Barometers

Barometers are used to measure air pressure. Air pressure is normally measured in millibars. 
Barometers are normally kept inside Stevenson screens to keep them safe. 
A barometer has a movable needle (pointer). 
The pointer can be moved to the current reading so that you can then make a comparison with the reading from the following day.

A Barometer

The History of the Barometer and How It Works