Here's why:

* Atmospheric Pressure: Barometers measure atmospheric pressure, which is the weight of the air above a certain point.

* Mercury's Density: Mercury is very dense, making it ideal for barometers. The weight of the air column above the barometer pushes down on the mercury, causing it to rise or fall.

* Mercury's Readability: Mercury is a liquid metal that is easily visible and its movement is easy to read on a calibrated scale.

However, due to environmental concerns regarding mercury's toxicity, modern barometers are increasingly using alternative fluids, such as water or alcohol.