The first stage in the history of these fiery particles is the formation among them, in some unaccountable way, of nuclei, or centres of aggregation, like the bright points that are now visible in some of the nebulae of the heavens. As soon as these centres are formed, gravity, one of the original principles of matter, begins to act, and the atoms in all the neighbouring parts of space are attracted towards the nucleus and heaped upon it. In this manner, a central sun of vast dimensions is formed, which soon assumes a motion of rotation upon its axis from the general law which gives a circular movement to all fluids that are drawn towards a common centre. The centrifugal force thus generated tends to throw off matter from the equatorial regions of the great orb, but is restrained by the attraction of gravitation, which would prevent any separation of the parts, if the sun itself did not now begin to cool down, and consequently to shrink in size. Under this cooling process, a crust is formed upon the surface, too rigid to yield to the force of gravity, and the parts within, continuing to shrink, separate from this envelope; so that there is now a central orb, revolving more rapidly from its greater density and smaller diameter, and surrounded by an exterior shell, or band, like Saturn's ring, rotating at its original speed. As we cannot suppose that the ring would usually be of uniform thickness and strength, it eventually breaks up into fragments, the larger of which attracts the smaller into itself, and the whole is formed by its revolving motion into an oblate spheroid circling round the contracted sun in the centre. In this manner, the planet Uranus was shelled off from our sun, which originally filled the whole of the vast sphere, of which the distance from Uranus to the centre of the present sun is but the radius. The planet itself, by the same process of cooling, shrinking, and thus forming exterior rings, threw off successively all its six satellites; and the sun, also, continuing to contract from the loss of heat, formed another ring, and thus constituted the planet Saturn. In this way were formed successively all the planets and satellites of the present solar system. The original diameter of our earth was equal, of course, to the present diameter of the moon's orbit. In the case of Saturn, the two rings formed around it happened to be of unusual homogeneity and equal thickness, so that they were not broken up, but have preserved their primitive shape. A ring was formed from the sun in the space between the present orbits of Mars and Jupiter; but when it was broken up, the fragments did not congregate into one, but spherified separately, so as to form the four smaller planets which now revolve in that opening.