A barometer is a simple instrument for measuring the atmospheric pressure. It consists (see figure) of a tube open at one end and closed at the other, which is inverted into a bath with a barometric liquid of specific weight, , so that the open end is submerged in the bath which is open to the atmosphere. A space above the liquid column allows the height, h , of the liquid column above the liquid level in the bath to vary as the atmospheric pressure varies. Ideally, the space above the liquid column should be a perfect vacuum, such the pressure on top of the barometric fluid would be zero; in practice, due to evaporation, the pressure on top of the barometric liquid column in the tube will be the vapor pressure of the barometric liquid. Similarly in the simplest analysis of a barometer, the effect of surface tension is neglected