The Earth in the Making.—The Earth on which we live and find so much that is interesting was once a part of our Sun just as the other planets were.

When the Sun was being made of star stuff great quantities of gases were set into mighty whirls, and when these acquired enough force they shot off into space like so many cannon balls, and they are still a-whirling.

But these new-born planets could only get a certain distance away from the Sun, as we have learned, for the force of his attraction offset the force of their motion with the result that they are still held in space around him.

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.

(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.

(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.

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.

The heavy pendulum, which after being started swung for hours, seemed to move away from the line in the direction of the hands of a watch. This showed that the floor of the Panthéon was really skewing around under the swinging pendulum. This is easily explained as due to the rotation of the earth because the northern edge of the floor was nearer the axis of the earth than the southern edge and therefore was carried more slowly eastward.

(2) A much simpler way to show the turning of the Earth on its axis, though this experiment only proves that either the Earth or the whole sky turns, is to make a photograph of the North Star and the stars in its neighborhood, as explained in Chapter XII.

During the time the sensitive plate is being exposed the camera will be carried round by the Earth turning on its axis and the fixed stars will leave bright trails on the plate in arcs of circles.

The Earth Turning on Its Axis Makes Day and Night.—The Earth, in turning round on its axis once every 24 hours, receives the light of the Sun on half of its surface at a time, making the day, while the other half is in the shadow, which makes the night.

If the equator of the earth was in a plane with the Sun, as shown in Pig. 74, it is easy to see that the days and nights all over the world would be of the same length, that is, each would be 12 hours long. Instead, the Earth is tilted a little, as shown in Fig. 75—to be exact, its axis is tilted 23½ degrees out of the perpendicular and this makes the day and the night at the equator each 12 hours long and the day and the night at the north and south poles each six months long.

The circle round the Sky which is in a plane with the Sun is called the ecliptic. The Sun seems to follow a path round the Earth in this plane, and this is called the path of the Sun.

To Show That the Earth Travels Round the Sun.—When we look at the Sun and the stars it is hard to believe that they are standing still and that it is the Earth which is whirling round on its own axis and also round the Sun.

We have given a couple of experiments to show that the Earth turns on its own axis, and here is one to make it clear how the Earth travels in a great circle, or rather ellipse, round the Sun and gives us our year.

Further, the Earth’s axis being tilted away from the axis of the orbit, its movement round the Sun causes the north pole to be turned toward the Sun half of the time, and then the south pole to be turned toward the Sun an equal length of time, which gives each of them a day and a night that is six months long.

In the center of a large room set a light so that it will be about as high as your eyes. Let this light represent the Sun; you must play now that you are the Earth, and think of the pictures on the wall as being the stars fixed in the sky away off in space. Now walk in a circle around the light toward the right and facing the light all the time. As you move around the light you will see that it seems to move in the opposite direction and that it seems to move past the pictures on the wall. The experiment is shown in Fig. 76.

This, then, is exactly what happens when we look at the Sun and the stars. The Earth moves round the Sun in a circle, nearly, and since the Sun is so much closer to us than any of the other fixed stars the Sun apparently moves by the stars, because the Earth changes its position relative to it and the stars.

The Earth Turning Round the Sun Makes the Seasons.—We have seen how the days and nights would be equal all over the world if the equator of the Earth was in a plane with the Sun, but since the Earth is tilted the days and nights are unequal except twice a year, and this is when the places where the ecliptic and the equator cross each other are facing the Sun.

Again, if the Earth’s equator was always in a plane with the Sun the day would be just as long as the night all through the year and there would be no seasons. But the Earth having its axis tilted, and which is always set in the same direction, together with the Earth speeding in a circle around the Sun, causes some curious things to happen and the seasons are one of them.

To make clearer the reason the axis of the Earth always stays in one position take a top and give it a good spin. The top, of course, turns round its axis very fast, and this is like the Earth turning round its axis every 24 hours.

Now, if you place the blunt end of a pencil on the upper axis of the spinning top, as shown in Fig. 77, and try to tilt it in some other direction than that it took when it began to spin, you will find it rather a hard thing to do. In other words, once a body is rapidly turning on its own axis it very strongly tends to keep its axis pointing in the same direction.

This rule also applies to the Earth, for having been tilted at an angle when it was thrown off by the Sun in the making, no other forces have ever been able to change the position of its axis to any great extent, though, the Earth spins easily on its axis and also revolves round the Sun.

To understand how the seasons are made you must first have clearly in mind the positions of the tilted Earth in different parts of its orbit round the Sun. The things needed for this experiment are a nice round apple, a candle, a piece of string, a safety-pin and a knitting needle.

Place the candle in the center of a table and call it the Sun; draw a circle a foot in diameter on the table top and around the candle with a bit of chalk. Mark one side September 22; at the next quarter of the circle mark December 21; directly opposite the September mark chalk in March 21, and finally between March and September mark June 21, all of which is shown in Fig. 78.

Push the knitting needle through the center of the apple, call the apple the Earth and call one end of the knitting needle the north pole and the other end the south pole. Fasten the safety-pin through the skin of the apple and tie the string to the pin so that when the string is tied to a tack in the ceiling or some one is holding it directly over the candle the knitting needle of the apple will be tilted to the perpendicular, as shown in Fig. 79.

Now grasp the top of the needle, which is the north pole, with the left hand and hold the apple away from the candle, as shown in Fig. 80. This is the position of the earth to the Sun on September 22, when the Sun passes directly over the Earth’s equator, and for us autumn is at hand.

Now pull the apple by its north pole toward you and around one quarter of the circle chalked on the table, as shown in Fig. 81, which is the position of the Earth to the Sun on December 21. You will see that the north pole is away from the light and heat and hence it is dark and winter there; but the south pole on the other end is getting plenty of light and heat and it is both day and summer there. This marks the beginning of our winter.

Push the apple by its north pole—always being careful to keep the knitting needle tilted in the same position—around another quarter of a circle and this is where the Earth is in its orbit, and its position to the Sun on about March 21. Once again the Sun is over the Earth’s equator and all parts or our world are then lighted and heated twelve hours out of the twenty-four, and we have the beginning of spring. See Fig. 82.

Again push the apple around another quarter circle and June 21 is reached. This time you will find the north pole is turned toward the Sun and this time it gets the light and heat for six months, while the south pole is away from the Sun and takes its turn of six months of night and winter. To us, however, it is the beginning of summer. Fig. 83 shows the position of the Earth to the Sun at this time.

Pull the apple one more quarter of the circle round the candle and you will have completed its orbit just as the Earth swings round the Sun in 365 days. The seasons are more clearly shown in Fig. 84.

The North Pole.—The north pole is not only one of the ends of the axis round which the Earth turns but close to it is the north magnetic pole as well. By magnetic we mean that the Earth behaves like a steel bar that has been magnetized.

A steel bar magnet like that shown in Fig. 85 is strongest at its ends. One end is positively magnetized, and we call this end its north pole, and the other end is negatively magnetized, and we call this end its south pole.

Magnetic lines of force stream from the south pole through the steel bar and reaching the north pole they stream through the air to the south pole, as shown by the curved lines, thus forming a magnetic circuit, just as two wires joined together may form an electric circuit.

If we place a compass needle near the steel bar magnet the needle will turn in the same direction as the magnetic lines of force are flowing, and it will, therefore, point to the north and to the south poles of the magnet.

Now the Earth is a great magnet with a positive pole, which we call the north pole at one end of its axis, and a negative pole, which we call the south pole, at the other end of its axis, as shown in Fig. 86.

Like a bar magnet, magnetic lines of force stream all over the Earth’s surface from the north pole to the south pole, and a compass needle placed anywhere on the Earth will swing round until it is in the same direction as the lines of magnetic force.

How to Make a Simple Compass.—Take a piece of watch spring about 3 inches long and straighten it. Heat the middle red-hot and let it cool slowly, when the temper will be taken out of it at this point. Place a center punch in the middle of the spring and strike it a sharp blow with a hammer; this will make a little dent in it. Bend it as shown in Fig. 87. To magnetize the needle rub one end on the north pole and the other end on the south pole of a steel magnet. Stick a sewing needle into a large cork and lay the magnetized needle on it, when it will point north and south, as shown in Fig. 88.

Boxing the Compass.—There are two kinds of compasses in general use, though both kinds use a magnetized needle. The first kind is the ordinary pocket compass, with either a pull-off cover or one of the watch-case pattern. In this kind of a compass the magnetic needle is fitted with a jeweled center which swings on a steel needle; the dial is fixed in the case and is marked with the cardinal points, that is with the chief points of a compass. Fig. 89 shows a pocket watch-case compass.

In the mariner’s compass, which is used at sea, the compass card and the magnetic needle are fastened together. The card is made of a circular sheet of mica and the points of the compass, which are called rhumbs, are marked on the edge. The needle and card float in a bowl of mercury.

The card is marked with 32 rays, forming a many-pointed star, and each of these points has a name, the names of the four cardinal points being north, east, south and west. To know all of the points by heart and be able to name them, beginning with the north and going round the card to the north again, is what sailors call boxing the compass. See Fig. 90.

Dial of a Mariner’s Compass

How to Make a Simple Dipping Needle.—When a compass needle is pivoted so that it can swing up and down, that is, to and away from the earth, it is called a dipping needle.

Such a needle will dip toward the nearest pole of the earth. At the magnetic equator there is no dip, that is, the needle will stand parallel with the Earth’s surface.

At the north magnetic pole the needle will stand straight up and down in a line with the axis of the Earth. The dip, therefore, of the needle at any place on the Earth’s surface is just about that of the latitude of the place where it is used. Dipping needles are also used by miners for finding iron ores.

To make a dipping needle, slip a small cork over a knitting needle and push a sewing needle through the cork at right angles to the knitting needle, as shown in Fig. 91. Now lay the sewing needle with its ends on the edges of two tumblers, and see that the knitting needle is perfectly balanced. This done, magnetize the knitting needle by rubbing one end on the north pole of a steel magnet and the other end on the south pole of the magnet. Make a little wood stand as shown in Fig. 92 and place the ends of the sewing needle on the wood supports.

The latitude running through the middle of the United States is about 40 degrees north of the equator and if you live in this latitude the dip of your needle will be about 40 degrees from the horizontal.

How to Find Latitude.—The latitude of a place on the Earth’s surface is its distance north or south of the equator. This distance is usually measured in degrees of a circle, instead of in miles.

The equator is called 0 (zero) degrees, and the north and south poles are 90 degrees from the equator, as shown in Fig. 93. If you are in Philadelphia, Pennsylvania, or Quincy, Illinois, or Tehama, California, you are in latitude 40 degrees. If you are in Bangor, Maine, St. Paul, Minnesota, or Portland, Oregon, you are in latitude 45 degrees, or just halfway between the north pole and the equator, as Fig. 94 shows.

(1) An easy way to find roughly the latitude of a place, that is, its distance from the equator, is to use a dipping needle and a protractor.

To make a protractor cut out a semi-circle of stiff, white cardboard, just the size shown in Fig. 93, and mark the figures on the edge and draw lines from the edge to the center point exactly as in the picture.

Now place your dipping needle on a level board or table and set the center of your cardboard protractor in line and on a level with the axis of the needle, as shown in Fig. 95. Whatever line on the protractor the dip of the needle takes the degree marked on the edge of the protractor will be the magnetic latitude you are in.

(2) A way to obtain true latitude is to take two smooth pieces of wood, about 1 foot long and ¼ inch thick, and hinge them together at one end with a screw, as in Fig. 96. Now set a bucket out-of-doors in an open space from which the North Star may be seen, fill the bucket with water and level it up until the water is parallel with the rim all the way round.

When night falls find the North Star and set your sticks across the rim, as shown in Fig. 97. Raise one of the sticks and sight it until it points straight at the North Star, and having done this you are through with the observation.

Take the sticks into the house, being very careful not to change their relative positions, so that the angle they form can be measured with a protractor. Tack a piece of paper on your starboard and draw a straight horizontal line on it.

Lay the stick that was on the bucket on the horizontal line, and draw a line along the edge of the other stick with which you sighted the North Star, as in Fig. 98.

Now measure the distance, in degrees, between the two lines with your protractor and the number of degrees you get will be roughly the latitude.

Shooting the Sun.—Another and very exact way to find the latitude, and which is also used to help find longitude when at sea, is by means of an instrument known as a sextant, so called from the fact that it is formed of a sixth part of a circle.

It is made with a metal frame A and having the degrees marked on its curved edge B like a protractor. On one end of a thin strip of metal, or arm, C (see Fig. 99), a mirror, D, called an index mirror, is rigidly fastened, and right under the center of this mirror the arm C is hinged to the frame A. The other end of this arm slides over the scale B.

To the left side of the frame also rigidly fastened is a second glass E called a horizon glass, and half of which is clear and half silvered. A telescope is also rigidly fastened to the frame A directly opposite but in a line with the horizon glass E.

Now to find the latitude by taking the Sun, or as sailors sometimes call it, shooting the Sun, in order to learn the position of the ship at sea, the sextant is held in both hands firmly, and the horizon which is sighted through the telescope is brought into view through the clear part of the horizon glass E.

The arm C, carrying the index mirror D, is now moved over the scale A until the light from the Sun just as it crosses the meridian at noon is reflected by its polished surface into the silvered part of the horizon glass E, and this reflects the sunlight into the telescope right in a line with the line of sight to the horizon. This forms an angle of the two beams of light just as an angle is formed of the two sticks of wood in the pail experiment and the number of degrees the end of the arm C points to on the scale B is the latitude in degrees or the distance of the ship north or south of the equator.

How Longitude is Found.—To find the longitude at sea, that is, the position of a ship east or west of a given place, is just as simple a matter as finding the latitude or its position north or south of the equator. Two instruments are used to find longitude, and these are the sextant, which has just been described, and a very accurate clock, called a chronometer (pronounced chro-nom´-e-ter).

As you know, imaginary lines running from the north pole to the south pole are called meridians of longitude. Now the Earth has been divided into 24 of these lines, the zero meridian, from which distances east and west are measured, running through Greenwich, England.

There are 24 of these standard meridians and hence they are 15 degrees apart—since there are 360 degrees in a circle—and they are 1 hour apart—since there are 24 hours in a day, and therefore 15 degrees equal 1 hour. (See Chapter X, The Time o’ Day.)

Now, since it is 12 o’clock noon when the Sun passes over any one of these standard meridians, it will be 11 o’clock A. M., 15 degrees west of it, and 1 o’clock P. M., 15 degrees east of it, and consequently there will be an hour’s difference in the time, either fast or slow, for every 15 degrees, depending on whether you count east or west from the noon meridian.

The purpose of a sextant in finding longitude on shipboard is to know when it is exactly noon by the Sun, and in this way the local time is found. The purpose of a chronometer is to carry exact Greenwich time, and the difference between the local time found each day by taking the Sun, and Greenwich time shown by the chronometer gives the distance in degrees the ship has traveled from Greenwich.

By knowing the latitude and longitude of a ship the distance in miles north and south and east and west from any port can be figured out without much trouble.

The reason that a very accurate clock has to be carried is that a difference of a few seconds either too fast or too slow will affect the calculations just that much, and this means that the ship will be thrown out of its calculated position by several miles.

To correct the observations with the sextant and the inaccuracy of the chronometer, etc., are a part of the business of the navigating officer, and he is provided with tables and things to make this work as easy and certain as is possible.

How to Know When You Are at the North Pole.—If you should ever reach the north pole where overhead is north and every other direction is south how would you know it?

Suppose you were standing on the exact top of the world during one of its long polar nights, then the North Star would be directly over your head and the Big Dipper and Cassiopeia would serve to mark the passing of days as well as of nights, that is, if you were at the north pole in the wintertime.

But if you were there during a long Arctic day, that is, in the good old summertime, you could see the Sun making a circle, or seeming to, once in 24 hours, parallel to the horizon, and never going higher or getting lower.

Explorers use a sextant to find out when they are at the north pole, and they sight the Sun’s height above the horizon at morning, noon and night. If the angle the Sun makes with the horizon is the same every time he is observed, the explorer knows he is standing on the very point round which the Earth turns. Then he plants a stick in the snow and ice on the north pole, hoists the American flag, and hurries home as fast as dogs and ships and trains can carry him to tell about it.

If you ever reach the north pole you can know it, too, even though you haven’t a sextant with you. All you need to do, when you think you are standing on the north pole, is to notice the length of your shadow five or six times in 24 hours. If the length of your shadow is exactly the same every time you look at it, as shown in Fig. 100, you are really and truly at the north pole.

Difference Between the True North Pole and the Magnetic North Pole.—The compass and dipping needle do not point to the true north pole, for the magnetic north pole and the true north pole are not located at the same place.

The magnetic north pole was found by Captain Ross in 1832. At that time the magnetic north pole was northwest of Hudson’s Bay in about latitude 70 degrees north and longitude 96 degrees and 45 minutes west, that is, west of the zero meridian which runs through Greenwich.