Persuasion for the undertaking, which removes the objections.

If you should say that the sinews and muscles of the bird are beyond comparison of greater power than those of man, inasmuch as all the fleshiness of so many muscles and the pulp of the breast are made for the benefit and increase of the motion of the wings, with that bone of one piece in the breast, which provides very great power to the bird, with the wings all woven of thick sinews and other very strong ligaments of cartilage and very strong skin with various muscles, here it is answered that such strength is provided so that, beyond its ordinary sustaining of the wings, it can at will double and triple its motion to flee from its predator or to pursue its prey. Whence in such an effect it needs to double or triple its force, and beyond this to carry through the air as much weight in its feet as is the weight of its own self. As one sees the falcon carry the duck and the eagle the hare, by which thing it is well shown where such excess force is distributed. But little force is needed for it to sustain itself and balance itself on its wings and to fan them over the course of the winds, and to steer the rudder along its paths; and little motion of the wings suffices, and the motion is the slower the larger the bird is.

Man too has a greater sum of force in his legs than his weight requires. And that this is true, set a man standing upon the sand and note how deep the print of his foot sinks. Then put another man on his back and you will see how much more it sinks. Then take the man off his back and make him leap upward straight as high as he can, and you will find the print of his foot to be more deeply sunk in the leap than with the man on his back. Thus here by two ways it is proved that man has double the force needed to sustain himself.

Bags by which a man falling from a height of 6 braccia may not hurt himself, falling into water as well as onto land. And let these bags, tied in the manner of rosary beads, be wound around the person.