MOTHER EARTH, OLD ADAM’S PLANET
=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.
[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.
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.
[Illustration: FIG. 73.—THE EARTH MOVES AWAY FROM THE SWINGING
PENDULUM.]
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.
[Illustration: FIG. 74.—IF THE EARTH’S EQUATOR WERE IN A LINE WITH THE
SUN.]
[Illustration: FIG. 75.—THE EARTH TILTED ON ITS AXIS.]
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.
[Illustration: FIG. 76.—LIGHT IN ROOM TO REPRESENT SUN.]
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.
[Illustration: FIG. 77—TOP SPRING ON PLATE.]
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.
[Illustration: FIG. 78.—CIRCLE AROUND CANDLE MARKED WITH SEASONS.]
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.
[Illustration: FIG. 79.—APPLE TO REPRESENT EARTH SUSPENDED IN AIR.]
[Illustration: FIG. 80.—POSITION OF THE EARTH AND SUN IN AUTUMN.]
[Illustration: FIG. 81.—POSITION OF THE EARTH AND SUN IN WINTER.]
[Illustration: FIG. 82.—POSITION OF THE EARTH AND SUN IN SPRING.]
[Illustration: FIG. 83.—POSITION OF THE EARTH AND SUN IN SUMMER.]
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.
[Illustration: FIG. 84.—CYCLE OF SEASONS.]
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.
[Illustration: FIG. 85.—LINES OF FORCE THROUGH AND AROUND A MAGNET.]
[Illustration: FIG. 86.—LINES OF FORCE AROUND THE EARTH.]
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.
[Illustration: FIG. 87.—WATCH SPRING—NEEDLE FOR COMPASS.]
[Illustration: FIG. 88.—COMPASS COMPLETE.]
=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.
[Illustration: FIG. 89.—POCKET WATCH-CASE COMPASS.]
[Illustration: FIG. 90.—DIAL OF MARINER’S 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.
Points on Names of Points
Compass Card
N North
N bE North by east
NNE North, northeast
NE bN Northeast by north
NE Northeast
NE bE Northeast by east
ENE East, northeast
E bN East by north
E East
E bS East by south
ESE East, southeast
SE bE Southeast by east
SE Southeast
SE bS Southeast by south
SSE South, southeast
S bE South by east
S South
S bW South by west
SSW South, southwest
SW bS Southwest by south
SW Southwest
SW bW Southwest by west
WSW West, southwest
W bS West by south
W West
W bN West by north
WNW West, northwest
NW bW Northwest by west
NW Northwest
NW bN Northwest by north
NNW North, northwest
N bW North by west
N North
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.
[Illustration: FIG. 91.—NEEDLE FOR DIPPING NEEDLE.]
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.
[Illustration: FIG. 92.—DIPPING NEEDLE COMPLETE.]
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.
[Illustration: FIG. 93.—PROTRACTOR SHOWING DEGREES.]
[Illustration: FIG. 94.—EARTH SURFACE DIVIDED INTO DEGREES.]
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.
[Illustration: FIG. 95.—PROTRACTOR SET BY DIPPING NEEDLE SHOWING
LATITUDE.]
=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.
[Illustration: FIG. 96.—TWO STICKS SCREWED TOGETHER.]
[Illustration: FIG. 97.—TWO STICKS ACROSS BUCKET OF WATER.]
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.
[Illustration: FIG. 98.—PROTRACTOR AND STICKS ON DRAWING PAPER.]
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.
[Illustration: FIG. 99.—SEXTANT IN USE. SHOOTING THE SUN.]
=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.
[Illustration: FIG. 100.—SHADOWS AT THE NORTH POLE.]
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.