A Body moving it self, will have so much the greater velocity, as the Centre of its Gravity or Line of Direction, is further removed from the Centre of that Part, on which the Body is sustained: This we mention principally, with regard to the Motion of Birds; which, without any wasting of their Wings, or any Assistance from the Wind, are frequently seen to Sail through the Air: Now, this happens, when the Centre of their Weight, is out of the Centre of their Support; that is, out of the Middle of their Wings: Since, if the Middle of the two expanded Wings, be behind the Middle, or Centre of the Bird's Weight, the Motion of the Bird, will, at the same Time, be both forwards and downwards; but then 'twill be so much the more, or less, forwards, than 'tis downwards, as the Centre of its Gravity, is more or less remote from the Middle of its Wings; That is, the Centre of Gravity, being removed from the Middle of the Wings, the Descent of the Bird is thereby render'd more Oblique; and on the Contrary, as that Centre approaches nearer the Middle of the Wings, the Direction of the Birds descent, will become more Perpendicular.