Physiography for high schools

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Preface

For a number of years the authors have felt the need of a text that presents physiography from the high school point of view, both in content and in treatment. Since more than nine tenths of the pupils who enter secondary schools complete their formal education in such schools, the needs of this great class cannot be neglected ; and subjects must be planned with the fact in mind that secondary pupils should know the scientific explanation of the common phenomena of nature.

A course in physiography in a college is naturally limited by the existence of parallel courses in astronomy, geology, and meteorology ; and the fear of overlapping has led college men to omit many valuable topics from their courses and from the texts which they have prepared. No such limitation is found in the high school, and teachers are at liberty to select such topics as will contribute most to the culture of the pupil.

High school pupils should know of the earth as a whole, its relation to the other heavenly bodies, and the influence of its size, shape, and motions upon our daily life. They should know of the sun and the moon and their influence, and lack of influence, upon us. We have, therefore, included in our course, such astronomical topics as are necessary to this end. The pupils should also know of the natural resources of our country and their importance, and should understand the influence of climate and physical environment upon a given region as well as upon the history and the development of our nation and of the civilizations of the world. We have, therefore, included topics usually treated only in geology, meteorology, and history.

The abstract discussion of processes, as processes, belongs to the college rather than to the High School ; we have, therefore, discussed such topics as diastrophism, erosion, and the like in connection with concrete instances of their work. -

IV PREFACE

We are not in accord with those who would make physiography in the high school a regional subject. The secondary student easily masters a scientific treatment of a general topic such as mountains when the mountains of various regions are studied and contrasted ; but he fails to do so if mountains are discussed in a fragmentary manner in connection with descriptions of various regions.

The treatment of the subject here presented has been in successful use in our class rooms many years, and we believe that it will be found equally satisfactory to others.

This text contains more matter than can be mastered by first-year pupils, and we have indicated by smaller type the paragraphs that it is our custom to omit with first-year classes ; it is expected, however, that each teacher will make his own selection of the topics to be omitted. Where the subject is taught in the fourth year, no omissions will be necessary.

The italicized words in the text are intended as a guide to the teacher and the pupil. Technical terms are italicized when first introduced, and are to be defined by the pupil. Sentences in italics are to be memorized. In descriptions of functions and properties, the important words are italicized for emphasis. This use of italics makes printed questions on the text unnecessary.

The questions found at the end of each chapter cannot be answered by quoting the text. Each requires the pupil to draw an inference from some fact or facts stated in the text, or to exercise his judgment in contrasting facts. Some of them call upon him to exercise his imagination, and here suggestions from the teacher may be in order. We have found them particularly valuable as a stimulant to independent thought on the part of the pupil.

Part I The Earth As A Planet Chapter Page

I. The Earth in Space 3

II. Latitude, Longitude, and Time. 17

III. The Moon 30

IV. The Solar System 36

V. Map Projection 50

VI. Terrestrial Magnetism 58

Part II The Air

VII. Properties and Functions of the Air 65

VIII. Temperature of the Air 73

IX. Weight and Density of the Air 91

X. Movements of the Air 99

XL Moisture of the Air 117

XII. Light and Electricity of the Air 129

XIII. Weather and Climate 139

XIV. Climate of the United States 160

Part III The Sea

XV. General Characteristics of the Sea 183

XVI. Movements of the Sea . 195 vi CONTENTS

Part IV The Land Chapter Page

XVII. The Mantle Rock 219

XVIII. The Bed Rock 246

XIX. The Ground Water 270

XX. The Work of Rivers 284

XXI. Life History of a River . 310

XXII. Lakes, Falls, and Rapids 315

XXIII. Glaciers . 328

XXIV. Plains and Plateaus 351

XXV. Mountains 376

XXVI. Volcanoes and Earthquakes 402

XXVII. Shore Lines and Harbors 423

Chapter I The Earth In Space

The earth is a ball nearly 25,000 miles around. It is composed of rock, with about three-fourths of its surface covered with oceanic waters having an average depth of only 2^ miles. The whole is surrounded by an envelope of air probably more than 200 miles thick.

On its surface we are unconscious of any motion because of the steadiness and freedom from jar, still we know that the familiar phenomena of the rising and the setting of the sun are due to the turning of the earth on its axis. At night the star dome appears to revolve about the earth, and gives us further evidence that we live on a ball that is turning in space uniformly and regularly.

We also learn that while the earth is whirling, it is rushing through space with inconceivable velocity. While the seconds' pendulum of a clock makes one swing, the earth moves 18)^ miles, which is a thousand times the speed of the fastest express trains.

In going this distance each second, the earth curves about one-ninth of an inch from a straight line. This slight rate curvature continued for a year, brings the earth around to the place of starting.

The absolute uniformity of turning of the earth on its axis, and the regularity of movement as a whole about the sun, are of great service to mankind. The former affords a convenient means of measuring the length of the day, and the latter marks off the year.

The earth is only one member of a family of rotating balls. This family, together with other bodies, is controlled by the sun and constitutes a system.

A conception of the earth in space, as a member of the solar system, and some knowledge of conditions on other worlds, may give us clearer views of our real insignificance in space, time, and matter.

Condition of the Interior of the Earth. — The interior of the earth is believed to be solid throughout, although the temperature is undoubtedly above the melting point of materials on the earth's surface. The melting point increases with the pressure, but it is believed that the pressure raises the melting point faster than temperature rises as the center is approached, so that the fusion point is never reached. Because the earth as a whole has a greater density than the material near the surface, the central portion is thought to be more dense than the so-called crust.

Careful studies made of the variation in the position of the earth's axis, the effects of tide-producing forces acting upon the earth, and the velocity of earthquake waves through the earth, have led to the conclusion that the earth is more rigid than steel.

Form, Size, and Weight. — Water and mud fly off from the fast rotating wheels of wagons and automobiles, when running on wet, muddy roads. This is due to the tendency of bodies to move in a straight line. When a body moves in a curved path it appears to be pulling away from the axis of rotation. This pull is called centrifugal force. The centrifugal force is greater, the greater the distance from the axis.

Because of the rotation of the earth, the excess of centrifugal force developed in equatorial regions causes it to bulge out there and to flatten in the polar regions. The resulting form is that of an oblate spheroid.

This does not necessarily indicate a one-time molten condition of the earth. The sea bulges at the equator and flattens at the poles, and the land wears down to sea level.

The axis of rotation of the earth, or the polar diameter, is 7,899.76 miles, and the equatorial diameter is 7,926.60 miles, the latter being nearly 27 miles more than the former. The ends of the earth's axis are called poles. The average of the different diameters of the earth is nearly 8,000 miles, and the circumference is about 25,000 miles.

The surface area of the earth is nearly 197,000,000 square miles, of which 54,000,000 square miles are land. The earth is 5.6 times as heavy as a sphere of water the same size.

Problem of Eratosthenes. — The first successful attempt to measure the size of the earth was made about 200 B.C. by Eratosthenes, an astronomer and geographer of Alexandria, Egypt. He learned that at Syene, the

Fig. 1. — Method of Eratosthenes

A, a vertical pillar at Syene, is 23)2° north of the equator at E, a point on the Tropic of Cancer. B, a vertical pillar at Alexandria is 7. 2° or 5,000 stadia further north. C is the center of the earth and CA and CB radii.

most southern city of Ancient Egypt, the gnomon or vertical pillar cast no shadow at noon on June 21st. At Alexandria, 5,000 stadia directly north of Syene, the sun's noon ray on the same day made an angle of 7.2 degrees with a vertical pillar. Assuming the earth to be a sphere, this angle of 7.2 degrees is equal to the angle formed at the center of the earth between radii to Alexandria and Syene. It follows that as these two places are on the same meridian, an arc of 7.2 degrees equals in length 5,000 stadia, so that 360 degrees, or the distance around the earth, equals 250,000 stadia. As the distance between Syene and Alexandria is about 500 miles, the circumference of the earth would be 25,000 miles, which is not far from the truth.

EVIDENCES THAT THE SURFACE OF THE EARTH IS CURVED

i. During every eclipse of the moon, that portion of the shadow of the earth cast on the moon always has a curved edge.

2. The circumnavigation of the earth proves that it is not flat. It does not prove that the earth has the form of a sphere. It might, for instance, have the oval form of a football.

3. As ships sail away their hulls disappear first, and as they come into port their masts appear first. This shows that the water surface is actually rounded up between us and the distant ship.

4. At sea, the circle known as the horizon seems both to sink and to increase in size with an increase in elevation above the surface of the water. If the water surface were an extended plane, our area of visibility would not increase with an increase of elevation.

5. That the weight of a body is about the same everywhere on the earth's surface shows that the earth is globular in form. The slight increase in weight noticed as one approaches the poles is in part due to a flattening of the earth's surface in those regions.

6. That the apparent shifting of the sky position of the stars is directly proportioned to the distance traveled north or south, shows that the earth's surface is curved along north-south lines like a sphere.

7. Places not on the same meridian have different times of day as a result of the curved shape of the earth's surface along east-west lines. If the earth were flat all places would have the same time.

8. On the shores of a calm lake, away from the tides and swells, the

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—£ curvature of the Earth's surface

Fig. 2. — Post Method for Measuring the Curvature of the Earth

curvature of the earth may be measured by erecting in a straight line three posts, A, B, C, at the same height above the surface of the water. When looking with a telescope from the top of post A to the top of post C, the top of post B will be above the line of sight. If the distance from A to B is a mile, top of post B will be 8 inches above the line of sight, or the curvature of the water surface is 8 inches to the mile. In two miles the curvature is 8 inches X 2 squared, or 32 inches; in three miles, 8 inches X 3 squared, or 72 inches — that is, the curvature for any distance is equal to 8 inches multiplied by the square of the number of miles.

THE MOVEMENTS OF THE EARTH

Rotation and Revolution. — The earth has two principal motions, a uniform spinning motion called rotation, and a forward movement in its path about the sun called revolution.

The angular rate of motion due to rotation is 15 an hour; and the absolute rate of motion of a particle on the earth's surface is greatest at the equator. Here it is 25,000 miles a day, or more than a thousand miles an hour, and decreases toward the poles, where it is nothing. The rate of motion of the earth as a whole, due to revolution, is about a degree a day. This amounts to 1,600.000 miles a day.

ROTATION AND ITS EFFECTS

i. The side of the earth that at any moment is turned toward the sun is in sunshine, and the side turned away from the sun is in darkness. The rotation of the earth from west to east produces a movement of illumination and shadow around the earth from east to west.

2. The eastern horizon is really sinking and the western horizon rising, which has the effect in the lower latitudes of making the sun, moon and stars appear to rise along the eastern and set along the western horizon.

3. The period of rotation, in respect to the sun, is 24 hours, and determines the length of day.

4. The slight bulging of the earth in the equatorial regions is due to rotation.

5. Winds and ocean currents, because of the earth's rotation, are deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This may be illustrated by pouring water upon a rotating globe.

6. Every particle of matter on the earth's surface describes each day a circle. These circles are largest at the equator, and particles there consequently have the greatest velocity. This motion has the effect of lessening the weight of bodies, due to the tendency of bodies to flyaway from the center of rotation. Bodies therefore, for this reason, weigh less at the equator than in higher latitudes.

Fig. 3. — Star Trails, Due to the Earth's Rotation, Made on a Photographic Plate.

7. Bodies falling from a considerable height fall to the east of a vertical line suspended from the point of starting.

Foucault Pendulum Experiment. — In 1851 Foucault, a French physicist, devised a remarkable proof of the earth's rotation by means of a pendulum. From the dome of the Pantheon in Paris he hung a heavy iron ball about a foot in diameter by a steel wire more than 200 feet long. The pendulum was set in motion and the plane of vibration seemed to rotate slowly toward the right. It can be easily shown by a simple

Fig. 4. — Foucault's Experiment experiment that the plane of vibration of a pendulum remains fixed. The true interpretation must then be that the floor of the Pantheon was actually turning under the plane in which the pendulum was swinging.

If the pendulum were suspended at the pole, the earth would turn around under it in twenty-four hours.

The time required for the earth to shift entirely around under the plane of the vibrating pendulum increases as the latitude decreases. At the equator there would be no tendency for the earth to shift.

REVOLUTION AND ITS EFFECTS

Stars Shift Westward.— The movement of the earth, in its path about the sun, causes the sun to appear to move eastward among the stars. This has the effect of making the stars appear to shift westward about a degree a day. The real path the earth travels each year is called its orbit.

The path which the sun appears to follow around the heaven once a year, as a result of the annual movement of the earth in its orbit about the sun, is called the ecliptic. It is so called because all the eclipses of the sun and moon occur when the moon is in the plane of this path.

The zodiac is the belt of the heavens, 16 degrees wide, 8 degrees on each side of the ecliptic. It is so called because the constellation or groups of stars in it are thought to resemble or outline the forms of animals. The 360 degrees of the zodiac are divided into twelve equal parts, each called a sign.

The Latin names, with the symbols used to represent them, are as follows :

The Spring Signs The Autumn Signs

f Aries, the Ram. — Libra, the Balance.

The Summer Signs The Winter Signs

2> Cancer, the Crab. V3 Capricornus, the Goat,

lo PHYSIOGRAPHY

Change of Seasons. — As the earth moves forward around the sun, its axis is always tipped 23 ^ degrees from a perpendicular to the plane of its orbit. The earth's axis is always inclined in the same direction, so that during a revolution the axis remains parallel to itself in all positions. It is because of the (1) inclination of the earth's axis and its maintenance of (2) parallelism during a complete (3) revolution, that the change of seasons occurs.

Cause of Unequal Days and Nights. — The same causes produce a shifting of the daily sky path of the sun during the year, and the consequent variations in length of daylight and darkness.

Ellipse, Perihelion, and Aphelion. — The orbit of the earth has the form of an ellipse, with the sun at the north focus. The earth is at perihelion, or nearest to the sun, on January 2, when it is 91,500,000 miles away, and at aphelion, or farthest from the sun, on July 3, when it is 94,500,000 miles away.

In the sketch (Fig. 5) the earth is shown in four positions as it makes its annual journey about the sun.

On December 21 the north pole is tipped away from the sun and in the middle of the long period of darkness. The noon tangent rays just reach the Arctic Circle, thus causing the whole area within that circle to be in darkness. At this time the sun's noon ray is vertical over the Tropic of Capricorn. It is winter in the northern hemisphere and summer in the southern. The area within the Antarctic Circle is lighted and the south pole is in the middle of the long period of sunlight. The days are shorter than the nights in the northern hemisphere.

On March 21 the noon ray is vertical over the equator and the rays are tangent at the poles. Day and night are equal all over the earth.

On June 21 the north pole is tipped toward the sun. The tangent noon rays just reach the Antarctic Circle, thus causing the area within that circle to be in darkness. At this time the sun's noon ray is vertical over the Tropic of Cancer. It is summer in the northern hemisphere and winter in the southern. The area within the Arctic Circle is lighted and the north pole is in the mid-

II

Fig. s — Four Positions of the Earth Corresponding to the Four Seasons

In the winter and summer positions two different views are shown, one looking along a line

vertical to the earth's orbit and the other looking parallel to the earth s orbit.

die of its long period of sunlight. The days are longer than the nights in the northern hemisphere.*

Direction of Sunrise and Sunset. — The sun rises directly in the east and sets in the west only twice a year, on March 21 and

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Fig. 6. — Showing Position of Sun's Apparent Daily Sky Paths at the Equator Paths are vertical to the horizon and days and nights always equal.

September 23. At these two dates, called the Equinoxes, the sun's noon ray is vertical at the equator, or as more commonly expressed, "the sun is crossing the line," and days and nights are everywhere equal.

From the March, or Vernal Equinox, to the September, or Autumnal Equinox, in the northern hemisphere the sun rises north of east and sets north of west, and the days are longer than the nights.

From the September to the March Equinox, in the northern hemisphere the sun rises south of east and sets south of west,

*See Chapter VII for more exact length of days in higher latitudes at different times of year.

and the nights are longer than the days. (Make corresponding statements for the southern hemisphere.)

The northern journey of the sun culminates on June 21, called the Summer Solstice. The southern journey culminates on December 21, called the Winter Solstice.

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Fig. 7. — Showing Position of Sun's Apparent Daily Sky Paths at Latitude 41 N. The paths are tipped toward the south, showing our long days in summer and our short days in winter. The direction of sunrise and sunset at different times of year may be read from the figure.

The Sun's Daily Sky Path. — Because of the inclined position of the earth's axis, the sun's daily sky path (not only the sun's vertical ray and the sunrise and sunset position, but also each corresponding position for every moment of the day), migrates northward for one half of the year and then southward for the other half of the year. The effect of this is to bring about the regular changes in the inequality of the lengths of day and night. From a study of the above sketches, the shifting position of the sun's daily sky path for the year may be seen in places of different latitudes. The middle position is the sun's daily path for the March and September equinoxes, and the position farthest north is the sun's path for the June solstice, and the position farthest south is the sun's daily path for the December solstice. The planes of all the sun paths are always inclined from a

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Fig. 8. — Showing Position of Son's Apparent Daily Sky Paths at the Arctic Circle,

Latitude 66^° N.

On June 21 the sun remains above the horizon and on Dec. 21 below the horizon for

the entire 24 hours.

vertical an amount equal to the latitude of the observer, for the' planes are parallel to each other. See Figs. 6, 7, 8, and 9.

1. How do we know that the earth is whirling uniformly in space?

2. Make a sketch of an oblate spheroid and draw in the axis and the equatorial diameters. Properly letter the sketch and locate the poles. Where is the centrifugal force due to rotation the greatest? The least?

3. What is the area of the water surface of the earth? What substances are as much as 5.6 times as heavy as water?

4. Why are not the so-called evidences proofs that the surface of the earth is curved? Which evidences are strongest? Which weakest?

5. What are the relative positions of daylight and darkness upon the earth? In what direction do they travel?

6. Try to picture in your mind, by using a globe, the actual path which a particle at the equator describes due to the combined motion

IS of rotation and revolution. Make a free-hand sketch to show the motion and describe it.

7. A degree of longitude in latitude 40 equals about 53 miles. How

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Fig o. — Showing Position of Sun's Apparent Sky Paths at the North Pole Paths are nearly horizontal. The year is divided into two periods of sunlight and darkness.

many miles an hour does a point in that latitude move? How does this result compare with the distance the earth as a whole moves in an hour, due to revolution?

8. When riding on a railroad train, in what direction does the outside view from the window appear to move? What does this illustrate in respect to the apparent motion of the sun, moon and stars?

9. What effect has centrifugal force, due to rotation, upon the weight of bodies at the surface of the earth? Where is this effect greatest? Where the least?

10. The stars that rise and set, rise about four minutes earlier each night. Why? In a month these stars appear to shift westward. How far? What effect has this in twelve months? What is the real cause of the apparent shifting of the star dome?

11. If the earth's axis were perpendicular to the plane of its orbit, would revolution cause a change of season? Suppose the inclination of the earth's axis varied during a single revolution, what effect would that have upon the change of seasons? Is revolution necessary for a change of seasons? Explain.

12. In what direction does the sun's daily path through the sky shift from December 21 to June 21? During this period of six months, which is growing in length, our period of illumination or the period of darkness? Why? How is it from June 21 to December 21?

13. Why is not the sun always the same distance from the earth?

14. In about what latitude is the noon ray of the sun vertical on January 1st? March 1st? July 1st? September 1st? At these different dates, which is the longer here, the daytime or the night? In what latitude approximately are the northern and the southern limits of illumination at these dates?

1 5. State whether at these different dates the sun rises north or south of east and sets north or south of west.

16. Why are the Tropics placed where they are?

17. What is meant by the expression "sun crossing the line"? How often does this occur? In what direction is the sun migrating at each time?

Chapter II Latitude

The equator is the circle extending around the earth midway between the poles. Circles parallel to the equator are called parallels. The planes of all parallels, as well as the plane of the equator, are at right angles to the earth's axis. The distance expressed in degrees, north or south of the equator, is called the latitude of a place.

The axis of the earth extended northward marks the position of the north pole of the heavens. The elevation of the celestial or sky pole above the equator equals the latitude of the observer. The angle between a vertical line and the plane of the earth's equator also equals the latitude of the observer.

Because the equatorial bulge makes the curvature of the surface of the earth grow gradually less from the equator toward the poles, degrees of latitude increase slightly in length toward the poles. Less curvature of the earth's surface in the higher latitudes means that the surface has the form of an arc of a larger circle. A degree, or 1-360 of the length of the circumference of a larger circle, is evidently longer than a degree of a smaller circle. A degree of latitude at any place is therefore 1-360 of the circle whose curvature is that of the meridian at that place.

The circle N E S E' represents a meridian section of the earth, N S being the axis and E E' the equator. H H' is the plane of the horizon with the observer at O.

O N' extends north and is parallel to the axis N S. The point Z is the zenith directly over the observer.

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The angle C E' is the latitude of the observer and equal to N' O H, the altitude of the north pole of the sky.

Proof. — Angle Z N', the zenith distance of the north pole of the sky plus the angle HON' equals a right angle, or 90 degrees, since Z O is the perpendicular to H H'.

Angle N C plus angle E' C equal a right angle, or 90 degrees, since the axis of the earth is perpendicular to the plane of the equator.

Fig. 10.— The Altitude of the North Sky Pole, Angle N' O E, Equals the Latitude of Observer 0, Angle C E'

Angles N C and N' Z are equal, being corresponding angles made by a line crossing two parallel lines.

Therefore the complementary angle E' C O, the latitude of the observer, equals HON, the altitude of the north pole of the heavens.

How to Find a North and South Line. — By the following methods, a north and south line may be located:

(a) On any clear night the direction of Polaris, when it is directly above or below the sky pole, is due north. This occurs twice in every twenty-four hours, when Polaris and Mizar, the star in the bend of the handle of the Big Dipper, are in a vertical line.

(b) The direction of a magnetic needle, when corrected for variation, will enable one to locate a north and south line.

(c) The direction of the shortest shadow cast on a horizontal plane by a vertical post is north and south. When the sun is at its highest point in the sky, shadows are shortest. This occurs at solar noon, which is approximately noon, local time.