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Aviation History
1911
1911 - 0798.PDF
[/LIGHT SEPTEMBER 16, 1911. jtfjSfadpefJ&ird OB3o By DrE.H.fiankin. MA. DSc (Cofiyrig h t Reser ved) CHAPTER XII.—Some Independent Evidence. AN article entitled " On the flight of birds "was published over the initials " A.O.H." in "Stray Feathers," vol. x, No. 4, p. 248, July, 1882. The author made his observations at Simla, at an elevation of about 7,000 ft. in the Himalaya mountains. He states that vultures start in the summer between 6 and 7 in the morning, but in winter not till nearly 9 a.m. Their usual speed of flight, presumably flex- gliding, he found to be from 12 to 15 miles an hour. He states that crows soar, rising in circles without flapping when the air is quite calm. My own observations made in the Himalayas at Naini Tal confirm this statement, but qualify it in so far that in my experience the presence of bright sunshine besides calm air is necessary for the soaring of crows. The author quoted puts forward a theory of the nature of soaring flight. He states that it is due to " levitation." This is a miracle or conjuring trick in virtue of which a man can remain unsupported in the air. He says that it consists in "so altering the magnetic polarity of the physical frame that in lieu of being attracted it is repelled by the earth." This power is achieved " by living an absolutely pure life and intense religious concentration." Birds are endowed with this power apart from 9uch mental exercise, unless, it may be suggested, the hill crow finds it helpful to indulge in <& dip the whole of the inside wing was momentarily depressed, and that the outside wing was also depressed, but to a lesser degree. In view of the desciiption of the effect of the dihedrally-down position given in Chapter IX, it appears probable that these adjust ments have the object of increasing speed. In leeward looping the windward dip may often be seen. At the leeward end of the glide a dip of the inside wing may occur, evidently with the object of turning the bird round to face the wind. After the windward gain of height, the windward dip may be seen. Then, when in consequence the bird turns round (to a direction facing away from the wind) to commence the next leeward glide, I have, in the case of the cheel and the white scavenger, frequently noticed a depression of both wings to a slightly marked dihedrally-down position. This adjustment only lasts for a second or two, and its function appears to be to produce or initiate the increased speed of the leeward glide. Several months' study was requisite to discover the above facts of directive movements in circling. After several more months' study I discovered smaller movements of the tip of the outside wing that occasionally occur. These movements appear to be of a somewhat complicated nature, and I propose discussing them on a future- occasion. On rare occasions, and if one is favourably situated for observation,. .\\S Fig. 20. — Outline of a parrot with wings advanced. Fig. 21. — Outline of a parrot with wings straight. Fig. 22.—Outline of a parrot with wings retired. irreligious sentiments when trying to descend to terra firma without the help of gravity. It is singular that the author should think that such a theory is an explanation, and also that he should think it right to publish such a theory without a particle of evidence in its favour. That an observer should be led to formulate such a theory is, however, of interest in showing that soaring flight, when seen at close range, is actually a mysterious phenomenon inexplicable by causes hitherto suggested. CHAPTER XIII.—Further Consideration of Circling. In view of the knowledge now gained of directive movements, a further description of circling may be attempted. The first steering movement in circling that I discovered is one that I propose to term the " windward dip." If a circling bird is carefully watched at the end of the windward gain of height, a slight downward dip of the wing tip of the wing that is nearest the centre of the circle may be observed. This windward dip may be seen occasionally in all species of soaring birds that have come under my observation, including both vultures, cranes, and adjutants. While watching a cheel circling a few feet over the roof of my house where I was sitting at the time, I noticed the windward dip in two or three circles. Then in the following circles, at the point in the track where one might expect to see the dip, a sudden wing depression occurred. That is to say, the dip was replaced by the other kind of steering movement that has been described. This steering movement was seen to be followed by a slight rotation of the bird, that is to say, by a slight change of course. In cases in which a leeward gain of height occurs, this gain may similarly be followed by a " leeward dip." Both with the white scavenger vulture and the black vulture I have on rare occasions noticed that at the time of the windward other dip movements of the inside wing tip may be seen. These are of leaser amplitude than the dip movements already described. The extraordinary fact about these different movements is that they do not always occur. Sometimes I have observed carefully, perhaps as many as a hundred birds without once seeing a windward dip. Sometimes at Jharna Nullah, out of thousands of birds circling together none show any such movements. Prestntiy a cluster of binis may be seen to detach themselves from the rest, and commence drifting to leeward. Every bird in this cluster that can be observed will be found to show strongly marked windward dips. Another cluster of birds drifting to leeward may show scarcely any such movements. The bird is always canted over towards the centre of the circle. If the air is fully soarable the wings of the vulture when circling are in the dihedrally up position. On the down-wind side, especially at the commencement of the down-wind side of the circle, the dihedrally-up angle is less, or the wings may for a short period be held nearly flat. When circling in less soarable air, the dihedrally- up angle also is less, and in this case along the down-wind side of the track the wings may be held flat. But whether the wings are flat or dihedrally-up, their centre of effort is always above the level of the centre of gravity. It might be suggested that the canted position in circling is connected with this fact, in that it might be due to an effect of centrifugal force. But this supposition is clearly negatived by the fact that the amouut of canting is greatest on the windward side of the track, where speed is least. Canting is least on the down-wind side where the speed is highest. On the windward side of the track the bird may be canted up to such an extent that the plane of its wings (supposing they were flat) may make an angle of between 30° and 6o° with the horizon. The amount of canting varies at different times without any very obvious cause. I have formed the impression that in ease-circling 800
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