AIR Balloons. A general name for bags of any light substance filled with inflammable air or other permanently elastic fluid, whose specific gravity is considerably less than that of common atmospheric air. The consequence of their being filled in this manner is, that, if they are of any considerable magnitude, they ascend in the air to an amazing height, and will not only ascend in this manner by themselves, but carry up great weights along with them; remaining in these elevated regions till the fluid escapes, either by bursting the bag in consequence of part of the external pressure being removed, or by its gradual evaporation through the pores.
By this invention, the schemes of transporting people through the atmosphere, formerly thought chimerical, are realized; and it is impossible to say how far the art of aerial navigation may be improved, or with what advantages it may be attended. In the last century, a scheme of navigating the atmosphere was proposed by Mr Hook, very much resembling that of air-balloons; but which did not succeed, because he was unacquainted with any vapour possessing equal elasticity with the common air, and less specific gravity. His project was to have a very large copper-globe, so thin, that when it was exhausted of air it would be specifically lighter than an equal bulk of the common atmosphere. Hence it would ascend, and carry up bodies along with it. But it is easy to see that an invincible objection lay against this scheme from the pressure of the external atmosphere, which would not fail to crush such a large globe made of such thin materials.—Though inflammable air, and its great inferiority in specific gravity to common atmospheric air, has been known for several years, yet we hear of no attempts to construct air-balloons till, in the year 1782, Mr Cavallo attempted to make paper bags and bladders, filled with inflammable air, ascend; but failed, the former being permeable to inflammable air, and the latter being too weighty in proportion to their bulk. Had he thought of varnishing his paper, or making bags of gold-beaters skin, he would certainly have succeeded.
The honour of this invention, however, is unquestionably due to the brothers Stephen and Joseph Mont-
(G) Such readers as have been little accustomed to speculations of this sort, will be at a loss to comprehend in what manner two holes, both of them in the roof of the room, and communicating with the air, without any valve, or other contrivance, for opening or closing of themselves, should yet answer the two very opposite purposes; one, of constantly bringing cool air into the room without emitting any warm air—and the other, of as constantly emitting warm and admitting no cool air. They will please to advert, that the one of these tubes communicates with the atmosphere at the bottom of the house, and the other towards the top: the opening of the one is beneath the level of the room, that of the other above it. Now, as the air is more dense at the surface of the ground than at any height above it, the warm rarefying air will naturally issue at that opening where it meets with least resistance, which must invariably be through that which opens to the external air at the greatest height; and as the cool air will naturally be pressed into the room by that opening where the air is most weighty, this must invariably be by that which is nearest the surface of the earth.
Air. Montgolfier, proprietors of a considerable paper-manufacture at Annonay, a town in the Vivarais, about thirty-six miles south of Lyons: and their invention is the more to be admired, as it is not the effect of the late discovery of a permanently elastic fluid lighter than the common air, but of properties of matter long known, and in the hands of the many acute philosophers of this and of the last century. They conceived, that the effect they looked for might be obtained by confining vapours lighter than common air, in an inverted bag or covering, sufficiently compact to prevent their evaporation; and so light, that, when inflated, its own weight, added to that of the inclosed vapour, might fall somewhat short of the weight of the air which its bulk displaces.
Review, 1833. On these principles, they prepared matters for an experiment. They formed a bag or balloon of linen cloth, lined with paper, nearly spherical, and measuring about 35 feet in diameter; its solid contents were about 22,000 cubic feet, a space nearly equal to that occupied by 1980 lb. of common air, of a mean temperature, on the level of the sea. The vapour, which, by conjecture, was about half as light as common air, weighed 990 lb. The balloon, together with a wooden frame suspended to the bottom, which was to serve as ballast, weighed 490 lb.; whence it appears, that the whole must have been about 500 lb. lighter than an equal bulk of common air. This difference of specific gravity, by which these bodies are made to rise, we shall henceforth, without warranting the propriety of the expression, call their power of ascension.
The 5th of June 1783 was fixed on for the display of this singular experiment. The states of Vivarais, who were then assembled at Annonay, were invited to the exhibition. 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, on a sudden, this immense mass ascended in the air with such a velocity, that in less than ten minutes it appeared to be about 1000 toises above the heads of the spectators. A breeze carried it about 1200 toises from the spot whence it departed; and then the vapour, either escaping through some loop-holes that had been accidentally left in the construction, or being condensed by the coldness of the circumambient air, the globe descended gradually on a vineyard, with so little pressure, that none of the stakes were broken, and scarce any of the branches of the vines bent.
The rumour of this successful experiment soon reached the metropolis, and roused the emulation of the Parisian philosophers. Without waiting for particular instructions from the inventors, they reflected on a method of their own; and resolved, instead of vapour, to use inflammable air; the specific weight of which, when pure, they knew to be to that of common air nearly as ten to one.
The process of producing this air being very expensive, a subscription was set on foot; and a sufficient sum being soon raised, M. Charles, professor of experimental philosophy, and M. Robert a mathematical-instrument maker, were set to work: and they constructed a globe of lutestring (tassetas), glazed
over with elastic gum dissolved in some kind of spirit or essential oil. After many difficulties and disappointments, which will ever attend first essays, they succeeded in two days to fill this globe with inflammable air, produced from 1000 lb. of iron-filings, and 498 lb. of vitriolic acid, diluted in four times its quantity of water. This globe measured 12 feet 2 inches in diameter, its solid contents were 943 feet 6 lines cubic, and its power of ascension was found equal to 35 lb.
The 27th of August 1783 having been fixed on for the exhibition of this experiment, the balloon was conveyed, in the preceding night, floating in the air, from a court near the Place des Victoires, where it had been constructed, to the Champ de Mars. The concourse of people, on foot and in carriages, was so immense, that a large body of troops were drawn out to prevent disturbances. At five o'clock in the afternoon, a signal having been given by the firing of a mortar, the cords that confined the globe were cut, and it rose in less than two minutes to a height of near 500 toises. It here entered a cloud, but soon appeared again, ascending to a much greater height; and at last it was lost among other clouds.
In this experiment, too much inflammable air, and even some common air, had been introduced into the globe, which being closed on all sides, left no room for the expansion of this elastic fluid when it should arrive to a more rarefied medium. We find, in fact, that it must have burst in consequence of this expansion; since, after having floated about three quarters of an hour, it fell in a field near Gonneffe, a village about five leagues N. N. W. of the Champ de Mars. It must be allowed, that the mere evaporation of the air could not well have been the cause of its descending so soon. Many periodical papers have already entertained the public with ludicrous accounts of the astonishment of the peasants who found it, and of the rough treatment it received at their hands.
It may easily be imagined, that these brilliant successes animated the zeal of all the curious in the metropolis, and that many essays were made to repeat the same experiments upon a smaller scale. A number of secondary attempts accordingly were made; upon which we shall dwell no longer, than only to observe, that they succeeded with globes made of gold-beater's skin, and only 12 inches in diameter; which being thought the least that could be made to ascend, considering that the proportionate weight of the materials increase as the bulk is diminished, were called mini-mums.
M. Montgolfier junior having arrived at Paris a few days before the experiment at the Champ de Mars, was desired by the Royal Academy of Sciences to repeat the experiment of Annonay. He accordingly constructed, in a garden in the Faubourg of St Germain, a balloon of an elliptical form, 70 feet high, and 40 feet in diameter. It was lined both inside and outside with paper. Its power of ascension was found upon calculation to be about 1250 lb. It was filled in ten minutes, by the burning of 50 lb. of straw and 10 lb. of chopped wool. It was loaded with a weight of 500 lb. and ascended, fastened to ropes, on the 12th of September, in the presence of the deputies of the
Royal Academy. But it proving a very rainy day, the whole apparatus was so essentially damaged, that it was not thought proper to set it loose.
We come now to the experiment made on the 10th of September, in the presence of the king and queen, the court, and all the Parisians who could procure a conveyance to Versailles. This balloon was 57 feet high, and 41 in diameter. Its power of ascension, allowing for a wicker cage, containing a sheep, a cock, and a duck, which was suspended to it, was equal to 696 lb. As only four days had been allowed for the making this machine, it could not therefore be lined with paper. M. M. had predicted, that it would remain in the air about twenty minutes, and with a moderate wind might float to a distance of about 2000 toises. But, beside some imperfection in the construction, owing to the great hurry in which it had been made, a sudden gust of wind, while it was inflating, made two rents seven feet long near the top, which could not but in some measure prevent the promised effect. It swelled, however, in eleven minutes sufficiently to raise it about 240 toises; it floated to the distance of nearly 1700 toises; and after having been in the air about eight minutes, it subsided gradually in the wood of Vauvres. 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.
M. Montgolfier determined now to repeat the experiment under more favourable circumstances, and more at his leisure. He therefore made a new balloon in a garden in the Faubourg St Antoine, which measured 70 feet in height and 46 feet in diameter. A gallery of wicker was contrived round the aperture at the bottom; under which an iron grate or brasier was suspended, and port-holes opened on the inside of the gallery towards the aperture, through which any person cui robur et as triplex circa pectus fuerit, who might venture to ascend, might feed the fire on the grate, and thus keep up the vapour, smoke, or, as we rather apprehend, the dilatation of the air, in this vast cavity.
On the 15th of October, M. Pilatre de Rozier, no doubt the most intrepid philosopher of the age, placed himself in the gallery, ascended about 80 feet from the ground, and there kept the balloon afloat for some time, by repeatedly throwing straw and wool upon the fire. In this experiment it was found, that the descent of a globe (provided no extraordinary accident happened to it) must necessarily be gradual; and that it will always light softly upon the ground, since, in fact, in every part of its descent, it enters a denser medium; whence its velocity in falling will rather be retarded than accelerated. On the 19th of October, M. P. Rozier ascended a second time about 250 feet. After continuing stationary about eight minutes, a gust of wind carried the balloon among some trees, where it entangled itself so as to endanger its being torn to pieces: but on M. R. throwing some fresh straw upon the fire, it immediately reascended, amid the loud acclamations of a vast multitude of people, who little expected to see so sudden a recovery. The balloon was then hauled down, and M. Giron de Villette placed himself in the gallery opposite to M. R.
They were once more let up; and for some time hovered over Paris, in the sight of all its inhabitants, at the height of 324 feet.
Matters seemed now ripe for a free aerial navigation. A smoke balloon, very similar to the one last described, was prepared to go off at La Muette, a royal palace in the Bois de Boulogne, where, we are informed, the king's children now usually reside. All things being ready, on the 21st of November M. Pilatre de Rozier took his post in the gallery, and the Marquis d'Arlandes, a major of infantry, placed himself on the opposite side of this gallery, as a counterpoise to preserve the equilibrium of the machine. After repairing some damage done to the balloon in a first essay, it was, at 54 minutes after one, absolutely abandoned to the element; and it ascended with great rapidity.
When these bold adventurers were about 250 feet in the air, they waved their hats to the astonished multitude; but they soon rose too high to be distinguished, and are thought to have soared to an elevation of about 3000 feet. The history of this navigation is, in fact, the history of the alarms of the Marquis d'Arlandes. When he found himself so high that he could no longer distinguish the objects upon earth, he thought both his ambition and his curiosity sufficiently gratified, and desired his companion to cease laying straw upon the fire, that they might descend. M. P. de Rozier, however, deaf to these remonstrances, continued his operations, and the Marquis continued murmuring. At length, being at the highest elevation above mentioned, the latter perceived some holes burnt in the sides of the balloon, and likewise heard some cracks near the top of the machine, which seemed to menace instant destruction. He then became outrageous; quickly clapped wet sponges to the burning holes; and vowed, that if his companion would now descend, he would take upon himself the whole blame of having thus shortened their navigation. M. Rozier at length listened to his urgent solicitations: but on approaching the earth, they found that they were descending immediately over the Seine; and fearing lest they might be carried away by the current of air that generally attends streams of water, the Marquis was glad to assist in throwing fresh straw upon the fire; and thus they rose again to a considerable height. On their next approach to the earth, the Marquis seeing the danger they were in of being spitted on the weather-cock of the Invalids, hastily threw a fresh bundle of straw upon the fire, and even spread it, in order to raise a greater blaze. This carried them over a great part of Paris, where they took care to clear all the steeples, &c. and passing the Boulevard, they landed safely in a field near Bicetre, without having experienced the least real inconvenience. The distance they went was between 4 and 5000 toises. They were in the air about 25 minutes. The collective weight of the whole apparatus, including that of the two travellers, was between 16 and 1700 lb. and when they landed, they had two thirds of their combustibles still left in store.
A more remarkable experiment than any of these hitherto mentioned, was made by Messrs Charles and Robert. The globe prepared for this expedition was made like that of the Champ de Mars, of gores of silk,
Air. silk, alternately red and white, and glazed with some sort of gum. It was spherical, and measured 26 feet in diameter. It was filled with inflammable air; the making of which alone cost 5000 livres. The expence of the whole apparatus amounted to no less than 10,000 livres. A net was spread over the upper hemisphere, which supported a hoop that surrounded the middle. To this hoop was suspended, by means of several cords, a boat, that swung at a small distance below the bottom of the globe, and which was so finely ornamented, as to deserve, in this respect, the name they gave it at Paris of a Triumphant Car. In order to prevent the bursting of the globe in a rarefied medium, an opening had been left, with a valve to it, which gave vent to the interior air, but suffered none of the exterior to enter. A long silken pipe or gut proceeded from this aperture; the farther end of which one of the navigators held in his hand, and thereby obtained a considerable command over the inflammable air. The car was ballasted with sand-bags. By these means they hoped, and in fact they succeeded, to guide themselves in point of elevation; for by letting some of the air escape, they naturally descended; and on discharging some of their ballast, they were sure to ascend.
The 1st of December 1783 was fixed upon for this pompous display. Two hundred thousand people assembled in and near the garden of the Thuilleries. The apparatus stood on a scaffolding raised for the purpose, in the middle of a piece of water, to prevent its being approached by the multitude. Upon this it rested, merely by the weight of the ballast in the car. The friends of the navigators had stored it with plenty of provision and cloathing; beside which, proper instruments were also embarked. A small balloon, which had been prepared for the purpose, was offered to M. Montgolfier, who, at the request of M. Charles, cut the string by which it was held, and, by this allegory, tacitly received the tributary homage due to him and his brother as the authors of the invention.
At forty minutes past one, Messrs Charles and Robert ascended the Car. They threw out 19 lb. of ballast, and instantly rose, with an accelerated velocity, to the height of about 300 toises. No acclamation, no sound was heard; for the multitude stood silent with fear and amazement. The navigators, however, gave signals of their security, by frequently waving two pennants; and M. Charles apprised his friends below that they were easy and happy, by a note he threw down among the crowd. After continuing a short time stationary, they perceived themselves moving nearly horizontally, in the direction of N. N. W. Finding that some of the inflammable air evaporated, they discharged some ballast; and soon after observing that the heat of the sun dilated the inflammable air, they suffered some of it to escape; and thus they kept pretty nearly in the same level. In this manner they floated twice across the Seine; and over many towns and villages, the surprise of whose inhabitants can more easily be conceived than described. About 56 minutes after their departure, they found themselves out of sight of Paris. They then descended so low as to skim along the surface of the ground, and conversed with several labourers in the fields. Seeing a hill before them, they cast some of their superfluous
cloathing out of the car, and thus cleared the eminence. They now made a comfortable meal. Finding themselves near the Isle d'Adam, where the prince of Conti has a palace, they again approached the ground, inquired after the prince, and were told that he was at Paris. At 45 minutes after three, they found themselves over Nefle, a small town about nine leagues from Paris. And there, after sliding a little way along the surface of the ground, they alighted gently, and without the least shock or concussion, in a field.
Of a great number of those who had galloped after the balloon from the Thuilleries, only the dukes de Chartres and Fitz-James, and Mr Farrer an English gentleman, who had relays posted in the direction of the wind, arrived a few minutes after the landing. The others either lamed or killed their horses, or grew tired of the pursuit. After the warmest congratulations, an affidavit was drawn up, and signed by all the parties present.
Mr Charles now declared his intention to reascend alone; but to this the duke de Chartres consented, only on condition that he would return in half an hour. M. Robert alighted; and by the diminution of his weight, the machine acquired a power of ascension equal to about 100 lb.
Mr Charles's account of this second ascent we shall give in his own words: "Thirty peasants held down the machine. I asked for some earth to ballast it, having not above four or five pounds left. A spade was not at hand, nor were there any stones in the meadow where the machine had alighted. The sun was near setting. I made a hasty calculation of the time requisite for the alteration of weight, and gave a signal for the peasants to quit their hold. I sprung up like a bird. In 20 minutes I was 1500 toises high, out of sight of all terrestrial objects. I had taken the necessary precautions against the explosion of the globe, and prepared to make the observations which I promised myself. In order to observe the barometer and thermometer placed at the ends of the chariot, without altering the centre of gravity, I kneeled down in the middle, stretching forwards my body and one leg, holding my watch and paper in the left, and my pen and the string of the valve in my right hand, waiting for the event. The globe, which at my first setting out was rather flaccid, swelled insensibly. The air escaped in great quantities at the valve. I drew the valve from time to time to give it two vents; and I continued to ascend, still losing air, which issued out hissing, and became visible, like a warm vapour in a cold atmosphere. The reason of this phenomenon is obvious. On earth the thermometer was seven degrees above the freezing point; after 10 minutes ascent, it was five degrees below. The inflammable air had not time to recover the equilibrium of its temperature. Its elastic equilibrium being quicker than that of the heat, there must escape a greater quantity than that which the external dilatation of the air could determine by its least pressure. For myself, though exposed to the open air I passed in 10 minutes from the warmth of spring to the cold of winter, I felt at first nothing disagreeable in the sudden change. When the barometer ceased to rise, I marked exactly 18 inches 10 lines; the mercury suffering no sensible oscillation.
Air. cillation. From this height I deduce an elevation of 1524 toises, or thereabouts, till I can be more exact in my calculation. In a few minutes more my fingers were benumbed by the cold, so that I could not hold my pen. I was now stationary, and moved only in an horizontal direction. I rose up in the middle of the chariot, to contemplate the scene around me. At my setting out, the sun was set on the vallies; but he soon rose for me alone, who was the only luminous body in the horizon, and all the rest of nature in the shade. The sun himself presently disappeared; and I had the pleasure of seeing him set twice in the same day. I beheld, for a few seconds, the circumambient air and vapours rising from the vallies and rivers. The clouds seemed to rise from the earth, and collect upon one another, still preserving their usual form, only their colour was grey and monotonous from the want of light in the atmosphere. The moon alone enlightened them, and showed me that I was tacking about twice; and I observed certain currents that brought me back again. I had several sensible deviations, and observed with surprise the effects of the wind, and saw the streamers of my banners point upwards. This phenomenon was not the effect of the ascent or descent; for I then moved horizontally. At that instant I conceived, perhaps a little too hastily, the idea of being able to steer one's own course. In the midst of my transports, I felt a violent pain in my right ear and jaw, which I ascribed to the dilatation of the air in the cellular construction of these organs, as much as to that of the external air. I immediately put on a woollen cap; yet the pain did not go off but as I gradually descended. For seven or eight minutes I had ceased to ascend; the condensation of the inflammable air rather made me descend. I now recollected my promise to return in half an hour; and, pulling the upper valve, I came down. The globe was now so much emptied, that it appeared only an half globe. When I was between 20 and 30 toises from the earth, I threw out hastily two or three pounds of ballast, and became for a moment stationary, till I descended gently on the field, above a league from the place whence I set out. The frequent deviations and turnings make me imagine that this voyage was about three leagues; and I was gone about 35 minutes."
These successful attempts soon produced the emulation of others. M. Montgolfier, whose method had been so successfully tried by M. Pilatre de Rosier, undertook the construction of a balloon, upon his own principles, much larger than any that had hitherto appeared; and it was given out by his friends, that with this machine he intended to set out from Lyons with a quantity of goods, either for Paris or Marseilles, as the wind should serve. This balloon was composed of double canvas, with three layers of paper between the folds; but, at the desire of M. Rosier, the upper part was changed for a simple cotton cloth. The balloon was 100 feet in diameter transversely, and 126 high, and weighed in all about 8000 pounds. The departure was announced for the 10th of January last; but when the attempt was then made, the globe swelled only partially, and could not be raised. Several other attempts were made on succeeding days, but with equally bad success; and in one of these unsuccessful attempts, part of the canvas was burnt. This damage,
however, was soon repaired; and on the 19th of January the projectors were ready for another trial, when the globe filled in 17 minutes. Seven persons embarked in the machine annexed to it, and ascended amidst the acclamations of 100,000 spectators. The globe first took a N.W. direction, but soon after took to the S. E. It had not floated fifteen, or according to some accounts five minutes, when it descended with a velocity that alarmed the spectators, and alighted in a field not far from the place whence it was set off.
In the next experiment we have to describe, an apparatus was to be applied to a balloon filled with inflammable air, for steering it both horizontally and vertically, and even against a current of air. M. Blanchard of Paris had some years ago announced that he was preparing wings, with which he meant soon to take a flight. The late discovery seemed to hasten the execution of his project. His globe is said to have been only 14 feet 2 inches in diameter: but this must certainly be a mistake; for the weight of air contained in such a globe cannot be supposed equal to that of two people, even of the smallest dimensions, with all their necessary apparatus. Be this as it will, a globe was prepared, which actually was sufficient to raise the necessary weight. To this globe was suspended a car; between the car and the globe was fixed an umbrella 12 feet in diameter, the intention of which was to break the fall in case of any accident: hence it was called parachute. To the car were adapted four wings, two on each side, and behind a rudder, all made of taffety distended by means of whalebone ribs. All this was to be worked by a machinery of M. Blanchard's invention. He was to ascend himself for the purpose of navigating the machine; and Dom. Pech, a Benedictine, was also to ascend for the purpose of making various aerological observations and experiments. Every thing was ready on the 2d of March, in the Champ de Mars. A party of M. Blanchard's friends had assembled at a country-house where he had promised to meet them through the air. The navigators were embarked, and ready to soar, when a young man, a pupil of the ecole militaire, suddenly rushed through the crowd, threw himself into the car, and insisted on sharing in the expedition. A scuffle ensued, in which Blanchard was wounded in the hand, the parachute and wings destroyed, and the purpose of the experiment entirely defeated. This madman was at last overpowered, and the two adventurers determined to take a chance flight. They rose accordingly; but their power of ascension was not sufficient to carry them to any height, for which reason they soon landed. Dom. Pech alighted, and Blanchard instantly ascended with great rapidity. The wind being east, he was carried to the westward. The account he gives of his navigation is, that he rose about 2000 toises from the ground; that he found himself at times stationary in a perfect calm, during which the heat of the sun was scorching; that he at different times felt currents of wind in different directions, in some of which the cold was intense; that during these cold intervals he felt an almost unconquerable desire to sleep; that clouds collected under his feet; and that it appeared to him that he was at different times carried towards different parts of the compass. He continued in the air one hour and a quarter; after which
Air. he landed safely near Seve, about five miles from the spot whence he ascended.
These are the most remarkable experiments that have yet been made with respect to this new and extraordinary navigation. The principles on which it is accomplished are those of common hydrostatics; but though the specific lightness of inflammable air easily shows why globes filled with it should rise, the principle on which Montgolfier's balloons ascend seem not to be so generally understood. In an original paper, said to be written by a foreign gentleman of great learning, (Appendix to Monthly Review, vol. lxix.), we find the following reasoning on the subject: "The ingredients used for inflating the machine that was let off at Versailles, consisted of 50 pounds of straw and 5 pounds of wool: and, in a work published by M. Faujas de St Fond on the subject, we are informed, that the gas which inflated the bag was produced by burning these ingredients. Now, in as far as simple combustion is here concerned, the experiments of Dr Priestley, since repeated in a thousand different ways, plainly evince, that the effect of combustion is indeed at first to produce a very light aeriform fluid, viz. inflammable air; but that this air, meeting immediately with atmospheric air, takes fire by its union with the dephlogisticated air; and that the residuum of this, being thus mixed with fixed air, does thereby become specifically heavier than it was before. Thus all that modern chemistry hath hitherto taught us concerning combustion, can point out nothing to us as the immediate result of M. Montgolfier's operation, but a mass of air specifically heavier than common atmospheric air, mixed for an instant with some fuliginous vapours which collect into foot on the sides of the bag, and some aqueous vapours, which likewise condense on those sides and become water. It is impossible therefore to discover a priori, in the process of M. Montgolfier, the formation of any gas, or of any aeriform fluid, which in its nature can be specifically lighter than atmospheric air; but, on the contrary, we perceive in his operation the cause of some augmentation in the specific weight of the atmospheric air produced by the combustion.
"These arguments a priori are so strongly corroborated by the fact, that no doubt can well be entertained concerning their validity. It appears, that the solid contents of the machine let off at Versailles measured 37,500 cubic feet; that the weight of the common air that is displaced, was 3192 pounds; and as the gas of M. Montgolfier was thought to be about one half lighter than common air, it is there said that this gas weighed only 1596 pounds. Weighed only!—But was it considered, that this gas must have been the produce of no more than 50 pounds of straw and 5 pounds of wool? Is it likely that substances weighing in all 55 pounds, should yield 1596 pounds of air? Is it not surprising that this philosophical paradox should have hitherto escaped unnoticed?
"But let us attend to a particular circumstance mentioned in the account of the experiment, and we shall perhaps find the true cause of the ascension of the machine. The manner in which this large bag was filled, is said to have been by piling it upon a circular stage, and contracting it as much as possible, that very little common air might remain in it. Its orifice was
spread over an aperture in the middle of the stage, and straw was burnt under it. The effect of this operation is thus described: "As soon as the machine began to swell, a rapid current of air immediately took place, which coming from without, rushed into the machine with such force, that before the necessary precautions could be taken, the cloth, which was spread upon the stage and round the fire, in the form of a cylindrical funnel, was agitated with great violence." This account itself clearly points out the cause that swelled and buoyed up the machine: it was filled with common air dilated by heat, in the same manner as if the current of air that passes up a chimney were collected in a large empty bag; which being let loose, would certainly rise and float in the air, until the warm air within, losing its heat, be reduced to the temperature of the surrounding air.
"The sanguine hopes that are entertained of the useful application of this discovery may be gathered from a table, in which the dimensions a machine must have to enable it to raise a certain weight, are set forth upon elaborate calculations. These calculations are founded on the principles, that the air produced by M. Montgolfier is one-half lighter than common atmospheric air, and that the weight of the bag is at the rate of two ounces for each square foot. A formula is hence deduced, in which the proportion between the solid contents and the surface of the sphere are the only elements.
"We must here observe, that if, as appears to us very probable, it be merely the heat that diminishes the weight of air in the experiment, the diminution of one-half seems to be greatly exaggerated. It follows from the observations made by M. de Luc on the dilatation of air by the heat of the atmosphere, that if a certain quantity of air passes from the temperature of 32° of Fahrenheit to that of boiling water, that is, 212°, the original bulk of that quantity will be increased parts of the whole; this therefore is the effect of 180° of Fahrenheit. That a given quantity of air may become one-half lighter, its bulk must be doubled; whence we infer, that a mass of air whose bulk is 500, in order to acquire a bulk of 1000, must increase in heat . Now, it is by no means probable, that the air contained in the machine of M. Montgolfier had ever acquired so considerable a degree of heat. This estimate, therefore, of a specific gravity, one-half less than that of the atmospheric air, manifestly requires to be farther investigated.
"Some inductions on this head may be derived from the circumstances of the experiment made at Versailles. According to the computation of M. Faujas, the weight of the lower air displaced by the inflated machine was 3192 lb. According to the observation of M. Gentil, made at the Royal Observatory, the machine must have ascended 240 toises. By an estimate, as nearly approaching to truth as possible (according to the rules of the measurement of heights by the barometer), of the density of the air at the elevation at which the globe ceased to ascend, this density must have been less than that of the place whence it first rose. Thus, at that elevation, the weight of the air displaced by the machine was only 3015 lb. The machine, together with the air contained in it, weighed
weighed therefore likewise 3015 lb. since it there ceased to rise. But of this total weight 900 lb. was the weight of the bag and its appendages; there remains therefore 2115 lb. for the air. But the air displaced by the machine near the ground weighed 3192 lb. The density therefore of the air contained in the machine, was to that of the air near the ground nearly as 2 to 3, instead of the supposed proportion of 1 to 2. And the excess of its heat above that of the outward air, altho' considerable, is thus found to have been one-half less than that above stated, namely, .
"This heat, it seems, may be much less still, and yet carry the machine very high, if such intrepid men as M. Pilatre de Rozier and the marquis d'Arlandes will ascend with it, and keep up the fire at its aperture. For, in fact, as the machine rises, it will undoubtedly, if the same degree of heat be constantly kept up in its cavity, continually emit part of the air it contains. Thus, as soon as the heat of the internal air shall be sufficient to raise the machine from the ground, it must continue to rise without any further increase of that heat, and consequently without there being much more difference between the temperature of the interior and exterior airs, than what is necessary to compensate for the weight of the bag and its appendages.
"But a second consideration, that appears to us materially to affect the table above-mentioned, (in which, on the supposition of a constant given specific gravity, nothing is considered but the proportion between the solid contents of the bag and its weight), is, that it can only be applicable to a real gas, and by no means to an air dilated by heat. For the effects of fire of a given intensity in heating its surrounding air, are in the inverse ratio of the squares of the distances: whence it appears that the fire must be increased, not in the simple ratio of the solid contents, but in a much greater one; which it will be very difficult to determine, since there are many other considerations that are to be taken into the computations. Every machine, of whatever kind and of whatever dimensions, may no doubt be inflated by conveying into its cavity the current of air produced by the flame; but this would only be a manner of inflating it similar to that which would be produced by opening the bag to the wind. Now this current of air, thus directed by the flame, would preserve but a small degree of dilatation; and a machine of so immense a size as that for which a subscription is said to be now open at Paris, will probably raise but a small weight, and certainly disappoint the expectations of those who have undertaken to construct it.
"If a farther proof were required, that the balloons of Messrs. Montgolfier rise only by the rarefaction of the interior air, we should find it in the account given of the descent of that which was launched at Versailles. "Two game-keepers, who were at about ten yards distance from the place where it subsided, assures us," says M. Faujas, "that it came down remarkably slow, gradually contracting itself as it came nearer the ground." Had this balloon been filled with gas lighter than the atmospheric air, it would not have been thus contracted in its descent: nothing but a change of air could have weighed it down; and in a balloon open at the bottom, this change
would have happened by the entrance of a heavier air, which would have driven the lighter air upwards, and forced it out through the rents in the bag, without suffering it to collapse. But a mass of air dilated by heat, contracts on cooling; and the bag, in that case, being compressed by the outward air, will naturally collapse, or be contracted in its dimensions."
In the Review for March 1784, we find the same kind of reasoning continued with regard to the great balloon at Lyons. "Thus much (says the author of the paper) appears certain, that the success was by no means adequate to the sanguine hopes of the projectors; and indeed, if we recollect how rapidly the effect of fire decreases in proportion to the increase of the distances, and that, in this instance, assuming the effect of the fire at the distance of one foot to be as unit, it could at the top of the balloon, when distended, be no more than of that unit, we shall not be surprised at the difficulties that occurred, nor at the sudden descent after the globe had reached a cold and rarefied stratum of the atmosphere (even though there had been no rent), in which the necessary dilatation of the internal air would have required a fire much more intense than would have been compatible with the safety of the apparatus."
On reading these observations it immediately occurs, that no fair argument can be drawn from the expansibility of common air to that of air which had passed through fire. The latter contains a great deal of fixed and phlogisticated air, along, no doubt, with some inflammable air which escapes during combustion; perhaps not without a mixture of some kinds of acid, as well as alkaline air, rendered incapable of union by their being so much enveloped with others. Besides all these, there must undoubtedly be also a very great quantity of phlogistic and aqueous vapours, the precise specific gravity of which is altogether unknown. We are not, however, to judge merely from the specific gravities: the degree of expansion to which each of these are subject by the same degree of heat must be taken into consideration; and this makes the calculation a priori of the force by which any balloon will ascend, very difficult, if not impossible. To understand this in some measure, however, let us compare the different powers of ascension of one cubic foot of fixed air with a cubic foot of common air, when both are heated to the degree of 212 of Fahrenheit, or to the boiling-point of water. With regard to the common air, it has been determined, that by the heat of boiling-water it is expanded one-third of its bulk. A cubic foot of this element (supposing it 800 times lighter than water, and that a cubic foot of water weighs 1000 ounces or 62 pounds) will weigh an ounce and a quarter, or 600 grains. By the third part of this weight, therefore, or 200 grains, the cubic foot of rarefied air will rise; and if we could form any vessel to contain it which would weigh less than 200 grains, such vessel would arise along with it. The specific gravity of fixed air is to that of atmospheric air, as 220 to 152, and must therefore weigh 868 grains. But the expansibility of fixed air is to that of common air, as 220 to 132. By the heat of boiling-water, therefore, a cubic foot of fixed air must lose 333 grains of its weight, which would reduce it to nearly 535 grains; and therefore a cubic foot even of this air, though so much
Air. much heavier than common atmospheric air, would have a power of ascension by means of the heat of boiling-water, almost equal to 65 grains.
Phlogisticated air is to common air in specific gravity, as 140 to 152; in expansibility, as 165 to 132: a cubic foot of it, therefore, will weigh only grains. By a boiling heat it would lose, according to the above-mentioned proportion, 250 grains. A cubic foot of phlogisticated air, then, heated to the degree of boiling-water, would weigh only between 202 and 203 grains, and consequently would have an ascensive power equivalent to 397 grains.
With regard to the other kinds of air which are lighter than phlogisticated air, the case is still more evident. However, supposing even the balloon to be filled by means of the fuel with equal quantities of fixed and phlogisticated air, it is plain that this mixture, when exposed to the heat of boiling-water, must have a considerably greater power of ascension than common atmospheric air; for the cubic foot of fixed air would have an ascensive power of 65, and the phlogisticated air of 397, which added together make 462; while two cubic feet of common air, heated to the same degree, would have only an ascensive power of 400.—It is impossible to ascertain the proportions of the different kinds of air which compose the elastic fluid issuing from burning fuel, but it is easy to see that they cannot be in the proportions assumed above; for the phlogisticated air is certainly in the greatest quantity; and hence our author's calculations must be vastly erroneous. The error, however, does not lie altogether here, but likewise in supposing heat to decrease when confined in a close place in the same proportion as in the open air. Without entering into any investigation of the causes, it is a well-known fact that heat ascends and accumulates in the upper part of any close place, as evidently as water descends and is accumulated in the lower part of any vessel into which it is poured. In any balloon, therefore, which is raised and kept up by fire, the upper part will constantly be hotter than the lower; excepting, indeed, those places which are immediately adjoining to the fire. The fire, therefore, is by no means to be increased in proportion to the cubical contents of the balloons, otherwise it would be impossible to preserve them from destruction. The following experiments were made with a view to determine these matters, as far as they can well be determined in miniature.
Three different parallelopipeds were prepared, by pasting writing paper over slight wooden frames. Externally the smallest measured 6 inches square, and 12 inches in height; the under-part being left quite open. Its cubical contents thus were 432 inches, or the fourth-part of a cubic foot; but by reason of the space occupied by the wood-work, it could not be estimated at more than 417 inches. The second measured exactly a cubic foot; and, deducting for the space occupied by the wood, measured 1710 inches. The third was one foot square, and two feet in height, and contained two cubical feet; and, deducting the wood as before, measured 3430 inches. The weight of air, therefore, contained in the least, was nearly grains; in the second, ; and in the third, . The first being suspended in a balance and exactly counterpoised, a lighted candle was held under the open part
at bottom. The machine instantly rose, and required 30 grains to bring it again into balance. A thermometer held within the cavity with the bulb upwards, stood, when taken out, at . The second being now suspended in like manner, and exactly counterpoised, required 75 grains to bring it into balance; the thermometer standing at within its cavity. The third, with the same candle, and treated in every respect in a similar manner, required 90 grains to bring it back to an equilibrium; the thermometer standing at .—On using a stronger heat, the first required 75, the second 255, and the third 405 grains to restore the equilibrium; the thermometer standing in the two first at , and in the last at .
From these experiments, it is evident, that the rarefaction of air which passes through fire, cannot in any manner of way be calculated from the rarefaction of common air by heat alone. In the first set of experiments, 417 cubic inches of air were rarefied one-fifth by the flame of a small candle; 1710 inches, by the same rule, were rarefied about an eighth; and 3430 inches were rarefied one-thirteenth. But in the common mode of computation, if the flame of the candle had rarefied the first quantity of air one-fifth, it ought to have rarefied the second, which was four times greater, only one-twentieth, and the third only one-fortieth part. Nay, according to the assertion above-quoted, that "the fire must be increased, not only in proportion to the cubical contents, but in a much greater proportion," the effects of the candle ought to have been still much less than we have calculated them. But this is not all. The room in which these experiments were made was warm, and the thermometer stood at . By an expansion of only, one-fifth part was rarefied; but if of Fahrenheit give very little more than one-third of rarefaction, 48 degrees ought to have given scarce a fourth-part of that third, or a twelfth part instead of a third. In like manner, when the strong heat was used, the weight raised by its means indicated a rarefaction of more than one-half, though by the thermometer it ought not to have been one-third.
It now remains only to give some instructions for making balloons. 1. For those who intend only to construct a small balloon merely to enjoy the sight of its ascent in the air, inflammable gas will be found most eligible. Indeed it is almost the only kind that can be used; for the other kinds are so little inferior in specific gravity to the common atmosphere, that it will require a very capacious bag to rise when filled with them. The lightest materials are to be procured; and it is a matter of no small difficulty to determine what these materials are. Thin silk covered with elastic gum dissolved in ether has been found to answer the purpose of confining inflammable air more perfectly than any other thing; but as this material is excessively dear, a varnish made of linseed oil has been tried, and found to answer tolerably well. Indeed, instead of silk, thin paper varnished may be used: for as neither silk nor paper will contain inflammable air by themselves, it is plain, that the only use of these materials is to support the varnish; and while this can be done, it is no matter by what means. In fact, the widest gauze may be covered with isinglass or gums properly dissolved,
Air. solved, in such a manner as to be capable of containing inflammable air; and the thinner and lighter the materials are, the less may be the diameter of the intended balloon.
With regard to the form, that of a sphere is most proper, as a body of this kind meets with less resistance in passing through the air than any other. A small aperture is always to be left, for the purpose of admitting the inflammable air with which the balloon is to be filled. From this aperture a gut or pipe ought to proceed, of such a length as to admit of being easily and closely tied when the vessel in which the inflammable air was produced shall be removed.
The balloon being fully prepared, its sides are to be squeezed together as completely as possible; but at the same time very gently, lest the varnish should be cracked. This is done to get out of it all the common air which it originally contained; for the less of this that remains in it, the more easily will the balloon ascend, and the greater weight it will be able to carry up. To fill it with inflammable air, a sufficient quantity of vitriolic acid and filings of iron or zinc must be procured. For a globe of three feet diameter the following proportions have been recommended; 40 ounces of vitriolic acid, 90 of iron-filings, and 450 of water. The most proper vessel for mixing these ingredients is a copper-still, perforated at top for the convenience of pouring in more acid or water as shall be necessary. This metal is to be chosen in preference to any other, on account of its not being easily affected by fire, and being likewise insoluble in diluted vitriolic acid. The still is to be fitted with a refrigeratory and worm, in order to condense any aqueous vapour that may arise along with the air. This condensed vapour must be received into a small vessel exactly fitted to the worm; and into the upper part of which must be closely inserted a tube of tin or some other solid material capable of conducting the air, without losing it, into the balloon. The acid diluted with the water is first to be put into the still, and the iron gradually added through the hole in the top. An effervescence will immediately ensue, and the balloon will swell considerably; but as the dilatation advances, the resistance of the external air rather increases at the same time that the propelling force of the inflammable air is lessened. Fire must therefore be applied to the still; which will soon expel all the inflammable air, and would even burst the balloon if long continued. Unless a valve is applied to let out part of the inflammable air when it ascends into the higher regions, it will be improper to fill the balloon very full; because of the danger of bursting in a more rarefied air. A method of making balloons ascend to a prodigious height, would be to have a valve opening outwards, and confined by a spring, the elasticity of which could be overcome by the expansion of the air, without bursting the globe. This valve ought to open into another flaccid balloon, by which means the air that escaped from the former would not be lost, but dilate the second; so that both would have the same propensity to ascend that the single one had. On their entrance into a region where the air was still more rare, both would be distended; and thus by connecting three, four, or more bags with one another, it seems practicable in theory to raise them to any height whatever.
2. The other kind of balloons, which are raised by means of air passing through fire, seem scarcely proper for experiments except of the higher kind, where some person ascends with them, and regulates their motion by increasing or diminishing the fire by which they are supported. With regard to the construction of these, little can as yet be said, excepting what may be inferred from the experiments of M. Montgolfier and Rosier. The descriptions of these already given sufficiently show the method of constructing fire balloons, as they are called. The only remark that can well be made on them at present is, that in these, as well as the others, the whole power of ascension lies on the top; because there the pressure of the atmosphere is considerably less than at bottom; whereas the expansive power of the air within continues the same over the whole. In the great balloon constructed at Lyons, which was 126 feet in height, the difference between the external and internal pressure must have been very great. Every cubic foot of air weighs about an ounce and a quarter. Of consequence, upon every cube of one foot, the air presses with one ounce and a quarter less weight on the upper than on the under part. Nevertheless, if we suppose the cube hollow, and filled with air, the internal air will press upon that which lies below it with a force equal to one ounce and a quarter; of consequence, the cube will neither have any inclination to ascend or descend; but if we lighten the internal air by any given quantity, suppose half an ounce, while we preserve the same elasticity with the external air, it is plain, that the upper and internal part of the cube will be pressed upwards by half an ounce more than it is pressed downward by the external air. Hence it is evident, that the upper part of these machines, instead of being weaker, ought to be stronger than the under parts. This is evident from what happened to the balloon at Lyons. At the height of 126 feet, there must have been a difference of no less than 9 pounds 13 ounces upon every square foot, between the internal and external pressure. From this indeed the weight of the internal column must be deducted; but supposing the air to have been but very little rarefied, the difference must have been immense. In fact, this difference is the very power of ascension itself; and we see that this was sufficient to overcome 8000 pounds weight, for the machine itself, besides that of seven people, whom we cannot well suppose to have weighed less than 1000 pounds. Now, as the diameter of the balloon was 100 feet, we see that the whole pressure rested upon a circle of cotton cloth, whose diameter was 100 feet, and area 7850. The pressure therefore was more than a pound upon every square foot, and how much more we do not know, unless the machine had been perfectly in equilibrium with the atmosphere. But this it was not: for it ascended with great velocity; and the power by which this velocity was occasioned, exerted itself upon the upper parts. The consequence was, that the cloth was torn; having been imprudently changed by M. Rosier from double canvas to single cotton.