THAT depends, first of all, on whether we are to make the requisite scientific discoveries. To do this we must penetrate a field of thought which Nature has hitherto held sacred from the tread of the most adventurous scientific explorer. What the human mind has been able to grasp belongs to a middle class of things between the infinitely great and the infinitely small. The universe made known to us by the telescope requires so many eons to go through a single stage of its growth that its origin and destiny are alike incomprehensible to a being who can observe it but for a few fleeting years. Its vastness defies comprehension and eludes investigation. The microscope has made known to us how active and busy a world may be bounded by the surface of a single drop of water, in whose crystal purity the unaided eye cannot distinguish a speck. But the power of the microscope has a limit set by the very nature of light itself. Far below that limit, within a single cell so minute as almost to elude vision, even with the most powerful microscope, are the myriad molecules of matter. We have evidence that a single one of these, so minute that its individual existence could never be made known to us by any process whatever, is a mechanism whose complexity evades description-- a seat of activity going through cycles of change millions of times in the millionth of a second.

Between these extremes lie two connecting links,­invisible bonds, making known their existence to our universal experience, and yet evading investigation, as do the infinitely great and the infinitely small. They are the luminiferous ether and the force of gravitation. The former, invisible and imperceptible as the optic nerve is imperceptible to its own sight, fills all space. And yet, its minutest parts are susceptible to the vibrations of light which number hundreds of millions of millions in a single second and are propagated with such speed as to fly to the moon in two beats of the clock. Gravitation binds every separate molecule of matter on the earth to every molecule on the planets and every molecule in the most distant star. Yet up to the present time the profoundest philosopher knows no more about its why and wherefore, if why and wherefore it has, than the child that knows it will fall to the ground if its foot slips.

It goes without saying that the science of to-day is not satisfied to accept any of these limitations longer than it is forced to do so. It is battering at every gate which Nature has closed against the entrance of its forces: Well knowing that the eye of man is never to see a molecule of matter, it is nevertheless investigating the phenomena associated with it, determined, if possible, to penetrate the mystery of its constitution. It is seeking to discover the cause of gravitation, the force which, coextensive with ether itself, may be in close association with it. From time to time philosophers fancy the road open to success, yet nothing that can be practically called success has yet been reached or even approached. When it is reached,­when we are able to state exactly why matter gravitates, then will arise the question how this hitherto unchangeable force may be controlled and regulated. With this question answered the problem of the interaction between ether and matter may be solved. That interaction goes on between ethers and molecules is shown by the radiation of heat by all bodies. When the molecules are combined into a mass, this interaction ceases, so that the lightest objects fly through the ether without resistance. Why is this? Why does ether act on the molecule and not the mass? When we can produce the latter, and when the mutual action can be controlled, then may gravitation be overcome and then may men build, not merely airships, but ships which shall fly above the air, and transport their passengers from continent to continent with the speed of the celestial motions.

The first question suggested to the reader by these considerations is whether any such result is possible; whether it is within the power of man to discover the nature of luminiferous ether and the cause of gravitation. To this the profoundest philosopher can only answer, "I do not know." Quite possibly the gates at which he is beating are, in the very nature of things, incapable of being opened. It may be that the mind of man is incapable of grasping the secrets within them. The question has even occurred to me whether, if a being of such supernatural power as to understand the operations going on in a molecule of matter or in a current of electricity as we understand the operations of a steam engine should essay to explain them to us, he would meet with any more success than we should in explaining to a fish the engines of a ship which so rudely invades its domain. As was remarked by William K. Clifford, perhaps the clearest spirit that has ever studied such problems, it is possible that the laws of geometry for spaces infinitely small may be so different from those of larger spaces that we must necessarily be unable to conceive them.

Let us now take up the question from a more immediately practical point of view. Can we decide whether the airship is or is not possible simply as a triumph of invention, unaided by any such revolutionary discovery as that we have suggested? If I should answer no, I should be at once charged with setting limits to the powers of invention, and have held before my eyes, as a warning example, the names of more than one philosopher who has declared things impossible which were afterward brought to pass. Instead of answering yes or no, I shall ask the reader to bear with me while I point out some general features of the progress of science and invention.

Invention and discovery have, notwithstanding their seemingly wide extent, gone on in rather narrower lines than is commonly supposed. If, a hundred years ago, the most sagacious of mortals had been told that before the nineteenth century closed the face of the earth would be changed, time and space almost annihilated, and communication between continents made more rapid and easy than it was between cities in his time; and if he had been asked to exercise his wildest imagination in depicting what might come­the airship and the flying machine would probably have had a prominent place in his scheme, but neither the steamship, the railway, the telegraph, nor the telephone would have been there. Probably not a single new agency which he could have imagined would be one that has come to pass.

For thousands of years mathematicians vainly grappled with three problems: to describe a square which should be equal in area to a given circle, to trisect an angle, to construct a cube having double the solid contents of a given cube. Vastly more complex problems had been solved, why should these evade their powers? When such advances in mathematical thought and methods were reached that the possibility of a solution could be inquired into, the answer was a negative one. That none of these problems could be solved was demonstrated by a process as rigorous, though not so accessible to the ordinary mind, as any proposition in Euclid. But this did not mean that mathematics had ceased to advance. The very demonstration of the impossibility was a triumph second only to what the solution itself would have been.

Great indeed have been the advances in our knowledge of the heavens. And yet, any inquirer can ask the astronomer questions to which he can only answer by avowing his ignorance. He has discovered revolving around the stars worlds which must remain forever invisible, even to the telescopic eye, and can tell what gases are bursting out from some blazing object in the most distant regions of the universe, as readily as a chemist can tell the composition of the most ordinary substance. And yet he remains dumb when asked about the surface of Jupiter, or called upon to tell the inquirer whether Mars is inhabited.

It is so with invention. The distinction between the possible and the impossible is not clear. A useful result may look entirely feasible on such consideration as we can give it, when, if we inquire into the case, we should see an absurdity in expecting it. Not many years ago the public was so much interested in the question of making it rain that Congress provided means to send a party all the way to Texas to see if rain could be brought down by bombarding the skies with dynamite bombs. The incredulous scientists who declared the attempt absurd were held up to ridicule by ardent spirits, while men of more caution held that the experiment was worthy of a trial, even if the chances of success were small.

Now, compare this problem with another, quite similar in principle. Every man who favored an attempt to bring down rain would have ridiculed a proposal to make it high tide in New York Harbor by blowing up the water with dynamite whenever a great ship had to go to sea. Why expect the one and not the other ? The answer would be that the one attempt was simply ridiculous while the other was not. No hydrographer would ever be expected to change the course of the Gulf Stream, or to vary its temperature. But it only needs a wide grasp of the subject to see that the problem of bringing to New York from some vaporous region an air current which shall deposit its moisture on an arid field is of the same kind as the problem of changing the Gulf Stream, or bringing a tidal wave into New York Harbor. Nor is it evident that to expect the air to condense its moisture without the necessary conditions being produced would be like expecting an engine to run without fuel.

No builder of air castles for the amusement and benefit of humanity could have failed to include a flying machine among the productions of his imagination. The desire to fly like a bird is inborn in our race, and we can no more be expected to abandon the idea than the ancient mathematician could have been expected to give up the problem of squaring the circle. The lesson which we draw from this general review of progress is that we cannot conclude that because the genius of the nineteenth century has opened up such wonders as it has, therefore the twentieth is to give us the airship. But even granting the abstract possibility of the flying machine or the airship, we are still met with the question of its usefulness as a means of international communication. It would, of course, be very pleasant for a Bostonian who wished to visit New York to take out his wings from the corner of his vestibule, mount them, and fly to the Metropolis. But it is hardly conceivable that he would get there any more quickly or cheaply than he now does by rail.

Another feature incidental to any aërial vehicle is very generally overlooked. In the absence of any such revolutionary discovery as I have pictured in the first part of this article-in the absence of the power to control gravitation­a flying machine could remain in the air only by the action of its machinery, and would fall to the ground like a wounded bird the moment any accident stopped it. With all the improvements that the genius of man has made in the steamship, the greatest and best ever constructed is liable now and then to meet with accident. When this happens she simply floats on the water until the damage is repaired, or help reaches her. Unless we are to suppose for the flying machine, in addition to everything else, an immunity from accident which no human experience leads us to believe possible, it would be liable to derangements of machinery, any one of which would be necessarily fatal. If an engine were necessary not only to propel a ship, but also to make her float--if, on the occasion of any accident she immediately went to the bottom with all on board-there would not, at the present day, be any such thing as steam navigation.

Let us look at the problem, and see what room there is for the airship among the inventions of the future. If we are to have an aërial machine of any kind, it must be one of two principles. Either we must control the law of universal gravitation, as I have already suggested, or the machine must be supported by the air.

Only two systems of air-support seem possible, or have ever been suggested. The vehicle must either float in the air, like a balloon, or it must be supported by the action of the air on moving wings, like a bird when it flies. The conditions of both of these methods can be made the subject of exact investigation. A floating vehicle to carry a certain weight must have a bulk corresponding to the volume of air which shall have this weight. With this bulk it must experience a certain resistance to its passage through the air, which resistance increases at least as the square of the velocity. To overcome this resistance requires a corresponding power to be exerted by an engine of some kind. The engine has weight. The best combination of all these conditions is a problem of applied science, of which the solution depends mainly on the strength and weight of materials. Solve it as we will, our floating ship must have a thousand times the bulk of a railroad train carrying an equal weight and experience a hundred times the resistance that the train does. It therefore seems quite evident that while the problem of a dirigible balloon may be within the power of inventive genius, we cannot hope that it will become a vehicle for carrying passengers and freight under ordinary conditions.

Now let us turn to the other alternative, that of the flying machine. If we can make a model of a bird with its wings, and set the wings in motion like those of a bird with no greater weight, the model will fly like a bird. To do this is, in a certain sense, a problem of nothing but applied mechanics. Yet it has its well-defined limitations. By experiments on the resistance of the air we can compute how large a wing or aëroplane, moving with a certain speed, will be required to support a given weight. We can also determine, or, at least, form some idea of, the power of the engine that will move the apparatus. There must be connecting machinery, by which the engine shall in some way act on the plane. Engine, machinery, and plane must all have a weight proportioned to, or at least increasing with, their size and efficiency. It is then a problem of strength of materials to form a combination in which the ratio of efficiency to weight will be enough to make the machine fly.

In studying the best combination, we meet two difficulties, one of which can be stated in a very simple mathematical form. Let us make two flying machines exactly alike, only make one on double the scale of the other in all its dimensions. We all know that the volume, and therefore the weight of two similar bodies are proportional to the cubes of their dimensions. The cube of two is eight. Hence the large machine will have eight times the weight of the other. But surfaces are as the squares of the dimensions. The square of two is four. The heavier machine will therefore expose only four times the wing surface to the air, and so will have a distinct disadvantage in the ratio of efficiency to weight.

Mechanical principles show that the steam pressures which the engines would bear would be the same, and that the larger engine, though it would have more than four times the horse power of the other, would have less than eight times. The larger of the two machines would therefore be at a disadvantage, which could be overcome only by reducing the thickness of its parts, especially of its wings, to that of the other machine. Then we should lose in strength. It follows that the smaller the machine the greater its advantage, and the smallest possible flying machine will be the first one to be successful.

We see the principle of the cube exemplified in the animal kingdom. The agile flea, the nimble ant, the swift-footed greyhound, and the unwieldy elephant form a series of which the next term would be an animal tottering under its own weight, if able to stand or move at all. The kingdom of flying animals shows a similar gradation. The most numerous fliers are little insects, and the rising series stops with the condor, which, though having much less weight than a man, is said to fly with difficulty when gorged with food.

We have also to consider the advantage which a muscle has over any motor yet discovered, in regard to its flexibility and the versatility of its application. It expands and contracts, pulls and pushes, in a way that no substance yet discovered can be made to do. It is also instantly responsive to a brain which cannot of itself act on external matter.

We may now see the kernel of the difficulty. If we had a metal so rigid, and at the same time so light, that a sheet of it twenty meters square and a millimeter thick would be as stiff as a board and would not weigh more than a ton, and, at the same time, so strong that a powerful engine could be built of it with little weight, we might hope for a flying machine that would carry a man. But as the case stands, the first successful flyer will be the handiwork of a watchmaker, and will carry nothing heavier than an insect. When this is constructed, we shall be able to see whether one a little larger is possible.

The cheapness of modern transportation is another element in the case frequently overlooked. I believe the principal part of the resistance which a limited express train meets is the resistance of the air. This would be as great for an airship as for a train. An important fraction of the cost of transporting goods from Chicago to London is that of getting them into vehicles, whether cars or ships, and getting them out again. The cost of sending a pair of shoes from a shop in New York to the residence of the wearer is, if I mistake not, much greater than the mere cost of transporting it across the Atlantic. I have shown that the construction of an aërial vehicle which could carry even a single man from place to place at pleasure requires the discovery of some new metal or some new force. Even with such a discovery, we could not expect one to do more than carry its owner.

Perhaps the main point I have tried to enforce in this paper is this-the very common and optimistic reply to objections, "We have seen many wonders, therefore nothing is impossible," is not a sound inference from experience when applied to a wonder long sought and never found. I have shown that the obvious and long-studied problems are not those that have been solved. The experience of the past leads us to believe that the progress of the twentieth century will be along lines that no one can anticipate, and will lead to results which, if a prophet could describe, might strike us as more surprising than the airship.

This article appeared in McClure's Magazine, 17, September 1901, pp. 432-435.