The motion of the earth, the immediate effects of which are the most obvious to the notice of men, is the daily rotation which takes place round an ideal axis passing through the two poles. The globe turns from right to left, or from west to east—that is, in a contrary direction to the apparent motion of the sun and stars, which seem to rise in the east and to set in the west. As the earth’s axis terminates at each pole, there is least surface-motion at those points, and the motion is the more rapid in any part of the surface of the globe the further it is from the central axis. At St. Petersburg, in 60° latitude, the speed of rotation is about nine miles a minute; in Paris, it exceeds eleven and a half miles during the same brief time; on the equatorial line, which may be looked upon as the ring of an immense wheel, the speed of the earth is twice as great as it is at 60° of latitude—that is, about eighteen miles a minute, or 528 yards a second—a rapidity equal to the flight of a 26-pound cannon-ball impelled by thirteen pounds of powder. By means of this rotatory motion, the earth presents toward the sun each of its faces alternately, and each also in turn toward the comparatively darker regions of space; the succession of day and night is thus constituted. In addition to this, the rotation of the earth is an important fact which must always be taken into account in determining the direction of fluids in motion on the surface of the globe, such as streams and rivers, also marine and atmospheric currents. The annual revolution which the earth performs round the sun follows the line of an ellipse, one of the _foci_ of which is occupied by the central star; the eccentricity of the ellipse is nearly equal to 17/1000th of the great axis. The distance between the sun and the earth always varies according to the particular point of its orbit which the latter is traveling over. At its _aphelion_, that is, at its greatest remoteness, this distance is about 93¾ millions of miles; at the period of its _perihelion_, when the two heavenly bodies are nearest to each other, it is approximately 90,259,000 miles. The mean distance, as estimated by astronomers since the corrections of Encke, Hansen, Foucault, and Hind, is 91,839,000 miles. This extent of space is traversed by the solar rays in 8 minutes, 16 seconds; sound would take fifteen years in passing through the same distance. As Kepler has laid down in his celebrated laws, our planet moves with an increased rapidity as it approaches nearer to the sun and travels more slowly in proportion to its distance from that luminary; but its mean speed may be estimated at nearly nineteen miles a second, or sixty times the rapidity of a ball from the cannon’s mouth. This speed, which makes one dizzy to think of, is to be added, as regards each point in the surface of the earth, to the rotatory motion which impels it round the polar axis.