The barometer is a device for measuring the current weight or pressure of the Earth's atmosphere. Invented by Torricelli in Florence, Italy in around 1643, it typically operates by balancing the weight of a column of mercury with the current weight of the atmosphere to obtain a measurement. This reading can then be used as a method of forecasting the weather.
Most barometers nowadays hang on the hallway walls of centrally heated houses where the varying temperatures render them largely useless, leading to an annoying habit of loudly insisting on 'Stormy' in the middle of heat waves and 'Fine and dry' during tropical floods.
It is commonly believed, strangely, that tapping the front of these mechanisms somehow improves their performance, which has become a social ritual which must be observed when passing. However, it is becoming more widely acknowledged that the only way to improve their accuracy would be to move the barometer's pointer to 'Changeable' and glue it there.