Elements of physical geography

Front matter

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Class \?> r

Book.

Contents

CHAPTER PAGE

I. The Earth as a Planet L

II. Groundwater and Rivers 40

Chapter I

Introductory.— Physical geography literally means a description of the natural features of the earth. The development of the subject during recent years has led to the subdivision as follows :

1. The earth as a globe or planet, its origin and relation to the other heavenly bodies.

2. The atmosphere or the surrounding gaseous portion.

3. The hydrosphere or the water, including the fresh water and the salt water of the ocean.

4. The lithosphere or the solid land portions, the causes producing the various topographic forms and the effects of these on climate and life.

5. Life geography or the effect of physical environment upon life and its effect on the earth features.

Physical geography includes a study of these subjects with reference to their influence upon man, his industries, civilization and relation to his surroundings. With this aim in view it leads one within the doorway of each of the natural sciences.

To the ancient philosopher's maxim, "Know thyself," the modern scientist adds "in relation to Nature." This is the foundation of modern physical geography. In gaining this knowledge man is better able to adapt himself to his surroundings, to utilize the various forces and products of Nature, to realize not only his dependence upon his fellow man and the lower forms of life, but his duty towards them as well, and consciously or unconsciously, he must gain respect if not love for the Omnipotent Power that rules over all.

Physical Geography is the science which treats of the

l natural features of the earth in their relation to man and the lower forms of life.

1. The Earth a Part of the Solar System.— The earth is a nearly round ball consisting of a large rock mass partly covered with oceanic waters, and entirely surrounded by the gaseous atmosphere. The whole mass solid, liquid and gaseous, rotates on its axis as it revolves in space around the sun. It is but one of a number of similar bodies called planets and is in no wise conspicuous among them. It is neither the largest nor the smallest; neither the farthest from nor the nearest to the sun. Because we live on the earth, it is most important to us, but if we could look on it from some distant point in the heavens we should not see anything to distinguish it particularly from the other planets.

2. What the Solar System Comprises.— The earth is an important member of the solar system which includes the sun at the center, the planets and their satellites, the planetoids or asteroids, and some comets. Besides the earth there are revolving around the sun seven other planets which are named in order beginning with the one nearest the sun, — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Four of these, Venus, Mars, Jupiter and Saturn, are plainly visible at certain periods. Two of them, Venus and Jupiter, are at times the brightest bodies in the heavens except the sun and moon. Part of the time they are morning stars ; at other times evening stars. The planets may be distinguished from the true stars by their steady light. The stars twinkle. Each of the planets, except Mercury and Venus, has one or more satellites or moons revolving around it. Saturn has besides the satellites several concentric bright rings surrounding it. The relative sizes, distances and other data concerning the planets are given in Appendix I.

The asteroids or planetoids, about 600 in number, are solid bodies much smaller than the planets, and revolve in orbits between Mars and Jupiter. One of the planetoids, Eros, about 20 miles in diameter, discovered in 1898, has a very eccentric orbit that sometimes brings it within 13% million miles of the earth. The student should learn to recognize the larger planets and observe their movements among the stars from season to season.

^X\V. SOLAR SYSTJ^,

Fig. 1. The solar system, showing the order of the planets, satellites, asteroids, and the orbits of a few comets.

3. Relation of the Solar System to the Universe.—

The Solar System, large and complex as it appears, is but one of a number of similar systems in the universe. Most of the bright stars in the heavens are suns similar to ours. They appear to be much smaller than our sun, but that is because they are so much farther away. In reality many of them are much larger. They probably have planets, satellites, comets, etc., like our own system, but they are so far away that these bodies, if they exist, are not visible from the earth. It is not known how many of these systems there are, nor how far out in space they extend, but certainly a great distance beyond our comprehension. It is estimated that with a large telescope one can see between 100 and 200 millions of stars, a large per cent of which lie in the Milky Way. With few exceptions all of these stars are so far away that it takes the light from them travelling at the rate of 186,000 miles a second many years to reach the earth. The moon is about 240,000 miles away or about ten times the distance around the earth ; the sun is nearly 400 times farther than the moon ; and the nearest fixed star or neighboring sun system is several thousand times farther than the sun. The light of the sun takes about 8 minutes to reach the earth. The light of the nearest star takes 3% years to cross the space separating it from the earth. Truly the earth is a very small part of the solar system and an exceedingly minute portion of the universe. 4. The Moon. — The earth has one satellite, the moon, which revolves around it once a month (27.32 days) and accompanies it through space in its journey around the sun. The moon is 2,163 miles in diameter and at an average distance of 238,840 miles from the earth, but it varies from 221,600 to 252,970 miles. It is because of its nearness to the earth that it is held in its orbit around the earth instead of pursuing an independent course around the sun. (The synodic month or the time from full moon to full moon is 29.53 days, but the sidereal month is 27.32 days.)

5. The Phases of the Moon.— The moon emits no light of its own. All the light that comes from it to the earth is reflected sunlight. When the moon is in that part of its orbit nearest the sun, it is nearly between the earth and the sun, and we see but a mere fringe of illumination; it is then the new moon. The sunlight reflected from the earth faintly illuminates its dark side giving what is called the earth shine. When it has completed a fourth of its circuit after new moon, it is at right angles to a line connecting the sun and the earth, and we see one-half of the illuminated face, that is, a fourth of the whole surface, and the phase is called the first quarter. When it has completed half a circuit and is on the opposite side of the earth from the sun, it

Fig. 2. The phases of the moon.

is full moon. At the third quarter the moon has completed three-fourths of its circuit and one-fourth of the whole surface is again reflecting light to the earth. The line separating the illuminated portion from the dark portion is called the terminator. Draw from observation a figure of the moon showing the light and dark portion every second night from one new moon to the next; arrange them in order around an ellipse and compare them.

6. The Sun. — The sun is the center of the solar system. All the planets of the system revolve about it and receive heat and light from it. It is much larger than all the planets combined, having a diameter of 866,000 miles, which would make it a million times the bulk of the earth ; but since its density is less, it has only 332,000 times the mass of the earth. Imagine the earth at the center of the sun and the moon revolving around it in an orbit the same size as the present one ; the moon would then be about half way from the center to the circumference of the sun. 7. The Sun's Energy.— Nearly all the heat, light, and other forms of energy on the surface of the earth come directly or indirectly from the sun. The radiant energy

from the sun, known as insolation, is thought to pass from the sun to the earth unaffected by intervening space until it reaches the earth where part of it, the part that we recognize, is perceptible as heat and light.

The part of the sun's insolation received by the earth is an exceedingly small part of the whole, and when one realizes that nearly all forms of heat and light come from the sun, the total quantity radiated into space is something beyond comprehension. All the energy used by man in heating and lighting, all that is used in running machinery everywhere, ^11 that is used in lifting the waters of the sea to the clouds to fall as rain, all that wonderful vital energy manifested in animals and plants,— all of these and probably other forms of energy as yet unrecognized are flashed like wireless telegrams across the vast space that separates us from the sun.

Fig. 3. Showing the relative size of the sun and the moon's orbit. What is the scale of the diagram?

8. Eclipses.— Since the sun is the source of the light received by the earth and the moon, when either of these

Annular Eclipse

Fig. 4. Solar and lunar eclipses.

latter bodies comes between the other and the sun, the light of the sun will be cut off. The shadow thrown by the intervening body on the other is known as an eclipse. The shadow of the moon on the earth produces an eclipse of the sun, and the shadow of the earth on the moon causes an eclipse of the moon. If the moon passes entirely into the earth's shadow, there is a total eclipse of the moon, if only part of it passes into the shadow, a partial eclipse is the result. There may be three kinds of solar eclipses: (1) a total eclipse when the moon passes centrally over the disc of the sun and so near the earth that the shadow reaches the earth; (2) an annular eclipse, produced when the moon passes centrally over the disc of the sun but is so far from the earth that the end of the shadow does not reach the earth ; then the moon appears as a black spot in the center of the sun surrounded by a ring of light which gives the name annular or ring eclipse; (3) a partial eclipse of the sun produced when the moon passes a little to one side of the line joining the earth and the center of the sun.

If the moon revolved about the earth in. the plane of the earth's orbit, there would be a total eclipse of the moon and sun once each month, but since the plane of the moon's orbit is inclined at an angle of five degrees to that of the earth's orbit, there is an eclipse only when the moon passes one of the nodes, that is, the points of intersection of the two orbits, at or near new moon or full moon. There may be an eclipse of the sun when there is none of the moon, and there must be at least two solar eclipses each year. Consult the almanac for several years and see how many eclipses of each kind there have been.

In 1907 there were four eclipses, two of the sun, one total and one annular, and two of the moon.

9. Comets. — Comets belong in part to the solar system. Several hundred of these bodies have been seen from the earth at .different times. Some of them travel in elliptical orbits which extend millions of miles out into space beyond the outermost planet in our system, hence the period of revolution is one of many years. Many comets travel in a parabola or a hyperbola and become visible once as they pass around the sun and away again, never to return. Whence they come and whither they go is not known. Prom fig. 5 it can be seen that parabolas and hyperbolas are curved lines, the ends of which never meet.

The comets differ in size and shape as widely as they do in their orbits. They are characterized by a nucleus or denser portion surrounded by a nebulous mass called the coma which streams out from the nucleus and forms the tail. The tail is single or double and of widely diverse

Fig. 5. Ellipse, parabola and hyperbola. The last two are diverging curves which never meet.

shapes and differs in length from that of the diameter of the nucleus to a length of 100,000,000 miles or more. As a comet approaches the sun, the tail streams out behind it, as it passes perihelion (the nearest point to the sun), the tail streams out ahead of it, that is, the tail keeps on the opposite side of the nucleus from the sun. Celestial photography has shown recently that the tails of several comets have been suddenly broken into two or more parts.

10. Historical Comets.— The comet of 1680 is an important one because it was the first whose orbit was determined by the principles of gravitation. The computation was made by Sir Isaac Newton who found that it passed within 140,000 miles of the sun travelling at the rate of 1,332,000 miles an hour. It had a tail 100,000,000 miles long.

Halley's comet (1682) is so called because Halley, a friend of Newton, computed its orbit and thus identified it with previous comets that had appeared at intervals of 75 years. He predicted that it would make its next appearance March 13, 1759. It passed perihelion within a month of that time. It appeared in 1835 and is due again in 1910. Watch for it.

Biela's comet (1826) was observed in the latter part of December, 1846, to elongate and divide into two parts which travelled in parallel orbits 160,000 miles apart. When they next appeared in 1852 the two portions were 1,500,000 miles apart. They have not been seen since.

The Comet of 1882 was the most conspicuous one in recent years. It approached the sun in perihelion close enough to pass through part of its gaseous envelope. Daniel's comet attracted attention in the summer of 1907.

11. Meteors and Shooting Stars.— Meteors and shooting stars are luminous bodies which are frequently observed in our upper atmosphere and are sometimes seen to strike the earth. The luminosity of these bodies is thought to be due to friction against the atmosphere and that before entering the atmosphere they are cold and non-luminous. Many of them are dissipated in the upper atmosphere, but probably the fragments in the form of invisible dust reach the earth in the course of time. Ten to twenty millions of meteors strike the earth's atmosphere every day. It is thought by some that the earth has been formed by the aggregation of such particles, which would mean that unless the earth is losing matter in some way it is still increasing in weight.

12. Meteorites.— Meteors which fall to the earth are called meteorites. They \jary in size from very minute fragments to bodies of many tons in weight. The great

Tent meteorite in New York City which Peary brought from Cape York, Greenland, weighs 36.5 tons. The Bacubirito meteorite in Mexico weighs about 27.5 tons. The Willamette meteorite, shown in fig. 6, weighs 15.6 tons. Some are composed of stone, some of metals and some of both. About four out of every hundred are nearly pure iron with a little nickel. The source of meteors and meteorites is not definitely known.

Fig. 6. Willamette meteorite, the third largest known, found near Oregon City, Oregon. Length 10 ft., height 6 ft. 6 in., weight 15.6 tons. (American Museum of Natural History.)

THE ORIGIN OF THE EARTH

All material things so far as we know have a beginning, a period of growth, decline, and death. This is not true of matter itself but of the forms which it takes. The fact is commonly recognized in regard to plants and animals but is probably no less true of many inanimate objects, except that the changes go on so much more slowly that they are frequently not recognized. It is now known that the hills are not "everlasting." They may be "rock-ribbed" but they are not as "ancient as the sun." The mountains have a beginning, and a period of growth, after which they begin to dwindle and gradually disappear. So it is with the earth, the sun, and the solar system; they did not always exist as such. When and how were they formed !

13. The Nebular Hypothesis.— Of the many attempts to explain the origin of the solar system none has met with more favor than that known as the nebular hypothesis, which assumes that at one time all the material in the solar system existed in the form of a rotating mass of nebulous gas that occupied all the space from the center of the present sun out to and beyond the limits of the orbit of the outermost planet — Neptune. Under the universal law of gravitation, by which every particle of matter in the universe attracts every other particle, these gas particles were attracted towards a common center. In the course of time a portion of the mass was separated in the form of a ring, or, as some say, as an irregular mass, which in time, by its rotation on its own axis, formed a spheroidal body revolving around the central mass. This was the planet Neptune, which continued to revolve around the central mass from which in turn the other planets and their satellites were separated, the earth being the sixth one and Mercury the last one. The residual central mass is the sun, which, according to the hypothesis, is still contracting. The planetary masses probably separated from the parent mass while still in the gaseous condition, but contiaued to contract until they became liquid and on further cooling, solid, at least on the surface.

This hypothesis, with sundry modifications, has been widely accepted because it seemed to account for so many things about the system. Recently many objections to this explanation have been raised, while another explanation has been growing in favor with some people because it appears to be free from some of the difficulties in the nebular hypothesis.

14. The Planetesimal Hypothesis.*— The planetesimal hypothesis, although it starts with a nebulous mass, differs radically from the nebular hypothesis in a number of particulars. Unlike the first, however, the nebula is not necessarily a gas nor is it highly heated and hence it need

Fig. 7. A spiral nebula in Ursa Major. (Ritchey, Yerkes Observatory)

not pass through a liquid state. The hypothesis starts with a spiral nebula, which is one of the most common forms in the sky at present. The knots or denser portions in the nebula are the nuclei of the future planets and satellites,

* Formulated by Professors Chamberlin and Moulton of the University of Chicago.

and the nebulous haze surrounding the nuclei consists of finely divided matter mostly solid, possibly some liquid and gaseous, which is later gathered in by gravitation, and added to the nuclei to form the planets. All of the material, first in the nebula and later in the planets and satellites, moves about the central mass in elliptical orbits. It all has a double motion, first around the central axis of its own nucleus or planet, and second around the central sun.

Pig. 8. The great nebula in Andromeda. (Ritchey, Terkes Observatory.)

The hypothesis supposes a relatively slow growth of the earth, as of the other planets, with increasing temperature in the central portions due to gravity. That is, a large body will have greater pressure by gravity at the center than a small one and, hence, will have greater heat induced by the pressure. In a body as large as the earth, the gravitative attraction of all the particles towards the center produces an enormous pressure on the central portions, a pressure sufficient to produce heat and raise the temperature of the interior.

For a long time after the earth niiclues began to grow it was too small to have an atmosphere or even a hydrosphere, both of which formed gradually as soon as the planet was large enough to hold them by the force of gravity. They would be increased by the parts expelled from the interior by gravity pressure, as well as the parts that would be drawn to the surface of the mass from the surrounding nebula. Hence, the accretion of the planetesimal matter of the outer half or more of the earth would be through an atmosphere and subject to the action of moisture. This hypothesis likewise makes possible a much longer period of time in which life was possible on the earth, or in which the conditions" favored the existence of life in the initial stages, than does the nebular hypothesis.

The principal points of difference between the two hypotheses are, that according to the first, the earth passes from a highly heated gaseous condition through a hot molten state to the present solid condition, while according to the second, the earth was never entirely gaseous, never necessarily molten and possibly never much hotter than at present. By the first, the earth was once larger than at present and included the moon which was later separated from it; by the second, the earth was never larger, probably not so large in the past as at the present. By the first, the outer planet, Neptune, is the oldest and the inner one, Mercury the youngest; by the second, the planets and their satellites are all of about the same age, that is, they were all in process of formation at the same time. According to the planetesimal hypothesis the moon is devoid of water and an atmosphere because it is too small to hold them on the surface by gravity, and not because it is so old that it has lost them as sometimes claimed in the nebular hypothesis.

15. The Shape of the Earth.— The earth is the shape of a ball that is flattened at the poles and bulged at the equator, so that the equatorial diameter is nearly 27 miles longer than the polar diameter. It approaches an oblate spheroid more nearly than any other mathematical figure.

Evidence that the earth has a curved and not a flat surface: (1) Its shadow on the moon is always a curved one. Could this be true of a flat surface? (2) New stars appear in front of the observer and old ones disappear behind him as he travels toward the north or south. How would

Fig. 9. Expansion of horizon with elevation indicates curvature of the earth.

it be on a flat surface? (3) The horizon expands rapidly as the observer ascends to higher altitudes. Wo,uld this be true on a flat surface? (4) At sea the slender toprigging of a vessel is visible farther than the larger but lower hull. Why? (5) There is a marked difference in time with a change of longitude ; thus the sun rises more than three hours later in San Francisco than it does in Philadelphia, and nearly nine hours later than it does at London. Tlow would it be if the earth were flat ? ( 6 ) The earth has been circumnavigated many times. (7) The flattening at the poles is indicated by the increased weight of a body in high latitudes over that of the same body at the equator, and by the greater length of a degree of latitude near the poles. (See sec. 28 and fig. 16).

16. Cause of the Shape of the Earth.— Nearly 200 years ago it was shown that an oblate spheroid is one of the figures of equilibrium for a rotating body, and the degree of oblateness or flattening is due to the rate of rotation. More recently it has been shown that the oblateness of the earth corresponds to the requirements of a rotating fluid mass of the size and rate of rotation of the earth. This was cited as evidence that the earth was fluid at one time in its history before reaching its present solid form. But there are good reasons for thinking that a solid earth would take the same shape after a long period of time, due to the shifting of materials on the surface, or according to the planetesimal hypothesis there would be more material accumulate at the equator than at the poles.

Gravitation shapes the material into a sphere. Rotation causes the flattening of the sphere into the oblate spheroid. It is gravitation that holds bodies on the earth, and the force increases with the mass of the planet. If the earth were the size of the moon, bodies would have much less weight on its surface. Gases would be so light that they would fly off into space and there would be no atmosphere, hence no water, and no life. If the earth were as large as Jupiter, bodies on the surface would be correspondingly heavier.

17. Size of the Earth.— The diameter of the earth through the poles is 7,899.6 miles; through the equator 7,926.6 miles. Compute the circumference, the area, the volume and the weight of the earth in tons from the following data :

1. The circumference equais the diameter multiplied by 3.14159.

2. The area equals the product of the diameter by the

circumference of a great circle. 2

3. The volume equals the area of the surface multiplied by one-third of the radius.

4. The mass equals the volume multiplied by the density. The mean density of the earth is 5.6. A cubic foot of water weighs 62.5 lbs.

18. Problem of Eratosthenes.— The diameter of the earth was not the dimension first determined, as there is no way of measuring it directly. The part that was actually measured was an arc of the circumference. This problem was first solved by Eratosthenes two centuries before the Christian era, He deter-

-Fig. 10. Illustrating the problem of Eratosthenes. The sun's rays, vertical at A, are inclined 7° 12' to the vertical sz' at s, which is the angle at the center of the earth, C, measured by the arc AS. Fifty times this arc equals the circumference of the circle or the distance around the earth.

mined that Syene in Egypt was close to the same meridian, hence to the same great circle as Alexandria. He had observed that at noon on the longest day in midsummer the sun's rays shone on the bottom of a deep well at Syene in Egypt. "What inference could he draw from this? He measured the angular distance of the sun from the zenith at Alexandria on the same day at noon and found it equaled 7 degrees and 12 minutes, or exactly one-fiftieth of a circle, which is the same as the angle at the center of the earth formed by the radii from these cities. Prove this. The distance between the two cities had been measured and found to be 5,000 stadia, hence by multiplying this distance by fifty he obtained the total distance around the earth as 250,000 stadia. Unfortunately we have no means of knowing at the present time the length of a stadium in any of our units of measurement, so that we have no certain means of comparing the accuracy of the result obtained by Eratosthenes with those obtained by similar means in more receDt times.

19. Structure of the Earth.— The earth is frequently divided for convenience of study into three parts or spheres : (1) The outer gaseous envelope, the atmosphere; (2) the liquid envelope, the water or hydrosphere which nearly surrounds (3) the solid rocky part, the lithosphere, the inner portion of which is sometimes called the centrosphere. A fourth is sometimes added called the biosphere, or life sphere. These are not true mathematical spheres, nor are they very sharply separated at times. Both the hydrosphere and the atmosphere penetrate the lithosphere and large quantities of the atmosphere are dissolved in the hydrosphere as well as large quantities of water as invisible vapor in the atmosphere. The life sphere is confined chiefly to the water and the contact of the atmosphere with the lithosphere. It is scattered through the water sphere to a greater depth, probably, than in either the gaseous or the rock portions, yet the greater portion of it lies close to the lower portions of the atmosphere. The air, water and land portions of the earth, which form the greater part of the subject of physical geography or physiography, are discussed in the following chapters.

20. Motions of the Earth.— The earth has (1) a daily rotation on its axis, and (2) a yearly revolution around the sun. Besides these, it has (3) an onward motion through space in company with the other parts of the solar system, but this is not so apparent as the other two and is not marked by such pronounced effects. There are a number of other minor motions of interest to the astronomer.