Popular Science Monthly/Volume 27/July 1885/Recent Progress in Aerial Navigation
|RECENT PROGRESS IN AËRIAL NAVIGATION.|
By Professor W. LE CONTE STEVENS.
BALLOONING has thus far been a French art: introduced a little over a century ago by a Frenchman, Montgolfier; rapidly developed by another Frenchman, Charles; more practiced in France than anywhere else in the world; and recently improved by Frenchmen to such an extent that it is quite possible now on any fair day to go an hour's journey through the air in any desired direction, even against the wind.
The history of the application of science to art has revealed a number of cases in which practical success was secured by inventors entertaining quite erroneous conceptions of the principles they were applying. Somewhat vague stories are transmitted in regard to Roger Bacon's suggestion, during the thirteenth century, of employing a thin hollow globe of copper, "to be filled with ethereal air or liquid fire and then launched forth from some elevated point into the atmosphere, where it will float like a vessel on water." Bacon gave no recipe for making "liquid fire," nor did he calculate the dimensions of a globe of copper to be filled with it that would have sufficient ascensive power to lift a human being. He assures us "there is certainly a flying instrument, not that I ever knew a man that had it, but I am particularly acquainted with the ingenious person who contrived it." His conception was never reduced to practice. It was merely a fair specimen of current science in his time. He believed that the aërial ocean around our earth had a definite boundary like the liquid ocean, and that a body of sufficient lightness, if it could only be found, would easily rise to this surface as a cork rises to the surface of water.
More than three centuries after the time of Bacon, Father Lana wrote out his idea of a vessel that might be made to rise in the air. Four hollow globes of copper, each having a diameter of about twenty-five feet, were to be carefully exhausted and then attached to a car. Torricelli and Pascal had already proved that the pressure of the atmosphere was nearly fifteen pounds per square inch at sea-level, and Lana's proposed method of exhausting his globes was to be an application of Torricelli's principle. Each globe was to be filled with water and lifted to a height of at least thirty-four feet. Beneath it should be fitted a tube with air-tight connections, which was to dip into water. On opening this tube the contents of the globe would be emptied into the vessel below, leaving a Torricellian vacuum above, while the tube would become a water-barometer. Lana seemed to know nothing about the specific gravity of gases. His copper globes were to be made very thin in order to secure lightness, but he failed to make any correct estimate of the rigidity they must have to sustain either the weight of the water they should hold, or the crushing force of the atmosphere after exhaustion. Unlike the product of Bacon's imagination, his conception was a correct application of demonstrated physical laws; but, had it been tested by experiment, he would at once have found that there were other laws which he had not taken into account. In his view, the only obstacle to success was that "the Almighty would never allow an invention to succeed by means of which civil government could so easily be disturbed." Air-ships floating beyond the reach of missiles, if only capable of being accurately directed, might well have been thought more terrible than dynamite is to-day.
Soon after the discovery of hydrogen gas by Cavendish and Watt in 1766, experiments were made with a view to utilizing it for the purpose of lifting bodies into the air. But, until 1783, nothing more substantial than a soap-bubble could be made thus to ascend. Joseph Montgolfier, who was a successful manufacturer of paper, tried bags of this material; but hydrogen was found to diffuse so rapidly through it that the idea was abandoned by him. Observing that clouds of vapor and smoke remained floating at various heights, he thought that, if they could be confined in bags of paper, these might be made to float in like manner. Since the experiments of Franklin in 1752 had proved the existence of atmospheric electricity, the idea gained currency that the lightness of clouds and of smoke was in some way due to electric charge. A paper bag was made, and, with its opening downward, a fire was kindled, "as well to increase the layer of electric fluid upon the vapor in the vessel as to divide the vapors into smaller molecules and dilate the gas in which they are suspended." The bag was carried up to a considerable height. Montgolfier seems not to have attributed the ascension to the effect of heat in diminishing the specific gravity of the contained air. The first successful experiment in ballooning was thus based on a misconception.
Montgolfier's first public exhibition of his invention was made on June 5, 1783. The news of his success was rapidly spread; and at Paris a balloon was soon constructed under the superintendence of M. Charles, who substituted hydrogen for smoke, confining it in a bag of varnished silk instead of paper and linen. The ascension was successfully accomplished on the 27th of August. Charles at once proceeded to the construction of a new and much larger balloon, in which he ascended with his colleague, Robert, on the 1st of December, making a journey of more than twenty-five miles. This balloon was provided with a safety-valve of Charles's invention, a hoop, to which the car was attached, and netting intended to equalize the distribution of weight upon the balloon. It was in all important particulars the same as the balloon almost universally employed throughout a century afterward. Montgolfier made several exhibitions of his hot-air balloons at Paris, and in one of these, on the 15th of October, M. Pilâtre de Rozier made the first ascent attempted by a human being. But to Charles is due the credit of making the balloon a moderately safe vehicle in which the aëronaut could ascend or descend at will by varying the relation between the amounts of ballast and of gas retained. Although many thousands of ascents have been made since 1783, the total number of lives recorded to have been lost does not exceed fifty.
It is somewhat remarkable that after the ascents made by Pilâtre de Rozier and others at Paris in the latter part of 1783, the first ascent accomplished elsewhere was in America, a country not only separated by a broad ocean from France, but at that time young in resources, and scarcely beginning to recover from the disastrous effects of the struggle for independence. It is true that in November of that year an Italian, Count Zambeccari, exhibited in London a small hydrogen balloon, which was sent into the air without any living freight; but no one rose from English ground in a balloon until a year after Charles had been successful in France. The news of Montgolfier's experiment of the 5th of June reached Philadelphia about the last of November, and the local newspapers of December 24th contained the accounts just received in regard to Charles's experiment of the 27th of August. David Rittenhouse, the friend of Franklin, and the most distinguished American astronomer of his time, was practicing his profession as a maker of philosophical instruments, and especially of clocks. One of his most intimate associates was Francis Hopkinson, an eminent jurist, whose interest in science was almost as great as in law. Both of these men were members of the American Philosophical Society which had been organized by Franklin. No sooner was the news from France received, than they began to test the use of hydrogen for balloons. On the 28th of December an ascent was made by the first American aeronaut, the account of which is perhaps best given in the language of an eye-witness, Francois Simonin, whose letter to the "Journal de Paris" was published May 13, 1784. In the "Gentleman's Magazine" of the following month a translation of it appeared, from which the following is an extract: "Messieurs Ritnose [Rittenhouse] and Opquisne [Hopkinson] began their experiments with bladders, and then with larger machines; they joined several together and fastened them round a cage, into which they put animals. The whole ascended, and was drawn down again by a rope. The next day, which was yesterday, a man offered to get into the cage, provided the rope was not let go. He rose about fifteen feet, and would not suffer them to let him go higher. James Wilcox, a carpenter, engaged to go in it for a little money. He rose twenty feet or upward before he made a signal to be drawn down. He then took instructions from Messieurs Ritnose and Opquisne, and after several repetitions on the ground consented to have the rope cut for fifty dollars. Dr. Jaune [Jones], the principal medical person in the city, attended in case of accident. The crowd was incredible, who shouted after the English fashion when they saw Wilcox rise crowded in the cage, surrounded by forty-seven balloons fastened to it, with astonishing coolness, nodding his head to express his satisfaction and composure. After all, he could not rise above ninety-seven feet, according to the measures taken by two other gentlemen of the Philosophical Academy. He was at least five minutes in the air, but, perceiving the wind to blow from the east and drive him toward the Scoulquille [Schuylkill], he was frightened, and, agreeably to his instructions, made several incisions with a knife in three of the balloons. This was not sufficient, though we saw him descend a little. He pierced three more, and, seeing the machine did not come, his fear increased. He cut five more in the greatest haste, and, unfortunately, all on the same side. He was then seen to tack about (chavirer), and, as if he had slid down (coulé bas), he fell on the edge of a ditch and a finse [fence], as they call the inclosures. Dr. Jaune ran up; the poor man had sprained his wrist, but received no other accident."
No sooner was the fact demonstrated that men could safely rise into the air at will, than inventors began to devise plans for directing aerial machines. So long as the balloon is completely at the mercy of the wind, it is practically useless as a means of conveyance. On rising high above the ground the direction of aërial currents is often found to differ greatly from that of the surface-currents. Rising above the clouds, the aëronaut may lose sight of the earth and be carried at a rate of which he is totally unconscious, there being no means of measuring his speed when borne along with a current to which the balloon opposes no sensible resistance. By noting the times and places of ascent and descent, the rate has been estimated to exceed that of our fastest railroad-trains; and more than one aëronaut has lost his life by being carried out to sea. In the early part of 1784 M. Robert, the colleague of Charles, attempted the propulsion of a balloon by means of oars, but in vain. He subsequently tried artificial wings, but with no better success. M. Blanchard, who crossed the English Channel in 1785 with Dr. Jeffries, tried a variety of similar devices without success.
For the directing of balloons one of the first suggestions based on correct principles was offered by Francis Hopkinson. In a letter to Franklin, dated at Philadelphia, May 24, 1784, he recommends that the balloon shall be made oblong instead of spherical, and provided with a large and light wheel at the stern. "This wheel should consist of many vanes or fans of canvas, whose planes should be considerably inclined with respect to the plane of its motion, exactly like the wheel of a smoke-jack. If the navigator turns this wheel swiftly round by means of a winch, there is no doubt but it would (in a calm at least) give the machine a progressive motion, upon the same principle that a boat is sculled through the water." (Sparks's "Life and Works of Benjamin Franklin," vol. x, p. 93.) This remarkable suggestion by Hopkinson shows that he had quite definite views about the application of the principle of the screw-propeller to the direction of aërostatic machines, though in his day screw-propellers had not yet been applied even to surface navigation.
Hopkinson's suggestion did not then find its way into print. Even had it been published, the means were wanting for any experiments on a large scale. It would have been a noteworthy step in the right direction, but the muscular power of his imaginary aëronaut would have been far from sufficient to control the propeller. Nearly seventy years elapsed before his idea, independently evolved by another, was put to the test; and during this interval ballooning was but rarely applied to any other purpose than that of public display. The fruitless attempts to direct balloons were often made the subject of caricature.
In 1852 a young French engineer, who subsequently won the highest distinction, M. Giffard, constructed an elongated balloon (Fig. 1),
Fig. 1.—Giffard's Aërial Steamer, 1852.
pointed at both ends and filled with illuminating gas. Suspended beneath it by cords was a longitudinal shaft, at the end of which was a triangular sail that could be turned about an almost vertical axis and be made to serve the purpose of a rudder. About twenty feet beneath the shaft was hung a framework of wood, on which rested a small steam-engine, whose piston gave motion to a screw-propeller. The weight of the machine, including furnace, boiler, coal, and water, was not quite fourteen hundred pounds. On the 24th of September Giffard ascended with it over Paris to the height of five thousand feet. The wind was quite strong; but he was able to make very perceptible headway against this, and by the aid of the rudder to turn the machine into any desired direction. He thus proved incontestably that the problem of directing an aërial steamer was by no means insoluble. A second ascent was accomplished by him in 1855, but under unfavorable conditions. He made no further attempts with this machine, the abandonment of these experiments being due chiefly to their danger. A steam-engine, sending forth sparks beneath a mass of thirty thousand cubic feet of inflammable gas contained within an envelope of thin cloth, is a source of peril to which few men would be willing to expose themselves, even if lofty elevations were not reached. Trouble also resulted from the fact that the weight of the balloon could not be kept constant. The loss of the products of combustion and of spent steam made it difficult to preserve the proper relation between the ascensive power and the weight to be sustained.
Not quite twenty years after Giffard's experiments the problem was again attacked by M. Dupuy de Lôme. His immense balloon, containing one hundred and twenty thousand cubic feet of pure hydrogen, was nearly similar in shape to that of Giffard. The car beneath was capable of carrying easily fourteen men, seven of whom at a time were employed in working a capstan which controlled the shaft of the propeller, each of the two blades of this being about ten feet in length. On February 2, 1872, Dupuy de Lôme ascended in this balloon, and attained a speed estimated at 2·8 metres per second, equivalent to about six miles per hour. By means of the rudder he changed the direction through an angle of 12°. Giffard had attained an estimated speed of four metres per second, or nine miles per hour. Muscular power was thus shown to be far too uneconomical, while steam was too dangerous, to be employed in the direction of aërostats.
It was not until 1881, the year of Giffard's death, that electricity was applied as a motive power in the attempt to solve the difficult problem with which he had grappled. His pupil, M. Gaston Tissandier, had early imbibed a passion for aëronautics, and made many successful ascents with spherical balloons. It was Tissandier who, in company with two friends, ascended in April, 1875, to the height of eight thousand six hundred metres, each of the three swooning on account of the rarefaction of the air, even before this limit was attained. The same aëronaut, in company with Fonvielle, was borne by the wind, in February, 1869, in thirty-five minutes, from Paris to Neuilly-Saint-Front, a distance of fifty miles. Keeping abreast with the progress of science, Tissandier conceived the idea of employing storage batteries instead of steam or hand power, as the immediate source of energy to actuate the propeller of an elongated balloon. He constructed a small experimental balloon, which was filled with hydrogen, the effective ascensional force being two kilogrammes. A motor, of the Siemens type (Fig. 2), weighing only two hundred and twenty grammes, was made to turn the propeller, which consisted of a pair of vanes, each ten centimetres long; storage-cell, motor, and propeller being supported on a light platform suspended by netting. This
Fig. 2.—Tissandier's Miniature Electric Motor and Propeller, 1881.
"dirigeable" aërostat was exhibited at the Electrical Exposition of 1881, and a bronze medal awarded to the inventor. It attained a speed of about three metres per second.
Encouraged by this success, Tissandier undertook the work of constructing an aërostat large enough to lift two or three persons in addition to the weight of the propelling apparatus and other accessories. The task was one which involved a heavy expenditure of money, aside from the time, labor, and thought bestowed by the inventor. He sought in vain to organize a company with a capital of two hundred thousand francs for the purpose of constructing an aërostat of three thousand cubic metres capacity; but the plan was not sufficiently promising of large dividends to be attractive to investors. No fortunes have been made thus far by the navigation of the air, and capitalists have not generally manifested a self-sacrificing spirit in behalf of pure science. The persevering aëronaut could find no one but his brother, M. Albert Tissandier, who was confident enough to join him in laying out capital for the promotion of what was generally regarded as a visionary scheme. The two brothers henceforward worked together, the one continuing to devote himself to the perfection of the electrical appliances on which reliance was to be placed, while the other, who is by profession an architect, gave his attention to the mechanical construction of the aërostat. M. Gaston Tissandier had found by experiments with his small aërostat that better results were to be had from a battery of cells, arranged in series, where a strong acid solution of potassium bichromate was the exciting liquid, than from a storage-battery the energy evolved during the first few hours being greater in proportion to the weight of the battery. He originated several ingenious contrivances by which great lightness was secured, and the liquid could be conveniently brought into contact with the zinc and carbon plates, or removed at will without disturbing the plates.
A Siemens electric motor was constructed, weighing but fifty-five kilogrammes. When excited by the current from a battery of twenty-four elements weighing one hundred and sixty-eight kilogrammes, this motor was found capable of doing work equivalent to that of twelve or fifteen men, that is, from seventy-five to one hundred kilogramme-metres per second, continued through three hours, the weight of battery and motor together being but little in excess of the weight of three men. Tissandier devised also important improvements in the method of generating pure hydrogen rapidly on a large scale. The ascensive force of this gas when pure is about seventy-five pounds per thousand cubic feet, or eleven hundred and eighty grammes per cubic metre; while that of coal-gas which has been most generally employed for ballooning purposes is not more than five eighths as much. By the substitution of hydrogen, the size, and consequently the expense, of the balloon is correspondingly diminished.
The aërostat constructed by M. Albert Tissandier is shown in Fig. 3. It is ninety-two feet long, thirty feet in its greatest diameter, with capacity of about thirty-eight thousand cubic feet, and ascensive power of twenty-eight hundred pounds. The propeller, nine feet in diameter, is in the rear of the cage. Above it, and farther back, is a triangular sail, to be manipulated as a rudder. On October 8, 1883, the first ascent was made. The air at the ground was calm, but on reaching a height of sixteen hundred feet the wind was blowing at the rate of rather more than six miles an hour. On putting the propeller into action, with a velocity of three revolutions per second, and turning the head of the aërostat against the breeze, it was kept motionless for some minutes; but the rudder soon proved to be insufficient to keep the direction constant, flapping like a sail, and at times
Fig. 3.—Tissandier's Balloon, 1883.
leaving. the aëronauts at the mercy of the wind. After stopping the propeller and waiting until the direction of the aërostat coincided with that of the wind, the action was renewed. A marked acceleration in speed was the immediate result, and deviations from the line of the wind were secured by very slight motion of the rudder, the aërostat keeping its stability perfectly. The descent was safely accomplished after remaining in the air a little more than an hour.
This first experiment in the use of electricity in practical aëronautics was about as successful as that of Giffard with steam in 1852, so far as relates to the attainment of speed; but it showed that such speed could now be secured without danger and without any uncontrollable variation in the weight of the mass propelled. Tissandier did not expect the attainment of complete success in a single trial; such as he did attain was enough to convince not only him but others that he had opened out a pathway which could be followed with en-tire confidence. He had not the means at hand sufficient to enable him to keep his aërostat inflated, so as to repeat his experiment on the first favorable day after imposing such modifications as were suggested by the experience of the first ascent. It was not until September 26, 1884, that this opportunity was presented. The velocity of the wind was about the same as during the first ascent, but the aërostat was propelled at a rate about one third greater, so as to make at times very perceptible headway against the wind.
Meanwhile the success achieved by the Tissandier brothers in 1881 and 1883 had inspired MM. Renard and Krebs, officers of the French army, who were stationed at Chalais-Meudon, near Paris. They had for several years been conducting experiments on the conditions requisite for directing balloons, being guided in their studies by the previous work of Dupuy de Lôme. An appropriation of one hundred thousand francs had been granted them, during Gambetta's brief administration, and their investigations were conducted with the utmost secrecy. The pecuniary resources at their command gave them a great advantage over Tissandier, in the ability to construct a balloon much larger than that with which Tissandier's success had been achieved; and this permitted the application of a motor nearly seven times as powerful as the one previously employed. Their balloon (Fig. 4) is one hundred and sixty-six feet long, twenty-eight feet in greatest diameter, its capacity sixty-seven thousand cubic feet, and ascensional power nearly five thousand pounds. The ratio of length to thickness is thus much greater than in Tissandier's balloon. The details of construction of the battery and motor have not been given to the public by Captain Renard. The rudder is almost a parallelogram in form, and thickest in the middle, the cloth being tightly stretched over a light framework so as to present a rigid surface to the air. The propeller is fixed to the extremity of a long shaft, and placed at the front, instead of rear, of the balloon. The front end of the machine is thicker than the rear end. This feature seems rather unaccountable. The balloon is filled with hydrogen, but within it is a subsidiary balloon, connected by a tube with the cage, where air can be pumped in or out at pleasure, thus varying slightly the specific gravity of the mass as a whole and enabling the aëronauts to vary their elevation at will.
On August 9, 1884, an ascent was accomplished with this balloon, the atmosphere being almost perfectly calm. A journey of nearly two miles was made in a southerly direction, then over a mile westward, after which the balloon was turned northward and eastward. Very slight motion of the rudder was needed to execute these curves. Twenty-three minutes after their flight was begun the aëronauts were immediately over their starting-point, having made a trip of not quite five miles. In descending it was necessary to move backward and forward several times in succession, alternately reversing the direction of rotation of the propeller. The return to the ground was at the very spot from which the departure had been made. This remarkable feat was thus accomplished almost exactly one hundred and one years after the ascent of the first hydrogen balloon, sent up by Charles from a point but a few miles distant.
A second ascent was made by Renard and Krebs on the 12th of September, but with only partial success, in consequence of an accident to the motor. On the 8th of November two successive journeys were taken, the balloon returning each time to its point of departure, and attaining a speed of nearly fifteen miles an hour, independently of the wind, which was blowing at the rate of five miles an hour.
In their communication to the French Academy of Sciences, on the 18th of August, Renard and Krebs accord to Tissandier the credit of
Fig. 4.—Renard and Kreb's Balloon, 1884.
priority in successfully applying electricity to the propulsion of balloons. Tissandier, on the other hand, equally freely accords to them the credit of making a pronounced success of what had been developed to only a limited extent in his hands on account of the want of funds. To each of the group the world must now give praise for the solution of a problem which was theoretically solved long ago, but involved practical difficulties that seemed almost if not quite insurmountable.
At best, however, the balloon as a means of locomotion is of more interest from a scientific than commercial standpoint. Increasing experience will determine the best disposition to be made in relation to a variety of points that are still open to discussion, such as the best methods of reducing resistances and increasing the efficiency of the motor. On the basis of the success already attained, calculations have been made which indicate that it may be quite possible in the near future to construct larger balloons that will travel in calm air at the rate of twenty-five or thirty miles an hour. Such air-ships, capable of ready direction at safe elevations, may serve important purposes in time of war. But for the public their use must long continue to be very limited. The enormous expense attendant upon the construction and manipulation of an aërostat capable of carrying even so few as a score of persons forbids competition with railroads and steamships. The high-tension battery, which is at present the most available source of energy to give motive power, has an effective life of only a few hours; and, even during this time, the cost of zinc and acid is far in excess of that of coal and water. For special purposes, where surface locomotion is impossible, and expenses can be sustained by great corporations or very wealthy individuals, the "dirigeable" balloon may, perhaps, win for itself an important place. The history of the application of science to art, especially during the last half-century, suggests caution in making sweeping denials, merely because our present knowledge does not enable us to grasp all the details of future development. Dr. Lardner's assertion, that no steamship would ever cross the Atlantic, may well remain fresh in our minds. The present competition between electricity and coal-gas as illuminating agents could scarcely have been foreseen in the days when coal-gas was itself comparatively a novelty. If we continue, as is probable, to attain cheaper methods of generating electricity, the balloon may grow in favor as the electric light has done, but without causing-the least commotion in the market for railroad stocks.
But, for the development of aëronautics as an art, we must continue to look to France, its earliest home. If stock companies are formed for the manufacture of air-ships, it must be first among those to whom the recent successes have already become a source of pride. Even as toys and as advertising media, balloons have always been more popular in France than elsewhere. The city of Paris has for years past included one or more large establishments devoted exclusively to the manufacture of them. Should "dirigeable" balloons ever become of commercial importance, enterprising Americans will be quick to imitate their French neighbors, and put upon our market all that the public may demand. The day is perhaps not far distant when at least a favored few in our own country may enjoy the luxury of summer afternoon excursions through the air, free from dust and cinders, and occasionally even vying with the birds in speed.
- The writer takes pleasure in acknowledging his personal indebtedness to M. Tissandier for the full accounts from which the facts set forth in the latter part of this article were obtained.