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.'

Then with reference to the resistance to the air of the wings he
explains: 'The air when struck offers resistance by its elastic
virtue through which the particles of the air compressed by the
wing-beat strive to expand again. Through these two causes of
resistance the downward beat of the wing is not only opposed,
but even caused to recoil with a reflex movement; and these two
causes of resistance ever increase the more the down stroke of