This uses water in manometers to show the pressure differences within the metal tube. With no air flow the columns of water are level within each tube. With air flow the water rises toward the open end of the tube, the difference in column height within each tube showing the pressure. Tube 1 is highest, 2 is lowest, and 3 and 4 are in the middle and equal. Rubber hoses may be connected between manometers to emphasize the pressure differences between the three areas of the tube.