was concerned, mechanical flight was theoretically possible with engines we could then build, since I was satisfied that boilers and engines could be constructed to weigh less than twenty pounds to the horse-power, and that one horse-power would, in theory at least, support nearly ten times that if the flight were horizontal. Almost everything, it will be noticed, depends on this, for if the flight is downward it will end at the ground, and if upward the machine will be climbing an invisible hill, with the same or a greater effort than every bicycler experiences with a real one. Speed, then, and this speed expended in a horizontal course, were the first two requisites. This was not saying that a flying-machine could be started from the ground, guided into such flight in any direction, and brought back to earth in safety. There was, then, something more than power needed that is, skill to use it, and the reader should notice the distinction. Hitherto it had always been supposed that it was wholly the lack of mechanical power to fly which made mechanical flight impossible. The first stage of the investigation had shown how much, or rather how little, power was needed in theory for the horizontal flight of a given weight, and the second stage, which was now to be entered upon, was to show first how to procure this power with as little weight as possible, and, having it, how by its means to acquire this horizontal flight in practice —that is, how to acquire the art of flight or how to build a ship that could actually navigate the air. One thing which was made clear by these preliminary experiments, and made clear nearly for the first time, was that if a surface be made to advance rapidly, we secure an essential advantage in our ability to support it. Clearly we want the advance to get from place to place; but it proves also to be the only practicable way of supporting the thing at all, to thus take advantage of the inertia of the air, and this point is so all-important that we will renew an old illustration of it. The idea in a vague sense is as ancient as classical times. Pope says: "Swift Camilla scours the plain, skater who can glide safely over the thinnest ice if the speed is sufficient. Think of a cake of ice of any small size, suppose a foot square. It possesses (like everything else in nature) inertia or resistance to displacement, and this will be less or more according to the mass moved. If the skater stands during a single second upon this small mass it will sink under him until he is perhaps waist-deep in the water, while a cake of the same width but twice the length will yield only about half as readily to his weight. On this he will sink only to his knees, we may suppose, while if we think of another cake ten times as long as the first-that is, one foot wide and ten feet long-we see that on this, during the same second, he will not sink above his feet. This is all plain enough; but now suppose the long cake to be divided into ten distinct portions, then it ought to be equally clear that the skater who glides over the whole in a second, distributes his weight over just as much ice as though all ten were in one solid piece. So it is with the air. Even the viewless air possesses inertia; it cannot be pushed aside without some effort; and while the portion which is directly under the airship would not keep it from falling several yards in the first. second, if the ship goes forward so that it runs or treads on thousands of such portions in that time, it will sink in proportionately less degree; sink, perhaps, only through a fraction of an inch. Speed, then, is indispensable here. Α balloon, like a ship, will float over one spot in safety, but our flying-machine must be in motion to sustain itself, and in motion, in fact, before it can even begin to fly. Perhaps we may more fully understand what is meant by looking at a boy's kite. Every one knows that it is held by a string against the wind which sustains it, and that it falls in a calm. Most of us remember that even in a calm, if we run and draw it along, it will still keep up, for what is required is motion relative to the air, however obtained. It can be obtained without the cord if the same pull is given by an engine and propellers strong enough to draw it, and Flies o'er the unbending corn, and skims along the light enough to be attached to and sus main." Now, is this really so in the sense that a Camilla, by running fast enough, could run over the tops of the corn? If she ran fast enough, yes; but the idea may be shown better by the analogous case of a tained by it. The stronger the pull and the quicker the motion, the heavier the kite may be made. It may be, instead of a sheet of paper, a sheet of metal even, like the plate of brass which has already been mentioned as seeming, when in rapid motion, to float upon the air, and, if it will takes the place of the string. A WING FROM A SOARING BIRD. THE BONES OF A BIRD'S WING AND THE BONES OF A HUMAN ARM, And now having the theory of the flight before us, let us come to the practice. The first thing will be to provide an engine of unprecedented lightness, that is to furnish the power. A few years ago an engine that developed a horse-power, weighed nearly as much as the actual horse did. We have got to begin by trying to make an engine which shall weigh, everything complete, boiler and all, not more than twenty pounds to the horse-power, and preferably less than ten; but even if we have done this very hard thing, we may be said to have only fought our way up to an enormous difficulty, for the next question will be how to use the power it gives so as to get a horizontal flight. We must then consider through what means the power is to be applied when we get it, and whether we shall, for instance, have wings or screws. At first it seems as though Nature must know best, and that since her flying models, birds, are exclusively employing wings, this is the thing for us; but perhaps this is not the case. If we had imitated the horse or the ox, and made the machine which draws our trains walk on legs, we should undoubtedly never have done as well as with the locomotive rolling on wheels; or if we had imitated the whale with its fins, we should not have had so good a boat as we now have in the steamship with the paddle-wheels or the screw, both of which are constructions that Nature never employs. This is so important a point that we will look at the way Nature got her models. Here is a human skeleton, and here one of a bird, drawn to the same scale. Apparently Nature made one THE SKELETON OF A MAN AND THE SKELETON OF A BIRD, DRAWN TO THE SAME SCALE, SHOWING repeats the radius and ulna, or two bones of our own forearm, while our wrist and fingerbones are modified in the bird to carry the feathers, but are still there. To make the bird, then, Nature appears to have taken what material she had in stock, so to speak, and developed it into something that would do. It was all that Nature had to work on, and she has done wonderfully well with such unpromising material; but any one can see that our arms would not be the best thing to make flying-machines out of, and that there is no need of our starting there when we can start with something better and develop that. Flapping wings might be made on other principles, and perhaps will be found in future flying-machines, but the most promising thing to try seemed to me to be the screw propeller. were spent by me on these, it was not possible to learn much about the balancing from them. Thus it appeared that something which could give longer and steadier flights than india-rubber must be used as a motor, even for the preliminary trials, and calculations and experiments were made upon the use of compressed air, carbonic acid gas, electricity in primary and storage batteries, and numerous other contrivances, but all in vain. The gas-engine promised to be best ultimately, but nothing save steam gave any promise of immediate success in supporting a machine which would teach these conditions of flight by actual trial, for all were too heavy, weight being the great enemy. It was true Some twenty years ago, Penaud, a Frenchman, made a toy, consisting of a flat, immovable sustaining wing surface, a flat tail, and a small propelling screw. He made the wing and tail out of paper or silk, and the propeller out of cork and feathers, and it was driven directly by strands of india-rubber twisted lamplighter fashion, and which turned the wheel as they untwisted. PENAUD'S FLYING TOY (ONE-EIGHTH OF ACTUAL SIZE). The great difficulty of the task of creating a flying-machine may be partly understood when it is stated that no machine in the whole history of invention, unless it were this toy of Penaud's, had ever, so far as I can learn, flown for even ten seconds; but something that will actually fly must be had to teach the art of "balancing." When experiments are made with models moving on a whirling table or running on a railroad track, these are forced to move horizontally and at the same time are held so that they cannot turn over; but in free flight there will be nothing to secure this, unless the airship is so adjusted in all its parts that it tends to move steadily and horizontally, and the acquisition of this adjustment or art of "balancing" in the air is an enormously difficult thing, and which, it will be seen later, took years to acquire. My first experiments in it, then, were with models like these, but from them I got only a rude idea how to balance the future aerodrome, partly on account of the brevity of their flight, which only lasted a few seconds, partly on account of its irregularity. Although, then, much time and labor also that the steam-driven model could not be properly constructed until the principal conditions of flight were learned, nor these be learned till the working model was experimented with, so that it seemed that the inventor was shut up in a sort of vicious circle. However, it was necessary to begin in some way, or give up at the outset, and the construction began with a machine to be driven by a steam-engine, through the means of propeller wheels, somewhat like the twin screws of a modern steamship, but placed amidships, not at the stern. There were to be rigid and motionless wings, slightly inclined, like the surface of a kite, and a construction was made on this plan which gave, if much disappointment, a good deal of useful experience. It was intended to make a machine that would weigh twenty or twenty-five pounds, constructed of steel tubes. The engines were made with the best advice to be got (I am not an engineer); but while the boiler was a good deal too heavy, it was still too small to get up steam for the engines, which weighed about four pounds, and could have developed a horse-power if there were steam enough. This machine, which was to be moved by two propelling screws, was labored on for many months, with the result that the weight was constantly increasing beyond the estimate until, before it was done, the whole weighed over forty pounds, and yet could only get steam for about a half horse-power, which, after deductions for loss in transmission, would give not more than half that gain in actual thrust. It was clear that whatever pains it had cost, it must be abandoned. This aerodrome could not then have flown; but having learned from it the formidable difficulty of making such a thing light enough, another was constructed, which was made in the other extreme, with two engines to be driven by compressed air, the whole weighing but five or six pounds. The power proved insufficient. Then came another, with engines to use carbonic-acid gas, which failed from a similar cause. Then followed a small one to be run by steam, which gave some promise of success, but when tried indoors it was found to lift only about one-sixth of its own weight. In each of these the construction of the whole was remodeled to get the greatest strength and lightness combined, but though each was an improvement on its predecessor, it seemed to become more and more doubtful whether it could ever be made sufficiently light, and whether the desired end could be reached at all. The chief obstacle proved to be not with the engines, which were made surprisingly light after sufficient experiment. The great difficulty was to make a boiler of almost no weight which would give steam enough, and this was a most wearying one. There must be also a certain amount of wing surface, and large wings weighed prohibitively; there must be a frame to hold all together, and the frame, if made strong enough, must yet weigh so little that it seemed impossible to make it. These were the difficulties that I still found myself in after two years of experiment, and it seemed at this stage again as if it must, after all, be given up as a hopeless task, for somehow the thing had to be built stronger and lighter yet. Now, in all ordinary construction, as in building a steamboat or a house, engineers have what they call a factor of safety. An iron column, for instance, will be made strong enough to hold five or ten times the weight that is ever going to be put upon it, but if we try any thing of the kind here the construction will be too heavy to fly. Everything in the work has got to be so light as to be on the edge of breaking down and disaster, and when the breakdown comes all we can do is to find what is the weakest part and make that part stronger; and in this way work went on, week by week and month by month, constantly altering the form of construction so as to strengthen the weakest parts, until, to abridge a story which extended over years, it was finally brought nearly to the shape it is now, Aerodrome, from words signifying air-runner, the running over the air being the essence of its plan. where the completed mechanism, furnishing over a horse-power, weighs collectively something less than seven pounds. This does not include water, the amount of which depends on how long we are to run; but the whole thing, as now constructed, boiler, fire-grate, and all that is required to turn out an actual horse-power and more, weighs something less than one one-hundredth part of what the horse himself does. I am here anticipating; but after these first three years something not greatly inferior to this was already reached, and so long ago as that, there had accordingly been secured mechanical power to fly, if that were all-but it is not all. After that came years more of delay arising from other causes, and I can hardly repeat the long story of subsequent disappointment, which commenced with the first attempts at actual flight. Mechanical power to fly was, as I say, obtained three years ago; the machine could lift itself if it ran along a railroad track, and it might seem as though, when it could lift itself, the problem was solved. I knew that it was far from solved, but felt that the point was reached where an attempt at actual free flight should be made, though the anticipated difficulties of this were of quite another order to those experienced in shop construction. It is enough to look up at the gulls or buzzards, soaring overhead, and to watch the incessant rocking and balancing which accompanies their gliding motion to apprehend that they find something more than mere strength of wing necessary, and that the machine would have need of something more than mechanical power, though what this something was, was not clear. It looked as though it might need a power like instinctive adaptation to the varying needs of each moment, something that even an intelligent steersman on board could hardly supply, but to find what this was a trial had to be made. The first difficulty seemed to be to make the initial flight in such conditions that the machine would not wreck itself at the outset, in its descent, and the first question was where to attempt to make the flight. It became clear without much thought, that since the machine was at first unprovided with any means to save it from breakage on striking against the ground, it would be well, in the initial stage of the experiment, not to have it light on the ground at all, but on the water. As it was probable that, while skill in launching was being gained, and until after practice had made |