Our planet is a spheroid; that is, a sphere flattened at the two poles and enlarged at the equator, so that all the circles passing through the extremity of the polar axis form ellipses. The presumed depression of each pole is about thirteen miles, nearly a three-hundredth part of the radius of the earth; but it is not altogether certain that the two poles are equally flattened. Perhaps a contrast exists between the two hemispheres, not only in the features of their continents and the distribution of seas, but also in their geometrical shape. Be this as it may, it appears to be proved that the curvature is not exactly the same at all points of the earth at an equal distance from the poles; the meridians appear without exception to be irregular ellipses. The dimensions of the earth, as we have already seen, are almost as nothing compared with the larger celestial bodies, and especially with the extent of space which can be explored by the telescope. If light, the speed of which has been adopted in astronomy as a term of comparison, could be diffused in a curved line, it would travel seven times round the globe in a second of time; this standard of measurement, therefore, the only one suited to the stellary field, is completely inapplicable to the surface of our globe. The isolated globule in the immensity of space which we call the earth is not motionless, as the ancients necessarily supposed, looking upon it, as they did, as the immovable base of the firmament of heaven. Hurried on in the vortex of universal vitality, our globe is ever actuated by ceaseless motion, describing in ether a series of elliptic spirals so complicated that astronomers have not yet been able to calculate their various curves. Besides rotating on its own axis, the earth describes an ellipse round the sun, and, under the influence of this body, is drawn along from one heaven to another toward distant constellations. It also oscillates and rocks on its axis, and deviates more or less from its path, to salute, as it were, every heavenly body which meets it. It is probable that it never passes a second time through the same regions of the air; yet, if it has again to traverse the spiral line of ellipses it has already described, it would be after a cycle of so many thousands of millions of years, that the earth itself, completely transformed, would be no longer the same planet.