On the fifth of June, 1783, the Montgolfiers' hot-air balloon rose at Versailles, and in its rising divided the study of the conquest of the air into two definite parts, the one being concerned with the propulsion of gas lifted, lighter than air vehicles, and the other being crystallized in one sentence by Sir George Cayley: 'The whole problem,' he stated, 'is confined within these limits, viz.: to make a surface support a given weight by the application of power to the resistance of the air.' For about ten years the balloon held the field entirely, being regarded as the only solution of the problem of flight that man could ever compass. So definite for a time was this view on the eastern side of the Channel that for some years practically all the progress that was made in the development of power-driven planes was made in Britain.
In 1800 a certain Dr Thomas Young demonstrated that certain curved surfaces suspended by a thread moved into and not away from a horizontal current of air, but the demonstration, which approaches perilously near to perpetual motion if the current be truly horizontal, has never been successfully repeated, so that there is more than a suspicion that Young's air current was NOT horizontal. Others had made and were making experiments on the resistance offered to the air by flat surfaces, when Cayley came to study and record, earning such a place among the pioneers as to win the title of 'father of British aeronautics.'
Cayley was a man in advance of his time, in many ways. Of independent means, he made the grand tour, which was considered necessary to the education of every young man of position, and during this excursion, he was more engaged in studies of a semi-scientific character than in the pursuits that normally filled such a period. His various writings prove that throughout his life aeronautics was the foremost subject in his mind; the Mechanic's Magazine, Nicholson's Journal, the Philosophical Magazine, and other periodicals of like nature bear witness to Cayley's continued research into the subject of flight. He approached the subject after the manner of the trained scientist, analysing the mechanical properties of air under chemical and physical action. Then he set to work to ascertain the power necessary for aerial flight, and was one of the first to enunciate the fallacy of the hopes of successful flight by means of the steam engine of those days, owing to the fact that it was impossible to obtain a given power with a given weight.
Yet his conclusions on this point were not altogether negative, for as early as 1810 he stated that he could construct a balloon, which could travel with passengers at 20 miles, an hour he was one of the first to consider the possibilities of applying power to a balloon. Nearly thirty years later in 1837 he made the first attempt at establishing an aeronautical society, but at that time the power-driven plane was regarded by the great majority as an absurd dream of more or less mad inventors, while ballooning ranked on about the same level as tight rope walking, being considered an adjunct to fairs and fetes, more a pastime than a study.
Up to the time of his death, in 1857, Cayley maintained his study of aeronautical matters, and there is no doubt whatever that his work went far in assisting the solution of the problem of air conquest. His principal published work, a monograph entitled Aerial Navigation, has been republished in the admirable series of 'Aeronautical Classics' issued by the Royal Aeronautical Society. He began this work by pointing out the impossibility of flying by means of attached wings, an impossibility due to the fact that, while the pectoral muscles of a bird account for more than two-thirds of its whole muscular strength, in a man the muscles available for flying, no matter what mechanism might be used, would not exceed one-tenth of his total strength.
Cayley did not actually deny the possibility of a man flying by muscular effort, however, but stated that 'the flight of a strong man by great muscular exertion, though a curious and interesting circumstance, inasmuch as it will probably be the means of ascertaining finis power and supplying the basis whereon to improve it, would be of little use.'
From this he goes on to the possibility of using a Boulton and Watt steam engine to develop the power necessary for flight, and in this, he saw a possibility of practical result. It is worthy of note that in this connection he made mention of the forerunner of the modern internal combustion engine; 'The French,' he said, 'have lately shown the great power produced by igniting inflammable powders in closed vessels, and several years ago an engine was made to work in this country in a similar manner by inflammation of spirit of tar.' In a subsequent paragraph of his monograph he anticipates almost exactly the construction of the Lenoir gas engine, which came into being more than fifty-five years after his monograph was published.
Certain experiments detailed in his work were made to ascertain the size of the surface necessary for the support of any given weight. He accepted a truism of to day in pointing out that in any matters connected with aerial investigation, theory and practice are as widely apart as the poles. Inclined at first to favour the helicopter principle, he finally rejected this in favour of the plane, with which he made numerous experiments. During these, he ascertained the peculiar advantages of curved surfaces, and saw the necessity of providing both vertical and horizontal rudders in order to admit of side steering as well as the control of ascent and descent, and for preserving equilibrium. He may be said to have anticipated the work of Lilienthal and Pilcher, since he constructed and experimented with a fixed surface glider. 'It was beautiful,' he wrote concerning this, 'to see this noble white bird sailing majestically from the top of a hill to any given point of the plain below it with perfect steadiness and safety, according to the set of its rudder, merely by its own weight, descending at an angle of about eight degrees with the horizon.'
It is said that he once persuaded his gardener to trust himself in this glider for a flight, but if Cayley himself ventured a flight in it, he has left no record of the fact. The following extract from his work, Aerial Navigation, affords an instance of the thoroughness of his investigations, and the concluding paragraph also shows his faith in the ultimate triumph of mankind in the matter of aerial flight: --
'The act of flying requires less exertion than from the appearance is supposed. Not having sufficient data to ascertain the exact degree of propelling power exerted by birds in the act of flying, it is uncertain what degree of energy may be required in this respect for vessels of aerial navigation; yet when we consider the many hundreds of miles of continued flight exerted by birds of passage, the idea of its being only a small effort is greatly corroborated. To apply the power of the first mover to the greatest advantage in producing this effect is a very material point. The mode universally adopted by Nature is the oblique waft of the wing. We have only to choose between the direct beat overtaking the velocity of the current, like the oar of a boat, or one applied like the wing, in some assigned degree of obliquity to it. Suppose 35 feet per second to be the velocity of an aerial vehicle, the oar must be moved with this speed previous to its being able to receive any resistance; then if it be only required to obtain a pressure of one-tenth of a lb. upon each square foot it must exceed the velocity of the
Current 7.3 feet per second. Hence, its whole velocity must be 42.5 feet per second. Should the same surface be wafted downward like a wing with the hinder edge inclined upward in an angle of about 50 deg. 40 feet to the current it will overtake it at a velocity of 3.5 feet per second; and as a slight unknown angle of resistance generates a lb. pressure per square foot at this velocity, probably a waft of a little more than 4 feet per second would produce this effect, one tenth part of which would be the propelling power. The advantage of this mode of application compared with the former is rather more than ten to one.
'In continuing the general principles of aerial navigation, for the practice of the art, many mechanical difficulties present themselves which require a considerable course of skillfully applied experiments before they can be overcome; but, to a certain extent, the air has already been made navigable, and no one who has seen the steadiness with which weights to the amount of ten stone (including four stone, the weight of the machine) hover in the air can doubt of the ultimate accomplishment of this object.'
This extract from his work gives but a faint idea of the amount of research for which Cayley was responsible. He had the humility of the true investigator in scientific problems, and so far as can be seen was never guilty of the great fault of so many investigators in this subject that of making claims which he could not support. He was content to do, and pass after having recorded his part, and although nearly half a century had to pass between the time of his death and the first actual flight by means of power-driven planes, yet he may be said to have contributed very largely to the solution of the problem, and his name will always rank high in the roll of the pioneers of flight.
Practically contemporary with Cayley was Thomas Walker, concerning whom little is known save that he was a portrait painter of Hull, where was published his pamphlet on The Art of Flying in 1810, a second and amplified edition being produced, also in Hull, in 1831. The pamphlet, which has been reproduced in extenso in the Aeronautical Classics series published by the Royal Aeronautical Society, displays a curious mixture of the true scientific spirit and colossal conceit. Walker appears to have been a man inclined to jump to conclusions, which carried him up to the edge of discovery and left him vacillating there.
The study of the two editions of his pamphlet side by side shows that their author made considerable advances in the practicability of his designs in the 21 intervening years, though the drawings, which accompany the text in both editions, fail to show anything really capable of flight. The great point about Walker's work as a whole is its suggestiveness; he did not hesitate to state that the 'art' of flying is as truly mechanical as that of rowing a boat, and he had some conception of the necessary mechanism, together with an absolute conviction that he knew all there was to be known. 'Encouraged by the public,' he says, 'I would not abandon my purpose of making still further exertions to advance and complete an art, the discovery of the TRUE PRINCIPLES (the italics are Walker's own) of which, I trust, I can with certainty affirm to be my own.'
The pamphlet begins with Walker's admiration of the mechanism of flight as displayed by birds. 'It is now almost twenty years,' he says, 'since I was first led to think, by the study of birds and their means of flying, that if an artificial machine were formed with wings in exact imitation of the mechanism of one of those beautiful living machines, and applied in the very same way upon the air, there could be no doubt of its being made to fly, for it is an axiom in philosophy that the same cause will ever produce the same effect.' With this, he confesses his inability to produce the said effect through lack of funds, though he clothes this delicately in the phrase 'professional avocations and other circumstances.’ Owing to this inability, he published his designs that others might take advantage of them, prefacing his own researches with a list of the very early pioneers, and giving special mention to Friar Bacon, Bishop Wilkins, and the Portuguese friar, De Guzman. But, although he seems to suggest that, others should avail themselves of his theoretical knowledge, there is a curious incompleteness about the designs accompanying his work, and about the work itself, which seems to suggest that he had more knowledge to impart than he chose to make public or else that he came very near to complete solution of the problem of flight, and stayed on the threshold without knowing it.
After a dissertation upon the history and strength of the condor, and on the differences between the weights of birds, he says: 'The following observations upon the wonderful difference in the weight of some birds, with their apparent means of supporting it in their flight, may tend to remove some prejudices against my plan from the minds of some of my readers. The weight of the humming-bird is one drachm that of the condor not less than four stone. Now, if we reduce four stone into drachms we shall find the condor is 14,336 times as heavy as the humming-bird. What an amazing disproportion of weight! Yet, by the same mechanical use of its wings the condor can overcome the specific gravity of its body with as much ease as the little humming-bird. But this is not all. We are informed that this enormous bird possesses a power in its wings, so far exceeding what is necessary for its own conveyance through the air, that it can take up and fly away with a whole sheer in its talons, with as much ease as an eagle would carry off, in the same manner, a hare or a rabbit. This we may readily give credit to, from the known fact of our little kestrel and the sparrow-hawk frequently flying off with a partridge, which is nearly three times the weight of these rapacious little birds.'
After a few more observations he arrives at the following conclusion: 'By attending to the progressive increase in the weight of birds, from the delicate little humming-bird up to the huge condor, we clearly discover that the addition of a few ounces, pounds, or stones, is no obstacle to the art of flying; the specific weight of birds avails nothing, for by their possessing wings large enough, and sufficient power to work them, they can accomplish the means of flying equally well upon all the various scales and dimensions which we see in nature. Such being a fact, in the name of reason and philosophy why shall not man, with a pair of artificial wings, large enough, and with sufficient power to strike them upon the air, be able to produce the same effect?'
Walker asserted definitely and with good ground that muscular effort applied without mechanism is insufficient for human flight, but he states that if an aeronautical boat were constructed so that a man could sit in it in the same manner as when rowing, such a man would be able to bring into play his whole bodily strength for the purpose of flight, and at the same time would be able to get an additional advantage by exerting his strength upon a lever. At first, he concluded there must be
Expansion of wings large enough to resist in a sufficient degree the specific gravity of whatever is attached to them, but in the second edition of his work, he altered this to 'expansion of flat passive surfaces large enough to reduce the force of gravity so as to float the machine upon the air with the man in it.' The second requisite is strength enough to strike the wings with sufficient force to complete the buoyancy and give a projectile motion to the machine. Given these two requisites, Walker states definitely that flying must be accomplished simply by muscular exertion. 'If we are secure of these two requisites, and I am very confident we are, we may calculate upon the success of flight with as much certainty as upon our walking.'
Walker appears to have gained some confidence from the experiments of a certain M. Degen, a watchmaker of Vienna, who, according to the Monthly Magazine of September 1809, invented a machine by means of which a person might raise himself into the air. The said machine, according to the magazine, was formed of two parachutes, which might be folded up or extended at pleasure, while the person who worked them was placed in the centre. This account, however, was rather misleading, for the magazine carefully avoided mention of a balloon to which the inventor fixed his wings or parachutes. Walker, knowing nothing of the balloon, concluded that Degen actually raised himself in the air, though he is doubtful of the assertion that Degen managed to fly in various directions, especially against the wind.
Walker, after considering Degen and all his works, proceeds to detail his own directions for the construction of a flying machine, these being as follows: 'Make a car of as light material as possible, but with sufficient strength to support a man in it; provide a pair of wings about four feet each in length; let them be horizontally expanded and fastened upon the top edge of each side of the car, with two joints each, so as to admit of a vertical motion to the wings, which motion may be effected by a man sitting and working an upright lever in the middle of the car. Extend in the front of the car a flat surface of silk, which must be stretched out and kept fixed in a passive state; there must be the same fixed behind the car; these two surfaces must be perfectly equal in length and breadth and large enough to cover a sufficient quantity of air to support the whole weight as nearly in equilibrium as possible, thus we shall have a great sustaining power in those passive surfaces and the active wings will propel the car forward.'
A description of how to launch this car is subsequently given: 'It becomes necessary,' says the theorist, 'that I should give directions how it may be launched upon the air, which may be done by various means; perhaps the following method may be found to answer as well as any: Fix a poll upright in the earth, about twenty feet in height, with two open collars to admit another poll to slide upwards through them; let there be a sliding platform made fast upon the top of the sliding poll; place the car with a man in it upon the platform, then raise the platform to the height of about thirty feet by means of the sliding poll, let the sliding poll and platform suddenly fall down, the car will then be left upon the air, and by its pressing the air a projectile force will instantly propel the car forward; the man in the car must then strike the active wings briskly upon the air, which will so increase the projectile force as to become superior to the force of gravitation, and if he inclines his weight a little backward, the projectile impulse will drive the car forward in an ascending direction. When the car is brought to a sufficient altitude to clear the tops of hills, trees, buildings, etc., the man, by sitting a little forward on his seat, will then bring the wings upon a horizontal plane, and by continuing the action of the wings he will be impelled forward in that direction. To descend, he must desist from striking the wings, and hold them on a level with their joints; the car will then gradually come down, and when it is within five or six feet of the ground the man must instantly strike the wings downwards, and sit as far back as he can; he will by this means check the projectile force, and cause the car to alight very gently with a retrograde motion. The car, when up in the air, may be made to turn to the right or to the left by forcing out one of the fins, having one about eighteen inches long placed vertically on each side of the car for that purpose, or perhaps merely by the man inclining the weight of his body to one side.'
Having stated how the thing is to be done, Walker is careful to explain that when it is done there will be in it some practical use, notably in respect of the conveyance of mails and newspapers, or the saving of life at sea, or for exploration, etc. It might even reduce the number of horses kept by man for his use, by means of which a large amount of land might be set free for the growth of food for human consumption.
At the end of his work, Walker admits the idea of steam power for driving a flying machine in place of simple human exertion, but he, like Cayley, saw a drawback to this in the weight of the necessary engine. On the whole, he concluded, navigation of the air by means of engine power would be mostly confined to the construction of navigable balloons.
As already noted, Walker's work is not over practical, and the foregoing extract includes the most practical part of it; the rest is a series of dissertations on bird flight, in which, evidently, the portrait painter's observations were far less thorough than those of da Vinci or Borelli. Taken on the whole, Walker was a man with a hobby; he devoted to it much time and thought, but it remained a hobby, nevertheless. His observations have proved useful enough to give him a place among the early students of flight, but a great drawback to his work is the lack of practical experiment, by means of which alone real advance could be made; for, as Cayley admitted, theory and practice are very widely separated in the study of aviation, and the whole history of flight is a matter of unexpected results arising from scarcely foreseen causes, together with experiment as patient as daring.