Jed A1
Air is a liquid. A liquid by definition is a substance that can resist shear only in motion. A fluid is a substance that deforms continuously under the action of an applied shear force or stress. The process of continuous deformation is called flowing, or as it is applied to air flow.
I would like to commend you on a well thought our and relevant analogy you presented with respect to the sand and the water.
212man
Your analogy of the lead pipes and balloons is again incorrect. If you increase the mass of a control volume the weight increases as weight = mass x acceleration (gravity).
Using your own example, if you were to seal off the container with the lead and balloons in it, and extract all the gas you would agree that the weight would increase. As you know the weight of the extracted gasses would have to added to the tube and lead weight to get the weight of the entire control volume (tube+lead+extracted gas).
