Aniriod Barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Instead of having a pool of mercury that the atmosphere is down on, they have a sealed, air-tight metal box inside the air pressure rises or falls, the box either squashes inward a tiny bit or flexes outward. A spring is cunningly attached to the box and as the box moves in and out it response to the changes in air pressure, the spring expands or contracts and moves the pointer on the dial. The dial is marked with numbers so you can read the air pressure instantly.
