For the elements have the property of moving back to their place in a straight line, but they have no properties which would cause them to remain where they are, or to move other-wise than in a straight line, These rectilinear motions of these four elements when returning to their original place are are of two kinds, either centrifugal,vziz.>the motion of the air and the fire; or centripedal,viz.> the motion of the earth, and the water; and when the elements have reached their original place, they remain at rest.