Borelli”s study is divided into a series of propositions in
which he traces the principles of flight, and the mechanical
actions of the wings of birds
Borelli”s study is divided into a series of propositions in
which he traces the principles of flight, and the mechanical
actions of the wings of birds. The most interesting of these
are the propositions in which he sets forth the method in which
birds move their wings during flight and the manner in which the
air offers resistance to the stroke of the wing. With regard to
the first of these two points he says: “When birds in repose
rest on the earth their wings are folded up close against their
flanks, but when wishing to start on their flight they first
bend their legs and leap into the air. Whereupon the joints of
their wings are straightened out to form a straight line at
right angles to the lateral surface of the breast, so that the
two wings, outstretched, are placed, as it were, like the arms
of a cross to the body of the bird. Next, since the wings with
their feathers attached form almost a plane surface, they are
raised slightly above the horizontal, and with a most quick
impulse beat down in a direction almost perpendicular to the
wing-plane, upon the underlying air; and to so intense a beat
the air, notwithstanding it to be fluid, offers resistance,
partly by reason of its natural inertia, which seeks to retain
it at rest, and partly because the particles of the air,
compressed by the swiftness of the stroke, resist this
compression by their elasticity, just like the hard ground.
Hence the whole mass of the bird rebounds, making a fresh leap
through the air; whence it follows that flight is simply a
motion composed of successive leaps accomplished through the
air. And I remark that a wing can easily beat the air in a
direction almost perpendicular to its plane surface, although
only a single one of the corners of the humerus bone is attached
to the scapula, the whole extent of its base remaining free and
loose, while the greater transverse feathers are joined to the
lateral skin of the thorax. Nevertheless the wing can easily
revolve about its base like unto a fan. Nor are there lacking
tendon ligaments which restrain the feathers and prevent them
from opening farther, in the same fashion that sheets hold in
the sails of ships. No less admirable is nature”s cunning in
unfolding and folding the wings upwards, for she folds them not
laterally, but by moving upwards edgewise the osseous parts
wherein the roots of the feathers are inserted; for thus,
without encountering the air”s resistance the upward motion of
the wing surface is made as with a sword, hence they can be
uplifted with but small force. But thereafter when the wings
are twisted by being drawn transversely and by the resistance of
the air, they are flattened as has been declared and will be
made manifest hereafter.”
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