the justice which is due to him, and only claim for ourselves
the merit of having furnished the experimental demonstration of
a truth already suspected.' In fact, all subsequent studies on
this subject concur in making Borelli the first investigator who
illustrated the purely mechanical theory of the action of a
bird's wings.

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.