Earth, the planet third in order from the sun. Mercury and Venus are nearer the sun. The general shape of the earth is spherical. Whenever the earth's shadow falls on the moon it has a circular outline. The curvature of the earth's surface is such that, if three stakes in a line be driven half a mile apart, so that the tops of the first and third are on a water level with the top of the second, it will be found, on sighting across, that a straight line from the top of the first stake to the top of the third cuts about eight inches below the top of the second stake. In other words the surface of a lake curves about eight inches per mile. Scientists have actually made numerous measurements-a score or more-of considerable portions of a meridian, that is to say, a north and south line. One measurement extended from Hammerfest, in the north of Norway, to the mouth of the Danube; another from the Himalayas to the Southern point of Hindustan. From these measurements it has been found that a degree of a meridian is 3,000 feet longer in Sweden than in southern India; in other words, that the earth is flattened at the poles. Various computations agree so well that it is believed the following dimensions of the earth are correct, to within a fifth of a mile. Equatorial diameter . . . . . 7,926.614 miles. Polar diameter . . . . . 7,899.742 miles. Difference . . . . . 26.872 miles. Average diameter . . . . . 7,920 miles. Equatorial circumference . . . . . 24,912 miles. Area of surface . . . . . 196,971,984 sq. miles. Weight in pounds including atmosphere . . . . . 6,666,225,819,600,000,000,000 The earth has three motions and possibly four: 1. It rotates on its axis once in each twenty-four hours. The rotary speed of the earth's surface varies from zero at the poles to one thousand miles an hour on the equator. 2. It revolves about the sun in an elliptical orbit once in each year, flying at a rate of about 66,600 miles an hour in its travels. This orbit is fairly regular, yet it is thought that the earth wobbles on its axis a trifle, and is jolted out of its path 4,000 miles or so. 3. It follows the sun in its travels at an estimated rate of 150,000,000 miles a year. 4. Very possibly the universe may be changing its position in space. The density of the earth's crust is about three times that of water. The average density of the earth is 5.5 times that of water, from which it is argued that the interior is about eight or nine times as dense as water. Inasmuch as pressure tends to solidify, and heat tends to liquefy, it is not known whether the interior is solid or fluid; but it is pretty well agreed that the interior is dense and intensely hot. See GEOGRAPHY; EQUATOR; ALTITUDE; LATITUDE; LONGITUDE; SEASON.