Draw a simple labelled diagram of a Fortin barometer and state how it is used to measure the atmospheric pressure.

To measure the atmospheric pressure, first the mercury in the leather cup is raised up or lowered down with the help of the screw S so that the mercury level in the glass vessel just touches the ivory point I. The position of mercury level in the barometer tube is noted with the help of the main scale and the vernier scale. The sum of vernier scale reading and the main scale reading gives the barometric height. The barometric height shows the atmospheric pressure.