His design had a central mast to which a sail was attached, and four masts which had thin copper foil spheres attached to them: the air would be pumped out of the spheres, leaving a vacuum inside, and so being lighter than the surrounding air, would provide lift . The airship would be steered like a sailing boat. Each sphere would have had a diameter of 7.5  m (24 ft 7 in). Terzi calculated that the weight of a sphere would be 180 kg (396 lb). He also calculated that the air in the sphere would weigh 290 kg (638 lb), and would provide enough lift to carry 6 passengers.