If we have found that in the rotations of the earth and of the sun, and in the revolutions of the planets round the sun, the conditions ultimately aimed at have not yet been reached, why should we feel surprised that in the outer planets of our system, Uranus and Neptune, the conditions which evolution tends to produce have not yet been fully attained? That the operation of the conservation of moment of momentum is in progress in the internal economy of the Uranian system, we have already had occasion to explain in Chapter XI. The fact which Newcomb demonstrated, that the four satellites revolve in the same plane, can only be accounted for by the supposition that in that system the conservation of moment of momentum, with declining energy, has gradually imposed this condition on the system belonging to Uranus. With reference to the position of the plane of the satellites, in the case of Uranus and Neptune, we would say, that though at present their arrangement appears anomalous, it will probably not always remain so. The fact that the satellites of Uranus are in a plane nearly perpendicular to the plane of the ecliptic really implies that there is a certain amount of energy still disposable in our system, if by readjustment of the plane of the Uranian satellites the necessary moment of momentum in the system is still preserved. [Illustration: Fig. 52.—TWO-BRANCHED SPIRAL (n.g.c. 7479; in Pegasus). (_Lick Observatory._)] The laws of dynamics tell us that the orbits of planets must be gradually, if with excessive slowness, tending still further to the same plane. In this process energy can be expended by the system, while the moment of momentum is unabated. We can at least suggest what seems to be at this moment in progress in the system belonging to Uranus. It will readily be admitted that there may be a difficulty in seeing how the movement of a planet, which is going in the wrong direction, could be stopped and turned into the right direction. But we need not suppose that so violent a change as this would imply is to be expected in our system. We are quite accustomed to find the planes of the orbits of all planets in gradual movement. The plane containing the orbits of the four satellites of Uranus is at this moment probably moving gradually upwards. It will in due course become actually at right angles to the ecliptic, and we may then reasonably assume that it will advance further in the same direction. At the moment the right angle is passed, this continuous movement will have the effect of changing the directions of the satellites’ movement from retrograde to direct. The present anomaly will then tend to become evanescent, for, as the exhaustion of the energy continues, the planes of the satellites of Uranus will gradually come into conformity with the plane of the ecliptic.