There are Difficulties in the Nebular Theory—The General Conformity of the Movements—Details of the Uranian Movements—The Anomaly in the Satellite of Neptune—Where the Difficulty Lies—The Fundamental Principle which Dynamics Offers for our Guidance—The Immense Contrast between the Nebula in its Original Form and its Final Form—Energy that could be Obtained by a rearrangement of our System—Probable Nature of the Present Change in the Plane of the Orbits of the Satellites of Uranus—The Similar Explanation in the Case of Neptune. NO one will deny that there are points in connection with the nebular theory which present difficulties which to some seem important. We shall endeavour to estimate the significance of these difficulties in this chapter. They are certain anomalous phenomena presented by the planets Uranus and Neptune. The satellites which attend upon the planets exhibit a general conformity with those movements of the planets themselves on which we have dwelt in Chapters XIV., XV., XVI. The planes in which the orbits of the satellites are contained are usually not much inclined to the plane of the ecliptic, and the directions in which the satellites revolve also agree with the general direction of the planetary movement. We find these conditions in the one satellite of the earth, in the two satellites of Mars, in the five satellites of Jupiter, in the eight or nine satellites of Saturn; but, when we come to Uranus and Neptune, the two outermost planets, we observe a striking but most instructive violation of the laws which we have found so consistently prevailing in the other parts of the solar system. Let me first mention the special circumstances of Uranus. It is now known that this planet has four satellites. Of these, Titania and Oberon were both discovered by Sir William Herschel on January 11th, 1787. The two remaining satellites, named Ariel and Umbriel, were not discovered for more than half a century later by Mr. Lassell, on October 24th, 1851. It is, however, just possible that they were previously seen by Sir William Herschel.