This is a clear scientific exposition of a plan for navigating the atmosphere on its upper surface, assuming a distinct upper surface to exist. In commenting on this passage, the Jesuit Schottus, in his _Magia Universalis_, uses an expression which indicates his belief that a vessel can be made to _float in the air_ by filling it with ether, or the element of fire. He says: “In such terms has this matter been treated by Mendoza (died 1626); nor is there any improbability involved in his view, whether the element of fire be placed above the air, or, what is still more credible, the ether—that is, the purest air. Although any wood, iron, copper, lead, and such like metals are weightier than an equal volume of water, and for that reason will sink in water when placed there alone, yet if fabricated into hollow shapes, and filled with our impure and heavy air, they swim upon waters, and are adapted to the construction of ships, and are sustained by water without danger of immersion; thus, although these bodies are of greater specific gravity than our air, nevertheless, when shaped into a boat and filled with that very light material, they can _float in the air_, and are suitable material for the construction of small ships, because the entire work composed of the little ship and the ether can be made lighter than an equal volume of our impure air, even in the highest region.” As Roger Bacon proposed a similar device in 1542, Mendoza’s was not entirely new and may not have been original. Bacon, describing his aërial vessel, says: “It must be a large, hollow globe of copper, or other suitable metal, wrought extremely thin, in order to have it as light as possible. It must then be filled with ‘ethereal air or liquid fire,’ and then be launched from some elevated point into the atmosphere, where it will float like a vessel on water.” In the year 1646 another learned Jesuit published a book, _Ars Magna Lucis et Umbræ in Mundo_, in which he relates an episode indicating that one of his order had made use of a hot-air balloon to intimidate some ignorant pagans. The following demonstration, if reported by a modern missionary, would be accepted as a matter of course; why, then, should we gravely question the story, since it describes an achievement quite possible at the time, assuming that the necessary materials were available? And even assuming the report to be fictitious, still it is a scientific description of a practicable hot-air balloon, presented and credited by a learned scholar and accomplished mathematician more than a century before the balloon was publicly exhibited by the illustrious Frenchmen. He writes: