Device to measure pressure, pressure being defined as the force exerted on an area. (See link) Now the top of the tube on the left has been closed. We imagine that there is a sample of gas in the closed end of the tube. The right side of the tube remains open to the atmosphere. The point A, then, is at atmospheric pressure. The point C is at the pressure of the gas in the closed end of the tube. >The point B has a pressure greater than atmospheric pressure due to the weight of the column of liquid of height h. The point C is at the same height as B, so it has the same pressure as B. And we have already seen that this is equal to the pressure of the gas in the closed end of the tube. Thus, in this case the pressure of the gas that is trapped in the closed end of the tube is greater than atmospheric pressure by the amount of pressure exerted by the column of liquid of height h. >