These bolides and uranoliths come to us from the depths of space; but they do not appear to have the same origin as the shooting stars. They may arise from worlds destroyed by explosion or shock, or even from planetary volcanoes. The lightest of them may have been expelled from the volcanoes of the Moon. Some of the most massive, in which iron predominates, may even have issued from the bowels of the Earth, projected into space by some volcanic explosion, at an epoch when our globe was perpetually convulsed by cataclysms of extraordinary violence. They return to us to-day after being removed from the Earth to distances proportional to the initial speed imparted to them. This origin seems the more admissible as the stones that fall from the skies exhibit a mineral composition identical with that of the terrestrial materials. [Illustration: FIG. 59.--A Uranolith.] In any case, these uranoliths bring us back at least by their fall to our Earth, and from henceforward we will remain upon it, to study its position in space, and to take account of the place it fills in the Universe, and of the astronomical laws that govern our destiny. CHAPTER VIII THE EARTH Our grand celestial journey lands us upon our own little planet, on this globe that gravitates between Mars and Venus (between War and Love), circulating like her brothers of the solar system, around the colossal Sun. The Earth! The name evokes in us the image of Life, and calls up the theater of our activities, our ambitions, our joys and sorrows. Does it not, in fact, to ignorant eyes, represent the whole of the universe? And yet, what is the Earth? The Earth is a star in the Heavens. We learned this much in our first lesson. It is a globe of opaque material, similar to the planets Mercury, Venus, Mars, Jupiter, etc., as previously described. Isolated on all sides in space, it revolves round the Sun, along a vast orbit that it accomplishes in a year. And while it thus glides along the lines of solar attraction, the terrestrial ball rotates rapidly upon itself in twenty-four hours. These statements may appear dubious at first sight, and contradictory to the evidence of our senses. Now that the surface of the Earth has been explored in all directions, there is no longer room to doubt that it is a globe, a sort of ball that we adhere to. A journey round the world is common enough to-day, and always yields the most complete evidence of the spherical nature of the Earth. On the other hand, the curvature of the seas is a no less certain proof. When a ship reaches the dark-blue line that appears to separate the sky from the ocean, it seems to be hanging on the horizon. Little by little, however, as it recedes, it drops below the horizon line; the tops of the masts being the last to disappear. The observer on board ship witnesses the same phenomenon. The low shores are first to disappear, while the high coasts and mountains are much longer visible.