Final answer:
Mercury is used in barometers and manometers because it is denser than most liquids, permitting the construction of manageable-sized devices, unlike water which would require impractically large instruments.
Step-by-step explanation:
Barometers and manometers are generally constructed using mercury because mercury is denser than most liquids. The high density of mercury allows for the construction of more compact and manageable devices. For example, in a barometer, the height, h, of the column of liquid mercury used to measure atmospheric pressure results in a manageable size. If water were used instead of mercury, the barometer would need to be more than 30 feet tall to measure the same pressure because water is much less dense than mercury. Similarly, mercury manometers are used to measure the pressure of gases, and due to mercury's high density, the difference in the height of the mercury columns provides an accurate measurement of gas pressure.