Here's how it works:

* The mercury column: The barometer consists of a glass tube filled with mercury, which is sealed at one end and inverted into a reservoir of mercury.

* Atmospheric pressure: The weight of the atmosphere presses down on the mercury in the reservoir, pushing the mercury up the tube.

* Height of the column: The height of the mercury column in the tube is directly proportional to the atmospheric pressure. Higher atmospheric pressure pushes the mercury higher, while lower pressure results in a lower mercury column.

Therefore, by measuring the height of the mercury column, you can determine the air pressure.