A barometer consists of an inverted tube containing a fluid and is used to measure atmospheric pressure. To create the barometer,the tube,which is sealed at only one end, is evacuated to zero absolute pressure. With open end down, the tube is immersed within a liquid-filled reservoir as shown in the illustration of the fortin barometer in figure 9.4. The reservoir is open to atmospheric pressure, which forces the liquid to rise up the tube.from equation 9.2and 9.3,the resulting height of the liquid column above the reservoir free surface is a measure of the absolute atmospheric pressure in equivalent head (equation 9.3 ) . Evangelista torricelli (1608-1647), a colleague of galileo, can be credited with developing and interpreting the working principles of the barometer in 1644.