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of proportions, as that at the surface of the earth; each 1,000 parts being 215 of oxygen.

50. The next voyage which we must notice by way of warning to future adventurers, was one of the few fatal ones. On the 7th April, 1806, M. Mosment ascended from Lisle, apparently under auspicious circumstances. During his ascent, he dropped a dog with a parachute, which came safely to the ground, and about one o'clock something was seen descending slowly through the air, and proved to be the flag of M. Mosment. Soon after, a murmur was circulated that the body of the æronaut was found in one of the fosses of the city, lifeless and covered with blood, which proved in the end but too correct. The misfortune has been attributed to the shallowness of the car, by which, he is supposed to have lost his balance in throwing out the dog, and precipitated to the earth.

51. On the 4th of August, M. Garnerin, ascended from Paris at eleven o'clock in the evening, for the purpose of making nocturnal observations, under the Russian flag. His balloon was illuminated by twenty lamps, and formed a very splendid appearance; rockets let off at Tivoli, seemed to him scarcely to rise above the earth, and Paris appeared studded with numerous stars. In forty minutes he rose to an elevation of 13,200 feet at twelve o'clock, when 3,600 feet from the earth, he heard the barking of dogs; at two o'clock, he saw several meteors flying around him; at half past three, he beheld the sun rising in magnificence and grandeur above the ocean of clouds, and the gas expanding, he rose to the height of 15,000 feet, and after a lapse of more than seven hours, descended at Loges, forty-five leagues from Paris.


52. He took a second nocturnal voyage, on the 21st September, in which he was exposed to great danger. He commenced his voyage as before from Tivoli, and was at first carried with unexampled rapidity to an immense height, when he began to prognosticate a storm. The balloon dilated to an alarming degree, and having neglected his apparatus for conducting the gas away from the lamps in its escape, he could not manage the balloon he therefore with one hand made an opening of two feet diameter, and with the other put out all the lamps he could, and was, without a regulating valve, tossed about from current to current; his ballast was gone, and in this condition, the balloon rose through thick clouds, then sunk, then struck the earth and rebounded to an amazing altitude: the storm dashed him against the mountains, and after many severe shocks he became for a time insensible. On recovering his senses, he found he had been carried to Tonnere, in a storm of thunder. His anchors shortly hooked in a tree, and he alighted in seven hours and a half from his ascension, 300 miles from Paris, having travelled a distance of forty miles in an hour. This, however, is not more than half the distance he travelled on one occasion in this country, when he went from London to Colchester, a distance of fifty miles, in three-quarters of an hour.

53. Since this period, the names of Sadler,

Green, Graham, and other adventurers have been connected with the ascent of balloons, for the purpose of emolument, in England: sometimes exhibiting the triumphs, and sometimes the paucity of scientific attainments in their conductors. But we know of no further efforts than the above, to increase the materials of science itself, by these voyages.

54. The utility of Aronautic studies and experiments has been very much questioned even by philosophical minds. M. Cavallo, well known in the philosophical world, suggested long ago that small balloons, especially those made of paper, and raised by means of spirit of wine, may serve to explore the direction of the winds in the upper regions of the atmosphere, particularly when there is a calm below; and we see the French æronauts adopted this idea, that they might serve also for signals in various circumstances, in which no other means can be used; and letters or other small things may be easily sent by them; for instance, from ships that cannot safely land on account of storms; from besieged places, islands, or the like. The larger ærostatic machines, he adds, may answer all the above mentioned purposes in a better manner; and they may, besides, be used as a help to a person who wants to ascend a mountain, or a precipice, or to cross a river; and perhaps one of the machines tied to a boat by a long rope, may be, in some cases, a better sort of sail than any that is used at present. Their conveying people from place to place with great swiftness, and without trouble, may be of essential use, even if the art of guiding them in a direction different from that of the wind should never be discovered. By means of those machines the shape of certain seas and lands may be better ascertained; men may ascend to the tops of mountains they never visited before; they may be carried over marshy and dangerous grounds; they may by that means come out of a besieged place, or an island; they may, in hot climates, ascend to a cold region of the atmosphere, either to refresh themselves, or to observe the ice which is never seen below; and, in short, they may be thus taken to several places, to which human art hitherto knew of no conveyance.

55. The philosophical uses, to which these machines may be subservient, are numerous indeed; and it may be sufficient to say, that hardly any thing which passes in the atmosphere is known with precision, and that, principally for want of a method of ascending into it. The formation of rain, of thunder storms, of vapours, hail, snow, and meteors in general, requires to be attentively examined and ascertained. The action of the barometer, the refraction and temperature of the air in various regions, the descent of bodies, the propagation of sound, &c. are subjects which all require a series of observations and experiments, the performance of which could never have been properly expected, before the discovery of erostatic machines.'

56. Such speculations have been for years on record: the reader will see from our preceding detail that they have but partially been fulfilled.

But we should not omit to state, that the French applied balloons to military purposes during the ate war, and ascribe to this in a great measure, the celebrated victory obtained over the Austrians, at Fleurus, 1794. A balloon was sent up under the direction of Mr. Coutel, accompanied by an adjutant and a general, who rose twice the same day to the height of 220 fathoms, and remained each time four hours suspended in the air, observing the movements of the enemy, and corresponding all the time by military signs with general Jourdan, commander of the French army. The enterprise was at last discovered, and a battery opened upon the æronauts, but they soon gained an elevation beyond the reach of the fire. Afterwards the French commonly prepared balloons to go with their armies, as into Egypt, &c. but we have never heard of any other practical result. We would now direct the reader to

57. SECT. III. OF THE PRINCIPLES OF ERONAUTIC MACHINERY.-The general principles of Erostation are so little different from those of hydrostatics, that it is almost superfluous to insist on them. It is a fact universally known, that when a body is immersed in any fluid, if its weight be less than an equal bulk of that fluid, it will rise to the surface; but if heavier, it will sink; and if equal, it will remain in the place where it is left. For this reason smoke ascends into the atmosphere, and heated air in that which is colder. Upon this simple principle depends the whole theory of ærostation.

58. A cubic foot of air has been found to weigh about 554 grains, and to be expanded by every degree of heat, about part of the whole. By heating a quantity of air, therefore, to 500 degrees of Fahrenheit, we just double its bulk when the thermometer stands at fifty-four in the open air, and in the same proportion we diminish its weight; and if such a quantity of this hot air be inclosed in a bag, and the excess of the weight of an equal bulk of common air weighs more than the bag with the air contained in it, both the bag and the air will rise into the atmosphere, and continue to do so until they arrive at a place where the external air is naturally so much rarefied, that the weight be comes equal; and here the whole will float.

59. From the frequent exhibition of ærostatic machines, the power of hot air in raising weights, or rather that by which it is itself impelled upwards, has been illustrated by a variety of experiments with which every one is familiar. From these it appears, that in the ærostatic machines on Montgolfier's plan, it must be an advantage to have them as large as possible; because a smaller quantity of fire will then have a greater effect in raising them, and the danger from that element, which in this kind of machines is chiefly to be dreaded, will be in a great measure avoided. On this subject it may be remarked, that as the cubical contents of a globe, or any other figure of which balloons are made, increase much more rapidly than their surfaces, there must ultimately be a degree of magnitude at which the smallest additional heat would raise any weight whatever. Thus, supposing any ærostatic machine capable of containing 500

cubic feet, and the air within it to be only one degree hotter than the external atmosphere; the tendency of this machine to rise, even without the application of artificial heat, would be near an ounce. Let its capacity be increased sixteen times, and the tendency to rise will be equivalent to a pound; and this may be done without making the machine sixteen times heavier than before.

60. It is certain, however, that all ærostatic machines have a tendency to produce or preserve heat within them, as Messrs Charles and Roberts found in their ærial voyage of 150 miles, when the external atmosphere was 63°. and the thermometer within the balloon at 104°. Such a difference of temperature affording a power of ascent equal to forty-one grains on every cubic foot, must, in a machine of 50,000 such feet, have amounted to almost 200 pounds. Hence we may easily account for Mr. Morveau's accident at Dijon, when a balloon filled only with common air made its escape to some distance by the spontaneous rarefaction of the air within. This difference between the external and internal heat, being so very considerable, must have a great influence upon ærostatic machines, and will undoubtedly influence those filled with inflammable air as well as the other kind. The rain, snow, and vapours, which condense upon them in the higher regions, may also occasion an evaporation, and consequently a very violent degree of cold, so as to make them specifically heavier than the atmosphere.

To this, probably, we may ascribe the descent of the balloon which carried M. Blanchard and Dr. Jeffries; and which seemed so extraordinary, that many had recourse to an imaginary attraction in the waters of the ocean, in order to solve the phenomenon. This supposition, however, is rejected by Cavallo; who explains the matter, by remarking, that in two former voyages made with the same machine, it could not long support two men in the atmosphere, so that we had no reason to wonder at its weakness on this occasion. In fact, it does not appear that the air over the sea is at all warmer than that above land; on the contrary, there is every reason to believe that the superior reflective power of the land renders the atmosphere above it warmer: but it is very natural to suppose, that the air above the sea is more moist than that above land; and consequently, by letting fall its moisture upon the balloon, must have occasioned an evaporation, that would deprive the machine of its internal heat; which it would partly recover after it entered the warmer and drier atmosphere over land.

61. Much attention has been directed to the shape most proper for the balloon, an object in aeronautics of considerable importance. The early balloons appear to have been elliptical or oval, but the spherical is now generally preferred because this figure admits the greatest capacity under the least surface. The conical or oblong figure has been proposed as an experiment; in which case the machine is supposed to proceed with its narrow end forward. This suggestion arose from the consideration that a round form, and that which is approximately so, presents a

greater surface to the opposition of air; and is, therefore, an insuperable obstacle to guiding it by the action of the oar, wings, or any other invention which might be contrived to turn it, either in the current of the atmosphere, or even in a perfect calm. Mr. Hoole, copying nature, suggests the shape of a fish, the head of which figure might serve to divide the fluid, and the tail as a rudder, to steer the course of the balloon. The traveller, he suggests, should sit in the centre of gravity of the whole mass; and thus he would not only render it perfectly horizontal, but be able, from his situation, to give an impulse of sufficient force to move the whole body he should also be provided with instruments analogous to the fins of fishes, says this author, and of sufficient texture to bear the whole force of his muscular strength. The oblong form is, however, attended with the disadvantage already named, as to its capacity being less under the same surface than the sphere; and consequently, its power of buoyancy must be calculated at a proportional diminution, to say nothing of the positive augmentation necessarily incurred by such a change of figure to its ponderosity, or the superadded consideration of the difficulty of balancing and governing such a figure in the open heavens, exposed to the action of violent winds and storms; the first blast of which might roll it sideways, sweep it away upon its bosom, or perhaps turn it over. Other advantages of the spherical form are as follow. The circumferences of spheres are as their diameters; their surfaces as their squares; their solid contents as the cubes of their diameters. The diameter of the circumference being as 7 to 22. If the diameter of a spherical balloon be 35 feet, the circumference will be 110: that diameter multiplied by the circumference, will give the surface of the sphere, and in the case above named 3850 feet. By knowing the weight of a given portion of the materials of which the balloon is formed, as of a square foot or yard, it is easy to calculate the whole; and, in order to find the capacity, take of the cube of the diameter, or multiply of the surface by the entire diameter.

62. Having found the contents and surface of the balloon, we calculate its buoyancy. If the cubic foot of air weighs 13 of an ounce, we take the number of cubic feet in the solid content of the balloon, and add to them part of themselves, considering the feet as ounces; and hence we have the weight of air displaced by the balloon. If the balloon contain 22,458 cubic feet, each of the weight of 14oz. we add to 22,458 ounces of that quantity; and hence have 26,9493 ounces the weight of the whole, supposing the dimensions of the same weight as air. This, however, is not the case. From the aggregate, then, of the 26,949 ounces, we deduct the weight of the materials of which the vehicle is made, and the remainder expresses the exact levity of the balloon, on the supposition that it contains common air. But inflammable air weighs from to of the weight of common air only, and, supposed in the above case at, will stand thus 16-280; also 1256-280-976ths power of ascension. The same general reasonings may be applied in other cases for calculating the buoyancy of

spherical balloons of all sizes, the levities being nearly as the cubes of the diameters, and consequently the diameters as the cube roots of the levities. But from the application of similar reasonings to the oblong figure, there appears, as we should expect, an augment of weight, and a decrease of capacity, under the same extent of surface.

63. The power of ascension in balloons differs considerably, according to the means of ærostation or rarefaction employed. If a perfect vacuum could be procured, a globe of 10 feet in diameter would rise, with a force of 40 pounds; one of 20 feet, 320 pounds; one of 30 feet, 1080 pounds, in the ratio of the cube of the diameter. We have already stated, the levities are as the cube of the diameters, and consequently diameters as the cube roots of the levities. But air expands by heat at least 450th part of its bulk; and it is impossible to apply caloric in a sufficient degree to obtain its whole ascensional power. That air within a balloon or vehicle, heated 50 degrees, would dilate by elasticity till one-ninth part was driven out; so that even then the tendency of the balloon to rise would be equal only to the ninth part of its whole buoyant force. That being the relation in which 50 stands to 450, and 50 degrees of heat is as much, perhaps, as could be supported. Humidity produces a dilation of air, amounting, in good weather to an eightieth part of the volume of fluid, and in the tropical regions would exceed one-twentieth: hence moist air thrown into a large bag, sufficiently waxed, would cause it to rise. But heat and moisture combined, produce a far greater rarefaction of the air than either of them is capable of producing alone. The smoke used by Montgolfier with so great success, was nothing scarcely but air charged with vapour, produced by burning of vine-twigs, chopped straw, &c. and was computed to be one-third specifically lighter than common air, and therefore must have pos sessed a degree of rarefaction which it would require 150 degrees of heat to produce operating alone. From these data it is evident that the force estimated at 12 pounds avoirdupoise in a globe of 10 feet diameter, would amount to 1562 pounds, if the diameter were 50 feet, and to 12,500, if it were a hundred, being as the cube roots of the levities, from which is to be deducted, the weight of the case, appendages, &c. which, estimated at two-fifths of a pound, for a sphere of one foot in diameter, demonstrates a balloon of 100 feet diameter, capable of exerting an ascending power of not less than 8500 pounds, independent of the cordage, car, ballast, &c. which still must be deducted; and one of 33 feet diameter would exert a power capable of producing a mere equilibrium between the weight of the canvass and the buoyant force of the rarefied air.

64. Balloons filled with gas remain to be consi dered. Hydrogen gas obtained from sulphuric acid, acting on iron filings, is six times lighter than common air, but gas evolved by a solution of zinc in that acid, is twelve times lighter. We may, therefore, independently of accidents, &c. consider, that on a general scale the hydrogen gas that fills a balloon, is six times specifically lighter than common air, the balloon must there

ore exert of the buoyant force corresponding to ts capacity, and will have an ascending power qual to of a pound avoirdupois for a globe of one foot diameter, 1124 pounds for a balloon of 15 feet diameter, 900 pounds for one of 30 feet, and 7200 pounds for one of 60 feet, the weight of the balloon, &c. to be deducted, which allowing, as has been calculated part of a pound for a varnished silk globe of one foot diameter, would be in a balloon of 15 feet diameter, 11 pounds, in one of 30 feet 45 pounds, in one of 60 feet 180 pounds, and the consequent power of ascension in such balloons would be 101, 855, and 7020 pounds, whilst a balloon of 11 feet would just float in the atmosphere. There being in that case an exact equilibrium between the weight of the materials and the ascensional power of the gas. The weight of the appendages, it must be considered, has a tendency to compress the gas embodied in the balloon, and thereby to render it more dense, but this effect is not considerable. A load of 6000 pounds in a balloon of 60 feet diameter, has been calculated to produce a compression amounting to of a pound; for a circle of one foot diameter in the horizontal section, the 979th part of the pressure of the atmosphere on a globe of those dimensions, and rating the weight of gas at 1200 pounds, there will be a diminution of buoyancy of 1 pounds. The balloon, according to its power of ascension, would rise with a motion perpetually accelerating, were not this velocity checked by the resistance of the air through which it passes, which according to the theory of dynamics, is to that which a falling body acquires in the same space of time, as the ascensional power is to the weight of the apparatus and fluid. Still, notwithstanding the opposition it meets with from the atmosphere, it will attain its final velocity in about double the time which would be otherwise required for that purpose, after which its ascent will be uniform, the resistance of the air being just equal to the buoyancy of the balloon. The resistance of a circle moving through any given fluid in a direction perpendicular to its plane, is measured by the weight of a column of that fluid of equal base with the circle, and at an altitude from which a heavy body falling will acquire the same celerity; a rule which arises out of the general principle that air presses equally every way. Near the level of the sea, at mean temperature, a column of air 17 feet high, incumbent on a circle of a single foot diameter, weighs a pound; the consequent resistance which such a circle would suffer, propelled at the celerity of 33 feet each second. Newton calculates the resistance of the atmosphere to be just half that of its generating circle, and therefore a velocity of 463 feet in a second, would create a resistance of one pound to a sphere of one foot diameter. In other circumstances the resistance will be proportionate to the squares of the velocities and densities. Consequently if the buoyant force were always the same, the velocity of ascent in a balloon would be inversely as its diameter. A balloon of 30 feet diameter and 100 pounds ascensional power, has the same effect as the ninth part of a pound for a globe of one foot diameter; and would therefore meet a resistance corresponding to the

velocity of 463 divided by 3, the square root of 9 or 15 each second, consequently the balloon would rise a mile in six minutes.

65. To determine the height to which a balloon will rise, the rule is, to compute the contents of the globe in cubic feet, and divide its restraining weight in ounces by the content, and the quotient will be the difference in the density of the atmosphere at the surface of the earth, and at the point to which the balloon will rise. Subtract this quotient from 13, the density at the earth, and the remainder will be the density at that height; after which, the height may be found by comparing the density thus obtained, with philosophical tables for that purpose. The balloon would indeed, at first, rise above this altitude, by reason of the rapid celerity acquired in its ascent, and afterwards sink below it; but this is the point at which it would ultimately settle and continue in the diffuse medium; the density of air, at that degree of elevation, just balancing the ascensional power.

66. Its equipoise cannot be stable so long as it remains in a loose flaccid state; but the circumstances of its ascent are to be determined by means of ballast, and the safety valve appointed for occasionally letting out a portion of the gas; for although the balloon designed for any considerable altitude, is not more than half filled with gas at first, still, by the increased rarefaction of the air as it ascends, and the consequent diminished pressure on the exterior, the expansion sometimes becomes so great as to endanger the bursting of the machine.

67. The rarefied air balloons have been commonly elevated or depressed by increasing or diminishing the fire; the inflammable air balloons, by throwing out the ballast, or letting out the gas, in suitable portions, through the valve. But these means will in time render the vessel incapable of floating; for in the air they can have no supply of ballast, and with great difficulty can procure gas. They will also rise or fall by the rarefaction of the inclosed air; and it has been proposed to effect these purposes, by means of annexing to the balloon a vessel of common air, by condensing and rarefying which, the machine may be lowered or elevated at pleasure. M. Meuvier proposed to inclose one balloon of common air in another of inflammable air; by this means, as the balloon ascends, the gas becomes expanded, and compressing the balloon of common air, diminishes its weight; and to increase the quantity of this air when needful, he suggested the use of bellows. Some propose to annex a small balloon with rarefied air, to one of inflammable air; and by altering the fire of the lower one, to raise or depress the machine.

68. Several inventions have been offered for the purpose of conducting these machines through the air, and giving them a direction to whatever point the aeronaut should wish to travel. Some have proposed to govern them by sails and rudder, in the same manner as ships at sea; but this, as might have been expected, was found impracticable, the course of balloons and ships being so exceedingly different. A vessel upon the water moves with less velocity than the wind, and therefore the wind acts powerfully

upon the sails; but ballous, having the same velocity as the surrounding air, feel no wind, and consequently can derive no benefit from sails. Oars and wings have been applied with as little success. The helm, indeed, was for some time encouraged, from the motion of fishes in the water; but from this no practical benefits have been derived. Professor Dauzel is said to have contrived a machine for the direction of the balloon, an account of which may be seen in the Philosophical Magazine, vol. iv. p. 108; but it seems to have fallen into disuse; and it is probable that these contrivances will never be brought to a state of perfection. The power which the aeronaut has of raising and lowering himself in the atmosphere, will, at present, best enable him to take advantage of the different currents of air, which are blowing at the same time in various directions; and, by committing himself to the breeze he wants, to supersede the necessity of contrivances, which would tend at best to encumber the machine. The parachute, however, is a useful appendage, by means of which the voyager can, at any height, leave the balloon, in case of danger, and escape to the earth without incurring the inconvenience of a rapid fall. A machine on the same principle is said to have been long used in the east, by the Vaulters, to enable them to jump from great heights. We have noticed its general principles of descent already.

69. Sec.V. OF MAKING AND FILLING BALLOONS. -With respect to the construction of the balloon or globe, the first thing to be considered, is the formation of the gores of which it is composed. In order to frame these, we must consider that the edges are not segments of circles, and cannot in consequence be described by compasses. To shape them, the following directions are necessary: first draw a right line AB, (see plate I. fig. 1.) equal to half the circumference, which divide by the line CD into two equal parts at the point E, each equal to the one-fourth of the circumference. Describe the line AB in feet and decimals; divide the quarter circumference AE into eighteen equal parts, and to the points of division apply the lines fg, hi, kl, &c: divide the whole circumference into twice the given number of pieces, and make CE and ED each equal to the quotient of this division: so that CD may be equal to the breadth of one of those pieces: multiply the above mentioned quotient by the decimals at the end of the lines fg, hi, kl, &c., and you have the length to which the line ought to be continued. Draw a curve along the extremities of these lines, and you have one quarter of the pattern complete, by which you may shape the rest. The reason of the rule is, that the several breadths of each slip, at the several distances from the point to the centre, are as the sines of those distances, the radius being the size of half the length of the slip. Having computed the circumference of the balloon, it is comparatively easy to form the gores after which nothing remains to complete the machine, but fitting them together, varnishing the exterior, and suspending the car, which is commonly of wicker work, covered with leather, to the balloon, by means

of ropes attached to a net, formed to fit the shape of the balloon. For the greater con venience of ærostation, the ropes from the net are attached to the circumference of a circle about twenty feet below the balloon, whence other ropes go to the edge of the boat. 70. The material universally employed for ærostatic machines of a larger size, to be filled with inflammable air, is varnished silk; and for those of the rarefied air kind, linen painted over with some size colour, or lined with paper; the best varnish for an inflammable air balloon, is that made with birdlime, and recommended by M. Faujas de Saint Fond, in a treatise published on the subject:-"Take one pound of birdlime, put it into a new proper earthen pot that can resist the fire, and let it boil gently for about one hour, viz., till it ceases to crackle; or, which is the same thing, till it is so far boiled, as that a drop of it being let fall upon the fire, will burn: then pour upon it a pound of spirits of turpentine, stirring it at the same time with a wooden spatula, and keeping the pot at a good distance from the flame, lest the vapour of this essential oil should take fire. After this, let it boil for about six minutes longer; then pour upon the whole, three pounds of boiling oil of nuts, linseed, or poppy, rendered drying by means of litharge; stir it well, let it boil for a quarter of an hour longer, and the varnish is made. After it has rested for twenty-four hours, and the sediment has gone to the bottom, decant it into another pot; and when you want to use it, warm and apply it with a flat brush upon the silk stuff, whilst that is kept well stretched. One coat of it may be sufficient, but if two are necessary, it will be proper to give one on each side of the silk, and to let them dry in the open air, while the silk remains extended.'

On this subject, however, M. Cavallo has made some improvements, as may be seen in his printed publications. Much likewise has been said in France of the elastic gum varnish, and its composition, which was kept a secret; but Mr. Baldwin, after many expensive trials, declares it to be made by melting in an iron ladle, properly heated, pieces of elastic gum, and afterwards stirring in a quantity of drying oil. M. Blanchard's method of making elastic gum varnish for the silk of a balloon, is the following: Dissolve elastic gum, cut small, in five times its weight of spirit of turpentine, by keeping them some days together; then boil one ounce of this solution in eight ounces of drying linseed oil for a few minutes; lastly strain it. It must be used warm. The pieces of silk, after the varnish is sufficiently dry, may be joined by laying about half an inch of the edge of one piece, over the edge of the other, and sewing them by a double stitch.'

71. Having thus completed the structure of the balloon mechanically, the next consideration is that of filling it. This is commonly done with gas, which may be obtained in different ways, according as inclination and circumstances may direct. The best way is by applying acids to certain metals; by exposing certain animal, vegetable, and mineral substances, to a strong rire, in a close vessel; or by transmitting the

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