A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Because a change in atmospheric pressure is a sign of changing weather, this instrument is one of the chief tools of the weather forecaster. The mercury barometer was invented in 17th century and consists of a hollow glass tube, about 1 m long, closed at one end and filled with mercury. It is a primary standard that shows absolute pressure in mmHg but it has fallen out of use due to mercury toxicity. Modern digital and analog barometers are usually aneroid type and do not rely on liquid filled tubes for measurement.