CHAPTER III
Directly after the first launching of human passengers in a crude aërostat, numerous schemes for controlling the course of a balloon were evolved. Apparently mere flotation afforded less contentment to the early pioneer aëronauts than to the free balloonists of the present hour. Many were eager to apply propelling mechanism to their gas bags, expecting thus to achieve practical locomotion through the air, even a generation before the advent of practical steam navigation. Magnificent dreams they had, indeed, but none the less futile. Few suspected the enormous power required to propel swift balloons of the very best shape and size; still fewer realized the impossibility of driving spherical bags at a practicable velocity.
On the other hand, it must be said, to the credit of that era of investigators, that certain noted scientists, after computing the power required to drive a balloon at high speed, promptly recognized the inadequacy to that task, of any motors then available. In conjunction with favorable aërial currents something might be effected; that they fully grasped; for they knew that the wind frequently has different directions at different levels. They believed, therefore, that by causing the craft to rise or fall to a suitable stratum, by use of various then known devices, it could be made to travel in any direction at the will of the pilot. Likewise they deemed that the rise and fall of a balloon, due to change of buoyancy, could be used to propel it, if sails attached to the vessel were set obliquely to the motion, so as to receive fair pressure; or if the balloon were made flat, or longish, so as to glide horizontally, like a kite or parachute.
Several devices for changing the altitude of the balloon were proposed or tried. If the vessel were a Montgolfière, the mere increase or lessening of the fire would promptly cause it to rise or fall. If a gas bag were employed it could be sent up or down by casting out ballast or opening the valve; or again, as proposed by Pilâtre de Roziere, by having a Montgolfière underneath the gas balloon, and lifting or depressing the whole by altering the intensity of the flame. Finally, an air balloon within a gas balloon was proposed by the Roberts, and a gas balloon within an air balloon was proposed by General Meusnier, in either of which combinations, a change of level could be effected by pumping air into, or letting it escape from, the air bag. All of these devices can be effected and practically operated by a competent balloon maker and pilot; and yet they have not enabled man to realize his dream of navigating the air in all directions without motive power.
The first attempts at balloon propulsion could not be seriously regarded by trained engineers, even at the inception of aëronautics; but still, as infantile steps in the new art, they may deserve passing notice.
Blanchard, on March 2, 1784, made the first real effort to steer a balloon, using for that purpose a spherical gas bag and car provided with aërial oars and a rudder. As he was about to ascend, however, from the Champs de Mars, a young officer with drawn sword persisted in accompanying the pilot, thus compelling Blanchard to leave his wings on earth to allow sufficient buoyancy for himself and his obtrusive guest. His first trial was, therefore, frustrated; but subsequent ones made with that inadequate contrivance also proved futile under the best circumstances; for the scheme was evidently puerile, though tried by various grown-up men besides M. Blanchard.
Fig. 13.—Blanchard’s Dirigible Balloon, 1784.
A no less simple and quaint device for propulsion was that of the two physicists, the Abbé Miolan and Janinet. The balloon was a Montgolfière with a large hole in one side, through which the hot air was to escape with such strong reaction as to drive the bag forward, on the principle of a lawn sprinkler, or of Newton’s reaction wagon. The projectors failed, however, to make an ascent, and the crowd becoming furious destroyed the balloon.
A more reasonable plan for practical navigation was devised and tried by the Robert brothers. A melon-shaped balloon, fifty-two feet long by thirty-two feet in diameter, was made of silk and inflated with pure hydrogen. Beneath was suspended a longish car of light wood covered with sky-blue silk. This elegant ship was to be rowed through heaven by means of six silken oars actuated by sturdy sailors. A silken rudder should guide her at pleasure when the winds were asleep, or softly playing in the placid sky. She was a fairy bark, indeed, a soaring castle lovely to behold.
After a preliminary trial, accompanied by their patron, the Duke de Chartres, they were ready for a substantial journey. On September 19, 1784, the vessel was inflated and taken to the Garden of the Tuileries, in front of the palace, where its cords were held by Marshall Richelieu and three other noblemen. At eleven forty-five the two Roberts and their brother-in-law arose and drifted beyond the horizon on a seven hours’ cruise. Before coming to earth, they plied the oars vigorously, and described a curve of one kilometer radius, thus deviating 22° from the feeble wind then prevailing. In a lighter wind they could deviate still more. They considered, therefore, that the experiment was a complete success. They had constructed the first elongated balloon, and had “solved the problem of aërial navigation.” In very happy mood, therefore, they landed at dusk among the delighted inhabitants of Artois, where they were graciously met and hospitably entertained by the Prince de Ghistelles-Richbourg.
Fig. 14.—Robert Brothers’ Dirigible, 1784.
The Robert brothers were the first to employ in practice an air bag inside a gas bag. This was held within the balloon by ropes and connected with the outer atmosphere by a tube, the idea being to regulate the internal pressure of the balloon by introducing air into, or withdrawing it from, the smaller bag. But during an ascension with their patron, the Duke de Chartres, they entered a violent eddy which tore away the oars and rudder, at the same time agitating the balloon so violently that the internal air bag broke its sustaining cords and fell upon the bottom of the gas bag, thus throttling the connection with the external atmosphere. The vessel rose swiftly and the gas expanded dangerously near to the bursting pressure. At a height of 16,000 feet the Duke de Chartres, perceiving the imminent danger of an explosion of the envelope, drew his sword and cut a rent ten feet long in its lower part. A part of the gas immediately rushed forth, and the balloon sank rapidly, but after the discharge of the ballast, landed safely without further mishap. The Duke acted wisely enough, but he was afterwards ridiculed for his apparent lack of courage. If he had possessed more bravery and less caution he might have allowed the balloon to burst and descend as a parachute, thus anticipating the spectacular performance of John Wise, in 1838.
Simultaneously other inventors were evolving designs of no less importance in the ultimate perfection of the dirigible. In a letter written to Benjamin Franklin on May 24, 1784, Francis Hopkinson of Philadelphia proposed to build a balloon of spindle shape and to drive it by means of a wheel-like propeller at the stern, consisting of vanes set at an angle to the line of progression, like the common smokejack. This proposed craft, the harbinger of the modern screw-driven motor balloon, far antedated the screw-driven boat and the submarine torpedo which it most resembles.[9]
While Blanchard and other aëronauts were paddling their globose bags in search of favorable winds, vainly hoping thereby to direct their course in the air, General Meusnier of the French army, and member of the Academy of Sciences, made a systematic study of the requirements for practical air navigation. After some research on forms suitable for aëronautic hulls, he designed a power balloon having a pointed car suspended from a bag of goose-egg form, this latter embodying his idea of the best shape for a balloon that must cleave the air swiftly and resist deformation. The propulsion was to be effected by means of three coaxial screw propellers, supported on the rigging between car and bag, and actuated by eighty men, for lack of a light artificial motor. He thus hoped to obtain a moderate velocity which, combined with skillfully selected air currents, would enable the ship to reach her destination in ordinary weather.
Fig. 15.—Gen. Meusnier’s Proposed Dirigible, 1784.
General Meusnier introduced important special features in the design of dirigibles for preserving their form and poise. He insisted that the bag and boat should be so rigidly connected that one could not swerve from alignment and relative position with the other. He also emphasized the necessity of preserving the vessel from deformation during flight, in order to diminish its resistance. To that end he proposed to provide the hull with a double envelope, the inner one thin and light but impermeable to hydrogen; the outer one strong and air-tight; the space between the two envelopes to be pumped full of air under pressure sufficient to preserve the form of the bag when beating its way swiftly against a buffeting wind. This was an important invention which in later years was adopted in many of the most powerful motor balloons—for all, indeed, except those of the rigid type. He also proposed the use of stabilizing planes to control the poise of the vessel, thus anticipating the Lebaudy brothers by more than a century. Like the Robert brothers he proposed to raise or lower the vessel in search of suitable currents, by altering the quantity of air in the space between the inner and outer envelope, by use of hand bellows.
Apparently General Meusnier and his colleagues were endowed with constructive genius sufficient to have developed a practical motor balloon, had they been able to secure a light engine. Lacking this the early aëronauts could do little more than describe their projects, and await the growth of the collateral arts and sciences. Accordingly no substantial advance in motor balloons beyond Meusnier’s designs was effected till after the middle of the nineteenth century; and until then the art of aëronautics remained in the hands of showmen. Hundreds of projects, indeed, were advanced, some exciting considerable interest and expectation, but nevertheless of such paltry value as hardly to deserve comment. One notable exception to these was the invention of Porter in America.
In 1820 Rufus Porter, a Yankee inventor, and later the original founder of the Scientific American, patented an air ship of very promising appearance for that early day. Its hull was a long, finely tapering symmetrical spindle, suspending a car of similar shape by means of cords, which were vertical at its middle but more and more slanting toward its ends. Midway between the hull and car was a large screw propeller actuated by a steam engine in the car. A model of this dirigible exhibited in Boston and New York, some years later, is reported to have carried its own power, at fair speed, and to have obeyed its helm satisfactorily.
Fig. 16.—Rufus Porter’s Dirigible, 1820.
The inventor, being too poor to develop his air ship alone, did little with the patent during its life; but in 1850 he organized a stock company to realize the needed funds. From the sale of 300 five-dollar shares he expected to raise $1,500, and with this sum build an “aëroport,” 150 feet long, capable of carrying five persons sixty miles an hour, the whole to be completed in six weeks. Once this was in operation he would easily command funds sufficient to build a full-sized vessel adapted to regular passenger service. For, after careful calculation, he reported that: “It appears certain that a safe and durable aërial ship (or aëroport) capable of carrying 150 passengers at a speed of ninety miles an hour, with more perfect safety than either steamboat or railroad cars, may be constructed for $15,000, and that the expense of running it would not exceed $25 per day.”
The language and project seem very modern, even at the present time, and might well be copied now by a promoter of that identical project. But it must be observed that the most successful European experimenters, after spending hundreds of thousands of dollars on giant air ships, have not yet attained one half the speed contemplated by that ambitious and chimerical Yankee. The picture was handsome and alluring, none the less. It may even be said to excel in outward design any of the air-ship plans produced in either hemisphere before the middle of the nineteenth century.
In 1850 a clockmaker and skillful workman, Jullien by name, exhibited in the Hippodrome, at Paris, a torpedo-shaped model balloon of gold-beater’s skin, provided with a screw propeller at either side of its bow, and a double rudder at its stern. It measured 23 feet in length and weighed 1,100 grammes complete. The propellers were actuated by spring power, and proved able to drive the tiny vessel against a moderate wind. The most suitable form for the bag was determined by towing models through water.
Fig. 17.—Jullien’s Model Dirigible, 1850.
Aërodynamically considered, this tiny motor balloon was by far the best in design of any that appeared during the first century of aëronautics. It may be regarded as the harbinger of the swiftest modern French balloons. It was also an inspiration to Henri Giffard who assisted Jullien in constructing his clever model, and shortly afterwards built the first dirigible ever driven by a heat engine.
The illustrious Henri Giffard was perhaps the first aëronautical engineer adequately endowed and circumstanced to realize, on a practical scale, General Meusnier’s well pondered and truly scientific plans for a motor balloon. He had studied in the college of Bourbon, and had worked in the railroad shops of the Paris and St. Germain railway. He had further equipped himself by making free balloon ascensions, under the auspices of Eugene Godard, for the purpose of studying the atmosphere; and by building light engines, one of which weighed 100 pounds, and developed three horse power. Finally in 1851 he patented an air ship, consisting of an elongated bag and car, propelled by a screw driven by a steam engine. He had not the means to build such a vessel, but he had the genius and training necessary to construct it, and at the same time enough enthusiasm and persuasive power to induce his friends, David and Sciama, to loan him the requisite funds.
Fig. 18.—Giffard’s Steam Dirigible, 1852.
Giffard’s first dirigible was successful in both design and operation. It consisted of a spindle-shaped bag covered with a net whose cords were drawn down and attached to a horizontal pole, from which the car and motor were suspended, and at the end of which was a triangular sail serving as a rudder. To guard against fire, the furnace of the vertical coke-burning boiler was shielded by wire gauze, like a miner’s lamp, and the draft, taken from its top through a downward pointing smoke pipe, was ejected below the car by force of exhaust steam, from the engine, thus obviating, as Giffard asserted, all danger from the use of fire near an inflammable gas. The car hung twenty feet below the suspension pole, and carried a three horse-power engine driving a three-blade propeller 11 feet in diameter, making 110 turns a minute. The motor complete, including the engine and boiler without supplies, weighed 110 pounds per horse power. The bag measured 143 feet long, 39 feet in diameter, and 75,000 cubic feet in volume. Giffard reports of his first voyage, made from the Hippodrome in Paris at five fifteen o’clock, September 23, 1852, that although he could not sail directly against the strong wind then blowing, he could attain a speed of six to ten feet per second relatively to the air, and he could easily guide the vessel by turning her rudder. He continued his journey till nightfall, then made a good landing, near Trappes, and by ten o’clock was back in Paris.
This vessel was but a prelude to mightier projects. After some further experience with dirigibles of moderate size, Giffard designed a colossal air ship calculated for a speed of forty-four miles an hour. Its hull was to be of torpedo shape, measuring 2,000 feet in length, 100 feet in diameter, and 7,000,000 cubic feet in volume. It was a most audacious project, one worthy of the genius and energy of that illustrious engineer, the most original and daring inventor known in the aëronautical world during the nineteenth century.
Stimulated by this huge enterprise, Giffard’s first step was to pay his debts and make a fortune. He soon acquired a hundred thousand francs from the sale of small high-speed engines of his own construction, and with this, settled his account with David and Sciama. Next he realized several million francs from his world-famous injector, a device by which steam flowing from a boiler is made to drive in feed-water against the same pressure.
He now made definite plans to build a motor balloon of one and a half million cubic feet capacity, driven by a condensing engine drawing steam from two boilers, one fired with oil, the other with gas from the balloon, so as to keep the vessel from rising with loss of weight. His designs were complete, and everything was provided for. He had deposited a million francs in the Bank of Paris to defray the estimated cost. But, in the words of Tissandier,[10] “above the human will and foresight are the fatal laws of destiny to which the strongest must submit.” The great inventor was visited with a painful affliction of the eyes; his sight waned, unfitting him for work; he became disconsolate, pined away with pain and grief, and in 1882 ended his life by taking chloroform.
Giffard was succeeded in France, first by Dupuy de Lome; then by Gaston Tissandier, well-meaning projectors of steerable balloons, but too cautious to effect an important advance in the art. The first of these gentlemen, an eminent marine engineer, in 1872, completed a gas balloon for the French government, resembling the one designed by General Meusnier in 1784, and like that also driven by muscular power actuating a screw, and kept rigidly inflated by use of an internal balloon, or ballonet. The car was suspended from the bag by a close fitting cover instead of a net, in order to lessen the resistance, and it was kept in alignment by use of crossed suspension cords. A speed of but six miles an hour was attained by the industrious work of eight men operating an ample screw propeller. A decade later Tissandier, with a balloon of like design, but driven by the power of an electric motor and bichromate of potash battery, attained a speed of six to eight miles an hour.
Fig. 19.—Dupuy de Lome’s Dirigible, 1872.
The two vessels were safe but of no practical value, for lack of sufficient power to cope with the wind. Their motors were fundamentally unadapted to the purpose of swift propulsion, and incapable of development to very great lightness and strength. Furthermore, the vessels themselves were unsuitably designed for speed; their shape being one of too much resistance, and their dynamic balance being that of a pendulum, or clumsy parachute, rather than that of a vessel adapted to cleave the air with celerity, grace and steadiness. If there had been danger of fire from placing the motor and screw near the gas bag, that might justify or excuse the clumsiness of design in the craft of De Lome and of Gaston Tissandier; but, having perfectly safe motors, it is astonishing that they did not place the center of mass and the line of thrust more nearly in the line of resistance. This obvious requirement was duly recognized by several of their contemporaries, notably by Hänlein in Germany, and by Captain Renard of the French War Department, and had been observed by Jullien.
Captain Charles Renard proved to be a worthy inheritor of the dreams, experience and inventions of the first century of aëronautical votaries. He did not, indeed, have the picturesque madness displayed by some of his predecessors; he did not project schemes of marvelous originality or boldness; but he manifested uncommonly good judgment and excellent scientific method in combining the researches and contrivances of others with those of himself and his collaborator, Captain Krebs. As a consequence they produced the first man-carrying dirigible that ever returned against the wind to its starting point, and the first aërial vessel whose shape and dynamic adjustment even approximated the requirements of steady and swift navigation in a surrounding medium presenting various conditions of turbulence or calm. Captain Renard had been studying and designing dirigibles since 1878 in coöperation with Captain La Haye and Colonel Laussedat, president of an aëronautic commission appointed by the Minister of War; and had endeavored to secure from the latter an appropriation sufficient to construct a dirigible; but his request was at first denied, owing to the waste of funds on similar projects in 1870. However, with the help of Gambetta, who promised a sum of $40,000, Renard was enabled to proceed. In the meantime he had been made director of the laboratory at Chalais Meudon, seconded by Captain Krebs.
Fig. 20.—Renard’s Dirigible, La France, 1884.
These officers first worked out the separate elements in the design of their motor balloon before proceeding to build on a practical scale. They chose the torpedo form for their gas bag, thereby ensuring in the hull itself, projectile stability, and diminution of resistance. They placed the car near the envelope, thus minimizing the disturbing moment of the screw thrust, and the resistance of the suspension cords. They employed an extraordinarily powerful electric motor actuating a large screw so as to obtain a strong thrust with the least effort. In addition they adopted the best ideas of their predecessors in aëronautical design; the internal ballonet of Meusnier, and the close fitting cover of De Lome, with crossed suspension cords. But unfortunately they used an electric motor instead of some light engine. Finally, having carefully computed its requisite dimensions, they proceeded to construct the elegant air ship, La France, which was tested in 1884 and aroused anew the hope of ultimately conquering the air.
Further details of this successful ship are of interest. Its hull was 165 feet long, 27.5 feet in greatest diameter, at one fourth the distance from its front end, and cubed 66,000 feet, thus having a buoyancy of two long tons. It was kept rigid under varying conditions, by means of a ballonet filled with air driven in by a common fan blower coupled to the motor. Beneath the envelope, a long narrow rectangular car made of bamboo, covered with silk, was suspended from the cords of the balloon cover which embraced the hull throughout nearly its entire length. The car was 108 feet long and 6 to 7 feet across, carried at its forward end the propeller, at its rear a rectangular rudder, and between them the aëronauts and the batteries and electric motor. A sliding weight was used to alter the poise of the ship, and a guide-rope to soften its descent.
The electric motor and battery which furnished the propulsive power were designed expressly for such use, and were considered at the time to be remarkably light and effective. The motor, which was designed with the assistance of M. Gramme, weighed 220.5 pounds, and developed nine horse power. The battery, composed of chlorochromic cells, was the result of the researches of Renard himself. Having made a careful study of the best geometrical arrangement of the parts of the cell, Renard found that this battery would deliver to the shaft one horse power for each eighty-eight pounds of its weight. Thus the power plant rivaled in lightness the steam engine of Giffard, and at the same time was free from danger; but apparently it could not be much reduced in weight, whereas Giffard’s steam-power plant could be reduced tenfold, as shown by Renard’s contemporaries.
The trials of La France in 1884–85 were most successful and encouraging; not that they represented or pointed to the complete mastery of aërial navigation, but because they so far surpassed all previous achievements. The vessel moved through the air as steadily as a boat on the water, and obeyed her rudder perfectly, heading against the wind, or at any angle to it, or turning entirely about, at the will of the aëronauts. On her first voyage from Chalais, August 9, 1884, she traversed a distance of four and one half miles in twenty minutes, made various evolutions in the air with the greatest ease, and returned to her point of departure. The following account of this voyage is given by Renard:
“As soon as we had reached the top of the wooded plateaus which surround the valley of Chalais, we started the screw, and had the satisfaction of seeing the balloon immediately obey it and readily follow every turn of the rudder. We felt that we were absolutely masters of our own movements, and that we could traverse the atmosphere in any direction as easily as a steam launch could make its evolutions on a calm lake. After having accomplished our purpose, we turned our head toward the point of departure and we soon saw it approaching it. The walls of the park of Chalais were passed anew, and our landing appeared at our feet, about 1,000 feet below the car. The screw was then slowed down, and a pull at the safety-valve started the descent, during which, by means of the propeller and rudder, the balloon was maintained directly over the point where our assistants awaited us. Everything occurred according to our plan, and the car was soon resting quietly on the lawn.”
Six other similar voyages were made within the two years following, and we have as a result, that in five out of the seven trials, the balloon returned to its point of departure. Its failure to return in the other two trials was due, in the one case, to the breaking down of the motor; in the other, to the resistance of a strong wind which made it necessary to land at a distance from the starting point. The last of these remarkable voyages was performed in presence of the Minister of War, on September 23, 1885. The balloon started from Calais and sailed against the wind directly to Paris, passed over the fortifications, described a graceful curve and returned to its place of departure, recording an average speed of 14.5 miles an hour.
The torpedo form of hull, chosen by Renard and Krebs, has two important advantages; one is projectile stability, the other is economy of propulsive power. Owing to the blunt bow and long tapering stern, the center of mass is well forward, while the center of side wind pressure is more to the rear. As a consequence, if the vessel should encounter a quartering wind-gust, or have her nose slightly turned from the course, she would promptly right herself like a dart or an arrow. If on the contrary, the hull were a symmetrical spindle, the vessel would move forward in unstable equilibrium, and, once slightly diverted from her course, would tend to deviate further, like an arrow with unloaded head.
The second advantage mentioned is also worth attention, viz.: that at ordinary transportation speeds a longish spindle has less resistance with a blunt bow than with a very sharp one. Renard and Krebs did not account for this fact; but the present writer, by determining separately the skin friction and the impactual resistance of the air, proved that in sharpening the bow beyond a certain best form, its friction increases faster than its head resistance diminishes, the most suitable shape being that of a torpedo whose nose has a radius of curvature of about two diameters, and its stern a radius of about twelve diameters.
While the successors of Giffard in France were thus engaged in developing dirigibles driven by muscular or electric power, a few German experimenters were applying gas and benzine engines to such vessels, with better promise of ultimate practical success and usefulness. The first of these was Hänlein, who in 1872 advanced the meritorious project of driving a well shaped balloon by means of a gas engine taking its fuel from inside the balloon, and making good the loss by pumping air into the ballonet. This balloon was of far better design for swiftness and kinetic stability than the contemporary one of Dupuy de Lome. Its hull was a well pointed cylinder 164 feet long, 30 feet in diameter and of 85,000 cubic feet capacity, made air-tight by a thick coating of rubber inside, and a thin one outside. The car was rigidly suspended near the envelope and carried a 6 horse-power Lenoir gas engine actuating a large screw. Notwithstanding that the buoyancy was small, owing to the use of coal gas, this air ship attained a speed of 15 feet per second. By employing hydrogen, a much larger engine could have been carried, entailing a much swifter speed. During its trial the balloon was kept near the earth’s surface, held loosely by ropes in the hands of soldiers. The air ship was remarkably successful for that early date, and had the potency of greater achievement than its contemporaries in France; but owing to lack of funds its capabilities were not fully developed. If it had been inflated with hydrogen, and propelled by use of gas and petrol, so that the loss of weight would compensate for the loss of buoyancy, it might have anticipated the speed and endurance of the best air ships built toward the close of the nineteenth century, or later.
PLATE II.
HAENLEIN’S GAS-DRIVEN DIRIGIBLE.
WÖLFERT’S BENZINE-DRIVEN DIRIGIBLE.
SANTOS-DUMONT’S DIRIGIBLE, NO. 16.
Photo E. Levick, N. Y.
In 1879, Baumgarten and Wölfert in Germany built a dirigible equipped with a Daimler benzine motor, but otherwise not possessing any special merit. An ascension was made at Leipsic in 1880, but owing to improper load distribution the vessel reared on end and crashed to earth. After further experiments, an ascension was made on the Templehofer field, near Berlin, in 1897, but this ended disastrously; for the benzine vapor ignited; the fire spread to the balloon, and the vessel fell flaming to the earth, killing Wölfert and his assistant. Baumgarten had died some years before.
In 1897, an aluminum air ship invented by an Austrian engineer, named Schwartz, was launched on the Templehofer field. Its hull was of cylindrical form with conical ends, made of sheets 0.008 thick, and stiffened with an internal frame of aluminum tubes. Being leaky and inadequately driven, it voyaged but four miles, drifting with the wind, then fell to earth with considerable shock. The pilot, a soldier of the Balloon Corps, escaped by jumping, before the vessel struck ground, but the frail unbending hull was soon demolished by the buffeting of the winds as it lay stranded on the unyielding earth. This was the second air ship built after the plans of poor Schwartz, the first having collapsed on inflation. He had, however, the credit of being the first to drive a rigid air ship with a petrol motor, and thus to inaugurate a system of aërial navigation capable of immense development, in the hands of sufficient capital and constructive skill. Thus the rigid type, conceived and crudely tried by Marey Monge and Dupuis Delcourt in the early part of the century, began to approach practical realization toward the end of the century.
The process of inflating with hydrogen such a rigid hull is interesting. Schwartz’s plan, carried out by Captain Von Sigsfeld, was to place the hydrogen in one or more sacs inside the hull, thus expelling the air and filling the space, then withdrawing the sacs and leaving the hydrogen within. A better plan is to have a single sac inflated with air just filling the hull like the lining of an egg, then to force the gas between the lining and metal wall of the hull, thus expelling the air from the sac, which when completely collapsed can be removed. Practically the same result can be obtained by use of a thin fabric covering one half the inner wall, like the lining of an egg. Further provision can easily be made for manipulating the ballonet in such a case.