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_.
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