ART. 50. _Aether: its Motions._--The question of the exact motions of the Aether is a question which has involved the attention of scientific men for many years, and which is at the present time receiving the attention of some of our most advanced scientists, not only in this country but in other countries also. Whether the Aether in space is at rest, or is moving along with all the bodies that float in it, so to speak, is a question of the greatest importance to scientists and philosophers generally, as the particular character of the motions of the Aether, which are either suggested or ascribed to it from the analogies of Nature, are sure to have a most important bearing not only on the motions of all the planets and satellites, but also upon such questions as the aberration of light, and such difficulties as presented by Lord Kelvin in his paper on "Clouds on the Undulatory Theory of Light" (_Phil. Mag._, July 1902). I need hardly point out that the hypothesis that Aether is gravitative, is bound to play a most important part in the consideration and development of this phase of the study of the universal aetherial medium. It is not my intention, however, at this stage of the work to go fully into the development of this aspect of the subject. The application of this principle will be considered at the right time, and in the right place. It is, however, generally assumed, that the Aether is at rest in space, and that the earth, the planets, and the sun and all stars, move through it with varying velocity, although, as Lord Kelvin points out, such an assumption is covered with a cloud which up to the present is "as dense as ever." Of course, if the Aether be at rest, and the planets and other heavenly bodies move through it with varying velocity, then the only assumption regarding the Aether is, that it is frictionless, but, as I have shown in Art. 45, this is opposed to all philosophical reasoning, and therefore to experience and observation. We have, therefore, to postulate for the Aether such motions as shall fulfil all the Rules of Philosophy, that is, shall be simple in conception, shall be in harmony with our experience and observation, and which shall satisfactorily account for the phenomena sought to be explained, that is, the universal Law of Gravitation; for it is by the properties, combined with the motions of the Aether, that the physical cause of Gravitation is alone to be explained. Let us revert to the question of a stationary Aether for a moment or two, and let us ask ourselves, where is the evidence for such an assumption? Has the sun ever ceased to shine, or to send its light-waves with their enormous velocity speeding through the solar system? So far as experience and observation go, I have never read of any record of such a fact, or that light-waves have ceased to proceed from the sun and fill the solar system with Aether-waves.