One of the earliest proposals for a flying machine based on rational principles was Francesco Lana de Terzi's design for a vacuum airship, c.1670. He had measured the pressure of air at sea level and based on this he proposed the first scientifically credible lifting medium in the form of hollow metal spheres from which all the air had been pumped out. His proposed methods of controlling height are still widely used; carrying ballast which may be dropped overboard to gain height, and venting the lifting containers to lose height. In practice de Terzi's spheres would have collapsed under air pressure, and further developments had to wait for more practicable lifting gases.