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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to conclude the history of those aerostatic machines which were elevated by means of fire; these having been soon after superseded by balloons in which inflammable air was enclosed.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

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

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-made by two brothers, the Messrs. 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 ascend-about thirteen English feet in diaed to the height of 210 feet. His meter. Many difficulties occurred next ascent was 262 feet; and in in filling it with the inflammable the descent, a gust of wind having air; but being at last set at liberty, blown the machine over some after having been well filled, it large trees in an adjoining garden, was thirty-five pounds lighter than M. Pilatre suddenly extricated an equal bulk of common air. It himself by throwing straw and remained in the atmosphere only wool on the fire, which raised him three quarters of an hour, during at once to a sufficient height; and which it traversed fifteen miles. on descending again, he once more Its sudden descent was supposed raised himself to a proper height to have been owing to a rupture by the same means. Some time which had taken place when it after he ascended with M. Girond ascended into the higher regions de Vilette to the height of 330 feet; of the atmosphere. The event of hovering over Paris at least nine this experiment, and the aerial minutes in sight of all the inhabi- voyage made by Messrs. Rozier tants, and the machine keeping all and Arlandes, naturally suggested the while a steady position. These the idea of undertaking something experiments having shown, that of the same kind with a balloon aerostatic machines might be raised filled with inflammable air. The or lowered at the pleasure of the machine used on this occasion was persons who ascended, M. Pilatre formed of gores of silk, covered and the Marquis d'Arlandes, on with a caoutchouc varnish, of a the 21st of November, 1783, under-spherical figure, and measuring took an aerial voyage, which last- 27 feet in diameter. A net was ed about 25 minutes, and during spread over the upper hemisphere, which time they passed over a land fastened to a hoop which

passed round the middle of the phere, rendered him again visi balloon. To this a sort of car was ble, though only for a short time. suspended, a few feet below the He saw for a few seconds vapours lower part of the balloon; and, in rising from the valleys and rivers. order to prevent the bursting of The clouds seemed to ascend from the machine, a valve was placed the earth, and collect one upon in it; by opening of which some the other, still preserving their of the inflammable air might be usual form, only their colour was occasionally let out. The car was grey and obscure, for want of of basket-work, covered with linen, sufficient light in the atmosphere. and beautifully ornamented; be- By the light of the moon, he pering 8 feet long, 4 broad, and 3 ceived that the machine was turndeep; its weight 130 pounds. Great ing round with him in the air, and difficulties again occurred in filling he observed that there were conthe machine; but these at last trary currents, which brought him being removed, the two adven-back again. He observed, also, turers took their seats at three-quar- with surprise, the effects of the ters after one in the afternoon wind, and that the streamers of of the 1st of December, 1783. his banners pointed upwards; They continued in the air an hour which, he says, could not be the and three-quarters, and alighted effect either of his ascent or deat the distance of 27 miles from scent, as he was moving horizon. Paris; having suffered no incon- tally at the time. At last, recol venience during their voyage, nor lecting his promise of returning to experienced any contrary currents his friends in half-an-hour, he of air, as had been felt by Messrs. pulled the valve, and accelerated Pilatre and Arlandes. As the bal- his descent. When within 200 loon still retained a great quantity feet of the earth, he threw out of inflammable gas, M. Charles two or three pounds of ballast, determined to take another voy- which rendered the balloon again age by himself. M. Robert ac- stationary; but, in a little time cordingly got out of the machine; afterwards, he gently alighted in a which now being 130 pounds field about three miles distant lighter, arose with such velocity from the place whence he set out; 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 warm atmosphere. In ten minutes, how-be ever, the thermometer indicated The expense in filling a balloon a great variation of temperature; with inflammable gas is very great, his fingers were benumbed with and it therefore became an object cold, and he felt a violent pain in of anxious inquiry how this might his right ear and jaw, which he be avoided or reduced. The first ascribed to the expansion of the attempt of this kind was made by air in these organs, as well as to the duke de Chartres: it consisted the external cold. The beauty of in having a smaller balloon within the prospect which he now enjoy the greater one, the smaller being ed, however, made amends for filled with common air, by means these inconveniences. At his de- of a pair of bellows, when neces parture the sun was set on the sary; viz. whenever it was thought valleys; but the height to which proper to descend, it being sup M. Charles was got in the atmos-posed that the machine would

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 Ætna. The subsequent aerial voyages differ so little from those above related, that any particular description of them would superfluous.

thus become heavier, and the air | twelve o'clock. Mr. Blanchard in the outer balloon condensed, having landed Mr.Sheldon at about and consequently, that the ascent 14 miles from Chelsea, re-ascended or descent might be effected at alone, and finally landed near pleasure. Another scheme was to Rumsey, in Hampshire, about put a small aerostatic machine, seventy-five miles distant from with rarified air, under an inflam- London, having gone nearly at the mable air-balloon, but at such a rate of twenty miles an hour. The distance, that the inflammable air wings used on this occasion, it of the latter might be perfectly seems, produced no deviation from out of the reach of the fire used the direction of the wind. for inflating the former; and thus Inflammable air for balloons may by increasing or diminishing the be obtained in several ways: but fire in the small machine, the ab- the best methods are, by applying solute weight of the whole would acids to certain metals; by exposbe considerably diminished oring animal, vegetable, and some 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 England 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 AFFECTED is a term made use of his voyage stood as low as 29°, of by algebraists, when speaking and he observed that the drops of of the signs or co-efficients of a water collected round the balloon quantity. Thus, if a quantity is were frozen. The second aerial preceded by the sign+, it is said voyage in England was performed to be affected with a positive by Mr. Blanchard, and Mr. Shel-sign; and if by, it is affected don, professor of Anatomy to the with a negative sign, &c. Thus Royal Academy. They ascended also, 2 a, 7 a, are said to be affectat Chelsea at nine minutes pasted with the co-efficients 2,7, &c.

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 4 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, æther, and animal substances, which yield air of different kinds and of various specific gravities.

AFFECTION, in Geometry and Physics, is a term signifying the same as property.

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.

mitted the principle, although he was under the 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 AFFIRMATIVE Sign, or sign of high, is sustained above its level addition, is+, and is read plus, or in the tube of a common pump, more, or added to: thus a+b sig- the same force would sustain a nifies that the quantity represent column of any other fluid, which ed by b, is to be added to the weighed as much as that column quantity represented by a. When of water; and hence he concluda single letter or quantity is pre-ed, that quicksilver, being about ceded by the sign+, as+a, it fourteen times as heavy as water, indicates that it is to be taken would not be sustained at a greater positively; and the same is always | height than twenty-nine or thirty 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 Physics, 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 properties of the air, are its fluidity, weight, and elasticity.

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 in. ches 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 incum bent 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 The Weight or Gravity of the Air. that the weight of the whole co-With this property of air the lumn of it, was equal to that of ancients were not totally unac-a like column of quicksilver thirty quainted, 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 vacuum; and Galileo himself ad

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 from 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.

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

came very popular. Father Mer- a given bulk of it compared with senne was apprised of it in 1644, that of water. To prove the exis and immediately conveyed an ac-tence of this property of air, varicount of it to the philosophers of ous experiments might be cited; France; and Pascal, after some it will, however, be sufficient to hesitation, adopted Torricelli's mention one; which is, simply idea, and devised several experi-inverting an open vessel full of ments for confirming it. One of air in water; in which case, the these was to procure a vacuum resistance it offers to further imabove the reservoir of quicksilver, mersion, and the height to which in which case he found the column the water ascends within it, in sunk down to the common level; proportion as it is further immersbut this appearing to him not sufed, are proofs of the elasticity of ficiently powerful to dissipate the the air contained in it. prejudices of the philosophers of With regard to the degree of the old school, he prevailed on this elastic force, it has been M. Perier, his brother-in-law, to shown, by the most satisfactory execute the famous experiment experiments, that with moderate of the Puy-de-domme, who found pressure it is always proportional that the height of the quicksilver, to the density of the air, and that half way up the mountain, was the density is always as the comconsiderably less than at the foot of pressing force; whence also the it, and still less at the top: by this elasticity of air is as the force by means the question was set to rest; which it is compressed. Thus, if as no doubt could any longer be air is confined in a bent tube, one entertained, that it was the weight end being open and the other herof the atmosphere that counterpois-metically sealed, and quicksilver ed the coluinn of quicksilver. The be poured in at the open end, it medium of all these is about as one to 832, or one to 833; when reduced to the pressure of thirty inches of the barometer, and the mean temperature of fifty-five degrees of the thermometer; or, by adding to the last number, the proportion becomes one to 833, or 3 to 2500; whence, upon It has been questioned, amongst the whole, we may conclude, that philosophers, whether this elastic the density of air is to that of power of the air is capable of bewater as 3 to 2500; and, conse-ing destroyed or diminished. Mr. quently, that a cubic foot of air Boyle could not discover that any weighs at a mean 1 ounces avoir-state of rarefaction, which he was dupois; the weight of a cubic foot of water being 1000 ounces, and of a cubic foot of quicksilver 13,600

ounces.

will be found that the spaces into which the air is compressed are always inversely as the weights by which it is loaded, and since these weights are the measures of the elasticity, therefore the elasticities are inversely as the spaces which the air occupies.

capable of producing, was sufficient for destroying this property. Colonel Roy has shown that the particles of air may be so far reElasticity of the Air.-It is ano- moved as to lose a very great part ther quality of this fluid, that it of their elastic force: the same will yield to any pressure, by con- experiments show that moist air tracting its dimensions; and that, possesses the greatest elastic force; upon removing or diminishing the and that common air, in its natural pressure, it will again return to state, is proportionally more elasits natural state or volume. This tic than when its density is conelastic force may be accounted the siderably augmented by pressure. distinguishing property of air, the The elasticity of air may be so others which we have mentioned effected by a violent pressure as above being common to all fluids. to require some time to return to Various experiments were soon its natural tone. The weight or instituted both on the continent pressure of the air, it is obvious, and in England, with a view of has no dependence on its elastiascertaining the specific gravity of city; but would be the the air, or the average weight of whether it possessed that property

same

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