One of these whirling bodies was the Earth, and when it had comfortably settled in its orbit and slowed down a bit it began to cool off and a crust was formed on its liquid surface, just as ice is formed when water is frozen. Then some of the gases condensed into water and others became air and when the Earth had cooled down still further some millions of years afterward it became a more or less suitable place for human beings to live on. While the Earth has cooled off until it is possible for us to live comfortably on its crust, it is still warm outside and very hot inside. This is proved by volcanoes which throw out white hot lava and gases when they are in eruption. If it had not been for the light and heat of the Sun in the past ages there never could have been any kind of life on the Earth. The Earth is just as dependent on the Sun now as it was in the past, and if he should fail to shine on our world for even a little time everything would die. Fig. 70 is a cross section of the Earth. =To Prove the Earth is Round.=—Every boy knows that the Earth is round, but the wisest of men did not know it for certain until about 400 years ago, when one of Magellan’s ships made a complete voyage round the world and returned to the place she started from. [Illustration: FIG. 70.—CROSS SECTION OF THE EARTH.] (1) One of the easy ways to show that the earth is round, or at least that the surface of the earth is curved, as shown in Fig. 71, is to watch a ship as she sails out to sea. All of the ship—hull, sails and smokestack, if she has one—can be seen until she sails over the horizon, and then her hull, which is the largest part of her, is lost to view, then her sails and stack, and finally only the tops of her masts can be seen. It is a pretty good sign that the Earth is a ball. [Illustration: FIG. 71.—SAILS OF SHIP CAN BE SEEN AFTER HULL HAS DISAPPEARED.] (2) A very pleasant way to prove the Earth is a ball is to take passage on a ship that makes a round-the-world cruise. If you leave New York and keep sailing east all the time you will finally land at San Francisco; keep on going east by rail and you will find yourself back in dear old New York, where you started from. See Fig. 72. =To Prove the Earth Turns on Its Axis.=—(1) Having proved that the Earth is round, the next thing to do is to prove that it turns about on its axis. (1) The best known experiment for showing that the Earth really turns on its axis was made by Foucault (pronounced Foo-ko´), a French philosopher, in 1851, who used a pendulum for the purpose. [Illustration: FIG. 72.—SAILING ROUND THE EARTH.] In the top of a great dome in a building in Paris, called the _Panthéon_, Foucault hung a large metal ball by means of a wire about 150 feet long. On the floor he made a mark, exactly under the ball, running due north and south. Then drawing back the ball, he let it go, when it swung directly over the line.