At the time no one could manufacture such thin copper foil and the pressure of the surrounding air would have collapsed the spheres. In addition, Francesco Lana de Terzi was aware that one could use such a vehicle as a weapon of war, and attack cities from air. He wrote: “"God will never allow that such a machine be built…because everybody realises that no city would be safe from raids…iron weights, fireballs and bombs could be hurled from a great height"".