The inner ring of Saturn is supposed to be composed of watery vapour, as it is somewhat transparent, but the outer ones are solid, which is shown by the shadow they cast upon the planet, and the shadow it casts upon them in different positions (figs. 22 and 23). Besides these rings Saturn has eight satellites or moons, which revolve in a plane nearly parallel to that of the rings and exterior to them. It has been calculated that Saturn weighs only 100 times more than the earth, although it is somewhere about 900 times larger, from which it is concluded that the substance of which Saturn is made must be about one-ninth the density of this earth, half the density of water, or about the same as cork. Saturn is very much flattened at the poles, so much so that the equatorial diameter is a tenth more than the axial diameter, which difference is distinctly visible through good glasses. Still further into space, at double the distance of Saturn, or nearly 1822 millions of miles from the sun, another great world or planet revolves round it, it is called Uranus. This planet is not so large as either Jupiter or Saturn, but is of considerable magnitude, being eighty times that of this world, it takes eighty-four of our years to complete its vast circle round the sun, which are therefore equivalent to but one year of Uranus. [Illustration: FIG. 22.] [Illustration: FIG. 23.] It has several satellites, four of which have been discerned perfectly, but it is doubtful whether there have not been two more seen. This orb was first recognised as a planet by Sir Wm. Herschell, after whom it was for some time named. Another, and the most remote planet in our system, is Neptune, revolving at the immense distance of 2850 millions of miles from the sun, and taking more than 164 years to perform the journey. This planet is only to be seen by the most powerful glasses, and was discovered under very peculiar circumstances, not (like other planets) by chance; its existence was recognised as necessary to account for certain "perturbations" or deviations in the orbit of Uranus, which was found to take a course differing from what it should, according to computation, and which were only to be accounted for by supposing that another planet existed far out in space, which affected the course of Uranus by its attraction. The idea of finding out where this orb should be, occurred to M. Le Verrier and Mr. Adams, independent of each other; they both arrived at nearly similar conclusions, for the positions assigned to the supposed planet so nearly agreed with each other and with its real position, that their calculations have been looked on as the greatest feat of astronomical research. It was discovered by Dr. Galle of Berlin, upon his hearing from Le Verrier the position in which at that time it should be sought for. THE PLANETOIDS.