Geography; grade 8A, mathematical and physical

Preface

The following brief treatise on the elements of mathematical and physical geography has been prepared to meet the requirements of elementary schools. Much of the material here given has already been taken up at intervals during the regular geographical course. Enough new matter has been added to provide for a term's work on these subjects. A number of astronomical facts have been given in the first chapter, not strictly related to geography but such as everyone ought to know. Chapters V and VI, dealing with the magnetism of the earth, the moon's phases, and eclipses, are also not defensible as geography, but are introduced as matters of general information. The remaining chapters of the book treat of the general features of the earth, of the forces at work in altering its surface, and of atmospheric and oceanic movements. The final chapters deal with the distribution of plant and animal life and with the races of man and their physical and social characteristics. It is believed that the book can be used as the basis for a half-year of profitable work at the close of the elementary school course.

Contents

Chapter Page

I. The Earth as a Planet i

II. Form and Size of the Earth ii

Chapter I

Fixed Stars and Planets. We have learned that the earth is a great round ball composed of land, water, and air, and that it is traveling rapidly onward in its neverending journey around the sun. But perhaps we have never thought of the earth as having any likeness to the stars which we see in the skies on a clear night. It is hard to believe that it is really one of that company of glittering lights that flash and twinkle in the far off depths of space.

If we observe the stars carefully night after night, we shall become familiar with the constellations, or stargroups, which were observed and named by astronomers thousands of years ago. Most of us can recognize the Great Bear, or Dipper, the v-shaped Hyades, and the Pleiades, or Seven Sisters, above them. Orion with the three stars in his belt, the Twins, Sirius, the Dog-Star, the brightest in the sky, Hercules, Polaris, or the North Star, are other constellations and stars with which we should become acquainted. With the eye alone we can see only a few thousand stars; but when a telescope is used, millions of others are brought into view.

Nearly all of the stars that we can see are fixed stars.

They occupy to-day nearly the same positions in the skies as when man first beheld them.

Other stars change their positions from night to night, until after a certain time they return to the positions where they were first observed.

There are seven principal stars of this class which we may observe from the earth. They are called planets, a word meaning wanderers. The earth also is a planet, and if we could see it from a great distance, as we see the other planets, it would appear to us like one of them.

The planets have been named after the Greek and Roman gods. Nearest the sun is Mercury, named for the messenger of the gods because of his swiftness. Next is Venus, the brightest of the planets, named after the goddess of beauty. Beyond the earth is Mars, which received the name of the war-god on account of its red color. Next is Jupiter, the largest planet, and hence named after the king of the gods. Most distant from the sun are Saturn, Uranus, and Neptune. Between Mars and Jupiter are several hundred smaller planets called asteroids. These and Neptune cannot be seen without a telescope.

Fig. I.

North star,

Some nf the constellations surrounding the

The planets revolve about the sun, but at different distances from it. The nearer a planet is to the sun, the faster it moves. Our earth makes one complete revolution in 365M days. This period we call a year. Mercury, the planet nearest the sun, makes the journey in 87 days. Mercury's year, therefore, is only about one fourth as long as ours. Neptune, the most distant planet, is 30 times as far away from the sun as the earth. He travels only one /asteroids mRs vemus

sixth as fast as the earth, and / earth me:rcur/\

requires 165 years as long as

ours to make one journey Ineptume saturm

around the sun.

The following table shows \ ^^^^^ ^^^^^

the chief numerical facts about the eight greater planets approximately correct in round pig. 2. Showing the relative sizes of

Y)- ^ ■ Average distance Time of Rev- Velocity in Name -i from the sun in olution miles per

millions of miles in days second

Mercury 3,ooo 36 87 29

The laws governing the motions of the planets were discovered by Johann Kepler, a German astronomer, in the early part of the 17th century. The first law is stated as follows: The orbit of each planet is an ellipse with the sun as one of its foci.

The method of drawing an ellipse is shown in Fig. 3. A loop of cord is passed around two pins driven into a piece of cardboard. The point

of a pencil is then inserted in the loop, and, keeping the string stretched taut, the ellipse is described about the pins in the manner indicated. The points marked by the pins are the foci.

If the distance between the pins be made less, or if the cord be lengthened, the form of the ellipse will more nearly approach a circle and the major axis and the minor axis (Fig. 4.) will be more nearly equal. The deviation of an ellipse from the circular form is called its "eccentricity" and depends upon the distance between the foci.

Fig. 3. Method of drawing an ellipse. A and B are the foci, CD the major axis, and EF the minor axis.

The orbits of the planets are nearly circular, as in Fig. 4. A planet when crossing the major axis at one of its extremities is at its greatest distance from the sun. This position is called its aphelion; when crossing the major axis at its other extremity it is in perihelion; that is, the position nearest the sun. The other laws of planetary motion are too difficult to be considered here.

Fig. 4. An ellipse with very slight eccentricity resembling the orbits of the planets.

Moons. All the planets, except Mercury and Venus, have bodies revolving about them called moons, or satellites. We have all seen the new moon as a slender crescent in the west just after sunset. If vv-e observe it for several nights at the same hour, we may notice that it is farther and farther above the horizon and that it appears to have moved toward the east. At the end of two weeks we may see it just after sunset as the full moon, near the eastern horizon; it has now moved all the way across the sky. Two weeks later it appears again in the west, having revolved entirely about the earth from west to east. All the moons revolve about their planets, and all the planets revolve about the sun in the same direction. With a telescope, or even with a good field-glass, we may watch the moons of Jupiter revolving about that planet. We may also watch the revolution of the planets about the sun, only these observations must be far more exact.

Comets and Meteors are strange, fiery bodies that occasionally appear in the heavens, and pass out of sight as mysteriously as they come. The comets circle about the sun, and then dart away on some unknown path into distant parts of the universe, sometimes reappearing after many years.

PATH OF

MALLEY*5

COME.T

?■ 5

A part of the path of Halle/'s comet, showing its progress around the sun, and its distance from the earth at different times.

Meteors, or "shooting" stars, flash suddenly through the

earth's atmosphere, some turning to vapor and others

faUing upon its surface.

One of the most remarkable comets whose period of revolution about the sun is known is that named after Edmund Halley, who predicted its appearance in 1759. This comet enters the solar system and passes about the sun every seventy-five years. It was first seen before the Christian Era. It was noticed again in 684, 1066, 1456, 1682, 1759, and 1835. Its last appearance was in 1910. A portion of its path about the sun is shown in Fig. 5.

The Solar System is a name given to the sun and all the planets with their moons that revolve about it. The sun is the center of the solar system. It is thought to be a white hot mass of matter enveloped in burning vapors. Great flames leap from its surface to a height of over 200,000 miles, a distance nearly as great as that from the earth to the moon. The diameter of the sun is 866,500 miles, or about one hundred times that of the earth. Its volume is 1,300,000 times as great as that of the earth and its average distance from the earth is about 93,000,000 miles. The sun supplies the earth with the light and heat necessary to support plant and animal life.

The sun is really one of the fixed stars and the one nearest to us. Like the planets, it rotates about its axis. By means of the dark spots which sometimes appear upon its surface, its period of rotation has been found to be about 25 days. The amount of heat given out by the sun is so great as to be beyond computation. The earth alone receives enough heat in one year to melt a layer of ice 137 feet thick and covering its entire surface, yet the amount of heat received by the earth is only ■2.T0 o.oV o.oTo P^rt of the total heat given out by the sun.

The revolution of the planets about the sun was discovered about four centuries ago by a German astronomer named Copernicus. He showed also that day and night are caused by the rotation of the planets upon their axes. The sun rotates on its axis like a planet, and with all its

URAMUS attending planets and their moons is moving slowly through space. The fixed stars are fiery suns like our own. Perhaps they, too, are centers of systems of planets; but they are so far away that even with our largest telescopes we can find out little about them.

The Nebular Theory. It is thought that the sun and all the planets were once one great mass of fiery vapor, filling all the space between the sun and the farthest planet. We do not know where this cloudy mass came from. But after a time it began to cool, and as it grew smaller, began to rotate slowly about its axis, just as the earth does now.

This motion caused it to bulge out at the equator and flatten at the poles. The motion at the equator became so rapid that a number of rings of vaporous matter

Fig. 6. The planets and their orbits, showing relative distances from the sun.

were thrown off. Tliese rings in some- way gradually became spheres and continued to revolve as planets about the central mass, or sun. Some of the planets themselves threw off rings which formed into moons.

The planets were once white hot, as the sun is now; but as they cooled at the surface, a hard crust formed, which grew thicker and thicker as the cooling continued. But how did this crust, which must have been smooth at first, become so rough and irregular.^ When melted rock cools it becomes about one seventh of its volume smaller; accordingly, as the hot rock within the crust went on cooling, it kept shrinking, and contracting. The crust, on account of its weight, pressed toward the center, and crumpled here and there in broad folds, like a garment that does not fit, or like a carpet that is too large for a floor.

The earth has been compared to an apple that has been baked. The heat makes the apple expand until the skin is almost bursting, but when the apple cools it shrinks until the skin becomes too large and lies in folds, or wrinkles. The upward folds formed by the cooling of the earth grew into the continents, while into the downward folds poured the waters that had been condensed from the clouds of

steam surrounding the earth.

There are several reasons for believing

\ this theory. First, the motions of the

sun, the planets, and their moons are all

in the same direction. Second, the planet

Saturn has still two rings circling about

• ■';■.' \y>. it. Again, the planet Jupiter is still so

-;;.".';-',''. hot that its atmosphere is filled with clouds

_-,.•'.•'• of steam. That the interior of our earth

ii^?-fe^"s"'':"' ^^ highly heated is shown by hot springs

tV,**li3?r..- • and volcanoes which break through its

■^mi^ -'^ surface. That the moon was once in a

"^'f heated condition is evident from the dead

volcanoes still visible upon it. We believe

that the sun is still cooling and contracting;

Fig. 7. Photoiraph of a ^^^ perhaps it may yet throw off rings of

baked apple. matter to be formed into planets. It is thought, also, that the earth, the planets, and the sun itself will some day grow cold like our moon, and that all life upon the earth will cease.

Gravitation. The members of the solar system as well as all the distant stars are held in their places by a force called gravitation; their motions also and the paths they follow are effects of the same force.

The law of gravitatioii as stated by Sir Isaac Newton, who discovered it, is as follows:

Every particle of matter in the universe attracts every other particle of matter, with a force which varies directly as the masses of the particles, and inversely as the square of the distance between them.

Gravitation is thus a form of attraction that tends to draw bodies of matter together. We do not understand the cause of it any more than we understand why a magnet attracts a piece of steel or iron. From the first part of the law it follows that if the amount of matter that two bodies contain should be doubled, the attractive force between them would be doubled. The second part of the law means that if the bodies should be twice as far apart, the attraction would be only one fourth as great; if the distance be three times as great then the attraction between them will be only one ninth as great. On the contrary, if the distance be divided by two, the attraction will be multiplied by four, etc. It is the force of gravitation which causes bodies to fall to the earth and which gives them weight. It is said that Newton's attention was first drawn to this subject by seeing an apple fall to the earth.

The onward motion of the planets causes a force, which tends to draw them away from the sun. This force is known as inertia, or centrifugal force. But the attraction between a planet and the sun (gravitation) is exactly equal lo PHYSICAL GEOGRAPHY

to the centrifugal force, and thus the planets are held in

their courses.

A familiar illustration of the two forces named above is furnished by tying a string to a body and whirling it about in a circle. The tendency of the body to fly ofi' in a straight line is the force of inertia, while the pull on the string may represent gravitation. These two forces are always equal. It may be added that if the speed of the body be increased the pull on the string will be correspondingly greater, and vice versa. This illustrates why the planets nearer the sun revolve more swiftly than those that are more remote.

REVIEW. I. Describe some of the leading constellations. 2. Which one is useful in determining direction? 3. How may we distinguish planets and fixed stars? 4. Name the eight principal planets. 5. How is the motion of the planet affected by its distance from the sun? 6. Compare the year of Mercury with that of the earth. Compare also the year of Neptune. 7. What is the first law of planetary motion? 8. Describe the method of drawing an ellipse. 9. What is meant by aphelion and perihelion? 10. Which of the planets have moons? 11. Describe the changes in the appearance of the moon. 12. Comets and meteors. 13. What is meant by the Solar System? 14. Describe the sun and the amount of heat that it gives out. 15. Who was Copernicus? 16. Write a paragraph about the Nebular Theory. 17. State the law of gravitation. 18. What is meant by inertia?

Chapter II

Fig. 8. In traveling from A toward B and C new stars appear to rise at N andO.

FORM AND SIZE OF THE EARTH

The shape of the earth as well as that of the other planets and heavenly bodies is spherical or nearly so. This fact is easily established in the case of the other planets by direct observation; but it is not so easy to prove the shape of the earth, because so little of it can be seen at one time.

By scientific methods, however a number of proofs have been accumulated, which show conclusively that the earth is spherical. The first of these that we shall mention was observed by travelers more than 2,000 years ago. To one who travels north or south, new stars are constantly coming into view, while the stars behind disappear below

the horizon. To an observer at the equator, the North star appears to be on the horizon.

But as one travels north it gradually rises, until on reaching the pole it would appear directly overhead.

The well-known fact also that the sun rises earlier at places east of us and later at

Fig. 9. The horizon seen from different elevations.

places west of us shows that the surface of the earth is curved; for if it were flat the sun would rise and set at the same time for all places on the earth. There would also be no apparent rising and setting

of the stars to a traveler going either north or south.

Another proof is the shape of the earth's shadow as seen upon the moon at the time of an eclipse of the moon. This shadow is always circular, no matter what the position of the earth may be, and only a sphere always casts a circular shadow. Among the more familiar pr.oofs of the earth's spherical shape is the circular appearance of the horizon when viewed from a great elevation. When ships are coming into port or sailing away, the tops of the masts are the first seen and the last to fade out of sight; whereas, if the surface of the earth were flat, the hull, being the most conspicuous part of a ship.

Fig. 10. the moon.

The earth's shadow upon

Fig. II. An orange and lemon, illustrating spheroidal forms.

FORM AND SIZE OF THE EARTH would be seen first. The circumnavigation of the earth is another familiar proof of its form. This, however, is not conclusive, as it must always be accomplished in an east and west direction, which would be possible if the earth were a cylinder.

Finally, from analogy with the other members of the solar system, we might reasonably conclude that the earth is spherical.

Fig. 12. Illustrating the effect of rotation on a plastic

The form and size of the earth have been accurately determined by scientific methods. The polar diameter has been found to be 7,900 miles, and the equatorial diameter 26 miles greater. The average circumference has been found to be 24,860 miles, and the circumference at the equator 24,900 miles. The earth therefore is not 'a perfect sphere, but is flattened at the poles. In exact terms, body, it is an oblate spheroid. The term applied to a sphere that has its polar diameter greatest is prolate spheroid. These two modifications of the spherical form are illustrated by an orange and a lemon as in Fig. ii.

The spheroidal form of the earth is supposed to have been caused by rotation when its was in a plastic condition. To illustrate the effect of rotation on a plastic body, take a strip of tin plate about two feet long and an inch wide; punch a hole in each end and also one in the middle. Bend the strip into a hoop and pass a wire through the holes and solder the two ends to the wire. Spin the metal hoop ^y rotating the wire between the fingers and note the effect indicated by the dotted lines in Fig. 12.

REVIEW. I. State three proofs of the spherical shape of the earth. 2. What is the difference between the polar diameter and the equatorial diameter? Define oblate spheroid; prolate spheroid. 3. Illustrate the effect of rotation on a plastic body.

Chapter III The Earth's Motions And Their Effects

Rotation and Revolution. These two motions of the earth may be illustrated by a spinning top which is moving about a center. If the axis of the top always points in the same direction and is inclined as the earth's axis is inclined, it will correctly represent the position and motions of the earth. The motion which the top would have if it remained in the same position is called rotation, while its onward motion around a center is called revolution. If the earth had no motion of rotation all places on its surface would have six months of daylight and six months of darkness during the time of one revolution. Again, if the earth remained always in one position and did not revolve about the sun, there would be no change of seasons. In this case, owing to the inclination of the earth's axis, the south frigid zone would havfe perpetual night if the position of the earth were at aphelion. If its position were at perihelion the north frigid zone would have perpetual night. If the position of the earth were at either equinox, or if the axis were perpendicular to the plane of the earth's orbit, all places on its surface would have days and

Fis*. I*?, Foucsult's cx~ nights of equal length throughout periment, proving the rotation

t>ip vpflr °^ ^^^ ^^^^^ ^'^ means of a

THE EARTH'S MOTIONS AND THEIR EFFECTS 15

The time required by the earth to make one rotation is a day of twenty four hours. An object at the equator, therefore, is moving at the rate of about one thousand miles per hour, while the velocity of the earth in its motion of revolution is, as we have seen, slightly greater than this.

An interesting proof of the rotation of the earth Is furnished by Foucault's experiment. He suspended a heavy pendulum from the dome in the cathedral of Notre Dame. Underneath it was placed a vessel containing sand. The pendulum was provided with a projecting tip at its lower end, as shown in Fig. 13. Such a long, heavy pendulum when set in motion will continue to vibrate for many hours in the same direction. As the earth rotates beneath it, the tip makes lines across the bed of sand, which extend through a complete circumference every twenty-four hours.

Direction on the earth's surface is determined by the direction of the axis and by the motion of rotation. The extremities of the axis are the north and south points, while the direction of rotation is called east. The north point may be found by observing the Polar star, or by means of the compass. The direction opposite to east is west. In the northern hemisphere the south point may be located by observing the position of the sun at noon.

Direction may also be determined by means of an ordinary watch. If the watch be held so that the hour hand points toward the sun the south point will be half way between this line and the twelve o'clock mark on the watch.

The Revolution of the Earth around the sun takes place in a trifle less than 365/^ days. Its orbit, or path, is an ellipse, but very nearly circular. The circumference of the orbit is 584,600,000 miles. The velocity, or rate of motion, as above stated is therefore about 18 miles a second. As the sun is located at one of the foci, its distance from the earth varies from 91,500,000 miles in December to 94,500,000 in June.

The Change of Seasons is not due to the varying distance of the sun from the earth, but to the direction in which the sun's rays fall upon it and to the varying length of the day. We have observed that the sun is low in the sky in i6

Fig. 14. The earth in its orbit, showing its position each month.

winter and high in summer; and also that the higher it is

the longer its daily path in the sky. We may say that the

length of the day varies according to the height of the sun.

In Fig. 15 the dotted curved lines represent the apparent path of the sun through the sky and the length of the day at the beginning of each season. On December 21st, winter begins and the sun is at its lowest position. This is the winter solstice (sun-stop). During the next three months it rises to the middle curve, when the days and nights are equal in length and the sun rises in the true east and sets in the true west This is the time of the vernal equinox. By June 21st, the summer solstice, the sun has reached the highest curve, and summer begins. After this date the sun returns to the autumnal

THE EARTH'S MOTIONS AND THEIR EFFECTS equinox on September 2ist, and three months later, to the winter solstice. It will be noticed that the place of sunrise and that of sunset in the northern hemisphere is south of the true east and west during the winter, and north of these points during the summer.

The longer the day is, the more heat the earth receives. At night the earth gives up its heat. When the nights are shorter than the days, the earth receives more heat than it gives up. Hence the heat accumulates. This explains why the month of August may be warmer than June although the days are shorter.

But the sun's rays are more direct in June than in August, and convey more heat to the ^^^ earth, because they are spread over a smaller surface and also because less of their heat is absorbed by the atmosphere.

The Explanation of the Sun's Apparent Motion and of the changes in the seasons and in the length of the day is found in the revolution of the earth about the sun and the inclination of its axis to the plane of its orbit. The amount of the inclination is 23^°. Since the axis always points in the same direction, it is plain that when the earth is at the position indicated by June 21,

SUNRISE

Fig. 15. The earth's daily path through the sky at different seasons. Notice the place of sunrise and sunset.

in Fig. 17, the north pole of its axis will be turned toward the sun. At this time the sun will appear north of the equator and long days and summer will prevail in the northern hemisphere. On December 21, precisely the opposite conditions will prevail. In March and September the axis will be perpendicular to a straight line connecting the center of the sun with the center of the earth. Hence both northern and southern hemispheres will be equally lighted up, and the days and nights will be equal all over the earth.

Varying length of Day and Night. The length of the day in the northern hemisphere increases as the sun passes

Fig. 16. Position of the earth at the time of the winter solstice.

Fig. 17. Position of the earth at the summer solstice.

from the winter solstice to the summer solstice, and decreases in returning.

Let Fig. 16 represent the position of the earth at the winter solstice, December 21. The rays are vertical at the tropic of Capricorn and the whole south frigid zone is lighted up. What is true in the northern hemisphere.'' What do the circles show about the length of day and night.'' On March 21, the time of the vernal equinox, the sun's rays are vertical at the equator. What shows the equality in the length of the days and nights at this time.? On June 21, the longest days are

THE EARTH'S MOTIONS AND THEIR EFFECTS found north of the equator and the shortest days are south of it. Which zones have summer? Which winter? The position of the axis would be precisely the same on September 21 as on March 21. Remembering this, how long would the day be at the north pole? At the equator?

What can you say of the length of the days and nights between the equator and the poles at this time?

The Earth and Man. Of all

the planets, the earth is the

one best fitted for human life WA ^7J ^^

as we know it. If it were too far from the sun the cold could not be endured; and if too near, the heat would be too

p-reat The mndpratp inrli Fig. 18. The position of the earth

great, ine moaerate men- at the time of the equinoxes. nation of the earth's axis allows

an agreeable change of season and makes the larger part of the earth habitable. What would be the effect if the inclination were 50 degrees.^ What if there were no inclination of the axis?