The person who is entitled to the honor of the discovery of the main principle of aeronautics--atmospheric buoyancy--is Roger Bacon, an English monk of the thirteenth century. This eminent man, whose uncommon genius was, in that superstitious and ignorant age, ascribed to his intercourse with the devil, was aware that the air is a material of some consistency, capable, like the ocean, of bearing vessels on its surface; and, in one of his works, he particularly describes the construction of a machine by which he believed it was possible to navigate the air. It is a large, thin, hollow globe of copper, or other suitable metal, which he proposes to fill with "ethereal air or liquid fire," and then to launch from some elevated point into the atmosphere, when he supposes it will float on its surface, like a vessel on the water. He afterward says, "There may be made some flying instrument, so that a man, sitting in the middle of the instrument, and turning some mechanism, may put in motion some artificial wings, which may beat the air like a flying bird." But, though Bacon knew the buoyancy of the atmosphere, he was very imperfectly acquainted with its properties. His idea seems to have been, that the boundaries of the atmosphere are at no great height, and that the aerial vessel, in order to its being borne up, must be placed on the surface of the air, just as a ship, in order to its being supported, must be placed on the surface of the water. And, whatever may be meant by his "ethereal air and liquid fire," there is no evidence that he, or any one living in that age, had any knowledge of the various and distinct gases. Bacon merely reasoned and theorized on the subject; he never attempted to realize these flying projects by actual experiment. It was not till the year 1782 that the art of aerial navigation was discovered, and the merit of the discovery is due to two brothers, wealthy paper manufacturers, at Annonay, not far from Lyons--Stephen and Joseph Montgolfier. This discovery they did not arrive at from any scientific reasoning founded on the elasticity and weight of the atmosphere, for, though attached to the study of mathematics and chemistry, they do not appear to have particularly turned their attention to aerostatics; but, from observing how clouds and smoke rise and float in the atmosphere, it occurred to Stephen, the younger of the two, that a light paper bag, filled with cloud or smoke, would, from the natural tendency of these substances to ascend, be carried by their force in an upward direction.