This mercury barometer was once used at the Royal Observatory, Greenwich. Its main dial consists of a brass scale with pointers to indicate the minimum and maximum readings since 2100 on the previous day. The glass tube and mercury (now drained) are held in a white wooden case that would have been set into a wall, with only the dial publicly visible. The barometer was first set up in 1864 when George Airy was Astronomer Royal. It was set into the wall just to the south of the main courtyard.