All these ascents, though they proved that the balloon had a certain utility for the exploration of the upper reaches of the atmosphere, did little or nothing for aerial navigation. The great vogue of the balloon distracted attention from the real problem of flight. That problem was not abandoned; a number of men, working independently, without any sort of public recognition, made steady advance during the whole course of the nineteenth century. By the end of the century, three years before flight was achieved, those who were most deeply concerned in the attempt knew that success was near. The great difficulty of scientific research lies in choosing the right questions to ask of nature. Every lawyer knows that it is easy to put a question so full of false assumptions that no true answer to it is possible; and many a laborious man of science has spent his life in framing such questions, and in looking for an answer to them. The contribution of the nineteenth century to the science of flight was that it got hold of the right questions, and formulated them more or less exactly, so that the answers, when once they were supplied by continued observation and experiment, were things of value.
The earliest of these pioneers was Sir George Cayley, a country gentleman with estates in Yorkshire and Lincolnshire, who devoted his life to scientific pursuits. He was born in 1773, and the balloons which excited the world during his boyhood directed his mind to the subject of aerial navigation. He invented many mechanical contrivances, and he laid great and just stress on the importance of motive power for successful flight. In 1809 he published, in Nicholson's Journal, a paper on Aerial Navigation, which has since become a classic, for although it stops short of a complete exposition, it is true so far as it goes, and contains no nonsense and no fantasy. He endeavoured, in the first year of Queen Victoria's reign, to establish an aeronautical society, but the ill repute of the balloon and the bad company it kept deprived him of influential support. He did his duty by his county, as a Whig magnate, and amused his leisure with science, till his death in 1857.
Cayley's work is difficult to assess. He had all the right ideas, though the means of putting them into practice did not lie ready to his hand. If he had been a poor man, he might have gone farther. He designed, so to say, both an airship and an aeroplane; there was no one to execute his designs, and the scheme fell through. He more than once anticipated later inventions, but he put nothing on the market. His mind was fertile in mechanical devices, so that if one proved troublesome, he could always turn his attention to another. He is content to enunciate a truth, and to call it probable. 'Probably', he says, in discussing engines of small weight and high power, 'a much cheaper engine of this sort might be produced by a gas-light apparatus and by firing the inflammable air generated with a due portion of common air under a piston.' This is an exact forecast of the engine used to-day in all flying machines. He has some good remarks on the shape that offers least resistance to the air in passing through it, that is, on the doctrine of the streamline. He knew that the shape of the hinder part of a solid body which travels through the air is of as much importance as the shape of the fore-part in diminishing resistance. He does not seem to have known that it is of more importance. He knew that the resistance of the air acting on concave wings, or planes, at a small angle of incidence was resolved chiefly into lift, and he suspected that the amount of the lift was greater than the mathematical theory of his day allowed. Above all, his treatise is stimulating, and suggests further inquiry and experiment along lines which have since proved to be the right lines.
Cayley's ideas were developed in practice by John Stringfellow, a manufacturer of lace machinery at Chard, in Somersetshire, and by his friend W. S. Henson, a young engineer. They constructed a light steam-engine, and designed an aeroplane, of which they entertained such high hopes that they took out a patent, and applied to Parliament for an Act to incorporate an Aerial Steam Transit Company. The reaction of public opinion on their proposals took the form of drag rather than lift, and they were thrown back on their own resources. In 1847 they made a model aeroplane, twenty feet in span, driven by two four-bladed airscrews, three feet in diameter, and they experimented with it on Bala Down, near Chard. It did not fly. Henson, completely discouraged, married and went to America; Stringfellow persisted, and in 1848 made a smaller model, ten feet in span, with airscrews sixteen inches in diameter. This machine, which had wings slightly cambered, with a rigid leading edge and a flexible trailing edge, made several successful flights, first in a long covered room at Chard, and later, before a number of witnesses, at Cremorne Gardens. After this success Stringfellow did no more for many years, until the foundation of the Aeronautical Society of Great Britain in 1866 roused him again to activity. At the society's exhibition of 1868, held in the Crystal Palace, he produced a model triplane, which ran along suspended from a wire, and, when its engine was in action, lifted itself as it ran.
The foundation of the Aeronautical Society, with the Duke of Argyll as president and with a council of men of science, attracted fresh minds to the study of flight, and gave the subject a respectable standing. Mr. Wenham's paper, read to the society on the 27th of June 1866, proved that the effective sustaining area of a wing is limited to a narrow portion behind the leading edge; that, in order to increase this area, the planes of a flying machine might advantageously be placed one above another—an idea which was borrowed and put into practice by Mr. Stringfellow in his triplane—and that a heavy body, supported on planes, requires less power to drive it through the air at a high speed than to maintain it in flight at a low speed. For some years the society flourished; then its energies declined, and it fell into a state of suspended animation. At its second exhibition, in 1885, there were only sixteen exhibits as against seventy-eight at the exhibition of 1868. The prospects of practical success seemed remoter than ever. At last, thirty years after its foundation, it sprang into renewed activity, and, with Major B. F. S. Baden-Powell as secretary, did an immense work, from 1897 onwards, in directing and furthering the study of aviation. The Aeronautical Journal, which was published quarterly by the revived society, is a record of the years of progress and triumph.
The cause of this sudden revival is to be sought in the extraordinary fermentation which had been going on under the surface, both in Europe and America. The public was careless and sceptical; inventors who were seeking practical success were shy of premature publicity; papers read to learned societies were more concerned with theory than with practice; but there was hope in the air, and hundreds of minds were independently at work on the problem of flight. Some idea of the variety of suggestions and devices may be gathered from Mr. Octave Chanute's Progress in Flying Machines, a reprint of a series of articles by him, which appeared, from 1891 onwards, in The Railroad and Engineering Journal of New York City. It was said in the ancient world that there is nothing so absurd but some philosopher has believed it; there is no imaginable way of flight that has not engaged the time and effort of some inventor. Yet among the multitude of attempts it is not difficult to trace the ancestry of the modern flying machine. Wing-flapping machines left no issue. Machines supported in the air by helicopters, that is, by horizontal revolving blades, can be made to rise from the ground, but cannot easily be made to travel. The way to success was by imitation of soaring birds; and it is worthy of note that some of the best minds were, from the first, fascinated by this method of flight, and were never tired of observing it. Cayley remarks that the swift, though it is a powerful flyer, is not able to elevate itself from level ground. Wenham records how an eagle, sitting in solitary state in the midst of the Egyptian plain, was fired at with a shotgun, and had to run full twenty yards, digging its talons into the soil, before it could raise itself into the air. M. Mouillard, of Cairo, spent more than thirty years in watching the flight of soaring birds, and devoted the whole of his book, L'Empire de l'Air (1881), to the investigation of soaring flight. The pelican, the turkey-buzzard, the vulture, the condor, have all had their students and disciples. M. Mouillard, indeed, maintains that if there be a moderate wind, a bird can remain a whole day soaring in the air, with no expenditure of power whatever. To those who have watched seagulls this may perhaps seem credible; but air is invisible, and soaring birds are skilful to choose a place, in the wake of a ship or in the neighbourhood of a cliff, where there is an up-current of air, so that when they glide by their own weight, though they are losing height in relation to the air, they are losing none in relation to the surface of the earth.
The parents of the modern flying machine were the gliders, that is, the men who launched themselves into the air on wings or planes of their own devising. The scientific investigators, who experimented with machines embodying the same principle, did much to assist the gliders, but in justice they must take a second place. The men who staked their lives were the men who, after many losses, were rewarded with the conquest of the air. There are stories of a certain Captain Lebris, how in 1854, near Douarnenez in Brittany, he constructed an artificial albatross, and tying it by a slip rope to a cart which was driven against the wind, mounted in it to a height of three hundred feet. But the first glider of whom we have any full knowledge is Otto Lilienthal of Berlin. He devoted his whole life to the study of aviation at a time when in Germany people looked upon such a pursuit as little better than lunacy. The principal professor of mathematics at the Berlin Gewerbe Academie, on hearing that Lilienthal was experimenting with aeronautics, advised him to spend no money on such things—a piece of advice which, Lilienthal remarks, was unhappily quite superfluous. In 1889 he completed, with the help of his brother, a series of experiments on the carrying capacity of arched, or cambered, wings, and published the results in a book entitled Bird Flight as the Basis of Aviation. In his youth every crow that flew by presented him with a problem to solve in its slowly moving wings. Prolonged study led him to the conclusion that the slight fore-and-aft curvature of the wing was the secret of flying. But he knew too much to suppose that this conclusion solved the problem. A dozen other difficulties, including the difficulty of balance, remained to be mastered. When German societies for the advancement of aerial navigation began to be formed, he at first held aloof from them, for the balloon, which he regarded as the chief obstacle to the development of flight, monopolized their entire attention. His insistence on the cambered wing did not convince others, who went on experimenting with flat planes. German and Austrian aviators, it is true, were induced by his book to put aside flat surfaces and introduce arched wings. 'However,' he remarks, 'as this was done mainly on paper, in projects, and in aeronautical papers and discussions, I felt impelled myself to carry out my theory in practice.' So, in the summer of 1891, on a pair of bird-like wings, with eighty-six square feet of supporting surface, stabilized by a horizontal tail and a vertical fin aft, he began his gliding experiments. His whole apparatus, made of peeled willow sticks, covered with cotton shirting, weighed less than forty pounds. He was supported in it wholly on his forearms, which passed through padded tubes, while his hands grasped a cross-bar. He guided the machine and preserved its balance by shifting his weight, backwards or forwards or sideways. In this apparatus, altered and improved from time to time, Lilienthal, during the next five years, made more than two thousand successful glides. At first he used to jump off a spring-board; then he practised on some hills in the suburbs of Berlin; then, in the spring of 1894, he built a conical hill at Gross-Lichterfelde to serve him as a starting-ground. Later on, he moved to the Rhinow hills. His best glides were made against a light breeze at a gradient of about 1 in 10; and he could easily travel a hundred yards through the air. 'Regulating the centre of gravity', he says, 'becomes a second nature, like balancing on a bicycle; it is entirely a matter of practice and experience.' His most alarming experiences were from gusts of wind which would suddenly raise him many metres in the air and suspend him in a stationary position. But his skill was so great that he always succeeded in resuming his flight and alighting safely. He continued to improve and develop his machine. He made a double-surface glider, on the biplane principle, and flew on it. He experimented with engines, intended to flap the extremities of the wings—first a steam-engine of two horse-power, weighing forty-four pounds, then a simpler and lighter type, worked by compressed carbonic acid gas. But he explains that these can be safely introduced only if they do not impair the gliding efficiency of the machine, and he does not seem to have made much progress with them. His last improvement was a movable horizontal tail, or elevator, worked by a line attached to his head, to control the fore-and-aft balance of the machine. This fresh complexity was perhaps the cause of his death. On the 9th of August 1896 he started on a long glide from a hill about a hundred feet high; a sudden gust of wind caught him, and it is supposed that the involuntary movements of his head in the effort to regain his balance made matters worse; the machine plunged to the ground, and he was fatally injured.
Lilienthal was a good mathematician, a careful recorder of the results of his experiments, and a disinterested student of nature. Complete success was denied to him, but his work informed and stimulated others. The Wright brothers, when they first took up the problem of flight, had the advantage of acquaintance with Professor S. P. Langley's aeronautical researches, but their gliding experiments were shaped and inspired by what they had read of Lilienthal's achievements.
The other pioneer, who has earned a place beside Lilienthal, is Percy Pilcher. In 1893, at the age of twenty-seven, he became assistant lecturer in naval architecture and marine engineering at Glasgow University. He devoted all his spare time to aeronautics, and in 1895 built his first glider, which he named 'The Bat'. The machine was built, with the help of his sister, in the sitting-room of their lodging in Kersland street, Glasgow, and was tested on the banks of the Clyde, near Cardross. Some defects were revealed by the tests; when these were remedied, and the glider was towed by a rope, Pilcher rose to a height of twenty feet, and remained in the air for nearly one minute. Thereafter he built, in rapid succession, three new gliders, all of different design, which he called 'The Beetle', 'The Gull', and 'The Hawk'. The professor of naval architecture at Glasgow, Sir John Biles, says of him, 'He was one of the few men I have met who had no sense of fear.... I was deterred from helping him as much as I ought to have done by a fear of the risks that he ran. He at one time talked to Lord Kelvin about helping him: Lord Kelvin spoke to me about it, and said that on no account would he help him, nor should I, as he would certainly break his neck. This was unfortunately too true a prophecy.' The Hawk was the best of his gliders; at Eynsford in Kent, on the 19th of June 1897, he made a perfectly balanced glide of 250 yards across a deep valley, towed only by a thin fishing line, 'which one could break with one's hands'. After this, Pilcher began to make plans for fitting an engine to his glider. Since the first appearance of the Otto engine in 1876, and of the Daimler engine eight years later, the oil-engine had steadily developed in lightness and power, but no engine exactly suitable for his purpose was on the market, so he resolved to build one. An engine of four horse-power, weighing forty pounds, with a wooden airscrew five feet in diameter, was, by his calculations, amply sufficient to maintain his glider in horizontal flight. The light engine has now been so enormously improved, that it comes near to developing one horse-power for every pound of weight. The violent have taken the kingdom of the air by force: in Pilcher's day the problem was more delicate. He worked at his engine in his leisure time, and, leaving the firm of Maxim & Nordenfeldt, by whom he had been employed from 1896 onwards, made, in 1898, his own firm of Wilson & Pilcher. In the spring of 1899 he was much impressed by Mr. Laurence Hargrave's soaring kites, exhibited by the inventor at a meeting of the Aeronautical Society, and it seems that he embodied some of Mr. Hargrave's ideas in his latest built machine, a triplane. He intended to fly this machine at Stanford Hall, Market Harborough, where he was staying with Lord Braye, but on the day appointed, the 30th of September 1899, the weather proved too wet. Nevertheless Pilcher consented to give some demonstrations on The Hawk, towed by a light line; during the second of these, while he was soaring at a height of thirty feet, one of the guy-wires of the tail broke, and the machine turned over and crashed. Pilcher never recovered consciousness, and died two days later. His name will always be remembered in the history of flight. If he had survived his risks for a year or two more, it seems not unlikely that he would have been the first man to navigate the air on a power-driven machine. He left behind him his gallant example, and some advances in design, for he improved the balance of the machine by raising its centre of gravity, and he provided it with wheels, fitted on shock-absorbers, for taking off and alighting.
Lilienthal and Pilcher are pre-eminent among the early gliders, for their efforts were scientific, continuous, and progressive. But there were others; and it is difficult, if not impossible, to determine the comparative value of experiments carried on, many of them in private, by inventors of all countries. Professor J. J. Montgomery, of California, carried out some successful glides, on machines of his own devising, as early as 1884; and Mr. Octave Chanute, the best historian of all these early efforts, having secured the services of Mr. A. M. Herring, a much younger man who had already learned to use a Lilienthal machine, made a series of experiments, with gliders of old and new types, on the shores of Lake Michigan, during the summer of 1896. About the same time some power-driven machines, attached to prepared tracks, were successfully flown. In 1893 Horatio Phillips flew a model, with many planes arranged one above another like a Venetian blind, on a circular track at Harrow; and in the same year Sir Hiram Maxim's large machine, with four thousand feet of supporting surface, was built at Baldwin's Park in Kent, and, when it was tested, developed so great a lift that it broke the guide rails placed to restrain it. Clement Ader, a French electrical engineer, worked at the problem of flight for many years, and, having obtained the support of the French Government, constructed a large bat-like machine, driven by a steam-engine of forty horse-power. In 1897 this machine was secretly tried, at the military camp of Satory, near Paris, and was reported on by a Government commission; all that was known thereafter was that the Government had refused to advance further funds, and that Ader had abandoned his attempts. When the Wrights had made their successful flights, a legend of earlier flights by Ader grew up in France; a heated controversy ensued, and the friends of M. Santos Dumont, who claimed that he was the first to fly over French soil, at length induced the French Government to publish the report on the trial of Ader's machine. The report proved that the machine had not left the ground.