Chinese aerostat. Historical artwork of a gas balloon picking up passengers. The balloon is filled with a gas that is less dense than air, and so is lifted by buoyancy. This balloon measured 40 metres in length, and had a volume of 7000 cubic metres. It flew at a height of 1 kilometre and never descended lower than 9 metres above the ground. Engraving from Astra Castra (Hatton Turner, London, 1865).