bular, with a bent tube, but in the form of a grotesque human figure, and the blast proceeds from the mouth.

AEOLUS'S harp, or EOLIAN harp, a musical instrument so named from its producing an agreeable harmony merely by the action of the wind. See ACOUSTICS.

ÆRA, a fixed point of time, from which any number of years is begun to be reckoned.

See CHRONOLOGY. AEROSTATION, in the modern application of the term, signifies the art of navigation through the air, both in its principles and practice. Hence also the machines which are employed for this purpose, are called aerostats, or aerostatic machines; and on account of their round figure, air balloons. The fundamental principles of this art have been long and generally known; although the application of them to practice seems to be altogether a modern discovery. It will be sufficient, therefore, to observe, in this place, that any body, which is specifically, or bulk for bulk, lighter than the atmospheric air encompassing the earth, will be buoyed up by it, and ascend; but as the density of the atmosphere decreases, on account of the diminished pressure of the superincumbent air, and the elastic property which it possesses at different elevations above the earth, this body can rise only to a height in which the surrounding air will be of the same specific gravity with itself. In this situation it will either float, or be driven in the direction of the wind or current of air, to which it is exposed. An air-balloon is a body of this kind, the whole mass of which, including its covering and contents, and the several weights annexed to it, is of less specific gravity than that of the air in which it rises. Heat is well known to rarefy and expand, and consequently to lessen the specific gravity of the air to which it is applied; and the diminution of its weight is proportional to the heat. One degree of heat, according to the scale of Fahrenheit's thermometer, seems to expand the air about one four-hundredth part; and about 400, or rather 435, degrees of heat, will just double the bulk of a quantity of air. If, therefore, the air inclosed in any kind of covering be heated and consequently dilated to such a degree, as that the excess of the weight of an equal bulk of common air above the weight of the heated air, is greater than the weight of the covering and its appendages, this whole mass will ascend in the atmosphere, till, by the cooling and condensation of the

included air, or the diminished density of the surrounding air, it becomes of the same specific gravity with the air in which it floats; and without renewed heat, it will gradually descend. If, instead of heating common air inclosed in any covering, and thus diminishing its weight, the covering be filled with an elastic fluid, lighter than atmospheric air, so that the excess of the weight of an equal bulk of the latter above that of the inclosed elastic fluid be greater than the weight of the covering and its appendages, the whole mass will, in this case, ascend in the atmosphere, and continue to rise till it attains a height at which the surrounding air is of the same specific gravity with itself. Inflammable air, or, as it is called, hydrogen gas, is a fluid of this kind. For the knowledge of many of its properties we are indebted to Mr. Henry Cavendish, who discovered that, if common air is eight hundred times lighter than water, infiammable air is seven times lighter than com-mon air; but if common air is eight hundred and fifty times lighter than water, then inflammable air is 10.8 times lighter than common air. The construction of airballoons depends upon the principles above stated; and they are of two kinds, as one or the other of the preceding methods of preparing them is adopted.

In the various schemes that have been proposed for navigating through the air, some have had recourse to artificial wings; which, being constructed like those of birds, and annexed to the human body, might bear it up, and by their motion, produced either by mechanical springs, or muscular exertion, effect its progress in any direction at pleasure. This is one of the methods of artificial flying suggested by Bishop Wilkins, in the seventh chapter of his "Dedalus, or Treatise on Mechanical Motions ;" but the success of it is doubtful, and experiments made in this way have been few and unsatisfactory. Borelli having compared the power of the muscles which act on the wings of a bird with that of the muscles of the breast and arms of a man, finds the latter altogether insufficient to produce, by means of any wings, that motion against the air, which is necessary to raise a man in the atmosphere. Soon after Mr. Cavendish's discovery of the specific gravity of inflammable air, it occurred to the ingenious Dr. Black, of Edinburgh, that if a bladder, sufficiently light and thin, were filled with this air, it would form a mass lighter than the same bulk of at

mospheric air, and rise in it. This thought was suggested in his lectures in 1767 or 1768; and he proposed, by means of the allantois of a calf, to try the experiment. Other employments, however, prevented the execution of his design. The possibility of constructing a vessel, which, when filled with inflammable air, would ascend in the atmosphere, had occurred also to Mr. Cavallo, about the same time; and to him belongs the honour of having first made experiments on this subject, in the beginning of the year 1782, of which an account was read to the Royal Society, on the 20th of June in that year. He tried bladders; but the thinnest of these, however scraped and cleaned, were too heavy. In using China paper, he found that the inflammable air passed through its pores, like water through a sieve; and having failed of success by blowing this air into a thick solution of gum, thick varnishes, and oil-paint, he was under a necessity of being satisfied with soapbubbles, which being inflated with inflammable air, by dipping the end of a small glass tabe, connected with a bladder containing air, into a thick solution of soap, and gently compressing the bladder, ascended rapidly in the atmosphere; and these were the first sort of inflammable air-balloons that were ever made. For balloons formed on a larger scale, and on the principle of rarefied air, we must direct our attention to France, where the two brothers, Stephen and Joseph Montgolfier, paper-manufacturers at Annonay, about 36 miles from Lyons, distinguished themselves by exhibiting the first of those aerostatic machines, which have since excited so much attention and astonishment. The first idea of such a machine was suggested to them by the natural ascent of the smoke and clouds in the atmosphere; and the first experiment was made at Avignon, by Stephen, the eldest of the two brothers, towards the middle of November 1782. Having prepared a bag of fine silk, in the shape of a paralelepipedon, and in capacity about 40 cubic feet, he applied to its aperture burning paper, which rarefied the air, and thus formed a kind of cloud in the bag, and when it became sufficiently expanded, it ascended rapidly to the ceiling. Soon afterwards the experiment was repeated by the two brothers at Annonay in the open air, when the machine ascended to the height of about seventy feet. Encouraged by their success, they constructed a machine, the capacity of which was about 650 cubic feet, which,

in the experiment, broke the ropes that confined it, and after ascending rapidly to the height of about 600 feet fell on the adjoining ground. With another machine, 35 feet in diameter, they repeated the expe riment in April 1783, when breaking loose from its confinement, it rose to the height of above 1000 feet, and being carried by the wind, it fell at the distance of about three quarters of a mile from the place where it ascended. The capacity of this machine was equal to about 23,450 cubic feet; and when inflated, it measured 117 English feet in circumference. The covering of

it was formed of linen lined with paper, its shape was nearly spherical, and its aperture was fixed to a wooden frame about 16 feet in surface. When filled with vapour, which was conjectured to be about half as heavy as common air, it was capable of lifting up about 490 pounds, besides its own weight, which, together with that of the wooden frame, was equal to 500 pounds. With this machine the next experiment was performed at Annonay, on the 5th of June 1783, before a great multitude of spectators. The flaccid bag was suspended on a pole 35 feet high; straw and chopped wool were burnt under the opening at the bottom; the vapour, or rather smoke, soon inflated the bag, so as to distend it in all its parts; and this immense mass ascended in the air with such a velocity, that in less than ten minutes it reached the height of about 6000 feet. A breeze carried it in an horizontal direction to the distance of 7668 feet; and it then fell gently on the ground. M. Montgolfier attributed the ascent of the machine, not to the rarefaction of the heated air, which is the true cause, but to a certain gas or aeriform fluid, specifically lighter than common air, which was supposed to be disengaged from burning substances, and which has been commonly called Montgolfier's gas, as baloons of this kind have been denominated Montgolfiers. As soon as the news of this experiment reached Paris, the philosophers of the city, conceiving that a new sort of gas, half as heavy as common air, had been discovered by Messrs. Montgolfier; and knowing that the weight of inflammable air was not more than the eighth or tenth part of the weight of common air, justly concluded, that inflammable air would answer the purpose of this experiment better than the gas of Montgolfier, and resolved to make trial of it. A subscription was opened by M. Faujas de St. Fond towards defraying the expense of the experiment. A sufficient

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sum of money having been soon raised, Messrs. Roberts were appointed to construct the machine; and M. Charles, professor of experimental philosophy, to superintend the work. After surmounting many difficulties in obtaining a sufficient quantity of inflammable air, and finding a substance light enough for the covering, they at length constructed a globe of lutestring, which was rendered impervious to the inclosed air by a varnish of elastic gum, or caoutchouc, dissolved in some kind of spirit or essential oil. The diameter of this globe, which, from its shape, was denominated a ballcon, was about thirteen feet, and it had only one aperture, like a bladder, to which a stop-cock was adapted its weight, when empty, together with that of the stop-cock, was 25 pounds. On the 23d of August, 1783, they began to fill the globe with inflammable air; but this, being their first attempt, was attended with many hindrances and disappointments. At last, however, it was prepared for exhibition; and on the 27th it was carried to the Champ de Mars, where, being disengaged from the cords that held it down, it rose before a prodigious concourse of people, in less than two minutes, to the height of 3123 feet. It then entered a cloud, but soon appeared again; and at last it was lost among other clouds. This balloon, after having floated about three quarters of an hour, fell in a field about 15 miles distant from the place of ascent; where, as we may naturally imagine, it occasioned much astonishment to the peasants. Its fall was owing to a reut, occasioned by the expansion of the inflammable air in that part of the atmosphere to which it ascended. When the balloon went up, its specific gravity was 35 pounds less than that of common air. In consequence of this brilliant experiment, many balloons were made on a small scale; goldbeaters skin was used for the covering; and their size was from 9 to 18 inches in diameter.

Mr. Montgolfier repeated an experiment with a machine of his construction before the commissaries of the Academy of Sciences, on the 11th and 12th of September. This machine was 74 feet high, and about 43 feet in diameter. When distended, it appeared spheroidical. It was made of canvas, covered with paper both within and without, and it weighed 1000 pounds. The operation of filling it with rarefied air, produced by means of the combustion of 50 pounds of dry straw, and 12 pounds of chopped wool, was performed in about nine

minutes; and its force of ascension, when inflated, was so great, that it raised eight men who held it some feet from the ground. This machine was so much damaged by the rain, that it was found necessary to prepare another for exhibition before the king and royal family on the 19th. This new machine consisted of cloth, made of linen and cotton thread, and was painted with watercolours both within and without. Its height was near 60 feet, and its diameter about 43 feet. Having made the necessary preparations for inflating it, the operation was begun about one o'clock on the 19th of September, before the king and queen, the court, and all the Parisians who could procure a conveyance to Versailles. In eleven minutes it was sufficiently distended, and the ropes being cut, it ascended, bearing up with it a wicker cage, in which were a sheep, a cock, and a duck. Its power of ascension, or the weight by which it was lighter than an equal bulk of common air, allowing for the cage and animals, was 696 pounds. This balloon rose to the height of about 1440 feet; and being driven by the wind, it descended gradually, and fell gently into a wood, at the distance of 10,200 feet from Versailles. After remaining in the atmosphere eight minutes, the animals in the cage were safely landed. The sheep was found feeding; the cock had received some hurt on one of his wings, probably from a kick of the sheep; the duck was perfectly well. The success of this experiment induced M. Pilatre de Rozier, with a philosophical intrepidity which will be recorded with applause in the history of aerostation, to offer himself as the first adventurer in this aerial navigation. Mr. Montgolfier constructed a new machine for this purpose in a garden in the Fauxbourg St. Antoine. Its shape was oval; its diameter being about 48 feet, and its height about 74 feet. To the aperture at the bottom was annexed a wicker gallery, about three feet broad, with a ballustrade about three feet high. From the middle of the aperture was suspended by chains, which came down from the sides of the machine, an iron grate, or brazier, in which a fire was lighted for inflating the machine; and port-holes were opened in the gallery, towards the aperture, through which any person, who should venture to ascend, might feed the fire on the grate with fuel, and regulate the dilatation of the inclosed air of the machine at pleasure. The weight of the aerostat was upwards of 1600 pounds. On the 15th of October, the fire

but on the earth, before he ascended, it stood at 30.16 inches. On the 23d of June, a large aerostat, on the principle of rarefied air, 91 feet high, and 79 feet in diameter, was elevated by Montgolfier at Versailles, in the presence of the royal family and the King of Sweden. M. Pilatre de Rozier, and M. Proust, ascended with it, and continued for 28 minutes at the height of 11,732 feet, and observed the clouds below them, that reflected to the region which they occupied the rays of the sun; the temperature of the air being 5o below the freezing point; and in three quarters of an hour they travelled to the distance of 36 miles. In consequence of this experiment, the king granted to M. Rozier a pension of 2000 livres. On the 15th of July the Duke of Chartres, the two brothers Roberts, and another person, ascended with an inflammable air balloon, of an oblong form, 55 feet long, and 3-4 feet in diameter, from the Park of St. Cloud: the machine remained in the atmosphere about 45 minutes. This machine contained an interior small balloon, filled with common air, by which means it was proposed to make it ascend or descend without any loss of inflammable air or ballast. The boat was furnished with a helm and oars, intended for guiding it. At the place of departure the barometer stood at 30.12 inches. Three minutes after ascending, the balloon was lost in the clouds, and involved in a dense vapour. An agitation of the air, resembling a whirlwind, alarmed the aerial voyagers, and occasioned several shocks, which prevented their using any of the instruments and contrivances prepared for the direction of the balloon. Other circumstances concurred to increase their danger; and when the mercury standing in the barometer at 24.36 inches indicated their height to be about 5100 feet, they found it necessary to make holes in the bottom for discharging the inflammable air: and having made a rent of between seven and eight feet, they descended very rapidly, and at last came safely to the ground. The first aerial voyage in England was performed in London, on the 15th of September, by Vincent Lunardi, a native of Italy. His balloon was made of oiled silk, painted in alternate stripes of blue and red. Its diameter was 33 feet. From a net which went over about two-thirds of the balloon, descended 45 cords to a hoop hanging below the balloon, and to which the gallery was attached. The balloon had no valve; and its neck, which terminated in the form of a pear, was the aperture through

which the inflammable air was introduced, and through which it might be let out. The air for filling the balloon was produced from zinc by means of diluted vitriolic acid. M. Lunardi departed from the Artillery Ground at two o'clock; and with him were a dog, a cat, and a pigeon. After throwing out some sand, to clear the houses, he ascended to a great height. The direction of his motion was at first north-west by west; but as the balloon rose higher, it fell into another current of air, which carried it nearly north. About half after three he descended very near the ground, and landed the cat, which was almost dead with cold: then rising, he prosecuted his voyage. He ascribes his descent to the action of an oar; but as he was under the necessity of throwing out ballast in order to re-ascend, his descent was more probably occasioned by the loss of inflammable air. At ten minutes past four he descended on a meadow, near Ware, in Hertfordshire. The only philosophical instrument which he carried with him was a thermometer, which in the course of his voyage stood as low as 29°, and he observed that the drops of water which collected round the balloon were frozen.

The longest and the most interesting voyage, which was performed about this time, was that of Messrs. Roberts and M. Collin. Hullin, at Paris, on the 19th of September. Their aerostat was filled with inflammable air. Its diameter was 27 feet, and its length 464 feet, and it was made to float with its longest part parallel to the horizon, with a boat of nearly 17 feet long attached to a net that went over it as far as its middle. To the boat were annexed wings, or oars, in the form of an umbrella. At 12 o'clock they ascended with 450 pounds of ballast, and after various mancnvres descended at 40 minutes past six o'clock near Arras, in Artois, having still 200 pounds of their ballast remaining in the boat. Having risen about 1400 feet, they perceived stormy clouds, which they endeavoured to avoid; but the current of air was uniform from the height of 600 to 4200 feet. The barometer on the coast of the sea was 29.61 inches, and sunk to 23.94 inches. They found that by working with their oars, they accelerated their course. In the prosecution of their voyage, which was 150 miles, they heard two claps of thunder; and the cold occasioned by the approach of stormy clouds made the thermometer fall from 77⚫ to 59°, and condensed the inflammable air in the balloon, so as to make it descend very