Barometers (Shire Library)

Many people consult a barometer before going out, to see what the weather is likely to be. Barometers measure the general pressure of the atmosphere, which changes with the weather: falling pressure generally heralds rain; rising pressure predicts fine weather. The simple mercury barometer originated in seventeenth century Italy and was soon copied in France and Britain. In the nineteenth century demand increased for domestic and scientific barometers. They were needed for weather forecasting, and surveyors carried them to measure height above sea level. Since 1850, mercury barometers have been gradually ousted by aneroids, which are smaller and more robust but nowadays equally accurate.