The Fortin barometer is used to measure atmospheric pressure accurately. The tube containing the mercury is itself enclosed in a brass tube, the upper part of which is made of glass so that the mercury surface is seen. The pressure is read by moving a millimetre scale alongside a vernier scale.
The zero of the scale is at the tip of an ivory pointer fixed to the lower end of the brass tube. The mercury reservoir is a leather bag which can be raised or lowered by a screw. Before taking a reading the screw is adjusted until the tip of the pointer just touches it's image in the surface of the mercury. The pressure due to a column of liquid of density
and height
is given by
where g=acceleration due to gravity. If we ignore the variation of the density of mercury with temperature and the variation of
over the surface of the Earth, the pressure is proportional to the height of the mercury column, It is sufficient to give the pressure in terms of the height of the mercury column.