Text: The phenomenon of planetary motion is expressed as resulting from the operation of two recognized forces, viz: centripetal and centrifugal - one directed to and one from the center. It is usual to say a body falling to the earth does so in obedience to the law of gravitation, or of centripetal force; and that in doing so, it falls in a line perpendicular to the surface of still water. [An Essay Upon Force and its Effect Upon Matter]

See Also:

Source: 159