Some of the mathematicians have demonstrated by formulæ, unsupported by facts, that there is a considerable amount of skin friction to be considered, but as no two agree on this or any other subject, some not agreeing to-day with what they wrote a year ago, I think we might put down all of their results, add them together, and then divide by the number of mathematicians, and thus find the average coefficient of error. When we subject this question to experimental test, we find that nearly all of the mathematicians are radically wrong, Professor Langley, of course, excepted. I made an aeroplane of hard rolled brass, 20 gauge; it was 1 foot wide and dead smooth on both sides; I gave it a curvature of about ¹⁄₁₆ inch and filed the edges, thin and sharp. I mounted this with a great deal of care in a perfectly horizontal blast of air of 40 miles an hour. When this aeroplane was placed at any angle between 1 in 8 and 1 in 20, the lifting effect was always just in proportion to its angle. The distance that the front edge was raised above the horizontal, as compared with the width of the aeroplane, was always identical with the drift as compared with the lift. On account of the jarring effect caused by the rotation of the screws that produced the air blast, we might consider that all of the articulated joints about the weighing device were absolutely frictionless, as the jar would cause them to settle into the proper position quite irrespective of friction. I was, therefore, able to observe very carefully, the lift and the drift. As an example of how these experiments were conducted, I would say that the engine employed was provided with a very sensitive and accurate governor; the power transmission was also quite reliable. Before making these tests, the apparatus was tested as regards the drift, without any aeroplane in position, and with weights applied that would just balance any effect that the wind might have on everything except the aeroplane. The aeroplane was then put in position and the other system of weights applied until it exactly balanced, all the levers being rapped in order to eliminate the friction in their joints. The engine was then started and weights applied just sufficient to counterbalance the lifting effect of the aeroplane, and other weights applied to exactly balance the drift or the tendency to travel with the wind. In this way, I was able to ascertain, with a great degree of accuracy, the relative difference between the lift and drift. If there had been any skin friction, even to the extent of 2 per cent., it would have been detected. This brass aeroplane was tested at various angles, and always gave the same results, but of course I could not use thick brass aeroplanes on a flying machine; it was necessary for me to seek something much lighter. I therefore conducted experiments with other materials, the results of which are given. However, with a well-made wooden aeroplane 1 foot wide and with a thickness in the centre of ⁷⁄₁₆ inch, I obtained results almost identical with those of the very much thinner brass aeroplane, but it must not be supposed that in practice an aeroplane is completely without friction. If it is very rough, irregular in shape, and has any projections whatsoever on either the top or bottom side, there will be a good deal of friction, although it may not, strictly speaking, be skin friction; still, it will absorb the power, and the coefficient of this friction may be anything from ·05 to ·40. These experiments with the brass aeroplane demonstrated that the lifting effect was in direct proportion to the angle, and that skin friction, if it exists at all, was extremely small, but this does not agree with a certain kind of reasoning which can be made very plausible and is consequently generally accepted.

Writers of books, as a rule, have always supposed that the lifting effect of an aeroplane was not in proportion to its inclination, but in proportion to the square of the sine of the angle. In order to make this matter clear, I will explain. Suppose that an aeroplane is 20 inches wide and the front edge is raised 1 inch above the horizontal. In ordinary parlance this is, of course, called an inclination of 1 in 20, but mathematicians approach it from a different standpoint. They regard the width of the aeroplane as unity or the radius, and the 1 inch that the front edge is raised as a fraction of unity. The geometrical name of this 1 inch is the sine of the angle—that is, it is the sine of the angle at which the aeroplane is raised above the horizontal. Suppose, now, that we have another identical aeroplane and we raise the front edge 2 inches above the horizontal. It is very evident that, under these conditions, the sine of the angle will be twice as much, and that the square of the sine of the angle will be four times as great. All the early mathematicians, and some of those of the present day, imagine that the lift must be in proportion to the square of the sine of the angle. They reason it out as follows:—If an aeroplane is forced through the air at a given velocity, the aeroplane in which the sine of the angle is 2 inches will push the air down with twice as great a velocity as the one in which the sine of the angle is only 1 inch, and as the force of the wind blowing against a normal plane increases as the square of the velocity, the same law holds good in driving a normal plane through still air. From this reasoning, one is led to suppose that an aeroplane set at an angle of 1 in 10 will lift four times as much as one in which the inclination is only 1 in 20, but experiments have shown that this theory is very wide of the truth. There are dozens of ways of showing, by pure mathematics, that Newton’s law is quite correct; but in building a flying machine no theory is good that does not correspond with facts, and it is a fact, without any question, that the lifting effect of an aeroplane, instead of increasing as the square of the sine of the angle, only increases as the angle. Lord Kelvin, when he visited my place, was, I think, the first to mention this, and point out that Newton’s law was at fault. Professor Langley also pointed out the fallacy of Newton’s law, and other experimenters have found that the lifting effect does not increase as the square of the sine of the angle. In order to put this matter at rest, Lord Rayleigh, who, I think we must all admit, would not be likely to make a mistake, made some very simple experiments, in which he demonstrated that two aeroplanes, in which we may consider the sine of the angle to be ¹⁄₄ inch, lifted slightly more than a similar aeroplane in which the sine of the angle was only ¹⁄₂ inch. Of course, Lord Rayleigh did not express it in inches, but in term of the radius. His aeroplanes were, however, very small. We can rely upon it that the lifting effect of an aeroplane at any practical angle, everything else being equal, increases in direct proportion to the angle of the inclination. In this little work, I have attempted to make things as simple as possible; it has not been written for mathematicians, and I have, therefore, thought best to express myself in inches instead of in degrees. If I write, “an inclination of 1 in 20,” everyone will understand it, and only a carpenter’s 2-foot rule is required to ascertain what the angle is. Then, again, simple measurements make calculations much simpler, and the lifting effect is at once understood without any computations being necessary. If the angles are expressed in degrees and minutes, it is necessary to have a protractor or a text-book in order to find out what the inclination really is. When I made my experiments, I only had in mind the obtaining of correct data, to enable me to build a flying machine that would lift itself from the ground. At that time I was extremely busy, and during the first two years of my experimental work, I was out of England fourteen months. After having made my apparatus, I conducted my experiments rather quickly, it is true, but I intended later on to go over them systematically and deliberately, make many more experiments, write down results, and prepare some account of them for publication. However, the property where I made these experiments was sold by the company owning it, and my work was never finished, so I am depending on the scraps of data that were written down at the time. I am also publishing certain observations that I wrote down shortly after I had succeeded in lifting more than the weight of my machine. I think that the experiments which I made with an aeroplane only 8 inches wide will be found the most reliable. All the machinery was running smoothly, and the experiments were conducted with a considerable degree of care. In making any formula on the lifting effect of the aeroplane, it should be based on what was accomplished with the 8-inch plane. Only a few experiments were made to ascertain the relative value of planes of different widths. However, I think we must all admit that a wide plane is not as economical in power as a narrow one. In order to make this matter plain, suppose that we have one aeroplane placed at such an angle that it will lift 2 lbs. per square foot at a velocity of 40 miles an hour; it is very evident that the air just at the rear of this aeroplane would be moving downward at a velocity corresponding to the acceleration imparted to it by the plane. If we wish to obtain lifting effect on this air by another plane of exactly the same width, we shall have to increase its inclination in order to obtain the same lifting effect, and, still further, it will be necessary to use more power in proportion to the load lifted. If a third aeroplane is used, it must be placed at an angle that will impart additional acceleration to the air, and so on. Each plane that we add will have to be placed at a sharper angle, and the power required will be just in proportion to the average angle of all the planes. As the action of a wide aeroplane is identical with that of numerous narrow ones placed in close proximity to each other, it is very evident that a wide aeroplane cannot be as efficient in proportion to its width as a narrow one. I have thought the matter over, and I should say that the lifting effect of a flat aeroplane increases rather faster than the square root of its width. This will, at least, do for a working hypothesis. Every flying machine must have what we will call “a length of entering edge”—that is, the sum of entering edges of all the aeroplanes must bear a fixed relation to the load carried. If a machine is to have its lifting effect doubled, it is necessary to have the length of entering edge twice as long. This additional length may, of course, be obtained by superposed planes, but as we may assume that a large aeroplane will travel faster than a small one, increased velocity will compensate in some degree for the greater width of larger aeroplanes. By careful study of the experiments which I have made, I think it is quite safe to state that the lifting effect of well-made aeroplanes, if we do not take into consideration the resistance due to the framework holding them in position, increases as the square of their velocity. Double their speed and they give four times the lifting effect. The higher the speed, the smaller the angle of the plane, and the greater the lifting effect in proportion to the power employed. When we build a steamship, we know that its weight increases as the cube of any one of its dimensions—that is, if the ship is twice as long, twice as wide, and twice as deep it will carry eight times as much; but at the very best, with even higher speed, the load carried by a flying machine will only increase with the square of any one of its dimensions, or perhaps still less. No matter whether it is a ship, a locomotive, or a flying machine that we wish to build, we must first of all consider the ideal, and then approximate it as closely as possible with the material at hand. Suppose it were possible to make a perfect screw, working without friction, and that its weight should only be that of the surrounding air; if it should be 200 feet in diameter, the power of one man, properly applied, would lift him into the air. This is because the area of a circle 200 feet in diameter is so great that the weight of a man would not cause it to fall through the air at a velocity greater than the man would be able to climb up a ladder. If the diameter should be increased to 400 feet, then a man would be able to carry a passenger as heavy as himself on his flying machine, and if we should increase it still further, to 2,000 feet, the weight of a horse could be sustained in still air by the power which one man could put forth. On the other hand, if we should reduce the diameter of the screw to 20 feet, then it would certainly require the power of one horse to lift the weight of one man, and, if we made the screw small enough, it might even require the power of 100 horses to lift the same weight. It will, therefore, be seen that everything depends upon the area of the air engaged, and in designing a machine we should seek to engage as much air as possible, so long as we can keep down the weight. Suppose that a flying machine should be equipped with a screw 10 feet in diameter, with a pitch of 6 feet, and that the motor developed 40 horse-power and gave the screw 1,000 turns a minute, producing a screw thrust, we will say, of about 220 lbs. If we should increase the diameter of the screw to 20 feet, and if it had the same pitch and revolved at the same rate, it would require four times as much power and would give four times as much screw thrust, because the area of the disc increases as the square of the diameter. Suppose, now, that we should reduce the pitch of the screw to 3 feet, we should in this case engage four times as much air, and double the screw thrust without using any more power—that is, assuming that the machine is stationary and that the full power of the engine is being used for accelerating the air. The advantages of a large screw will, therefore, be obvious. I have been unable to obtain correct data regarding the experiments which have taken place with the various machines on the Continent. I have, however, seen these machines, and I should say when they are in flight, providing that the engine develops 40 horse-power, that fully 28 horse-power is lost in screw slip, and the remainder in forcing the machine through the air. These machines weigh 1,000 lbs. each, and their engines are said to be 50 horse-power. The lifting effect, therefore, per horse-power is 20 lbs. If the aeroplanes were perfect in shape and set at a proper angle, and the resistance of the framework reduced to a minimum, the same lifting effect ought to be produced with an expenditure of less than half this amount of power, providing, of course, that the screw be of proper dimensions. It is said that Professor Langley and Mr. Horatio Philipps, by eliminating the factor of friction altogether, or by not considering it in their calculations, have succeeded in lifting at the rate of 200 lbs. per horse-power. The apparatus they employed was very small. The best I ever did with my very much larger apparatus—and I only did it on one occasion—was to carry 133 lbs. per horse-power. In my large machine experiments, I was amazed at the tremendous amount of power necessary to drive the framework and the numerous wires through the air. It appeared to me, from these experiments, that the air resisted very strongly being cut up by wires. I expected to raise my machine in the air by using only 100 horse-power, and my first condenser was made so that it did actually condense water enough to supply 100 horse-power, but the framework offered such a tremendous resistance that I was compelled to strengthen all of the parts, make the machine heavier, and increase the boiler pressure and piston speed until I actually ran it up to 362 horse-power. This, however, was not the indicated horse-power. It was arrived at by multiplying the pitch of the screws, in feet, by the number of turns that they made in a minute, and by the screw thrust in pounds, and then dividing the product by the conventional unit 33,000. I have no doubt that the indicated horse-power would have been fully 400. On one occasion I ran my machine over the track with all the aeroplanes removed. I knew what steam pressure was required to run my machine with the aeroplanes in position at a speed of 40 miles an hour. With the planes removed, it still required a rather high steam pressure to obtain this velocity, but I made no note at the time of the exact difference. It was not, however, by any means so great as one would have supposed. From the foregoing, it will be seen how necessary it is to consider atmospheric resistance. Although I do not expect that anyone will ever again attempt to make a flying machine driven by a steam engine, still, I have thought best to give a short and concise description of my engine and boiler, in order that my readers may understand what sort of an apparatus I employed to obtain the data I am now, for the first time, placing before the public. A full description of everything relating to the motor power was written down at the time, and has been carefully preserved. An abridgement of this will be found in the Appendix.

CHAPTER II.
AIR CURRENTS AND THE FLIGHT OF BIRDS.

In Mr. Darwin’s “Voyage of the Beagle” I find:—

“When the condors are wheeling in a flock round and round any spot their flight is beautiful. Except when rising from the ground, I do not remember ever having seen one of these birds flap its wings. Near Lima I watched several for nearly half an hour, without taking off my eyes; they moved in large curves, sweeping in circles, descending and ascending without giving a single flap. As they glided close over my head I intently watched from an oblique position, the outlines of the separate and great terminal feathers of each wing, and these separate feathers, if there had been the least vibratory movement, would have appeared as if blended together; but they were seen distinct against the blue sky.”

Man is essentially a land animal, and it is quite possible if Nature had not placed before him numerous examples of birds and insects that are able to fly, he would never have thought of attempting it himself. But birds are very much in evidence, and mankind from the very earliest times has not only admired the ease and rapidity with which they are able to move from place to place, but has always aspired to imitate them. The number of attempts that have been made to solve this problem has been very great; but it was not until quite recently that science and mechanics had advanced far enough to put in the hands of experimenters suitable material to attack the problem. Perhaps nothing better has ever been written regarding our aspirations to imitate the flight of birds than what Prof. Langley has said:—

“Nature has made her flying machine in the bird, which is nearly a thousand times as heavy as the air its bulk displaces, and only those who have tried to rival it know how inimitable her work is, for ‘the way of a bird in the air’ remains as wonderful to us as it was to Solomon, and the sight of the bird has constantly held this wonder before men’s eyes, and in some men’s minds, and kept the flame of hope from utter extinction, in spite of long disappointment. I well remember how, as a child, when lying in a New England pasture, I watched a hawk soaring far up in the blue, and sailing for a long time without any motion of its wings, as though it needed no work to sustain it, but was kept up there by some miracle. But, however sustained, I saw it sweep, in a few seconds of its leisurely flight, over a distance that to me was encumbered with every sort of obstacle, which did not exist for it. The wall over which I had climbed when I left the road, the ravine I had crossed, the patch of undergrowth through which I had pushed my way—all these were nothing to the bird—and while the road had only taken me in one direction, the bird’s level highway led everywhere, and opened the way into every nook and corner of the landscape. How wonderfully easy, too, was its flight. There was not a flutter of its pinions as it swept over the field, in a motion which seemed as effortless as that of its shadow.”

During the last 50 years a great deal has been said and written in regard to the flight of birds; no other natural phenomenon has excited so much interest and been so imperfectly understood. Learned treatises have been written to prove that a bird is able to develop from ten to twenty times as much power for its weight as other animals, while other equally learned works have shown most conclusively that no greater amount of energy is exerted by a bird in flying than by land animals in running or jumping.

Prof. Langley, who was certainly a very clever observer and a mathematician of the first order, in discussing the subject relating to the power exerted by birds in flight and the old formula relating to the subject, expresses himself as follows:—

“After many years and in mature life, I was brought to think of these things again, and to ask myself whether the problem of artificial flight was as hopeless and as absurd as it was then thought to be. Nature had solved it, and why not man? Perhaps it was because he had begun at the wrong end, and attempted to construct machines to fly before knowing the principles on which flight rested. I turned for these principles to my books and got no help. Sir Isaac Newton had indicated a rule for finding the resistance to advance through the air, which seemed, if correct, to call for enormous mechanical power, and a distinguished French mathematician had given a formula showing how rapidly the power must increase with the velocity of flight, and according to which a swallow, to attain a speed it is known to reach, must be possessed of the strength of a man.

“Remembering the effortless flight of the soaring bird, it seemed that the first thing to do was to discard rules which led to such results, and to commence new experiments, not to build a flying machine at once, but to find the principles upon which one should be built; to find, for instance, with certainty by direct trial how much horse-power was needed to sustain a surface of given weight by means of its motion through the air.”

There is no question but what a bird has a higher physical development, as far as the generation of power is concerned, than any other animal we know of. Nevertheless, I think that everyone who has made a study of the question will agree that some animals, such as hares and rabbits, exert quite as much power in running, in proportion to their weight, as a sea-gull or an eagle does in flying.

The amount of power which a land animal has to exert is always a fixed and definite quantity. If an animal weighing 100 lbs. has to ascend a hill 100 feet high, it always means the development of 10,000 foot-lbs. With a bird, however, there is no such thing as a fixed quantity. If a bird weighing 100 lbs. should raise itself into the air 100 feet during a perfect calm, the amount of energy developed would be 10,000 foot lbs. plus the slip of the wings. But, as a matter of fact, the air in which a bird flies is never stationary, as I propose to show; it is always moving either up or down, and soaring birds, by a very delicate sense of feeling, always take advantage of a rising column. If a bird finds itself in a column of air which is descending, it is necessary for it to work its wings very rapidly in order to prevent a descent to the earth.

I have often observed the flight of hawks and eagles. They seem to glide through the air with hardly any movement of their wings. Sometimes, however, they stop and hold themselves in a stationary position directly over a certain spot, carefully watching something on the earth immediately below. In such cases they often work their wings with great rapidity, evidently expending an enormous amount of energy. When, however, they cease to hover and commence to move again through the air, they appear to keep themselves at the same height with an almost imperceptible expenditure of power.

Many unscientific observers of the flight of birds have imagined that a wind or a horizontal movement of the air is all that is necessary to sustain the weight of a bird in the air after the manner of a kite. If, however, the wind, which is only air in motion, should be blowing everywhere at exactly the same velocity, and in the same direction—horizontally—it would offer no more sustaining power to a bird than a dead calm, because there is nothing to prevent the body of the bird from being blown along with the air, and whenever it attained the same velocity as the air, no possible arrangement of the wings could prevent it from falling to the earth.

It is well known that only a short distance above the earth’s surface, say 30 or 40 miles, we find an extremely low temperature sometimes referred to as interstellar temperature or absolute zero. In order to illustrate the extremely low temperature of space, I would cite the following instance:—

One evening, in the State of Ohio, a farmer saw a very brilliant meteor; it struck in one of his fields not more than 100 feet from his house. He at once rushed to the spot, and, pushing his arm down the hole, succeeded in touching it; but he very quickly withdrew his hand, as he found it extremely hot. Some of the neighbours rushed to the spot, and he told them what had occurred, whereupon one of them put his hand in the hole, expecting to be burnt, but, much to his surprise, the tips of his wet fingers were instantly frozen to the meteor. The meteor had been travelling at such an exceedingly high velocity that the resistance of the intensely cold and highly attenuated outer atmosphere was sufficient to bring its temperature up to the melting point of iron; but the heat did not have time to pass into the interior, it only extended inwards perhaps ¹⁄₈ inch, so that when the meteor came to a state of rest, the heat of the exterior was soon absorbed by the intensely cold interior, thus reducing the surface to a temperature much below any natural temperature that we find at the surface of the earth.

Nothing can be more certain than that the temperature is extremely low a slight distance above the earth’s surface. As the air near the earth never falls in temperature to anything like the absolute zero, it follows that there is a constant change going on, the relatively warm air near the surface of the earth always ascending, and, in some cases, doing sufficient work in expanding to render a portion of the water it contains visible, forming clouds, rain, or snow, while the very cold air is constantly descending to take the place of the rising column of warm air. I have noticed a considerable degree of regularity in the movement of the air, especially at a long distance from land, where the regularity of the up and down currents is, at times, very marked.

On one occasion while crossing the Atlantic in fine weather I noticed, some miles directly ahead of the ship, a long line of glassy water. Small waves indicated that the wind was blowing in the exact direction in which the ship was moving, and as we approached the glassy line, the waves became smaller and smaller until they completely disappeared in a mirror-like surface, which was about 300 or 400 feet wide, and extended both to the port and starboard in approximately a straight line as far as the eye could reach. After passing the centre of this zone, I noticed that small waves began to show themselves, but in the exact opposite direction to those through which we had already passed, and these waves became larger and larger for nearly half an hour. Then they began to get gradually smaller, when I observed another glassy line directly ahead of the ship. As we approached it, the waves again completely disappeared, but after passing through it, the wind was blowing in the opposite direction, and the waves increased in size exactly in the same manner that they had diminished on the opposite side of the glassy streak (Fig. 4).

This, of course, shows that directly over the centre of the first glassy streak, the air was meeting from both sides and ascending in practically a straight line from the surface of the water, and then spreading out high above the sea, setting up a light wind in both directions.

I spent the winter of 1890-91 on the Riviera, between Hyères les Palmiers and Monte Carlo. The weather for the most part was very fine, and I often had the opportunity of observing the peculiar phenomena which I had already noticed in the Atlantic, only on a much smaller scale. Whereas, in the Atlantic, the glassy zones were from 8 to 15 miles apart, I often found them not more than 500 feet apart in the bays of the Mediterranean. This was most noticeable at Antibes (Fig. 5), very good photographs of which I obtained. It will be observed that the whole surface of the water is streaked like a block of marble.

At Nice and Monte Carlo this phenomena was also very marked. On one occasion, while making observations from the highest part of the promontory of Monaco on a perfectly calm day, I noticed that the whole of the sea presented this peculiar effect as far as the eye could reach, and that the lines which marked the descending air were never more than 1,000 feet from those which marked the centre of the ascending column. At about three o’clock one afternoon, a large black steamer passed along the coast in a perfectly straight line, and its wake was at once marked by a glassy line, which indicated the centre of an ascending column. This line remained almost straight for two hours, when finally it became crooked and broken. The heat of the steamer had been sufficient to determine this upward current of air.

In 1893 I spent two weeks in the Mediterranean, going and returning by a slow steamer from Marseilles to Constantinople, and I had many opportunities of observing the peculiar phenomena to which I have referred. The steamer passed over thousands of square miles of calm sea, the surface being only disturbed by large patches of small ripples (Fig. 6), separated from each other by glassy streaks, which, however, were not straight as on the Atlantic, and I found that in no case was the wind blowing in the same direction on both sides of these streaks, every one of which indicated the centre of an ascending or a descending column of air. If we should investigate these phenomena in what might be called a dead calm, we should find that the air was rising very nearly straight up over the centres of some of these streaks and descending in a vertical line over the centres of others. But, as a matter of fact, there is no such thing as a dead calm. The movement of the air is the resultant of more than one force. The air is not only rising in some places and descending in others, but at the same time, the whole mass is moving forward with more or less rapidity from one part of the earth to another, so we must consider that, instead of the air ascending directly from the relatively hot surface of the earth and descending vertically in other places, in reality the whole mass of rotating air is moving horizontally at the same time.

Suppose that the local influence which causes the up and down motion of the air should be sufficiently great to cause the air to rise at the rate of 2 miles an hour, and that the wind at the same time should be blowing at the rate of 10 miles an hour, the motion of the air would then be the resultant of these two velocities. In other words, it would be blowing up an incline of 1 in 5. Suppose, now, that a bird should be able to so adjust its wings that it advanced 5 miles in falling 1 mile through a perfectly calm atmosphere, it would then be able to sustain itself in an inclined wind, such as I have described, without any movement at all of its wings. If it were possible to adjust its wings in such a manner that it could advance 6 miles by falling through 1 mile of air, it would then be able to rise as relates to the earth while in reality falling as relates to the surrounding air.

In conducting a series of experiments with artillery and small guns on a large and level plain just out of Madrid, I often observed the same phenomena, as relates to the wind, that I have already spoken of as having observed at sea, except that the lines marking the centre of an ascending or a descending column of air were not so stationary as they were over the water. It was not an uncommon thing, when adjusting the sights of a gun to fire at a target at a very long range, making due allowances for the wind, to have the wind change and blow in the opposite direction before the word of command was given to fire. While conducting these experiments, I often noticed the flight of eagles. On one occasion a pair of eagles came into sight on one side of the plain, passed directly over our heads, and disappeared on the opposite side. They were apparently always at the same height from the earth, and in soaring completely across the plain they never once moved their wings. These phenomena, I think, can only be accounted for on the hypothesis that these birds were able to feel out with their wings an ascending column of air, that the centre of this column of air was approximately a straight line running completely across the plain, that they found upward movement more than sufficient to sustain their weight in the air, and that whereas, as relates to the earth, they were not falling at all, they were in reality falling some 4 or 5 miles an hour in the air which supported them.

Again, at Cadiz in Spain, when the wind was blowing in strongly from the sea, I observed that the sea-gulls always took advantage of an ascending column of air. As the wind rose to pass over the fortifications, the gulls selected a place where they would glide on the ascending current of air, keeping themselves always approximately in the same place without any apparent exertion. When, however, they left this ascending column, it was necessary for them to work their wings with great vigour until they again found the proper place to encounter a favourable current.

I have often noticed that gulls are able to follow a ship without any apparent exertion; they simply balance themselves on an ascending column of air, where they seem to be quite as much at ease as they would have been roosting on a solid support. If, however, they are driven out of this position, they generally commence at once to work their passage. If anything is thrown overboard which is too heavy for them to lift, the ship soon leaves them behind, and in order to catch up with it again they move their wings very much as other birds do; but when once established in the ascending column of air, they manage to keep up with the ship by doing little or no work. In a calm or head wind we find them directly aft of the ship; if the wind is from the port side they may always be found on the starboard quarter, and vice versâ.

One Sunday morning, while living at Kensington, I noticed some very curious atmospheric effects. The weather had been intensely cold for about a week, when suddenly the atmosphere became warm and very humid. The earth being much colder than the atmosphere, water was condensing on everything that it touched. I went to the bridge over the Serpentine in Hyde Park, and was not disappointed in finding a large number of sea-gulls waiting about the bridge to be fed. On all ordinary occasions these birds manage to move about with the expenditure of very little energy, but on this occasion every one of them, without a single exception, no matter in what position he might be, was working his passage like any other bird, just as I had expected. It is only on very rare occasions that the surface of the earth is sufficiently cold as relates to the atmosphere to prevent all upward currents of air.

Everyone who has passed a winter on the northern shores of the Mediterranean must have observed the cold wind which is generally called the mistral. One may be out driving, the sun may be shining brightly, and the air warm and balmy, when suddenly, without any apparent cause, one finds himself in a cold descending wind. This is the much-dreaded mistral, and if at sea it would be marked by a glassy line on the surface of the water. On land, however, there is nothing to render its presence visible. The ascending column of air is, of course, always very much warmer than the descending column, and this is taking place in a greater or lesser degree everywhere and at all times. A decided upward trend of air is often encountered by those who are experimenting with kites, the kite often mounting higher than can be accounted for on the hypothesis that the wind is moving in a horizontal direction. I have heard this discussed at considerable length. When a kite is flown in an upward current, it behaves in many respects like a soaring bird.

From the foregoing, I think, we may safely draw the following conclusions:—

First, that there is a constant interchange of air taking place, the cold air descending, spreading itself out over the surface of the earth, becoming warm, and ascending in other places.

Second, that the centres of the two columns are generally separated from each other by a distance which may be from 500 feet to 20 miles.

Third, that the centres of greatest action are not in spots, but in lines which may be approximately straight, but sometimes abound in many sinuosities.

Fourth, that this action is constantly taking place over both the sea and the land; that the soaring of birds, the phenomenon which has heretofore been so little understood, may be accounted for on the hypothesis that the bird seeks out an ascending column of air, and while sustaining itself at the same height in the air, without any muscular exertion, it is in reality falling at a considerable velocity through the air that surrounds it.

It has been supposed by some scientists that birds may take advantage of some vibratory or rolling action of the air. I find, however, from careful observation and experiment, that the motion of the wind is comparatively steady, and that the short vibratory or rolling action is always very near to the earth and is produced by the air flowing over hills, high buildings, trees, etc.

Tools and instruments used by mechanicians are very often made of the material most used in their profession; for instance, a blacksmith’s tools are generally of iron, a carpenter’s tools largely of wood, and a glass-blower uses many things made of glass, and so on. Mathematicians are no exception to this general rule, and seem to imagine that everything can be accomplished by pure mathematical formulæ.

It appears that Prof. Langley was at times considerably puzzled by the extraordinary behaviour of birds, and was led to believe that they took advantage of some vibratory or oscillating movement of the air; he called it “the internal work of the air.” I have been very much amused in a recent mathematical work that I have read, in which the writer seeks to solve all questions by pure mathematics. In this case, notwithstanding that all of the factors are unknown and unknowable, still, with the use of about two pages of closely written algebraic formulæ, he appears to have solved the whole question. Just how he arrived at it, however, is more than I am able to understand.

If a kite is flown only a few feet above the ground, it will be found that the current of air is very unsteady. If it is allowed to mount to 500 feet the unsteadiness nearly all disappears, while if it is allowed to mount further to a height of 1,500 or 2,000 feet, the pull on the cord is almost constant, and, if the kite is well made, it remains practically stationary in the air.

I have often noticed in high winds that light and fleecy clouds come into view, say, about 2,000 feet above the surface of the earth, and pass rapidly and steadily by preserving their shape completely. This would certainly indicate that there is no rapid local disturbance in the air in their immediate vicinity, but that the whole mass of air in which these clouds are formed is practically travelling in the same direction and at the same velocity. Numerous aeronauts have also testified that, no matter how hard the wind may be blowing, the balloon is always practically in a dead calm, and if a piece of gold-leaf is thrown overboard, even in a gale, the gold-leaf and the balloon never part company in a horizontal direction, though they may in a vertical direction.

Birds may be divided into two classes. First, the soaring birds, which practically live upon the wing, and, by some very delicate sense of touch, are able to feel the exact condition of the air. Many fish which live near the top of the water are greatly distressed by sinking too deeply, while others which live at great depths are almost instantly killed by being raised to the surface. The swim-bladder of a fish is in reality a delicate barometer provided with sensitive nerves which enable the fish to feel whether it is sinking or rising in the water. With the surface fish, if the pressure becomes too great, it involuntarily exerts itself to rise nearer the surface and so diminish the pressure, and I have no doubt that the air cells, which are known to be very numerous and to abound throughout the bodies of birds, are so sensitive as to enable soaring birds to know at once whether they are in an ascending or a descending column of air.

The other class of birds consists of those which only employ their wings occasionally for the purpose of taking them rapidly from one place to another. Such birds do not expend their power so economically as the soaring birds. They do not pass much of their time in the air, but what time they are on the wing they put forth an immense amount of power and fly very rapidly, generally in a straight line, taking no advantage of air currents. Partridges, pheasants, wild ducks, geese, and some birds of passage may be taken as types of this kind. This class of birds has relatively small wings, and carries about two and a half times as much weight per square foot of surface as soaring birds do.

We shall never be able to imitate the flight of the soaring birds. We cannot hope to make a sensitive apparatus that will work quick enough to take advantage of the rising currents of air, and he who seeks to fly has this problem to deal with. A successful flying machine, moving at a high velocity, is likely at any time to encounter downward currents of air, which will greatly interfere with its action. Therefore flying machines must, in the very nature of things, be provided with sufficient power to propel them through various currents of air, after the manner of ducks, partridges, pheasants, etc.

CHAPTER III.
FLYING OF KITES.

It was said of Benjamin Franklin that when he wished to fly a kite in order to ascertain if lightning could be drawn down from the clouds, he managed to have a boy with him in order to avoid ridicule. It was considered too frivolous in those days for grown-up men to amuse themselves with kites, and a good many besides Benjamin Franklin have feared to face the ridicule that was inevitable if they took up or even discussed the question of artificial flight. Nineteen years ago, when I commenced my own experiments, I was told that my reputation would be greatly injured, that mankind looked upon artificial flight as an ignis-fatuus, and that anyone who experimented in that direction was placed in the same category as those who sought to make perpetual-motion machines or to find the philosopher’s stone. Although I had little fear of ridicule, still I kept things as quiet as I could for a considerable time, and I had been working fully six months before anyone ascertained what I was doing. When, however, it became known that I was experimenting with a view of building a flying machine, the public seemed to think that I was making honest and praiseworthy scientific investigations; true, I might not succeed, still it was said that I would accomplish something, and find out some of the laws relating to the subject. No one ridiculed my work except two individuals, and both of these were men whom I had greatly benefited. As is often the case, those whom you find in difficulties and place on their feet seek to do you some injury as compensation for the benefits they have received.

At the present time it is not necessary for any man to take a small boy with him as a species of lightning-rod to ward off ridicule when he flies a kite. I have been one of a committee on kite-flying at which some of the most learned and serious men in England were my colleagues in investigating the subject. The behaviour of kites is certainly very puzzling to those who do not thoroughly understand the subject. A kite may be made with the greatest degree of perfection, and placed in the hands of one of considerable experience; nevertheless, it may behave very badly, diving suddenly to the ground without any apparent cause. Then, again, this same kite will sometimes steadily mount in the air until it reaches a height difficult to account for. If the surface of the earth should be perfectly smooth, and the wind should always blow in a horizontal direction, kites would not show these eccentric peculiarities, but, as a matter of fact, the air seldom moves in a horizontal direction; it is always influenced by the heat of the surface of the earth. Heated air is continually ascending in some places only to be cooled and to descend in other places. If one is attempting to fly a kite where the air is moving downwards, he will find it an extremely difficult matter, whereas, if he is fortunate enough to strike a current of air which is rising, the kite will mount much higher in the air than can be accounted for, except we admit of the existence of these upward draughts of air. On one occasion many years ago, I was present when a bonded warehouse in New York containing 10,000 barrels of alcohol was burnt. It was nine o’clock at night, and I walked completely around the fire, and found things just as I had expected. The wind was blowing a perfect hurricane through every street in the direction of the fire, although it was a dead calm everywhere else; the flames mounted straight in the air to an enormous height, and took with them a large amount of burning wood. When I was fully 500 feet from the fire, a piece of partly burnt 1-inch board, about 8 inches wide and 4 feet long, fell through the air and landed very near me, sending sparks in every direction. This board had evidently been taken up to a great height by the tremendous uprush of air caused by the burning alcohol. It is very evident that a kite made of boiler iron could have been successfully flown under these conditions providing that it could have been brought into the right position.