To escape the danger of the fall. The ruin of such an instrument can happen in two ways, of which the first is the breaking of the instrument; the second will be when the instrument turns onto its edge, or near that edge, because it must always descend at great obliquity and almost along the line of equality. As for the remedy against the instrument's breaking, it will be remedied by making it of the greatest strength along whatever line it might turn over; that is, whether edge-on, falling either head or tail foremost, or with the tip of the right or left wing, or by the halves or quarters of the said lines, as the drawing shows.

As for the turning onto any edge whatever, one must remedy this from the start by building the instrument in such a way that, in its descent by whatever aspect it may make, the remedy is found anticipated. And this will be done by giving the centre of its gravity above the centre of the weight it carries, always in a straight line and quite distant one centre from the other. That is, in an instrument 30 braccia in width these centres should be 4 braccia apart one from the other, and the one, as has been said, should be beneath the other, and the heavier below, because in descending the heavier part should always act as a guide of the motion. Besides this, if the bird should wish to fall head-downward with a degree of obliquity that would carry it upside-down, this cannot happen, because the lighter part would be below the heavier and the light would come to descend before the heavy, which thing in a long descent is impossible, as is proved in the fourth of the Mechanical Elements.

And if the bird should fall with its head below, with part of the body's obliquity turned toward the earth, then the undersides of the wings should be turned flat against the earth and the tail raised toward the loins, and the head, or rather the underside of the jaws, should also turn itself toward the earth; whence immediately there will arise in such a bird its deflected motion, which will throw it back toward the sky. For which reason such a bird would come, at the end of that deflection, to fall backward, if in its climbing it did not somewhat lower one of the wings, which ...