No impetus made by any motion whatever can be immediately consumed; and if it meets an object of great resistance, it is consumed by reflex motion. The impetus acquired by the beats of the wings in the oblique descent of birds is the cause that those birds descend for a long space without beating their wings and along the said obliquity. Define what impetus is and what oblique motion is, and which is the greater or lesser obliquity, and how the reflex motion of birds becomes more or less oblique according to the greater or lesser opening of tail and wings. The acquired impetus will be the more permanent the less oblique the motion of the descent is.

b f a - g - e - h - d c

If the bird a departs with impetus from a along the line a b, it has lost the impetus at f; and if the impetus is made along the line a c, the impetus will end at g; and along the line a d it will end at h; and if it is made along the line a c, it will pass 7/8 of the incident motion beyond the centre of the world. Impetus is a power of the mover applied in its movable, which is the cause of moving the movable after it is separated from its mover. There will never be impetus where the resistance of the movable is not much surpassed by its mover; and especially when the dense friction of the moving bodies resists the movable. This is seen in the beam dragged by oxen, which moves only as much as the motion of those oxen, and, the cause ceasing, the effect fails—that is, the motion of the beam ends together with the motion of the oxen. When the friction of the movable upon the place where it moves is of little density, then the power of the mover will be joined to the movable, after it is separated from the mover, for as much space as the friction is of less density. As is seen in the boat drawn through the water, which, after it is separated from the power of its mover, moves by itself for some space; and the bird, after it has beaten its wings upon the condensed air, without further beating of the wings moves, carried by the impetus, for a long way. Movables and movers are of three natures: of which the one moves for as much space as is the motion made by its mover, and this is little surpassed by the power of the mover; the other moves for much less space than that in which the mover moves, and this is in itself of greater resistance than the power of its mover, making no mention of the medium interposed between the mover and movable; the third moves for a much greater space in equal time than is the space through which its mover moves, and this movable is in itself, making no mention of the medium, of much less resistance than that of its mover. There are other powers of movers and movables, of which the movable follows or is followed by its motion for as much space and straightness as is the motion of its mover; and after the mover remains without motion, the movable applies to itself the power of the mover and with that moves for a long space of way, like the arrow shot from the string of the bow, which long moves by itself after the string of the bow stops.