It is thus that, when we stand in the middle of an extensive plain, we judge distant objects—those that border the horizon—to be immovable; while we instantly perceive the slightest change of place in the near ones. It is true that when we ourselves move, the real movements become complicated with the apparent movements, but the former must be distinguished, if we wish to have an exact idea of the actual course traveled. This complication of the apparent movements of the planets—a necessary consequence of the movement of the earth—is one of the most striking testimonials to the reality of the latter; but it must also be added, that this was precisely the stone of stumbling of ancient astronomy until the time—and that not long ago—when the real movements were made known. Movements of rotation, movements of revolution, around the common centre, the duration of these movements, distances, forms and dimensions, distribution of light and heat, all change in passing from one planet to another. And yet, marvelous thing, the same laws govern, all in such a way that the unity of plan is not less marked than the astonishing variety of the phenomena. One circumstance common to all the bodies of the solar system forcibly strikes the imagination. It is, that these enormous masses—these globes, many of which are much heavier than the earth, and lastly, the earth itself—are not only suspended in space, but move through the ether with velocities truly stupendous. Imagine yourself a spectator, standing immovable in space. A luminous body appears in the distance, little by little you see it approach and increase in size; its immense circumference, which exceeds a hundred thousand leagues, is in rapid rotation, which makes each point on its periphery travel through nine miles a second. The globe itself passes before you, carried through space with a velocity twenty-four times greater than that of a cannon-ball. In such a way Jupiter would appear to you traveling in its orbit. This headlong course would banish it forever to the most remote regions of the visible universe, if it were not subdued and held by the powerful attraction of a globe a thousand times larger than its own—by the sun himself. Not only does astronomy show, by undeniable proofs, the reality of these marvelous movements—not only has she arrived at the knowledge of their invariable constancy, at least during thousands of centuries; but she has found in their very rapidity the cause of the stability of all the celestial bodies.