Professor Challis points this out very conclusively in the _Phil. Mag._ of 1859, where, writing on this point, he states: "It may also be remarked, that if the Law of Gravity be absolute, there is no security for the stability of a system of stars, whether the system be a Milky Way or a nebulous cluster. For, however small the mutual attraction between the constituent bodies may be, in the course of ages it must produce a general movement towards the centre or densest region. But the form of the Milky Way and of certain nebulae seems to present an utter contradiction to any such tendency." With the conception, however, of a physical centrifugal force or motion due to the pressure of a physical medium, the stability of even the Milky Way may be physically conceived and understood. Again, when we consider the sun as a star, we find that it has two motions of its own, one of rotation on an axis, and the other of translation in an orbit, such rotation being due to the fact that it is a magnet and has ever circulating round it electro-magnetic Aether currents (Art. 91). By inference, therefore, we arrive at the fact that every star is a magnet, as suggested by Professor Schuster, and possesses rotation on an axis, such rotation being due to exactly the same cause as produces the rotation of any other planetary or solar body (Art. 92). Not only has each star a rotation on its axis, but it must also possess translational motion in an orbit, and that orbital motion must be due to exactly a similar cause as that which produces the orbital motion of the sun. Are there any indications given by astronomical observations which lead us to the conclusion that stars do possess such orbital motions? The answer is unanimously in the affirmative; for, although all the stars and the constellations retain apparently the same relative position to each other, yet they are all in motion. The actual translational motion of the stars is termed proper motion, and has been calculated with more or less success in relation to many of the stars nearest to us. There are other motions of the stars known as apparent motions, which are easily noted by any observer. These apparent motions are due to the rotation of the earth on its axis, and its orbital motion round the sun. Nothing is more certain, however, than that careful astronomical observations have revealed the fact that stars have actual orbital motions of their own through space. In many cases the orbital velocity has been approximately ascertained. Halley discovered proper motions of certain stars as far back as 1715, when he found out, by comparing different observations, that Sirius, Arcturus, and Aldebaran had moved during the period which had elapsed since the respective observations were taken.