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AER0 NAUTIC W 0Y AGES

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ww.11ti A VIEW TO THE ADVANCEMENT OF SCIENCE.

Translated for the Smithsonian Institution from the works of Francis Arago, late secretary of the French Academy of Sciences, &c.

I.-The iNVENTION OF BALLooNs.

MAN, by reason of his weight, and the weakness of his muscular power, seemed doomed to creep on the surface of the earth, and to have been disqualified for studying the physical properties of the higher regions of our No. except through the toilsome ascent to mountain summits. But what difficulties are there over which genius, united with perseverance, will not eventually triumph : From the most remote times the idea of soaring into the air, far above all terrestrial objects, by means of machines which the imagination endowed with properties unfortunately of impossible attainment, has never ceased to occupy the human mind. Who has not heard of the attempts of Dedalus and Icarus, of the projects of Roger Bacon, and of Fathers Lara and Galen But, until 1783, it had been granted to no one to realize the dream of so many ages. Joseph Michel Montgolfier, who was born in 1740, at Annonay, in the department of the Ardéche, and who died, a member of the Academy of Sciences, in 1810, had calculated that through the rarefaction, by means of heat, of the air contained in a paper balloon of a certain extent, an ascensional force might be given it sufficent for elevating men, animals, and any desired instruments. So much confidence had he in his theory that he did not hesitate to undertake, June 5, 1783, a public and formal exhibition before the deputies of the provincial estates of Vivarais, assembled at Annonay. Montgolfier has himself described, in the following terms, this first experiment, which forms an epoch in the history of the most important discoveries: “The aerostatic machine was constructed of canvas, lined with paper, and covered by a network of twine attached to the canvas. It was nearly of a spherical form, and of a circumference of 110 feet, (35".73;) a frame of wood, 16 feet square, steadied it on its base. Its capacity was about 22,000 cubic feet. It, therefore, displaced, supposing the mean weight of the air equal to god of the weight of water, a mass of air equivalent to 1,980 pounds, (969 kilograms.)

“The weight of the gas (heated air) was nearly half that of the air, for it equalled 990 pounds, and the machine, with the frame, weighed 500 pounds. For the rupture of equilibrium there remained, therefore, 490 pounds, as was found conformable to the experiment. The different pieces of the balloon were fastened together simply by means of button-holes and buttons. Two men sufficed to lift and fill it with gas, but it required eight to retain it. When released, at a given signal, it mounted with an accelerated velocity, though less rapid towards the end of the ascension, to the height of 1,000 toises, (upwards of 6,000 feet.) A wind, scarcely perceptible at the surface of the earth, bore it to the distance of 1,200 toises from the place of departure. It remained ten minutes in the air. The loss of gas by the button-holes, needle punctures, and other imperfections, prevented any longer suspension. The wind was, at the time, southerly, with rain. The balloon descended so lightly that it broke neither the branches nor frames of the vineyard on which it finally rested.” The gas employed in this experiment was nothing but air dilated by heat, but its nature was not stated in the report of the ascension published in the journals. Without waiting for further indications, the artist Robert and the physicist Charles, by means of a national subscription, which was readily ad. vanced, constructed, of lutestring coated with gum elastic, a balloon four meters (13.12 feet) in diameter, which they filled with hydrogen gas, procured by the action of diluted sulphuric acid on iron filings. This balloon ascended from the Champ de Mars, August 27, 1783, at five o'clock in the afternoon, in |. of an immense crowd, and heralded by salvos of cannon. It remained ut three-quarters of an hour in the air, and fell at Gonesse, near Econen, five leagues distant from Paris. Thus was demonstrated the possibility of making balloons of varnished material, nearly impermeable by hydrogen, the lightest of known gases, and possessing great advantages over the heated air. Yet this means of obtaining very considerable ascensional force with balloons of limited dimensions was not immediately adopted, and sundry experiments were sue. cessively made with very large aerostats inflated with air heated by a fire of straw mixed with a little wool. It was with such a balloon, having an oval form, a height of 23 meters, a diameter of 15, and a capacity of 2,056 cubic meters, that Pilatre de Roziers and d'Arlandes made the first aerial voyage which man had ventured to undertake in balloons wholly detached and unconfined. Ascending from the Chateau de la Muette, November 21, 1783, they traversed a distance of two leagues at an elevation of about 1,000 meters, having, in their transit, hovered over Paris for 20 or 25 minutes. The 1st of December following, Charles and Robert ascended from the Tuilleries in a spherical balloon, made of lutestring coated with gum elastic, and having a diameter of only 8.50 meters, which was inflated with hydrogen. After a passage of about nine leagues the balloon touched the earth at Nesles, where Robert left the car, while Charles reascended and reached an elevation of abcut 2,000 meters, alighting finally two leagues further on, after having experienced a cold of –5°, or + 23 Fah., when the thermometer indicated on the ground +7°, 44; Fah. From this day dates the demonstration of the practical possibility of balloon voyages— voyages always adventurous, but which have become, at a later period, a pastime with persons of leisure. I shall not speak here of the attempts which have been made to derive advantage from aerostats in military expeditions, nor of the numerous contrivances to direct their course through the air, nor of the unfortunate experiment of uniting the action of fire with the employment of hydrogen, for which Pilatre de Roziers atoned with his life, nor of the substitution of illuminating gas for hydrogen, a substitution which renders these enterprises less costly, but which diminishes the ascensional force of apparatus of a determinate dimension. I must restrict myself to aeronautic voyages, performed with a view to the advancement of science. We must refer to the old Academy of Sciences if we would find an account of the first voyages by which science was benefited through the employment of balloons, in which hydrogen gas was used as an agent. The expeditions of MM. Biot and Gay Lussac, made in 1804, were preceded by the ascensions of Robertson, Lhoest, and Sacharoff, which yielded some interesting results; but not until after nearly half a century were the remarkable voyages of MM. Barral and Bixio undertaken, followed shortly afterwards by those of Mr. John Welsh.

II.-RESEARCHES TO BE MADE IN AEROSTATIC ASCENSIONS.

Those who propose to undertake aerial voyages form, in general, no idea of the number of questions to be resolved, nor of the difficulties to be surmounted in order to furnish science with certain elements of discussion. The instruments requisite for investigating, as well the temperature as the hydrometric

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state of the air, the phenomena of the magnetic needle, the proportions of polarized light contained in the light of the atmosphere, the diaphaneity, the color more or less blue of the different strata of air, &c., do not exist at all, or else require important modifications before being applied to the research of the laws by which the phenomena vary with the height, which is itself not determined with entire precision by barometrical observations. For half a century many learned bodies—the French Academy of Sciences, that of St. Petersburgh, the British Association for the advancement of science, the Academy of Dijon, &c.—have directed inquiry to the means of supplying the defect of which I speak, and of furnishing aeronauts with adequate instruments of investigation. But the problem has been by no means considered under all its aspects, and is very far from having received a complete solution; at all events, the suggestions which have been derived from the voyages of Biot and Gay Lussac, and especially from those of Barral and Bixio, should be taken into serious consideration by those whose zeal shall hereafter prompt them to encounter the perils of such enterprises, in the view, particularly, of reaching the most highly rarefied aerial regions, and traversing the atmosphere under its most variable conditions. The principal questions on which the attention of such explorers should be fixed are the following: 1. The law of the decrease of atmospheric temperature with the elevation. 2. Influence of the solar radiation in the different regions of the atmosphere, deduced from observations made upon thermometers whose bulbs are coated with very different absorbing substances. 3. Determination of the hygrometric state of the air in the several atmospheric strata, and comparison of the indications of the psychrometer with the dew-point at very low temperatures. 4. Analysis of the air from different heights. 5. Determination of the quantity of carbonic acid contained in the higher regions of the atmosphere. 6. Examination of the polarization of light by clouds. 7. Observation of different optical phenomena produced by the clouds. 8. Observation of the diaphaneity, and of the intensity of the blue color of different strata of air. 9. Observation of the declination and inclination of the magnetic needle, and of the intensity of magnetism. 10. Study of the electric state of different atmospheric strata. 11. Experiments on the transmission and reflection of sound in different strata of air in a serene state of the sky, and in a sky containing clouds. 12. Physiological observations on the effects produced by the rarefaction of the air, very low temperatures, extreme dryness, &c. The instruments at the disposal of the voyagers should be the same as those which, by my own advice, and that of my illustrious colleague, M. Regnault, were carried by MM. Barral and Bixio in their expeditions, and which they would have continued to use had they been able to make other ascensions, to wit: 1. Two siphon barometers, graduated on glass, of which the aeronaut need observe only the upper meniscus, the position of the lowermeniscus being given by a table constructed after direct observations made in the laboratory. Each of these barometers should be provided with a thermometer divided in centigrade degrees, so as to present a scale extending from +35° to —399. It is now known that the aeronaut may encounter strata of air having a temperature lower than that of the congeation of mercury; hence the ordinary barometer will not answer, and an instrument should therefore be furnished, founded on the pressure exerted by the atmosphere on an elastic spring, and tested at very low temperatures under feeble pressures obtained by the pneumatic machine. 2. A vertical thermometer, of arbitrary graduation, the cylindrical reservoir of which is placed in the axis of several concentric envelopes of bright tin, open at their bases to admit the circulation of air. This arrangement has been devised in order to obtain, at least approximately, the temperature which a thermometer would indicate in the shade. 3. Three thermometers, having arbitrary scales, attached to a metallic plate 5 centimeters apart. The reservoir of the first of these thermometers should have a vitreous surface; the surface of the second should be coated with lampblack; and the reservoir of the third should be covered with a cylinder of polished silver, which must also envelop a portion of the stem. The reservoirs should be narrow cylinders, much elongated. Immediately below the reservoirs the metallic plate should support another o brightly coated with silver. The plate bearing these thermometers should be arranged horizontally on one of the sides of the car, with a view to its remaining constantly exposed to the solar radiation. 4. A psychrometer formed by two thermeters of an arbitrary scale. 5. One of Regnault's condensing hygrometers. 6. Tubes of caustic potash, and also of pumice, wet with sulphuric acid, for the determination of the carbonic acid of the air. The air should be drawn in by means of a pump of the capacity of one litre (1,760 pint) accurately gauged. 7. Two balloons of one litre capacity, furnished with stop-cocks of steel, for collecting the air of the higher regions. These balloons, enclosed in tin boxes, should be scrupulously exhausted of air before the ascent. 8. A minimum thermometer of M. Walferdin, which should be enclosed in a tin cylinder, pierced with holes. It is best that this instrument should be placed under seal, as was done by MM. Barral and Bixio, since the control of the personal observations by means of a mute instrument imparts considerable value when they come to be verified, and affords a triumphant reply to objections which, through a natural tendency of the human mind, always oppose themselves to results which cannot be immediately verified by new experiments made under the same conditions. In the event, moreover, of the ascension of the balloon to heights where the temperature falls below—40°, the point of congelation for mercury, it will be necessary to have thermometers of alcohol or sulphuret of carbon, graduated below that point of the thermometric scale, so that the observations may not be interrupted by a circumstance which has ceased to be considered as of impossible occurrence. 9. It is from the considerations just stated, that I would recommend also the use of the apparatus devised by M. Regnault, and intended to indicate the minimum of barometric pressure, and consequently the maximum of elevation to which the balloon has attained. This apparatus should be enclosed in a tin case pierced with numerous small openings. The lid of this case should be secured with a seal like the minimum thermometer. 10. Polariscopic telescopes, such as I have described, Astronomie populaire, ii, p. 101. 11. Instruments for showing the declination, inclination, and intensity of magnetism, suspended in such a manner as not to be affected by the movements of rotation of the balloon in its ascent, as has been observed by MM. Biot, Gay Lussac, Barral, and Bixio. 12. Electrometers so constructed as to be capable of indicating at once the kind and the intensity of the electricity of different atmospheric strata. It is scarcely probable that in an ascension, observers will be able to embrace at one time so many subjects of study, or use successively and opportunely so many instruments. The aeronaut should, on each occasion, limit himself to a small number of important inquiries. It is only in a series of aeronautic expeditions that a collection of records can be made corresponding to the great number ...” which the constitution of the terrestrial atmosphere presents for solution.

It is impossible to frame a programme which will embrace all the points worthy of examination; we are constrained to admit that the unforeseen will always play a principal part in aeronautic expeditions. We know little at present of the constitution of clouds, of the phenomena of refrigeration produced by their evaporation, of the mixture of strata of air differently saturated with humidity and derived from very different sources, of the action of electricity which traverses great aerial spaces, &c. In every case it is desirable that, during the progress of aerial voyages, there should be made, at least from hour to hour, in the principal terrestrial observatories, observations analogous to those which the aeronauts propose to undertake. This was advised in 1841 by a committee of the British Association, in a report relative to the advantages which science might derive from aerostatic ascensions, a report signed by Brewster, Herschel, Lubbock, Robinson, Sabine, Whewell, and Miller, and the advice was observed by MM. Barral and Bixio, who were thus enabled to connect the phenomena noticed in the higher regions of the air with those which occurred at the same time on the surface of Europe.

Barometric observations in connexion with those of temperature yield, by means of a formula which we owe to the genius of Laplace, the measure of the elevation to which balloons ascend above the level of the sea. This formula has been reduced into the usual tables which are found in the Annuaire du bureau des longitudes. The considerations on which the illustrious geometer founded his analysis led him to employ in his admirable formula a coefficient whose determination Ramond had arrived at, by comparing a great number of the measurements of the height of mountains taken with the barometer with their trigonometric measurements. Now, as Ramond operated chiefly under the parallel of 45°, and upon mountains whose elevation scarcely reached 3,000 meters, there is nothing to prove that the undetermined coefficient of Laplace's formula is susceptible of being applied to the measurement of much more considerable heights, and made in other latitudes. It would not be superfluous to measure directly, by observations made from several astronomical stations situated at known distances, the heights to which aeronauts attain, and to compare the results obtained with the barometric determinations. No doubt these operations will present numerous difficulties, and may be not unfrequently tried without success, because the balloons may disappear in the clouds or be carried in directions which will not permit the terrestrial telescopes to follow them with any advantage. But the problem to which I here call attention merits by its importance the sacrifices which may be encountered in giving it a satisfactory solution.

III.-AERONAUTIC VOYAGES OF LHOEST, ROBERTSON, AND SACHAROFF.

The first aeronautic voyage to which science was indebted for some useful indications was that performed at Hamburg, July 18, 1803, by the physicist, Robertson, accompanied by his countryman, Lhoest. They remained suspended in the air five hours and a half, and descended at Hanover, twenty-five leagues distant from the place of departure.

At the moment of the ascension the barometer on the earth stood at 28 inches, and the thermometer at-H16°. Reaumur; at the greatest height to which they attained the barometer showed 12.4 inches, and the thermometer–5°.5 Reaumur. These observations, reduced to metric and centigrade measurements, give 758 millimeters for the barometric height, and +20° for the temperature at starting; 336 millimeters and—6°.9 at the highest point reached. Hence, according to the formula of Laplace, we deduce 6,831 meters as the maximum height to which the balloon ascended.

The two aeronauts thought that at that height they observed the oscillations of the magnetic needle to be much less rapid than at the surface of the earth, and that consequently the magnetic intensity diminishes rapidly as the elevation

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