Hi, The air pressure at the surface is about 1,00,000 N/m2. It means that on average, a virtual column of air one square meter [m2] in cross-section, measured from sea level to the top of the earth's atmosphere, has a mass of about 10,000 kilograms and weight of about 1,00,000 newtons. That weight (across one square meter) is a pressure of 1,00,000 N/m2. Let's look at the air molecules in that virtual column of air. Let's imagine that the column is closed at one end by earth's surface where we also have a pressure sensor and the other end at the top of earth's atmosphere by some other material. At any instant only a fraction of molecules will be hitting the bottom of earth's surface downward, some of them will be hitting the other end upward, some of them will be hitting the column laterally and others will be in the space between the column of box. Although at any instant only a fraction of molecules will be hitting the bottom of earth's surface downward, the pressure sensor will still read 1,00,000 N/m because the molecules strike the bottom of the column more frequently and harder than the top of the column. The sum total of the greater number of collisions and the greater strength of the collisions will be that the average force against the bottom will be 1,00,000 N. Please have a look on the attachment. The cylinder is filled with air having the mass of 1.2 kg. Please also notice that at 25 C° the density of air at sea level could also be taken to be 1.2 kg/m3. The valves 1 and 2 are closed. When valves 1 and 2 are opened simultaneously what would happen? I think that outside air pressure will press the mercury downward because outside air pressure is greater than that of the cylinder. What do you think? Thank you for your help.