The third planet from the sun is the Earth, distant about 95,000,000 miles, with a diameter of 7925 miles, so that the most lofty mountains (five miles high) bear about the same proportion to it as would an elevation one-fourteenth of an inch in height to a ball of ten feet in diameter; therefore, with all its valleys and high hills, the surface of the earth is smoother in proportion to its bulk than the rind of an orange. The earth is not a perfect sphere, but has a slightly flattened form, as though it had been compressed at the poles. The diameter at the poles is the smallest, and is 7899 miles, while that at the equator (its greatest) is 7925 miles, being a difference of twenty-six miles; but this is so small, when compared with the size of the earth, that if an exact model of it were made, four inches in diameter, it would require the most accurate measurement to determine that it was not a perfect sphere. [Illustration: FIG. 7.] The form of the earth has been demonstrated by accurate experiments and calculations, but there are many things we may observe, convincing us of its rotundity; one of the most evident of these is the fact that as a ship at sea goes from the land, the hull first disappears, then the lower sails, and lastly the tops, while a ship approaching the shore shows first the topsails, and lastly the hull; see fig. 7, where A shows a ship on the horizon, B hull-down, C out of sight. In the same way, the tops of distant mountains are seen at sea long before the lower lands. Another proof is drawn from the fact, that ships have been sailed quite round the earth; by steering as nearly as possible in one direction, they have arrived at the place from which they started. A third proof of the earth's rotundity is found in the form of its shadow, when the moon is eclipsed by it; this shadow, as thrown by the earth upon the moon, is circular, in whatever position the earth may be, and a sphere is the only solid form which can in all positions cast a circular shadow. The earth turns upon its axis every 23 hours, 56 minutes, and 4 seconds, which constitutes a day, and makes one revolution round the sun in every 365-1/4 days, which make up a year; but the earth's axis is not at a right angle to an imaginary line drawn from the earth to the sun, but at an angle of 23 deg. 28 min. to it, as shown at fig. 8, where the line from N to S is the earth's axis of rotation, the dotted line leading to S is the sun's direction, and E Q is the equator. [Illustration: FIG. 8.]