A bag made of silk, paper, or other light material, for containing heated air, or gas, of less specific gravity than the atmosphere. If the weight of the balloon, and of the heated air or gas which it contains, be less than that of an equal bulk of atmospheric air, the balloon will ascend in consequence of the equilibrium of atmospheric pressure being disturbed; the pressure of the column of which the balloon forms a part, being less than that of the ordinary atmosphere. The ascending, or upward pressure, therefore, carries the lighter body up until it arrives at a more attenuated medium, which is of the same specific gravity. After the discovery of hydrogen gas, the lightest of all ponderable substances, it occurred to Dr. Black, that a thin bag filled with this would ascend in the air. He suggested the employment of the allantois of a calf for this purpose, but Mr. Cavallo, who performed various experiments on this subject, found that this, and the lightest bladders he could procure, were all too heavy. Chinese paper was also tried, but the gas escaped rapidly through it.

By filling soap-sud bubbles with hydrogen gas he succeeded, and these balloons rapidly ascended to the ceiling.

Within these few years small experimental balloons have been formed of the crop of a turkey, which, when the fat, etc. have been separated, are sufficiently light to ascend if filled with hydrogen gas, although some of them do not contain more than a pint. At about the same period of time, two brothers, named Montgolfier, natives of Annonay, in France, were trying experiments with balloons filled with heated air, the specific gravity of which is less than that of air at the ordinary temperature.

They were led to these experiments by observing the rapid ascent of smoke from chimneys, and imagined that if the smoke were confined in a large and light bag, it would be borne upwards. A bag of fine silk, in the shape of a paral-lelopiped, was constructed, and burning paper was held under the aperture, until the balloon contained a sufficient quantity of rarefied air to carry it up to the ceiling of the apartment. When this experiment was repeated in the open air, the vessel ascended to an altitude of about 70 feet. Other balloons on a larger scale were constructed, and a public exhibition of one containing 23,000 cubic feet was made at Annonay, on the 5th of June, 1783. This was formed of linen lined with paper, and when filled with heated air, was capable of raising 500 lbs. appended to the extremity. One of the Montgolfiers shortly after this visited Paris, and as the affair had excited much attention, he was invited by the Academy of Sciences to repeat the experiment at the expense of the Society.

A large balloon, of elliptic shape, was filled with heated air, in the presence of the members, and in this state it would have ascended with a weight of 500 lbs. if the cords which prevented it had been liberated.

As it was intended to exhibit the ascent before the king of France and his court, this was not done, and the balloon was subsequently so much damaged by wind and rain, that it became necessary to construct another. This was nearly 60 feet in height, and 43 in diameter, and a wicker cage was attached to it, containing a sheep, a cock, and a duck.

The balloon, when duly prepared, ascended with these to an altitude of about. 1,400 feet, and would have ascended to a greater height had not a violent gust of wind torn the cloth, and permitted the heated air to escape. The facility of ascending in the atmosphere being thus fully established, Pilatre de Rozier offered personally to ascend in another balloon, to be constructed by Montgolfier. Accordingly another balloon was completed, of enormous magnitude. The height was 74 feet, and the diameter 48; and its weight, including the car and necessary fuel for continuing the fire, was 1600 lbs. By this the intrepid Pilatre de Rozier made several ascents to an altitude of from 200 to 330 feet, but in these preliminary attempts the balloon was confined by cords. Subsequently he ascended in company with the Marquis d'Arlandes, with the balloon unrestrained, and they continued in the air about 25 minutes, and descended at a distance of about five miles from the place of their departure. The difference between the specific gravity of air heated by the means employed, and that of air at ordinary temperatures, not being very considerable, the buoyant power of a balloon thus filled, was comparatively small, and therefore it was indispensable to employ a large quantity, and, consequently, a large machine to contain it.

To double the volume of air by heat, requires a temperature of nearly 480° Fahr. Messrs. Charles and Roberts therefore tried the experiment of filling a silk bag only 13 feet in diameter, with hydrogen gas, which is only 1/15. the weight of ordinary air.

The experiment was perfectly successful. This small balloon was capable of raising 35 lbs., and on being liberated, it ascended to a considerable height, and after remaining three-quarters of an hour in the air, descended at a distance of fifteen miles from the spot where it had ascended. A balloon formed of silk, and varnished with a solution of Indian rubber, was then constructed, which was filled with hydrogen gas. The diameter was 271/2 feet, and it was covered with a net-work, to which a car, capable of holding two persons, was attached. Messrs. Charles and Roberts ascended from Paris in this, in December 1783, and after remaining in the air an hour and three quarters, they alighted at a distance of 27 miles without accident. A sufficient quantity of gas still remained in the balloon to carry up one person, and Mr. Charles again ascended alone, and attained an altitude of more than 10,000 feet. Shortly after this successful effort, attempts were made to guide or impel a balloon in any required direction; and if this could have been accomplished the invention would have introduced a new era in science.