During my visit to the Southern States of America, I have had several opportunities of watching, under favorable conditions, the flight of the buzzard, the scavenger of Southern cities. Although in most respect this bird's manner of flight resembles that of the various sea-birds which I have often watched for hours sailing steadily after ocean steamships, yet, being a land bird, the buzzard is more apt to give examples of that kind of flight in which a bird remains long over the same place. Instead of sailing steadily on upon outstretched pinions, the buzzard often ascends in a series of spirals, or descends along a similar course. I have not been able to time the continuance of the longest flights during which the wings have not once been flapped, for the simple reason that, in every case where I have attempted to do so, the bird has passed out of view either by upward or horizontal traveling. But I am satisfied that in many cases the bird sweeps onward or about on unflapping wings for more than half an hour.
Now, many treat this problem of aerial flotation as if it were of the nature of a miracle--something not to be explained. Explanations which have been advanced have, it is true, been in many cases altogether untenable. For instance, some have asserted that the albatross, the condor, and other birds which float for a long time without moving their wings--and that, too, in some cases, at great heights above the sea-level, where the air is very thin--are supported by some gas within the hollow parts of their bones, as the balloon is supported by the hydrogen within it. The answer to this is that a balloon is not supported by the hydrogen within it, but by the surrounding air, and in just such degree as the air is displaced by the lighter gas. The air around a bird is only displaced by the bird's volume, and the pressure of the air corresponding to this displacement is not equivalent to more than one five-hundredth part of the bird's weight. Another idea is that when a bird seems to be floating on unmoving wings there is really a rapid fluttering of the feathers of the wings, by which a sustaining power is obtained.
But no one who knows anything of the anatomy of the bird will adopt this idea for an instant, and no one who has ever watched with a good field-glass a floating bird of the albatross or buzzard kind will suppose they are fluttering their feathers in this way, even though he should be utterly ignorant of the anatomy of the wings. Moreover, any one acquainted with the laws of dynamics will know that there would be tremendous loss of power in the fluttering movement imagined as compared with the effect of sweeping downward and backward the whole of each wing.
There is only one possible way of explaining the floating power of birds, and that is by associating it with the rapid motion acquired originally by wing flapping, and afterward husbanded, so to speak, by absolutely perfect adjustment and balancing. To this the answer is often advanced that it implies ignorance of the laws of dynamics to suppose that rapid advance can affect the rate of falling, as is implied by the theory that it enables the bird to float.
Now, as a matter of fact, a slight slope of the wings would undoubtedly produce a raising power, and so an answer is at one obtained to this objection. But I venture to assert, with the utmost confidence, that a perfectly horizontal plane, advancing swiftly in a horizontal direction at first, will not sink as quickly, or anything like as quickly, as a similar plane let fall from a position of rest. A cannon-ball, rushing horizontally from the mouth of a cannon, begins to fall just as if it were simply dropped. But the case of a horizontal plane is altogether different. If rapidly advancing, it passes continually over still air; if simply let fall, the air beneath it yields, and presently currents are set up which facilitate the descent of the flat body; but there is no time to set up these aerial movements as the flat body passes rapidly over still air.
As a matter of fact, we know that this difference exists, from the difference in the observed behavior of a flat card set flying horizontally through the air and a similar card held horizontally and then allowed to fall.
I believe the whole mystery of aerial flotation lies here, and that as soon as aerial floating machines are planned on this system, it will be found that the problem of aerial transit--though presenting still many difficulties of detail--is, nevertheless, perfectly soluble.--R.A. Proctor, in Newcastle Weekly Chronicle.