In view of such observations as that of Prof. E.E. Barnard, in 1892, showing that a satellite passing through the shadow of Saturn's rings does not entirely disappear--a fact which proves that the rings are partially transparent to the sunlight--one might be tempted to ask whether Saturn itself, considering its astonishing lack of density, is not composed, at least in its outer parts, of separate particles of matter revolving independently about their center of attraction, and presenting the appearance of a smooth, uniform shell reflecting the light of the sun. In other words, may not Saturn be, exteriorly, a globe of dust instead of a globe of vapor? Certainly the rings, incoherent and translucent though they be, reflect the sunlight to our eyes, at least from the brighter part of their surface, with a brilliance comparable with that of the globe of the planet itself. As bearing on the question of the interior condition of Saturn and Jupiter, it should, perhaps, be said that mathematical considerations, based on the figures of equilibrium of rotating liquid masses, lead to the conclusion that those planets are comparatively very dense within. Professor Darwin puts the statement very strongly, as follows: "In this way it is known with certainty that the central portions of the planets Jupiter and Saturn are much denser, compared to their superficial portions, than is the case with the earth."[13] [Footnote 13: The Tides, by G.H. Darwin, p. 333.] The globe and rings of Saturn witness an imposing spectacle of gigantic moving shadows. The great ball stretches its vast shade across the full width of the rings at times, and the rings, as we have seen, throw their shadow in a belt, whose position slowly changes, across the ball, sweeping from the equator, now toward one pole and now toward the other. The sun shines alternately on each side of the rings for a space of nearly fifteen years--a day fifteen years long! And then, when that face of the ring is turned away from the sun, there ensues a night of fifteen years' duration also. Whatever appearance the rings may present from the equator and the middle latitudes on Saturn, from the polar regions they would be totally invisible. As one passed toward the north, or the south, pole he would see the upper part of the arch of the rings gradually sink toward the horizon until at length, somewhere in the neighborhood of the polar circle, it would finally disappear, hidden by the round shoulder of the great globe. URANUS, NEPTUNE, AND THE SUSPECTED ULTRANEPTUNIAN PLANET