1/ Like density in water change with depth, the number of chocs (from temperature) is not the same all around the bubble. The bubble have an additional up force I think. But the weight is directly the volume of bubble by the density of water. The weight can be change with temperature ? Maybe it's possible to study easily a vacuum object. How temperature compensate up force in water ? 2/ In addition, it's possible to have different temperatures in water (like Oceans have), this change the number of chocs, so the up force can be bigger (or down force if hot temperature is at top). Bubble can be an object with vacuum and a shape like we want. If temperature is hot at top, the number of choc at bottom is lower. 3/ It's possible to study in gas, I think there is the same problem. No gas outside, just gas inside with a vacuum object attached with a rope for example. I consider in this case force from gravity at k/d² not k/d. It's important because if I isolate a part of gas somewhere, the difference of pressure inside object is not the same.