How does a wall barometer work?

The Barometer is a scientific instrument used to measure air pressure in a certain environment. The barometer has a flattened metal cylinder that looks like a silver disc inside its construction, which is filled with a partial vacuum. As the pressure in the air increases, the disc gets crushed and as air pressure decreases the disc expands. This movement of the disc is transformed into the reading of the black barometer needle under the glass.