A EROSTATION, in the modern appli cation of the term signifies the art of na vigation through the air, both in its kin ciples and practice. Hence also the ma chines which are employed for this pur pose are called aerostats, or aerostatic machines; and on account of their round figure, air balloons.
The principles of this art have heed long and generally known; al though the application of them to prac tice seems to he altogether a modern dis covery. 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 encom passing the earth, will be buoyed up by it, and ascend; but as the density of the atmosphere decreases, on acconnt of the diminished pressure of the superinetim bent air, and the elastic pmperty which it possesses at different elevations above the earth, this body can rise only to a height, in which the surrounding am will be of the same specific gravity with itself. In this situation it will either float, or be driven in the direction of the vvinci orcur rent 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 sev eral weights annexed to it, is of less specific gravity than that of the air in which it rises. neat is well known to rarefy and expand, and conse quently to lessen the specific gravity of the air to which it is applied; and the di minution 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. lf, 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 wei,ght of the covering and its appendages, this whole mass will ascend i n the atmospl ere, till, by the cool ing and condensation of the included air, or the diminished density of the surround ing air, it becomes of tile same specific gravity with the air in vs-hich it floats; and without renewed heat it will gradually descend. If, instead of heating common air inclosed in any covering, and thus di minishing its weight, the covering be filled -with an elastic fluid, lighter than atmo spherIc 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 con tinue to rise, till it attains a hei gilt at which thc surroundingair is of tile 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 ure indebted to Mr. Den ry Cavemlish, who discovered, that if common airis eight Inindredtimes lighter than water, inflammable air is seven times lighter than common air ; but if common air is eight hundred and fifty- times er than water, then inflammable air is 10.8 times lighter than common air. Tliecon struction of air-balloons depends upon the principles above stated ; and they are of two kinds, as one or the otherof the pre ceding 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-, mighthear it up, and by theirmotion, pro duced 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 Wil kins, in the sev-enth chapter of his " Dedalus, or 'Treatise on Zslecha nical Motions;" but the success of it is doubtful, and experiments made in this way hare 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 arid 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 atmo sphere. Soon after Mr. Cavendish's dis covery of the specific gravity of infltun mable air, it occuiTed 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 atmospheric air, and rise in it. This thought was suggested in his lectures in 1767 or 1768; and lie propos ed, by means of the alantois of a calf, to try tile experiment. OdieremploymentF, however, prevented the execution of his design. The possibility' of constructing a vessel, V6 111C111, %%hen filled with inflam mable air, would ascend in the atmo sphere, had occurred also to Mr. Cavallo, about the same time; and to him belongs the honour of halting first made experi ments on this subject, in the beginning of the year 1782, of which an account was read to the Itoy-al Society-, on the 20th of ,Iiinc in that year. Ile tried bhuldets ; but the thinnest of these, however, scrap ed and cleaned, were too heavy-. In using China paper, he found that the inflamma ble air passed throug-h its pores, like wa ter 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 soap-bubbles, which being inflated with inflammable air, by dipping the end of a small glass tube, connected with a bladder containing air, into a thick solution of soup, and gently compressing the bladder, ascended rapidly in the at mosphere; 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' rare fied air, we nmst direct our attention to France, where the two brothers, Stephen and Joseph Montgolfier, paper-manufactu rers at Annonay, about 36 miles froni Ly ons, distinguished themselves, by exhibit ing the first of those aerostatic machines, which have since excited so much atten tion 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 Ste phen, the eldest of the two brothers, to wards the middle of November 1782. Having prepared a bag of fine silk, in the shape of a parallelopipedon, and in capa city 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 b ecame sufficiently expanded, it ascended rapidly to the ceil ing. Soon afterwards the experiment was repeated by the two brothers at Annonay in the open air, v..hen the machine ascend ed to the height of about seventy feet. Encouraged by their success, they con structed 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 ofabout 600 feet, fell on the adjoin ing ground. With another machine, 35 feet in diameter,they repeated the expe riment in Apri11783, whembreaking loose from its confinement, it rose to the h eight of above 1000 feet, and being carried by the w-ind, 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 23,430 cubic feet: and when inflated, it measured 117 English feet in circumference. The co veting Of it was Rimed of linen lined with paper, its shape was nearly spherical, and its aperture was fixecl to a wooden frame about 16 feet in surface. When filled with vapour, which wasconjectured 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 ma chine the next experiment was performed at Annonay, on tile 5th of Jiine 1783, be fore a great multitude of spectators. The flaccic bag was suspended on a pole 35 feet high; straw and chopped wool A ere burnt under the opening at the bottom ; the vapour, or rather smok e, soon inflated the bag, so as to distend it in all its parts; and this immense mass ascended in the air with such a elocity, that in less than ten minutes it reached the height of about 6000feet A breeze carried it in an hori zontal direction to the distance of 7668 feet; and it then fell gently on th e ground. M. Montgolfier attributed the ascent of the machine, not to the rarefaction of the heatedair, which is the true cause; but to a certain gas or aeriform fluid, specifically lighter than conunon air, which was sup posed to be disengaged from burning sobstances, and which has bce n common ly called Montgollier's gas, as balloons of this kind have been denominated Mont golfiers. As soon it.s the news of this ex periment 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 inflam mable air was not more than the eighth or tenth part of the weight of common air, justly concluded, thaf,inflammable air would answer the purpose of this experi ment better than the gas of' Montgolfier, and resolved to make trial of it. A sub scription WAS opened by M. Faujas de St. Fond towards defraying the expense of the experiment. A sufficient sum of mo ney having been scion raised, Messrs. Ro berts were appointedtoconstructthe ma chine; and M. Charles, professor of expe rimental philosophy, to superintend the work. Mter surmounting many difficul ties in obtaining a sufficient quantity of inflammable air, and finding a substluice light enough for the covering., they at leng-th constructed a globe of lutestring, which was rendered impervious to the in closed air by a varnish of elastic gurn, or caoutcholic,eissolved in some kind of spirit or esseiCtial oil. diameter of this globe, which, from its shape, was de nominated a balloon, was about thirteen feet, and it hacl only one aperture, like a bladder, to which a stop-cock was adapt ed : its weight, when empty, together with that °Nile stop-cock, was 23 pounds. On the 23d. of August 1783, they began to fill the globe with inflammable air ; but this being their first attempt, was attend ed with many hindrances and disappoint nients. At last, however, it was prepared for exhibition; and on the 27th it was carried to the Champ de Mars. where, beingdisengagedfrom the cords thatheld it down, it rose before a prodigious con course of people in less than two minutes to the height of 3123 feet. It then enter ed 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 ima gine, it occasioned much astonishment to the peasants.lts fall was owing to a rent, occasioned by the expansion of the inflam mable air in that part of the atmosphere to which it ascended. When the balloon up, its specific gravity was 35 pounds less than that of common air. In conse quence of this brilliant experiment, many balloons were made on a small scale ; gold-beaters skin was used forthe cover ing; and theirsizewasfrom 9 to 18 incises in diameter.