CHAPTER II.

FORM AND ROTATION OF THE EARTH.

15. The shape of the Earth.-Several facts prove that the earth is round or spherical.

1. When there is nothing to obstruct or extend the view, the part of the earth's surface seen from any point

is a circle. The circle is made larger by raising the observer, as on a spire, on a mountain, or in a balloon. At ordinary heights, a person can see no farther on the earth, with a telescope than without; he only sees more distinctly.

2. The surface of the sea is curved, as is shown by the way in which a ship disappears when it sails from

Fig. 6.

the shore. First the hull goes down behind the horizon, then the sails, finally the mast-heads. If the ship moved on a flat surface, hull, sails, and masts would all dwindle to a point, and vanish together; they would appear again in a telescope.

A monastery, three stories high, stands on the top of Mt. Toro, in the center of the island of Minorca, in the Mediterranean Sea. As vessels come to the island in any direction, the first thing seen by the sailors is the roof of the monastery; then the windows of each story in succession; then the whole building, as if standing on the Presently the mountain appears to rise, and at last the island and its coasts are seen. Similar or opposite facts are observed whenever a vessel leaves or approaches the land.

3. The world has been circumnavigated by many mariners from the days of Magellan until now.

4. The shadow of the earth, as seen in eclipses of the moon, is always round. A sphere is the only solid whose direct shadow is always round.

16. The size of the earth.-From the height of a mountain and the distance at

which its top is visible at sea, the size of the earth may be computed approximately. Let DBC be a great circle of the earth; A, the top of a mountain; B, the farthest point on the circle, from which the mountain may be seen. Then AB is tangent to the circle at B; AD is a secant, and AC its external segment. Therefore, Geom. 333,

Fig. 7.

Example. Suppose the mountain, 2 miles high, is seen

at a distance of 126 miles,

AC 7936 miles.

=

17. We do not hesitate to say that an orange is round although its rind is rough, yet its roughness in proportion to its size is much greater than that of the earth's surface where broken by the loftiest mountains. If the highest peaks of the Andes or the Himalayas were accurately represented on an 18 inch globe, they would project from the general surface only about .013 of an inch, the thickness of a sheet of paper, or of a grain of sand.

18. The apparent revolution of the sky.-The most casual observer sees an apparent daily motion of the heavens. The sun rises in the east, passes over through

At night, the stars rise

the south, and sets in the west. and set, in like manner. In the northern sky, the stars seem, to the people in our latitude, to move about a fixed point, which is about half-way between the horizon and the zenith. Patient watching from hour to hour and from night to night, shows that all the stars, in the south as well as in the north, appear to move about this point, at greater or less distances. The whole circular path of a star is above the horizon, and is visible, only when the distance of the star from the fixed point, that is, the radius of the apparent motion, is less than the altitude (13) of the point.

.19. The earth rotates.-The apparent motion of the heavens from east to west is caused by the actual rotation of the earth from west to east, once in twenty-four hours. All the appearances are the reverse of facts. The sun does not rise, but the horizon sinks below the

sun. A star does not come to the meridian, but the meridian sweeps by the star.

is not felt, because it is When gliding in a boat

20. The motion of the earth uniform, and we move with it. on a smooth stream, we often seem to be at rest, while men, trees, and houses pass swiftly by us, yet our reason · teaches that we move and that the land is still. Persons who ascend in a balloon see the ground fall quickly from under them, and when they descend the earth seems to rise up to meet them. So, though our senses tell us that the sun and stars rise and set, it is more reasonable that this seeming is caused by the actual rotation of the earth, than that the sky, with all the heavenly bodies, immensely distant from us, moves about the earth in so short a time.

21. Galileo. The doctrine of the rotation of the earth was taught by several ancient philosophers. Copernicus revived it in 1543, and Galileo believed and taught it in the next century. In 1637, at the age of 70, Galileo was forced to read and sign a denial of this belief. It is related that when he rose from his knees after this abjuration, he struck the earth with his foot, and said in an undertone: E pur si muove, "and yet it moves," but there is no evidence that he was so imprudent.

DEFINITIONS.

22. The axis of the earth is the line about which it rotates. The points where the axis meets the surface are the north and south poles. The direction in which the earth turns is east; that from which it moves is west.

If a plane pass through the center of the earth perpendicular to the axis, the line in which it cuts the surface is called the equator. The plane of the equator divides the earth into two halves, the northern and south

ern hemispheres. Meridians are circles on the surface of the earth which pass through the north and south poles; they are perpendicular to the equator.

23. The longitude of a place on the earth is the distance of its meridian, east or west from an assumed meridian; it is measured in degrees on the equator. English astronomers reckon from the meridian of Greenwich observatory; French, from the observatory at Paris; American, from Washington. Astronomers have never agreed upon a common meridian, and the vanity which makes each nation adopt a meridian of its own, only creates confusion. Our globes, maps, and tables usually refer to the meridian of Greenwich.

24. The latitude of a place on the earth is its distance north or south from the equator, measured in degrees on a meridian. The latitude of the poles is 90°. Parallels of latitude are small circles on the earth's surface, parallel to the equator. Places which have the same distance north or south of the equator are said to be on the same parallel; those which are on the same line from the equator to the poles have the same meridian.

FOUCAULT'S EXPERIMENT.

25. The rotation of the earth made visible.-M. Foucault fastened one end of a fine steel wire to the under surface of a high ceiling; to the lower end of the wire he hung a heavy copper ball, carrying below it a steel pointer. This pendulum swung over a place so hollowed out that the pointer would move just over the surface; about the edge of the hollow was laid a ridge of fine sand which the pointer should pass through at each vibration. That the pendulum might not be moved by any other impulse than the simple attraction of the earth, he drew it aside from its vertical position and tied it by a thread;