A mercury barometer reads 75 cm. Now 3 cc of atmospheric air is introduced into the tube. The mercury falls to a height of 65 cm and the length of air column above mercury is found to be 15 cm. Calculate the cross sectional area of the tube. This question in my book has been solved by boyle's law. 10cm of Hg is the pressure which the air column exerts over mercury. Isnt it? Now how can we say that the air is at 10cm of Hg pressure. The pressure on air should be equal to the pressure exerted on it by the wall of the tube and the free mercury surface. Please explain!