BALLOON. (Fr. ballon, a little ball.) The name of a machine, which, consist ing of an envelope containing a gas speci fically lighter than common air, rises into the atmosphere with a greater or less de gree of aseensional force. A car, sup ported by a net-work which extends over the balloon supports the aeronaut: and a valve, usually placed at the top, to which a string is attached reaching to the car, gives him the power of allowing the gas to escape, and of descending atpleasure.
During the dark ages, and for some time after the revival of science, numer ous projects were entertained for navigat ing the air ; but it is only in very recent times, since 1783, that any of them have been realized. - The first idea was to em ploy some mechanical contrivance resem bling the wings of birds ; bnt Borelli de monstrated that all attempts on the part of man to fly must necessarily fail, from the utter disproportion of his muscular power to the force that would be neces sary to give impulsion to wings of such enormous magnitude as would be required to sustain his weight in the air.
The principle by which a balloon rises in the atmosphere is exactly the same as that which causes the ascent of a cork from the bottom of a vessel filled with water. The weight of the volume of air which it displaces must exceed the weight of the balloon and all that it carries with it. That bodies must rise and remain suspended in a fluid denser than them selves was proved by Archimedes ; but the weight of the air is a modern discov ery ; and it was only in the latter half of the last century that chemistry detected the nature and differences of specific gra vities of ariforin fluids. Mr. Cavendish, in 1766, by some ingenious experiments, recorded in the Philosophical Thinsactions, vol. lvi., found hydrogen gas to be from about seven to eleven times lighter than common air, according to the mode of its preparation. In its pure state it is found to be nearly sixteen times lighter than common air. This substance, therefore, if prevented from diffusing itself, and al lowed to obey the force by winch it is impelled upwards, will continue to mount till it arrives at a stratum of the atmos phere sixteen times more attenuated than at the surface of the earth. Accordingly, no sooner had Cavendish announced his discovery, than it occurred to Dr. Black that a very thin bag filled with hydrogen gas would mount to the ceiling of a room. Through some imperfection, the experi ment when lie attempted to execute it failed : and it was several years later be fore an envelope was thought of suffici ently light, and at the same time imper meable to the gas. Cavallo made a series
of experiments on this subject in 1782, but did not succeed in raising any thing heavier than a soap-bubble. The expense attending the preparation of the gaspro bably prevented the experiment from being made on a great scale.
Knowing the specific gravities of atmos pheric air, of the gas with which the bal loon is to be filled, and the weight of the envelope in which it is confined, it is not difficult to compute the size the balloon must have in order to rise from the ground, or carry a given weight to a given height in the atmosphere. A globe of air, one foot in diameter, at the level of the sea and tinder the ordinary pressure, weighs about 1-25th of a pound avoirdu pois. An equal globe of hydrogen gas, obtained in the usual way by dissolving iron filings in dilute sulphuric acid, may be assumed (making every allowance for imperfect preparation) to be about six times lighter than atmospheric air ; con sequently 5-6ths of its whole buoyant force will act in impelling it upwards : that is to say, the force with which a sphere of such gas, one foot in diameter, will tend to rise in the atmosphere, will be of a pound avoirdupois. The ascensional forces of different spheres be proportional to their magnitudes, that is to the cubes of their diameters : therefore a sphere 12 feet in diameter would rise with a force of 57 pounds, and one of 24 feet' in diameter with a force of 8 X57= 456 pounds. But these determine ations must be diminished by the weight of the envelope. The best material for the purpose at present known is thin silk varnished with elasticgum, or Indian rubber. The quantity of this material required to-cover a globe one foot-in di ameter, weighs about 1-20th Now for a globe of a greater size, the i quantity required will increase with the square of the diameter ; hence the cover of a balloon 12 feet in diameter must weigh about 7 pounds, and of one 24 feet in diameter 28 pounds. It follows, there fore, that a balloon of 12 feet diameter will only raise from the ground a weight of 50 pounds, and one of 24 feet 428 pounds. Computing in the same man ner, it is found that a balloon 60 feet in diameter would raise a weight equal to about 6,950 pounds • ' and that one of a foot and a half would barely float, the weight of the bag being just equal to that of the imprisoned gas.