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Narrative
Pressure exerted by a vapor can be measured using barometers. Mercury is poured into a dish below three barometer tubes. A vacuum pump is used to create a vacuum at the top of each tube, and draw the mercury into it. When the tube is evacuated, the valve is closed. Here the height of the mercury in each tube is 736 millimeters.

Discussion
Three barometers are constructed by evacuating vertical glass tubes whose lower ends are below the surface of a pool of mercury. The three barometers read 736 mmHg.