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pheric air, the whole mass will ascend as in the former case, and continue to rise till it attains an altitude at which the surrounding air is of the same specific gravity with itself. The idea of flying by means of wings and other contrivances, was certainly entertained by the ancients; and some accounts relate exploits of this kind having been performed ; but still there is reason to suppose they are mere fictions, and that no means were ever possessed for accomplishing this undertaking till the invention of balloons, which dates no further back than the conclusion of the last century. Soon after Mr. Cavendish’s discovery of the specific gravity of inflammable air, it occurred to the ingenious Dr. Black, that if a bladder sufficiently light and thin were filled with this air, it would form a mats lighter than the same bulk of atmospheric air, and rise in it. This thought was suggested in his lectures, in 1767 and 1768; and he proposed, by means of the allantois of a calf, to try the experiment. Other em. ployments, 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, 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. But while aerostation seemed thus on the point of being made known in Britain, it was unexpectedly announced in France by two brothers, Stephen and John Montgolfier, natives of Annonay, and masters of a considerable papermanufactory there, who had turned their thoughts to this project as early as the middle of the year 1782. Their idea was to form an artificial cloud, by inclosing smioke in a bag, and making it carry up the covering along with it. In that year the experiment was made at Avignon with a fine silk bag; and opplying burning paper to

an aperture at the bottom, the air was rarefied, and the bag ascended to the height of 70 feet. Various experiments were now made upon a large scale, which excited the public curiosity very greatly. An immense bag of linen, lined with paper, and containing upwards of 23,000 cubic feet, was found to have a power of lifting about 500 pounds, including its own weight. Burning chopped straw and wool under the aperture of the machine, immediately occasioned it to swell, and afterwards to ascend into the atmosphere. In ten minutes it had risen 6000 feet; and when its force was exhausted, it fell to the ground at the distance of 7668 feet from the place it ascended.—Soon after this one of the brothers, invited by the Academy of Sciences to repeat his experiment at their expense, constructed a large balloon of an elliptical form ; and in a preliminary experiment, this machine lifted from the ground eight, persons who held it, and would have carried them all off, if more had not quickly come to their assistance. Next day the machine was filled by the combustion of 50 pounds of straw and 12 pounds of wool. The balloon soon swelled and sustained itself in the air, together with the burden of between 400 and 500 pounds weight. It was designed to repeat the experiment before the king at Versailles; but a violent storm of rain and wind happening to damage the machine, it became necessary to prepare a new one ; and such expedition was used, that this vast balloon, near 60 feet in height and 43 in diameter, was made, painted within and without, and finely decorated, in the course of four days and four nights. Along with it was sent a wicker cage, containing a sheep, a cock, and a duck, which were the first animals ever sent on such a voyage. The full success of the experiment was, however, prevented by a violent gust of wind, which tore the machine in two places near the top, before it ascended. Still it rose 1440 feet; and after remaining in the air about eight minutes, fell to the ground, at the distance of 10,200

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feet from the place of its ascent, with the animals in perfect safety. —As the great powers of those aerostatic machines, and their very gradual descent, showed they were capable of transporting people through the air with all imaginable safety, M. Pilatre de Rozier offered himself to be the first aerial adventurer, in a new machine constructed in a garden in the Fauxbourg of St. Antoine. It was of an oval shape, 48 feet in diameter and 74 in height, elegantly painted with the signs of the zodiac, ciphers of the king’s name, and other ornaments. A proper gallery, grate, &c. enabled the person who ascended to supply the fire with fuel, and thus keep up the machine as long as he pleased: the weight of the whole apparatus was upwards of 1600 pounds. On the 15th of October, 1783, M. Pilatre placing himself in the gallery, the machine was inflated and permitted to ascend to the height of 84 feet, where he kept it afloat about four mi. nutes and a half; after which it descended very gently: and such was its tendency to ascend, that it rebounded to a considerable height after touching the ground. On repeating the experiment, he ascended to the height of 210 feet. His next ascent was 262 feet; and in the descent, a gust of wind having blown the machine over some large trees in an adjoining garden, M. Pilatre suddenly extricated himself by throwing straw and wool on the fire, which raised him at once to a sufficient height; and on descending again, he once more raised himself to a proper height by the same means. §: time after he ascended with M. Girond de Vilette to the height of 330 feet; hovering over Paris at least nine minutes in sight of all the inhabitants, and the machine keeping all the while a steady position. These experiments having shown, that aerostatic machines might be raised or lowered at the pleasure of the persons who ascended, M. Pilatre and the Marquis d’Arlandes, on the 21st of November, 1783, undertook an aerial voyage, which lasted about 25 minutes, and during which time they passed over a

space of above five miles. From the account given by the marquis, they met with several different currents of air, the effect of which was to give a very sensible shock to the machine, and the directions of the motion seemed to be from the upper part downwards. It appears, also, that they were in some danger of having the balloon burnt altogether; as the marquis observed several round holes made by the fire in the lower part of it, which alarmed him considerably, and indeed not without reason. However, the progress of the fire was easily stopped by the application of a wet sponge, and all appearance of danger ceased.— This voyage of M. Pilatre and the marquis may be said to conclude the history of those aerostatic machines which were elevated by means of fire ; these having been soon after superseded by balloons in which inflammable air was enclosed. The first experiment was made by two brothers, the Messrs. Roberts, and M. Charles, a professor of experimental philosophy. The bag was composed of lutestring varnished over with a solution of gum caoutchouc ; and was about thirteen English feet in diameter. Many difficulties occurred in filling it with the inflammable air; but being at last set at liberty, after having been well filled, it was thirty-five pounds lighter than an equal bulk of common air. It remained in the atmosphere only three quarters of an hour, during which it traversed fifteen miles. Its sudden descent was supposed to have been owing to a rupture which had taken place when it ascended into the higher regions of the atmosphere. The event of this experiment, and the aerial voyage made by Messrs. Rozier and Arlandes, maturally suggested the idea of undertaking something of the same kind with a balloon filled with inflammable air. The machine used on this occasion was formed of gores of silk, covered with a caoutchouc varnish, of a spherical figure, and measuring 274 feet in diameter. A net was spread over the upper hemisphere, and fastened to a hoop whicle

assed round the middle of the alloon. To this a sort of car was suspended, a few feet below the lower part of the balloon; and, in order to prevent the bursting of the machine, a valve was placed in it; by opening of which some of the inflammable air might be occasionally let out. The car was of basket-work, covered with linen, and beautifully ornamented ; be: ing 8 feet long, 4 broad, and 3% deep; its weight 130 pounds. Great difficulties again occurred in filling the machine; but these at last being removed, the two adventurers took their seats at three-quarters after one in the afternoon of the 1st of December, 1783. They continued in the air an hour and three-quarters, and alighted at the distance of 27 miles from Paris; having suffered no inconvenience during their voyage, nor experienced any contrary currents of air, as had been felt by Messrs. Pilatre and Arlandes. As the balloom still retained a great quantity of inflammable gas, M. Charles determined to take another voyage by himself. M. Robert accordingly got out of the machine; which now being 130 pounds lighter, arose with such velocity that in twenty minutes he was almost 9000 feet high, and entirely out of sight of terrestrial objects. The globe, which had been rather flaccid, soon began to swell, and the inflammable air escaped in great quantity. He also drew the valve, to prevent the balloon from bursting; and the inflammable gas being considerably warmer than the external air, diffused itself all around, and was felt like a warn atmosphere. In ten minutes, however, the thermometer indicated a great variation of temperature; his fingers were benumbed with cold, and he felt a violent pain in his right ear and jaw, which he ascribed to the expansion of the air in these organs, as well as to the external cold. The beauty of the prospect which he now enjoyed, however, made amends for these inconveniences. At his departure the sun was set on the valleys; but the height to which M. cho was got in the atmos

phere, rendered him again visible, though only for a short time. He saw for a few seconds vapours rising from the valleys and rivers. The clouds seemed to ascend from the earth, and collect one upon the other, still preserving their usual form, only their colour was grey and obscure, for want of sufficient light in the atmosphere. By the light of the moon, he perceived that the machine was turning round with him in the air, and he observed that there were contrary currents, which brought him back again. He observed, also, with surprise, the effects of the wind, and that the streamers of his banners pointed upwards; which, he says, could not be the effect either of his ascent or descent, as he was moving horizontally at the time. At last, recollecting his promise of returning to his friends in half-an-hour, he pulled the valve, and accelerated his descent. When within 200 feet of the earth, he threw out two or three pounds of ballast, which rendered the balloon again stationary ; but, in a little time afterwards, he gently alighted in a field about three miles distant from the place whence he set out; though by making allowance for all the turnings and windings of the voyage, he supposes that he had gone through nine miles at least. By the calculations made, it appears that he rose at this time not less than 10,500 feet; a height somewhat greater than that of Mount AEtna. The subsequent aerial voyages differ so little from those above related, that any particular description of then would be superfluous. The expense in silling a balloon with inflammable gas is very great, and it therefore became an object of anxious inquiry how this might be avoided or reduced. The first attempt of this kind was made by the duke de Chartres: it consisted in having a smaller balloon within the greater one, the smaller being filled with common alr, by means of a pair of bellows, when necessary; viz. whenever it was thought proper to descend, it being supposed that the machine would

thus become heavier, and the air in the outer balloon condensed, and consequently, that the ascent or descent might be effected at pleasure. Another scheme was to put a small aerostatic machine, with rarified air, under an inflammable air-balloon, but at such a distance, that the inflammable air of the latter might be perfectly out of the reach of the fire used for inflating the former; and thus by increasing or diminishing the fire in the small machine, the absolute weight of the whole would be considerably diminished or augmented. This scheme was unhappily put in execution by the celebrated M. Pilatre de Rozier and M. Romaine. Their inflammable air-balloon was about thirtyseven feet in diameter, and the power of the rarified air one was equivalent to about sixty pounds. They ascended without any accident; but had not been long in the atmosphere, when the machine took fire at the height of about three quarters of a mile from the ground. No explosion was heard; and the silk of the balloon seemed to resist the atmosphere for about a minute, after which it colapsed, and descended along with the two unfortunate travellers so rapidly, that both of them were killed. The first aerial voyage in Fo was performed on the 15th of September, 1784, 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 thirty-three feet. M. Lunardi de parted from the Artillery-ground at two o'clock; and at ten minutes past four, he descended in 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 collected round the balloon were frozen. The second aerial voyage in England was performed by Mr. Blanchard, and Mr. Sheldon, professor of Anatomy to the Royal Academy. They ascended at ches;" at nine minutes past 7

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twelve o’clock. Mr. Blanchard having landed Mr.Sheldon at about 14 miles from Chelsea, re-ascended alone, and finally landed near Rumsey, in Hampshire, about seventy-five miles distant from London, having gone nearly at the rate of twenty miles an hour. The wings used on this occasion, it seems, produced no deviation from the direction of the wind. Inflammable air for balloons may be obtained in several ways: but the best methods are, by applying acids to certain metals; by exposing animal, vegetable, and some mineral substances, in a close vessel, to a strong fire; or by transmitting the vapour of certain fluids through red-hot tubes. In the first of these methods, iron, zinc, and vitriolic acid, are the materials most commonly used : the vitriolic acid must be diluted with about five or six parts of water. Iron may be expected to yield, in the common way, about 1700 times its own bulk in gas; or .# ounces of iron, the like weight of oil of vitriol, and 224 ounces of water, will produce one cubic foot of inflammable air: six ounces of zinc, and an equal weight of oil of vitriol, and thirty ounces of water, are necessary for producing the same quantity. Inflammable air may also be obtained at a much cheaper rate, by the action of fire on various substances; but the gas thus obtained is not so light as that produced by the effervescence of acids and metals. The substances proper to use for this purpose are, — pit-coal, asphaltum, amber-rock oil, and other minerals; wood, and especially oak ; camphor oil, spirits of wine, aether, and animal substances, which yield air of different kinds and of various specific gravities. AFFECTED is a term made use of . algebraists, when speaking of the signs or co-efficients of a quantity. Thus, if a quantity is preceded by the sign +, it is said to be affected with a positive sign; and if by —, it is affected with a negative sign, &c. Thus also, 2 a, 7 a, are said to be affected with the coelene 2,7, &c.

AFFECTION, in Geometry and Physics, is a term signifying the $anne as §§§

AFFIRMATIVE Quantity, or Positive Quantity, in Algebra, is one that is to be added or taken effectively, in contradistinction to a negative quantity, which is to be subtracted.

AFFIRMATIVE Sign, or sign of addition, is +, and is read plus, or more, or added to ; thus a + b signifies that the quantity, represented by b, is to be added to the quantity represented by a. When a single letter or quantity is preceded by the sign +, as + a, it indicates that it is to be taken positively; and the same is always to be understood if there be no sign prefixed. Before the introduction of this sign into algebra, the word plus, or its equivalent, was used to imply the addition of quantities; as was the word minus, to indicate the subtraction of them.

AIR, in ... an elastic, transparent, ponderous, compressible, and dilatable fluid body surrounding the globe to a considerable height. The ancients considered air as an element; but, taken in its popular sense, it is very far from the simplicity of an elementary substance. The most important physical or mechanical pronerties of the air, are its fluidity, weight, and elasticity.

Of the Fluidity of the Air.— That the air is a fluid is evident from the easy passage it affords to bodies moving through it. That air is a fluid, is also proved front this circumstance, that it is found to exert an equal pressure in all directions; an effect that could not take place otherwise than from its extreme fluidity.

The Weight or Gravity of the Air. —With this property of air the ancients were not totally unacquainted, though their sentiments on the subject were confused and unsatisfactory. The effects which are now known to result from the weight and elasticity of the air, were for a long time attributed to an imaginary principle called fuga vacui, or Nature's abhorrence of a

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mitted the principle, although he was under thc necessity of assigning a limit to it, corresponding to the weight of a column of water thirty-four feet high. In the year 1643 it occurred to Torricelli, the disciple of Galileo, that whatever might be the cause by which a column of water, thirty-four feet high, is sustained above its level in the tube of a common pump, the same force would sustain a column of any other fluid, which weighed as much as that column of water; and hence he concluded, that quicksilver, being about fourteen times as heavy as water, would not be sustained at a greater height than twenty-nine or thirty inches. He then made the experiment, called after him the Torricellian experiment. He took a glass tube, of several feet in length, and having sealed it hermetically at one end, he filled it with quicksilver; then inverting it, and holding it upright by pressing his finger against the lower or open orifice, he immersed that end in a vessel of quicksilver; then removing his finger, and suffering the fluid to run out, the result verified his conjecture: for, the quicksilver, faithful to the laws of Hydrostatics, descended till the column of it was about thirty inches high above the surface of that in the open vessel : and hence he concluded, that it was no other than the weight of the air incumbent on the external surface of the quicksilver, which counterbalanced the fluid contained in the tube. By this experiment he not only proved, as Galileo had done, that the air had weight, but that its weight was the cause of the suspension of water in pumps, and of quicksilver in the tube; and that the weight of the whole column of it, was equal to that of a like column of quicksilver thirty inches high, or of water thirty-four or thirty-five feet high ; but he did not ascertain the weight, or any particular quantity of it, as a gallon, or a cubic foot; nor its specific gravity to water, as had been done, though inaccurately, by Gaiileo.

Torricelli's experiment soon be

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