer and vice versa.
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