The barometer is an instrument used to measure atmospheric pressure in weather studies or for lab experiments involving pressures close to normal air pressure. A Fortin's barometer consists of a narrow glass tube of length about 90 cm, about 6mm bore  This tube is closed at one end. Usually, the glass tube is protected by enclosing it in a brass tube. The upper part of the brass tube has a slit that enables the level of the mercury in the glass tube to be seen. A scale graduated in millimetres is attached to the brass tube. This functions as the main scale. For accurate measurement, a vernier scale that can slide over the main scale is also fixed to the barometer. The vernier scale can be moved up and down using a screw. Complete apparatus mounted on a wooden base. Supplied without mercury.