If you have a tank with compressed air. Say this tank is a cube of 1x1x1m. One of the walls are "mobile" but sealed so no air can excape. A spring on the outside is pushing on this wall so it will remain in its position at for example 10 bar pressure inside the tank. Now, atmospheric air has a density of approx 1.3 kg pr. 1000 litre. If we compress this air to a density of 13 kg pr 1000 litre (Say 10 bar pressure), would the spring push on that wall with 100 000 kg + the extra 13 kg. of air? Or how can we determine the correct pressure the spring must counterforce? ( No, this is not a school project - I'm just curious) Vidar.