The following is a continuation of a fascinating SSSF forum debate going on in Australia. It will be a great opportunity to see if Canadians and Americans can out-think the Aussies. The thought experiment is this: A very long sealed cylinder is pressurised to X psi with air. At horizontal, the pressure is tested at each end of the cylinder at test point A, which stays at sea level, and point B, which is to be elevated. The pressures, as expected, are identical, X psi. By miraculous means, (remember it's a thought experiment), the very long cylinder is hoisted into the air until its top is level with say Mt Everest, around 28,000 ft. We test the pressure at the low end, A, and at the same time, at B, the high end. Will the pressures be the same, or different. Reply with your reasoning. Cheers from Australia !