e, 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.'

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 the wing is maintained and accelerated. On
the other hand, the impulse of the wing is continuously diminished and
weakened by the growing resistance. Hereby the force of the wing and the
resistance become balanced; so that, manifestly, the air is beaten by
the wing with the same force as the resistance to the stroke.'

He concerns himself also wi