Isolated in space like a child's balloon floating in the air, and more absolutely too, for the balloon is carried by aerial waves, while worlds gravitate in the void, the Earth is a toy for the invisible cosmic forces which it obeys,--a real soap-bubble, sensitive to the faintest breath. Besides, we can easily judge of it by looking at the same time at the whole of the _eleven_ principal motions of the Earth, by which it is moved. Perhaps they will help us to find that "fixed point" which our philosophical ambition asks for. Thrown around the Sun at a distance of 37,000,000 leagues, and making at this distance its annual revolution around the luminous star, it consequently moves at the rate of 643,000 leagues per day; that is, 26,800 leagues an hour, or 29,450 metres per second. This speed is eleven hundred times more rapid than an express train going at the rate of a hundred kilometres an hour. It is a ball, going with a rapidity seventy-five times greater than that of a bomb, always hurrying on, but never reaching its goal. In 365 days, 6 hours, 9 minutes, and 10 seconds, the terrestrial projectile has returned to the same point of its orbit relative to the Sun, and continues its flight. The Sun, on its part, is moving in space, following a line oblique to the plane of the Earth's annual motion,--a line drawn towards the constellation of Hercules. The result is, that instead of describing an exact circle, the Earth describes a spiral, and has never passed over the same road twice in its existence. To its motion of annual revolution around the Sun there is added perpetually, as a second motion, that of the Sun itself, which draws it, with all the solar system, into an oblique descent towards the constellation of Hercules. During all this time our little globe pirouettes around itself every twenty-four hours, and gives us the daily succession of days and nights,--diurnal rotation: third motion. It does not turn upright upon itself, like a top, which would be vertical on a table, but is inclined, as everybody knows, by 23° 27'. This inclination, too, is not always the same; it varies from year to year, from age to age, oscillating slowly by secular periods. That is a fourth kind of motion. The orbit in which our planet yearly travels around the Sun is not circular, but elliptical. This ellipse itself also varies from year to year, and from century to century; sometimes it approaches the circumference of a circle, sometimes it lengthens out to a great eccentricity. It is like an elastic ring, which can be bent more or less out of shape. Fifth complication in the Earth's motion. This ellipse itself is not fixed in space, but revolves in its own plane in a period of 21,000 years. The perihelion, which at the beginning of our era was at 65 degrees of longitude, starting from the vernal equinox, is now at 101 degrees. This secular displacement of the line of the apsides brings a sixth complication to the motion of our abiding-place.