The next principle is of a different character. Half a century ago celestial mechanics dealt with bodies chiefly as points. The Earth was treated as a weighted point, and so was the Sun. This was possible because a sphere acts upon outside bodies as if all its mass were collected at its centre, and the Sun and many of the planets are practically spheres. But when it came to nicer questions of their present behavior and especially of their past career, it grew necessary to take their shape into account in their mutual effects. One of the results was the discovery of the great rôle played in evolution by tidal action. Inasmuch as the planets are not perfectly rigid bodies, each is subject to tidal deformation by the other, the outside being pulled more than the centre on one side and less on the other. Bodily tides are thus raised in it analogous to the surface tides we see in the ocean, only vastly greater, and these in turn act as a brake on its rotation. Now the retrograde motions occurring in the outermost parts of all the systems, principal and subsidiary, only and always there: the retrograde rotations of Neptune and Uranus, the retrograde revolutions of the ninth satellite of Saturn and of the eighth of Jupiter, point to something fundamental. For when we consider that it is precisely in its outer portions that any forces shaping the development of the system have had less time to produce their effect, we perceive that apparent abnormality now is really survival of the original normal state, only to be found at present in what has not been sufficiently forced to change. It suggests that the pristine motion of the constituents of the scattered agglomerations which went to form the planets was retrograde, and that their present direct rotations and the direct revolutions of most of their satellites have been imposed by some force acting since. Let us inquire if there be a force competent to this end, and what its mode of action. Let us see how tidal action would work. Tidal force would raise bulges, and these, not being carried round with the planet’s rotation except to a certain distance, due to viscosity, must necessarily act as brakes upon the planet’s spin. In consequence of the friction they would thus exert, energy of motion must be lost. So long, then, as tidal forces can come into play, the energy of the system is capable of decrease. According to the last principle we considered, the system cannot be in stable equilibrium until this superfluous energy is lost or until tidal forces become inoperative, which cannot be till all the bodies in the system turn the same face to their respective centres of attraction.