Neither round nor rectilinear objects therefore rebound at right angles, because the perpendicular divides the objects in motion into two parts depthways, and there cannot be several perpendiculars to the same plane surface cutting one another, which will happen if a perpendicular is formed at the moment of their impact at the point where the object in motion strikes the plane surface; so that the original perpendicular alo which it travelled must necessarily be cut by the new perpendicular.