I'm confused when i consider about this picture in my head. ... imagine that there is a ''closed-box'' on the floor. And ball A is in this closed-box. Pressure acted on the box = Patm. I'm wondering, how much pressure act on ball A ? With my sense, i think it is Patm. ... but i can't explain! it's a closed-box, so air mass above the box should have no effect on the ball in box ?!