A mercury barometer is used for measuring

A mercury barometer is used for measuring atmospheric pressure.

Here's a simple explanation:

A barometer is an instrument designed to measure the pressure exerted by the atmosphere. The atmosphere exerts pressure because of the mass of air above the Earth, and this pressure changes as you move up and down in altitude or due to weather systems.

A mercury barometer consists of a long glass tube closed at one end and filled with mercury. The open end of the tube is placed in a bowl of mercury, called a reservoir. The pressure from the air pushing down on the mercury in the bowl forces the mercury up the tube. When the atmospheric pressure is high, the mercury in the tube rises, and when the atmospheric pressure is low, the mercury level falls.

Thus, by measuring the height of the mercury column in the tube, we can determine the atmospheric pressure. In summary, a mercury barometer provides a way to directly observe changes in the atmospheric pressure.