spaces in every variety of form and condition. To account for the phenomena of meteors as above described, we must suppose that there are both detached masses, each revolving in an independent orbit, and giving rise to sporadic meteors, and also connected systems, forming rings or zones around the sun. The intersection of the earth's orbit by such zones or streams would account for the periodic swarms of meteors; and if we suppose the asteroids composing it to be irregularly grouped, we see a reason why the same stream should not be always of equal intensity. There might even be periodicity in this respect too.
What causes the luminous and ignited condition of aërolites? Terrestrial magnetism was at one time suggested as the exciting cause. It is now recognized, however, that the atmosphere extends to a very great height, and the ignition is believed to be caused by friction between the rapidly moving body and the air. As to meteorites that do not fall on the earth, we may suppose that some are merely deflected from their path by the proximity of the earth, are rendered luminous through a short arc, and continue their course with altered orbit, while the greater part are soon burnt up and fall to the earth in impalpable dust. See Meteors.
A'ËROMAN'CY. See Superstition.
A'ËRONAUT'ICS ( Gk. ὰήρ, aēr, air + ναύτης, nautēs, sailor). The art of aërial navigation. It is of comparatively recent development, as the ancients seem to have been convinced that the navigation of the air was impossible to human beings, and to have made no attempt to accomplish it. Grecian mythology, however, furnishes us the fable of Dædalus, who made wings of feathers cemented with wax for himself and his son Icarus, and endeavored to escape by flight from King Minos. The story of how Icarus, by forgetting the injunctions of his father and soaring so high that the sun melted the wax of his wings, was precipitated into the sea, while Dædalus accomplished his flight in safety, is familiar to all as a fanciful legend of ancient mythology. A more comprehensible tale, but yet one which is based entirely on tradition, is that told of the wooden dove invented by the Greek mathematician Archytas. According to the tradition, this dove could maintain sustained flight and was set in motion by "hidden and inclosed air." Passing to the Middle Ages, we find the field scarcely more fruitful in facts relating to aërial navigation. There are recorded a few actual and usually disastrous attempts at gliding flight, which will be noted further on, but generally speaking the consideration of the problem of flight by human beings was confined mostly to surmise and speculations which in many cases were nearly as fanciful as the earlier Grecian fables. The statement of these meagre facts brings us to the invention which for the first time placed the art of aërial navigation upon a more practical basis than mere speculation, namely, the discovery of the balloon.
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MONTGOLFIER BALLOON.
Balloons. The germ of the invention of balloons is to be found in the discovery by the English chemist and physicist, Henry Cavendish, in 1766, of the remarkable lightness of hydrogen gas, then called inflammable air. Professor Black, of Edinburgh, seems to have been the first who conceived the idea that a light envelope containing this gas would rise of itself. He requested Dr. Monro, the professor of anatomy, to give him some thin animal membrane for the experiment; but for some reason or other the experiment was never made. The first practical attempts were made by Cavallo, who in 1772 filled swine's bladders and paper bags with the gas, but found the former too heavy and the latter too porous, and only succeeded in raising soap-bubbles inflated with the gas. The invention of the balloon is due to the two brothers Étienne and Joseph Montgolfier, paper-makers at Annonay, in France, whose names are as distinguished in the development of their own industry as in the history of aeronautics. It occurred to these brothers, on reading Cavendish's Different Kinds of Air, that the air could be rendered navigable by inclosing a light gas within a covering of inconsiderable weight. Led by their vocation, they fixed upon paper as the most fitting material for the purpose, and first attempted to make balloons of paper filled with inflammable air. Finding that these emptied themselves almost as soon as they were filled, instead of abandoning the paper as an unsuitable covering for the gas, they sought after another gas more suited to the paper. They thought that the gas which resulted from the combustion of slightly moistened straw and wool would answer the purpose, since it had, as they imagined, an upward tendency, not only from its being heated, but from its electrical properties, which caused it to be repelled from the ground. It is hardly necessary to say that this so-called Montgolfier gas possessed no advantages for raising balloons other than that possessed by heated air of any kind; in fact, the abundant smoke with which it was mixed, by adding to its weight, rather detracted from its merits. At Avignon, in November, 1782, Étienne Montgolfier first succeeded in causing a silk parallelopiped, of about 50 cubic feet, to rise to the ceiling of a room. Encouraged by this success, the brothers made experiments on a larger scale at Annonay with an equally happy result; and finally, in June, 1783, in the presence of the assembly of the estates of Vivarais and of an im-
