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 with the most difficult problem that
confronts the flying man of to-day, namely, landing effectively,
and his remarks on this subject would be instructive even to an
air pilot of these days: 'Now the ways and means by which the
speed is slackened at the end of a flight are these. The bird
spreads its wings and tail so that their concave surfaces are
perpendicular to the direction of motion; in this way, the
spreading feathers, like a ship's sail, strike against the still
air, check the speed, and so that most of the impetus may be
stopped, the wings are flapped quickly and strongly forward,
inducing a contrary motion, so that the bird absolutely or very
nearly stops.'

At the end of his study Borelli came to a conclusion which
militated greatly against experiment with any heavier-than-air
apparatus, until well on into the nineteenth century, for having
gone thoroughly into the subject of bird flight he states
distinctly in his last proposition on the subject that 'It is
impossible that men should be able to fly craftily by their own
strength.' This statement, of course, remains true up to the
present day for no man has yet devised the means by which he can
raise himself in the air and maintain himself there by mere
muscular effort.