Henry W. Ruoff

THE EARTH AS A PLANET

ITS STRUCTURE: Interior, Crust, Rocks, Fossils, Heat

GEOLOGICAL VIEW OF GROWTH OF THE EARTH

SURFACE OF THE EARTH: Land Forms: Continents, Islands, Mountains, Plains; Water Forms: Springs, Rivers, Lakes, Oceans

CELEBRATED MOUNTAIN PEAKS AND RANGES

ATMOSPHERE, CLIMATE AND WEATHER

NATURAL WONDERS AND FORCES: Volcanoes, Earthquakes, Geysers, Caverns, Waterfalls, Whirlpools, Tides, Deserts, Ocean Depths, Clouds, Seasons, Glaciers, Icebergs, Snow, Rain, Hail, Dew, Coral Islands and Reefs

DICTIONARY OF MINERAL PRODUCTS

TABLES FOR THE IDENTIFICATION OF MINERALS

GEMS AND PRECIOUS STONES

PRONOUNCING DICTIONARY OF SCIENTIFIC TERMS ABOUT THE EARTH

NUMEROUS ILLUSTRATIONS, CHARTS AND MAPS

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Life Ages of the Earth	Pictorial Diagram Showing the Corresponding Forms of Animal and Plant Life, and Rock Strata in the Earth’s Crust.	Rocks and Strata to which they belong
Cenozoic, or Recent Life. Age of Mammals.		Alluvium, Gravel, Mud, Sand, Clay, Marl, Limestone.	Ceno- zoic
Mesozoic, or Middle Life. Age of Reptiles.		Chalk, Gault, Green Sand, Oolite, Clays and Limestone, China Clay, Shales, Cement, Sandstone, Pervian.	Meso- zoic
Paleozoic, or Old Life. Age of Invertebrates. Age of Fishes. Age of Acrogens.		Coal Massives, Upper and Lower. Millstone, Grit, Mountain, Limestone, Old Red Sand Stone, Iron Ore, Gypsum, Gas, Lead, Zinc, Phosphate, Marble, Sandstone, Shales, Copper.	Paleo- zoic
Proterozoic, or Earlier Life. Earliest Forms of Life.		Copper, Silver, Lake Superior Iron Ores, and many Metals. Granite, Schists. Emery, Gems, and Building Stone.	Protero- zoic
1. Sivatherium, (siv-a-the´-ri-um). 2. Mastodon, (mas´to-don). 3. Elephas, (el´e-fas). 4. PalÆotherium, (pa-le-o-the´-ri-um). 5. Pterodactyl, (ter-o-dak´til). 6. Ammonites, (am´mo-nitz). 7. Plesiosaurus, (ple-zi-o-saw´rus). 8. Ichthyosaurus, (ik-thi-o-saw´rus). 9. Carboniferous, (kÄr’bon-if´er-us) fern. 10. Lepidodendron, (lep-i-do-den´dron). 11. Calamites, (kal´a-mits or kal´a-mi´tez). 12. Labyrinthodon, (lab-i-rin´tho-don). 13. Acanthodus, (a-kan-tho´dus). 14. Diplacanthus, (dip-la-kan´thus). 15. Lepidosteus, (lep-i-dos´te-us). 16. Climatius, (cli-mai´te-us). 17. Zosterites, (zos-ter-i´tez). 18. Goniatites, (go-ni-a-ti´tez). 19. Strophomena, (stro-phom´e-na).

Large illustration (465 kB)

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BOOK OF THE EARTH

Science tells us that the Earth was once a shining star, a globe of liquid fire. As it cooled down, a crust formed over its surface, composed chiefly of rocks and metals. This crust was rent by the force of the gases shut up within, and thus the mountains, valleys, gorges, and volcanoes were formed. The Earth, indeed, is still upheaving and subsiding, but so slowly that we rarely feel it. Through these agencies the distribution of land and water on the surface of the earth has undergone great changes. The shape of the Earth is that of a sphere somewhat flattened at the poles, and it has a diameter of about 8,000 miles. The solid crust is called the lithosphere—which is surrounded by an envelope of air—the atmosphere—and in part by an envelope of water—the hydrosphere.

HOW THE EARTH WOULD APPEAR IF CUT THROUGH THE CENTER

Beneath the rocky crust of the earth, thirty-five miles in thickness, there is a broad belt of heavier material to a depth of nine hundred miles. Within this shell lies the great metallic core.

OUR EARTH: ITS STRUCTURE AND SURFACE

Our first glimpse of the earth as a planet shows it as a nebulous star, still intensely hot, and with no solid nucleus, rotating on its own axis, and at the same time revolving around the sun in a nearly circular orbit.

WHAT THE HEAT OF THE
EARTH SHOWS

At first it seems hardly possible that the earth could have been a star. But, if we go down beneath the surface of the earth, we find that at a depth of forty or fifty feet there is very slight variation in temperature. When we go yet deeper, as in mines, we find that the earth grows hotter as we descend. The temperature increases on an average about one degree Fahrenheit for every sixty-four feet descent. But this amount is variable according to the locality, geological formation, and dip of strata. In the Calumet and Hecla Mine, observations show an increase of one degree in about every one hundred and twenty-five feet. At Paris, the water from a depth of 1794 feet has a temperature of eighty-two degrees; at Salzwerth, in Germany, from a depth of 2144 feet, a temperature of ninety-one degrees. Natural hot springs, rising from unknown depths, are sometimes scalding hot. One in Arkansas has a temperature of one hundred and eighty degrees.

At a depth of twenty miles, with this continual increase of temperature, the ground must be fully red-hot; and not very much farther down the heat must be sufficient to melt every known substance. The solid earth, then, is merely a thin crust, covering a sea of liquid fire below. The streams of lava poured forth from volcanoes are a proof of the existence of this molten mass beneath our feet.

WHAT CAUSES THE INTERNAL
HEAT OF THE EARTH

If we examine the solid crust of the earth we shall not long be at a loss in regard to the origin of this internal heat. We are all familiar with the burning of coal. Now coal is mainly a substance called carbon, and when it burns it unites with oxygen, one of the gases in the air. Many rarer substances, such as silicon, and the metals magnesium, calcium, and sodium, are even more inflammable than carbon, and in burning give rise to solid products. Now the rocks in the earth are found to be made up almost wholly of these very inflammable substances combined with oxygen. The solid portions of the earth, then, are nothing but the ashes and cinders of a great conflagration. Even the waters are made up of hydrogen, one of the most inflammable substances, united with this same oxygen, and, strange as it may seem, they too, are the products of combustion. When, therefore, the materials of which the earth is formed were burning, our planet must have been a fiery star, and the great heat must have reduced all the products of the conflagration to a liquid state.

HOW THE EARTH’S CRUST
WAS FORMED

When the fire went out for lack of fuel the mass began to cool at the surface, and a solid crust was finally formed, which with the lapse of time became thicker and thicker. This crust shut in the steam and gases generated in the fiery ocean underneath; and these, acting upon the crust with enormous pressure, heaved it into ridges. At times the strain caused the crust to crack, and forced the melted mass up through it, and in this way hills and mountains were formed. The thicker the crust the greater the strain it would bear before it gave way, and the greater the amount of molten matter driven out through the rent. The highest mountains, then, are the last that were uplifted. In some cases the openings thus made in the crust were never completely closed, and thus volcanoes were formed. These act like safety-valves, and prevent the forces within from accumulating sufficiently to cause fresh rents. But notwithstanding the relief thus given to the pent-up forces, they still manifest themselves in earthquakes.

SHAPE OF THE EARTH
A SPHEROID

Like all other planets, the earth is a solid sphere that has undergone a slight flattening at the opposite extremities or poles of the axis of revolution. More accurately, it is an oblate spheroid generated by the rotation of an ellipse about its minor axis. Such a figure would be assumed by a sphere of liquid rotating about a diameter, centrifugal force acting most vigorously at the equator, and tending to overcome the internal forces that keep the molecules together.

SIZE AND DENSITY OF
THE EARTH

The smallest diameter of the earth is that measured from pole to pole along the axis of rotation; this is 7,899.6 miles, or about 500,000,000 inches. The greatest diameters are those measured between opposite points on the equator; these are 7,926.6 miles, and, therefore, show that the eccentricity of the earth, or the extent of its departure from the perfect sphere, is very slight.

The circumference of the earth, measured along the equator, is 24,899 miles; the area is 197,000,000 square miles; and the volume is 260,000,000,000 cubic miles. Experiments on the comparative attraction of the earth show that its density is about five and one-half times that of pure water. Its mass is, therefore, approximately six thousand trillion tons.

HOW WE KNOW THE EARTH
IS A SPHERE

The ordinary proofs of the sphericity of the earth are: (1) It can be circumnavigated; (2) the appearance of a vessel at sea always indicates a nearer convexity of the earth’s surface; (3) the sea-horizon is always depressed equally in all directions when viewed from an elevation; (4) the elevation of the pole star increases as we travel northwards from the equator; (5) the shadow of the earth on the moon during a lunar eclipse is spherical.

THE ROTATION OF
THE EARTH

The earth rotates uniformly about its axis. The time taken to make a complete revolution of three hundred and sixty degrees is called a sidereal day, for it is the interval of time between consecutive transits of any distant star across any meridian of the earth. The time between consecutive transits of the sun across any meridian is called a solar day; the average of these throughout the whole year is called a mean solar day, and is the practical standard of time adopted by civilized nations. The ordinary proofs that the earth rotates are: (1) Bodies falling from a great height have an easterly deviation; (2) Foucault’s pendulum experiment; (3) a gyroscope delicately balanced so as to be free to change the direction of its axis in any way will, if rotated, exhibit an apparent deviation; (4) in northern hemispheres a projectile deviates to the right, in southern hemispheres to the left; (5) the trade winds; (6) Dove’s law of wind-change.

The speed of a body on the equator, due to the diurnal rotation, is about 1,000 miles an hour. The centrifugal force due to this speed diminishes the weight of bodies; if the earth rotated in an hour, they would be thrown off from the surface at the equator.

The axis of the earth is not perpendicular to the ecliptic, but at angle of 66° 32' to it; the equator is, therefore, inclined to it at an angle of 23° 28'. This unsymmetrical placing of the bulging portions of the earth causes a slow wobbling, or precession of its axis, in the same sort of way as a spinning top will wobble when pushed over on one side. There is also a slight vibration or “nodding” motion of the earth’s axis, known as nutation. The period of each precession is about twenty-one thousand years; if the earth’s orbit occupied a constant position in its plane, the periods would be twenty-six thousand years each. These motions have considerable influence on climate, the modern theories of the Ice Age being connected with the known facts of precessional motion.

THE EARTH A SERIES OF
SHELLS OF MATTER

The great bulk of the earth consists of the lithosphere, or solid globe of rocks, with which geology properly deals. It is on the part of this lithosphere, composing a little more than a quarter of the earth’s whole area—55,500,000 square miles—which rises above the seas and is called land, that mankind lives.

The central core is a globe of about 7600 miles in diameter, which is composed of iron and other elements, probably not forming compounds, in the gaseous state, but exposed to such tremendous pressure that it behaves as a solid and extremely rigid body. Outside this core is a shell of liquid matter which consists of all the rocks which we know at the surface in a state of fusion, perhaps one hundred miles in thickness. Upon this magma floats the solid crust, thirty or forty miles thick, which is composed of various rocks, breaking down at the surface into soil. Three-fourths of the surface of this crust are covered by the water of the oceans, the hydrosphere, the rest being dry land. Outside all comes the atmospheric mantle, chiefly composed of air, which supports life, acts as a blanket to keep the earth warm, and as a shield against the blows of meteorites.

HOW THE EARTH’S CRUST
IS CONSTRUCTED

An examination of the Earth’s crust shows us that it is constructed of numerous strata of rocks, some of limestone, some of sandstone, and some of clay; and some are very hard, others soft and crumbling, and readily worn away by the action of running streams or the waves of the ocean. To these several substances which form the materials of the earth’s crust we give the name rock. Hence we see that while in ordinary language the word rock denotes a great mass of hard stone, in geology a rock is any mass of natural substance forming part of the earth’s crust. In this sense, loose sand, gravel, and soft clay are as much rocks as hard limestone and granite.

MATERIALS OF WHICH ROCKS
ARE COMPOSED

Rocks are formed of various materials called minerals. If we take a piece of sandstone rock, or a piece of granite, we shall probably be able to notice that the rock is made up of different substances.

On looking at a piece of sandstone, for example, especially if we use a magnifying glass, we see that it is composed of little rounded grains of a glassy-looking substance cemented together. In some specimens these grains are larger than in others. This cementing material is not the same in all sandstones, but in our specimen it is formed of calcium carbonate, for when we drop a little diluted hydrochloric acid on the rock there is an effervescence. The cementing material is dissolved, but the little rounded grains, which consist of quartz, are not affected by the acid. The sandstone, then, consists of quartz grains cemented together by calcium carbonate. It is called a calcareous sandstone.

Now take a piece of granite, and break it with a hammer to get a clean-cut face. On looking at this face we see that the rock is made up of three different substances.

One of these has a glassy appearance like the grains in the sandstone, and is so hard that we cannot scratch it with a knife. This is quartz. Another of the substances is of a dull white or pinkish color. It lies in long, smooth-faced crystalline patches, which easily break along a number of smooth parallel surfaces having a pearly lustre. It can be scratched with difficulty by the point of a knife. This substance is called felspar. The third substance consists of bright glistening plates, sometimes of a dark color, which can be easily scratched, and which readily split into transparent leaves. This is mica. Notice that these substances do not occur in any definite order, but are scattered about through the stone irregularly, the felspar occurring in some specimens in larger crystals than in others.

WHAT A
MINERAL IS

Hence we see that granite consists of a mixture of three substances, called quartz, felspar, and mica, the felspar being in greatest quantity. Each of these substances possesses properties more or less peculiar to itself, such as hardness, solubility in acids, specific gravity, crystalline form, way of splitting, etc. Hence, each of these substances has a definite chemical composition and constant physical properties which define them as minerals.

This definition may be understood to include such substances as coal and chalk, which are the mineralized remains of plants and animals respectively. Even water and gases of the atmosphere may be said to belong to the mineral kingdom of nature, as plants and their parts are said to belong to the vegetable kingdom, and animals and their parts to the animal kingdom.

CHIEF ROCK-FORMING
MINERALS

The total number of rock-forming minerals is very large, but many of them are very rare, and form but a very small part of the earth’s crust.

The most abundant materials or earths of which rocks are composed are silica, lime and aluminum. Silica or flint is very universally diffused. It is found almost pure in quartz, opal, chalcedony, rock crystal, and the flinty sand of the sea-shore. Lime is also a very generally distributed earth, and is usually found in the form of carbonate. Under the several names of marl, limestone, oolite, and chalk it constitutes mountains, and even ranges of mountains. Aluminum is likewise very abundant, and of great importance to mankind. It enters largely into the clayey or argillaceous earths, and forms part of various kinds of rock which possess the property of not permitting water to pass through its substance—a property which renders it of inestimable value both for natural and artificial reservoirs of water.

CHIEF CHEMICAL ELEMENTS WHICH
FORM MINERALS

The larger number of elements play so small a part in the constitution of the earth that they may be neglected by the geologist. The following list includes the elements of which ninety-nine per cent of the earth’s crust, as known to us, is composed, with their relative proportions, as indicated by Clarke’s laborious analyses of a very large number of typical rocks:

The ten elements given above form 99.24 of the earth’s solid crust.

HOW ROCKS ARE
CLASSIFIED

The beds or layers which form the crust of the earth are divided into three classes: (1) Sedimentary, or stratified; (2) Igneous, or unstratified; (3) Metamorphic, or transformed.

SEDIMENTARY OR
STRATIFIED ROCKS

Sedimentary rocks are such as give evidence of having been formed by successive deposits of sediment in water. They include sandstones or freestones, limestones, clays, etc. The material for these must have been derived from some original source, and in many instances this may be traced to the disintegration of older rocks. Thus gneiss appears to be formed by the disintegration of granite. The great class of sedimentary rocks may be divided into three smaller divisions. These divisions, with the chief rocks of each division, may be tabulated as follows:

(a) Mechanically formed rocks from detrital sediments: Conglomerates, sandstones, clay, and shale.

(b) Organically formed rocks from animal and plant remains: Limestones, chalk, coral, peat, and coal.

(c) Chemically formed rocks from material once in solution: Limestones, stalactites, gypsum, rock-salt and sinter.

Most of the stratified rocks contain fossils; and since each group contains certain kinds peculiar to itself, it is by means of these organic remains that their relative ages have been determined.

Although the lowest stratified rocks are more ancient than those which have been deposited above them, the layers or beds do not always retain a horizontal position. Were such the case, it could only be by deep cuttings that we should arrive at the older strata. We however find that, owing to some convulsion of nature, stratified rocks have been thrown out of their original position, and thus crop out to the surface. Not only is facility thus afforded us to become acquainted with the nature of the lower rocks, but many of the most valuable products of the earth are by this means rendered accessible to man.

The greater number of these beds contain organic remains, i. e., the remains of animals and plants, which are termed fossils. Among these the most numerous are the remains of marine animals, and in some instances shells and corals occur in such abundance as to form the principal part of extensive beds. Every part of the earth exhibits similar, or nearly similar formations; and not only are marine fossils met with in the interior of continents, and at great elevations above the sea, but a vast variety of plants, corals, shells, fish, reptiles, etc., are found, of species dissimilar to any at present on the land or in the waters. Besides rocks, we meet with earthy formations on the surface. These include such loose materials as are disintegrated or worn away from rocks, and form, when combined with decayed animal and vegetable matter, the soil of meadows and arable lands.

Igneous, or Unstratified Rocks are such as appear to be of igneous origin, or to have been formed by the action of fire or intense heat. They are called unstratified, because instead of having been deposited in successive layers, like the stratified rocks, they seem to have been formed by the fusion or melting of the materials of which they are composed, and the subsequent cooling and hardening of the melted matter into one great mass. Granite, basalt, lava, etc., are examples of this class of rocks, and represent respectively the sub-classes of plutonic, trap, and volcanic rocks. Plutonic rocks are those which have cooled under the pressure of overlying rocks; trap rocks, those which have cooled under that of deep water; and volcanic rocks, such as have cooled in the air.

Though granite is the most useful of the igneous rocks, basalt is probably the most interesting because of the wonderful formations it discloses. It is a dense basic lava of a dark color, that breaks with a conchoidal or shell-like fracture, and shows a finely grained or hemi-crystalline texture in a glassy base. The basalt rocks are found both as intrusive masses and as sheets that have been poured out on the surface. Many of these lava sheets of basalt in slowly cooling and solidifying acquired a columnar structure, the columns often having a more or less hexagonal shape, though the number of sides varies. Fine examples of these columnar basalts occur at Fingal’s cave in the island of Staffa, at the Giant’s Causeway in the north of Ireland, and on the shores of Lake Superior.

Metamorphic, or Transformed rocks, include altered rocks of either sedimentary or igneous origin, in which the acquired are more prominent than the original characteristics. Igneous rocks have, in many cases, forced their way up through stratified rocks. These igneous formations, while still in a molten state, in coming in contact with the aqueous or stratified rocks, have usually changed the character of those portions immediately near them. The chief changes of structure effected by metamorphic action are crystallization and foliation. Examples of metamorphic rocks are marble, quartzite, slate, gneiss, and the schists.

HOW THE METALS
ARE FOUND

In some localities fissures in rocks are found to contain metallic substances. Such fissures are frequently found partially filled with calcareous spar which forms the matrix in which the metals are inclosed.

Metallic veins are supposed to be partially filled by mechanical means, the particles of metallic substances being conveyed into them by the action of water or some other power, and partly by chemical action, or by sublimation or fumes rising from below.

Some metallic deposits appear to occur in situations where igneous rocks have intruded themselves. Gold is supposed to be found almost invariably under such circumstances. Such appears to be the case in the rich deposits near the Ural mountains, and also in California and in Australia. In all these places it is met with in quartz. It is in pebbles or sand of the same rock that it occurs in the beds of rivers, and in some cases is found spread over a large extent of country.

Copper, though frequently met with in veins, is also found in extensive masses or beds, interposed between layers of rock. The same remark applies to tin, lead, and silver. Iron is also met with in beds, and also in nodules or rounded masses, which occur in great abundance among some kinds of rock. The last-named is the most universally diffused of all metals, and the most useful.

A GEOLOGICAL VIEW OF THE GROWTH OF THE EARTH

Giving the geological ages, rock systems, strata and the development of life, with their relative positions and order of succession, according to the latest scientific knowledge. Many attempts have been made to compute from geological, physical, and other data the length of the period during which the earth has been in a solid state.

Geologists, however, are disinclined to accept any period much less than 100,000,000 years as sufficient for the elaboration of the present structure of the earth. It is indisputable that many millions of years, probably thirty or forty, must have elapsed while the great sedimentary rocks were being deposited. With respect to the larger features of the earth’s surface, it is likely that two different kinds of movement are responsible. Where the contraction of the earth has caused a lessening of the support below the surface, there has been a subsidence of great areas. In the second place, where the rigid crust has been able to contract into a smaller space, great mountain ridges and folds have been formed. The subsidences which caused the ocean took place at different ages. The Atlantic Ocean probably dates from middle Cenozoic times; the Indian Ocean may be older; the Pacific suffered great modifications in comparatively recent times.

Large illustration (310 kB)

THE SURFACE OF THE EARTH

Large illustration (222 kB)

LAND FORMS OF THE WORLD

The proportion of land to water upon the earth is as 27 to 72, or roughly one-fourth to three-fourths; the land covering fifty-three million square miles, the sea one hundred and forty-four million. The land consists of six great bodies called continents, and a multitude of small fragments called islands, which skirt the shores of the continents or dot the broad expanse of the sea.

THE DISTRIBUTION OF
LAND AND WATER

By far the greatest proportion of land is in the northern hemisphere, and in temperate latitudes. Broadly speaking, the northern hemisphere is the hemisphere of land, and the southern hemisphere is the hemisphere of ocean. The earth could be bisected in such a way that one hemisphere contained almost no land, while the other was composed almost equally of land and water.

LOCATION OF THE
CONTINENTS

The greater part of the land on the earth’s surface is grouped into two great hemispheres, the Old and the New World. The former and far larger of these consists of Eurasia in the north, separated by ill-defined boundaries from Europe to the west and Asia to the east, and of Africa in the south, united to Eurasia by the narrow neck of the isthmus of Suez. The hemisphere of the New World is divided into North America and South America, united by the long, narrow isthmus of Central America. The island of Australia is also reckoned as a continent. It is believed that an island continent, Antarctica, surrounds the South Pole. Of islands not reckoned as continents, the largest is the polar island of Greenland.

CERTAIN RESEMBLANCES OF
THE CONTINENTS

In comparing the continents, we at once notice certain resemblances. The first is the tapering to the south, which is seen in Greenland, North and South America, Africa, and Australia (Tasmania). Another is the southward-running peninsulas which characterize Europe and Asia. We may notice, too, that the general lines of the Old World, broad in the north, tapering in the south, resemble those of the New World, especially if we include Australia (Tasmania), and compare its position with that of South America. There is also a certain uniformity in the distribution of relief. Notice the so-called Mid-World and Pacific Mountain systems, which may be traced in the mountains of Central Europe, North Africa, Central Asia, the islands of the Pacific from Japan to New Guinea, and the lofty mountains of North, Central, and South America.

COMPARISON OF THE CONTINENTS

THE SHAPING OF
THE COAST

The coast line, or margin of sea and land, is an area rapidly wearing away under the ceaseless influence of the waves, and of the sand and rock, they are perpetually hurling to and fro. Coasts may be either flat or high, composed either of hard or soft rock, and either submerged or raised. A submerged coast is one where the land has sunk or the sea has risen, so that the low grounds and valleys are flooded. A raised coast is one where the land has risen or the sea has retired, and what was formerly the sea bottom is bared.

A flat coast is usually sandy, often bordered by sandhills and lagoons. It may be carved into cliffs, as in the clay cliffs of Norfolk, England. A raised coast is usually flat from the long-continued action of the waves during the period when it was submerged. Flat coasts have no good harbors.

A submerged coast differs according to the nature of the submerged region. If this was hilly or mountainous, with valleys running parallel to the shore, the coast will be ironbound and harbor-less unless the sea-level has risen sufficiently to give access to the valleys behind the first range of heights. If this happens, T-shaped gulfs are formed. Where the valleys open at right angles to the sea, they become bays, usually with excellent harbors. The hills between the valleys rise as peninsulas, or islands. If the land was flat before submerging took place, a flat coast is the result.

Where the land is composed of soft rocks, a more uniform coast-line results than where it is composed of harder rocks, or of hard and soft rocks mixed. The waves, in eating out the softer rocks, often form magnificent sea-caves, natural arches, and pinnacles.

THE COASTLINE OF THE
VARIOUS CONTINENTS

Europe surpasses all the other continents in the magnitude of its indentations and projections. Three great peninsulas—the Balkan peninsula, Italy, and Spain, project into the Mediterranean; while Brittany, Denmark, and Scandinavia jut into the shores of the Atlantic. Even the British Isles are scarcely more than a projection of the continent.

Asia is a second in the relative extent of its peninsula. Asia Minor on the west, Arabia, India, and Indo-China on the south, and China, Manchuria with Corea and Kamchatka, advancing into the waters of the Pacific, form a wide border of projecting lands, containing the richest regions of the continent.

North America is considerably less indented. Florida, Nova Scotia and Labrador are more prominent on the Atlantic coast, and California Peninsula and Alaska on the Pacific.

The southern continents on the contrary, are nowhere deeply penetrated by the waters of the ocean. The Gulf of Arica in South America, the Gulf of Guinea in Africa, and the Great Australian Bight, are merely gentle bends in the coast line.

LOCATION OF THE GREAT
PLAINS OF THE WORLD

Plains occupy nearly one-half of the surface of the continents. They are most extensive and unbroken on the Arctic slopes of the Old World, and in the interior of the two Americas.

Treeless plains, whose vegetation consists of grasses and other herbaceous plants, or stunted shrubs, occur in every continent, and are designated by a variety of terms. Wherever treeless plains are subject to periodical rains, they lose their verdure in the season of drought, and assume the aspect of a desert; but they resume their freshness on the return of the rain, and many are adorned with a great variety of beautiful flowers.

Plains of the Old World. The great Siberian plain extends from the northeastern extremity of Asia to the Ural Mountains and Caspian Sea; and the European plain stretches from the Ural westward, through Russia and North Germany, to the lowlands of Holland.

The plains of the Caspian Sea and western Siberia are dreary steppes, covered with coarse grasses, often growing in tufts, alternating with patches of heather, furze, dwarf birch, and other stunted shrubs; or old sea bottom, covered with salt efflorescence. Immense reaches of flat country, near the Arctic shores of Asia and Europe, consist of frozen marshes, called tundras, where mosses and lichens are almost the only vegetation. Those of eastern Europe and Asia are denominated steppes; while more limited treeless regions in western Europe are called landes and heaths.

On the alluvial plains of the Old World, civilization began and developed; and their inexhaustible fertility supplied the wants of the most populous nations of antiquity. The great centers of ancient civilization in Egypt, China, India and Babylonia, all had their growth in alluvial plains, built up and fertilized by the mighty rivers which traverse those countries.

Plains of the New World. In North America the great Central Plain extends, with but slight interruptions, from the Arctic shores to the Gulf of Mexico. The fertile, treeless plains are termed “prairies” (meadows), while the sterile ones, east of the Rocky Mountains, are known as “the plains.” There are vast cane fields and forests in the lower Mississippi Valley.

In South America the plains of the Orinoco basin, the Selvas of the Amazon, and the Pampas of the La Plata, form an uninterrupted series of lowlands which, continued by the plains of Patagonia to the southern extremity of the continent, extend over a distance of three thousand five hundred miles from north to south. The Spanish term “llano” (plain), and the Peruvian “pampa,” designate the treeless plains of the Orinoco and La Plata basins. The Llanos of the Orinoco, during one-half of the year are covered by the richest pasturage, bright with flowers, but during the other half are a parched waste. The Selvas of the Amazon, a luxuriant forest, cover more than a million square miles; and the treeless Pampas, with their tall grasses and thickets of clover and thistles, illustrate the endless richness and variety of nature.

Alluvial and marine plains generally have but a slight altitude, while the undulating plains are sometimes considerably elevated. The Mississippi Valley, at St. Louis, one thousand miles from the ocean, is hardly four hundred feet above the sea-level; and the Amazon, at an equal distance from the sea, does not reach two hundred and fifty feet. The marine plains adjacent to the Caspian and Aral seas are still lower, the larger portion being below the sea-level.

SITUATION, ELEVATION AND
SOIL OF PLATEAUS

Plateaus are situated either between two lofty mountain chains, which form their margins, or descend by successive terraces to the nearest seas; or they pass, by gradations, from the base of high mountains to the low plains in the interior of the continents.

The Great American Basin, between the Rocky and Sierra Nevada Mountains, and the plateau of Tibet, between the Himalaya and Kuenlun mountains, are examples of the first position; and the table-land of Mexico, of the second. The third is seen in the high plains at the eastern foot of the Rocky Mountains, which descend from an altitude of five thousand or six thousand feet, at the foot of the mountains, to the low plains of the Mississippi basin.

The plateaus most remarkable for their elevation are, Tibet, from ten thousand to eighteen thousand feet above the sea; and the elongated valley-like highlands, from ten thousand to thirteen thousand feet high, between the two chains of the Andes, in South America. East Turkestan and Mongolia, in central Asia; the plateau of Iran, in western Asia; Abyssinia, and the vast plateau which occupies all the southern part of Africa; and the broad table-land which fills the western half of North America with a continuous mass of high land, range in height from four thousand to eight thousand feet.

The great peninsulas of Deccan, Arabia, Asia-Minor and Spain, the central plateau of France, and those of Switzerland, Bavaria, and Transylvania, vary from one thousand to four thousand feet in elevation.

SOIL AND CLIMATE
OF PLATEAUS

The nature of the soil and climate of great plateaus is in general such as to render them the least useful portions of the continents. Sahara, with an average altitude of 1,000 feet, and the higher plateaus of Mongolia, Iran and parts of the American Basin, may serve as types.

Their surface consists of hardened sand and rock; of hillocks and plains of loose sand constantly shifting by the wind; and of immense tracts, as in Mongolia, covered with pebbles varying from the size of a walnut, or even less, to a foot in diameter: all indicating the original transporting, grinding and depositing of these materials by water.

Salt lakes without outlet occur in each, and salt efflorescence often covers the ground. A lack of rain to wash from the soil substances injurious to vegetation, and supply the water necessary for the growth of plants, leaves these plateaus generally sterile, and some of the most extensive are in part, if not wholly, deserts.

MOUNTAINS AND THEIR STRUCTURES

Mountains rise in long and comparatively narrow lines or ridges, the tops of which are often deeply indented, presenting to the eye the appearance of a series of peaks detached one from another. As each of these peaks or distinct elevations is called a mountain and often receives a separate name, the common designation chain or range of mountains is naturally applied to the whole.

The top of the ridge, from which the waters descend on opposite sides, is called the crest; and the notches between the peaks, from which transverse valleys often stretch like deep furrows down the slopes of the chain, are called passes.

HOW MOUNTAIN CHAINS
FORM SYSTEMS

Mountain chains are seldom isolated, but are usually combined into systems, consisting of several more or less parallel and connected chains, with their intervening valleys,—as the Appalachian system, the Alps, and the Andes.

Most mountain chains seem to have been produced by tremendous lateral pressure in portions of the Earth’s crust, causing either long folds, or deep fissures with upturned edges rising into high ridges, the broken strata forming ragged peaks.

TWO TYPES OF MOUNTAIN
CHAINS

Mountains by folding are generally of moderate elevation, while mountains by fracture include the highest chains of the globe. The Appalachian Mountains in North America, and the Jura in Europe, are examples of the first; the Rocky Mountains, Andes, Alps and Himalayas, of the second.

Folded mountains are curved into long arches, either entire or broken at the summit and forming a system of long, parallel ridges, of nearly equal height, separated by trough-like valleys. Here and there, however, deep gaps, or gorges, cut the chains allowing the rivers to escape from one valley to another.

In systems of mountains produced by fracture, there is usually one main central chain, with several subordinate ranges. They have, however, less regularity and similarity among themselves than the parallel chains of mountains by folding.

The crests are deeply indented, cut down one-third or one-half the height of the range, forming isolated peaks and passes which present to the eye the appearance of a saw, called in Spanish Sierra; in Portuguese, Serra. Such ranges are frequently distinguished by these terms, as the Sierra Nevada, in North America; and the Serra do Mar, in Brazil.

HOW VALLEYS ARE
FORMED

Valleys among mountains owe their existence primarily to folds or fissures in the Earth’s crust, produced in the upheaving of the ranges; but they are subsequently deepened, widened and otherwise changed in form and extent, by the action of rains and frosts, and the streams to which they furnish a pathway. Most of the Alpine lakes, celebrated for their picturesque beauty, occupy deep basins at the outlet of transverse valleys.

Valleys in plains and plateaus are mainly, if not entirely, the result of the erosion, or wear of the surface, by running water.

Little rills, formed by the rains or issuing from springs, set out on their course down the slope of the ground, each wearing its small furrow in the surface. Uniting they form a rivulet which wears a broader and deeper channel; and the rivulets in turn combining, form rivers which produce still greater effects.

The great basin of the Mississippi for example, is one grand central valley, cut by the main stream in the line of lowest level, towards which the valleys of the Missouri, the Arkansas, the Ohio, and a multitude of smaller streams, all converge.

COMBINATION OF VOLCANIC
AND CORAL ISLANDS

A large number of volcanic islands in the Pacific are encircled by coral reefs, which, when near the shore, are called fringing reefs. When at a considerable distance, leaving a lagoon of quiet water between them and the volcanic island, they are termed barrier reefs.

CORAL REEFS AND THEIR
BUILDERS

Coral reefs are masses of limestone originally secreted, in the form of coral, by minute polyps which live in countless numbers in the tropical seas. The coral produced by a single community of polyps grows chiefly upward; but multitudes of distinct communities often live so near together that the small lateral growth of each brings them into contact.

Their separate, fragile structures, gradually broken up and compacted by various means, are in time transformed into a solid mass, forming walls of coral rock frequently of enormous extent. The great barrier reef near the northeastern shores of Australia, the longest known, is not less than one thousand two hundred and fifty miles in length.

Reef-building polyps do not live below the depth of one hundred or one hundred and twenty feet, and hence require a foundation near the surface. This is supplied by submarine mountains and plateaus, or the slopes of those volcanic cones which form the high islands.

Growing vertically, the reefs repeat at the surface the outlines of their bases, which fact gives rise to the circular figure both of atolls and reefs in mid-ocean, and to the elongated, wall-like form of reefs adjacent to the continents, like those of Florida and of Australia.

DISTRIBUTION OF
CORALS

Reef-building polyps are confined to the tropical seas, where the winter temperature is not below sixty-eight degrees. Coral formations are most extensive in the Pacific Ocean, especially south of the Equator, and in the two great archipelagoes of the East and West Indies; but a large number of coral islands also occur in the Indian Ocean. The Coral Sea, east of northern Australia, is particularly remarkable for the great extent of its coral reefs.

THE ATOLL FORM
OF ISLAND

The usual form of coral islands is that of a broken ring, numerous channels affording entrance into the lagoon. Such a group of islands is called an atoll, a Malay term, which has been adopted to designate these singular structures. The central lagoon enclosed by an atoll, is invariably shallow, seldom exceeding a few scores, or at most hundreds, of feet in depth; while the outer sea reaches a depth of thousands of feet at a short distance from the shore, showing that the atoll rests upon a submarine mountain.

Atolls are often clustered together in large numbers, forming extensive archipelagoes. Paumotu, or Low Archipelago, numbers eighty coral islands, nearly all of which are atolls; the Caroline, Gilbert and Marshall islands together contain eighty-four atolls, while the Laccadive and Maldive islands form two long double series of atolls extending eight hundred miles from north to south.

MAP SHOWING COMPARATIVE SIZE OF ISLANDS

(See next page for the Area, Population and Countries to which these islands belong).

Large illustration (404 kB)

MOST NOTED ISLANDS OF THE WORLD—WESTERN HEMISPHERE

MAP SHOWING COMPARATIVE SIZE OF ISLANDS

ISLANDS OF EASTERN HEMISPHERE

THE WATERS OF THE EARTH

Water is found in Nature in three states or conditions—as ice, vapor or steam, and as simple water. These three forms have the same chemical composition—the substance being a compound of oxygen and hydrogen, represented by the formula H₂O; but the physical condition depends entirely on its temperature. Under ordinary atmospheric conditions water is a solid below 32 degrees Fahrenheit; a gas above 212 degrees Fahrenheit, and a liquid between these temperatures.

The purest form of water which exists in nature is rain water, though this always contains a little oxygen and carbon dioxide dissolved from the air. To obtain pure water artificially, any ordinary water is distilled, when all the solids dissolved in it are left behind. River water and spring water always contain a small quantity of solid matter, the amount and nature of the dissolved solids depending on the nature of the rocks over which the water has flowed.

Geographically it may be considered under the four heads of springs, rivers, lakes, and the ocean, which taken together forms the hydrosphere of the earth.

WHERE SPRINGS HAVE
THEIR SOURCE

Springs, or the natural fountains of water, take their rise from reservoirs stored under ground. Water maintains a level, and hence the height to which a spring will rise depends on that of the level from which it is supplied. If the internal reservoir be on a hill, and the spring should gush out in a valley, the water may rise to a considerable height and form a natural fountain; but, on the other hand, if the reservoir be at some depth below the surface, the water may never reach the surface, and mechanical aid may be required to obtain it.

These internal reservoirs are in a great measure supplied by moisture derived from rain, snow, mist, and dew. The atmospheric water enters the earth through porous rocks, or by means of fissures, and continues to sink until arrested in its progress by rocks, such as clay, which will not permit the water to pass, or by faults which check it from spreading. The waters will then gush forth as a spring, of greater or less size, according to the supplies it may have received.

HOW MINERAL SPRINGS
ARE FORMED

All springs contain a certain portion of air and gas, and also some solid matter, usually in the form of salts. When these salts are abundant, mineral springs are the result, which may be classified according to the character of their several properties, as acidulous, chalybeate, sulphurous, saline, calcareous, and silicious.

Acidulous or acid springs are those surcharged with carbonic acid gas.

Chalybeate springs are those in which iron, in the form of carbonate or sulphate, is held in solution.

Sulphur, in the form of sulphureted hydrogen or sulphate of lime, is the distinguishing ingredient in Sulphurous springs.

Saline springs are of two kinds—brine and medicinal; brine when containing a greater or less amount of chloride of sodium or common salt, and medicinal when containing other salts, as sulphate of soda, etc.

Calcareous springs are those highly charged with the salts of lime, and which have the property of petrifying substances placed within their reach, and also of depositing their contents, forming the stalactites and stalagmites of caverns, etc.

Silicious springs are so called from holding silica or flint in solution. The last-named are all hot or thermal as well as mineral springs, deriving their heat either from the natural heat of the earth at great depths, or from volcanic action. When occurring near volcanoes, they are frequently charged with bitumen, petroleum, naptha, asphaltum, etc.

WHY WATER FLOWS FROM
ARTESIAN WELLS

An important class of artificial springs or wells is known as Artesian Wells. Where bent pervious beds of rock lie between two bent impervious beds, so as to make a basin-shaped depression, lower in the middle than at the edges, the rain which sinks into the pervious rock where it reaches the surface will begin to gather in the central part of the porous rock as in a reservoir.

If a hole be now bored in the hollow of the upper impervious bed till it reaches the water-bearing stratum, the water will flow out at the top. The water thus obtained may have fallen a distance of many miles several months previously, and if the gathering-ground be high the issue at the well may be forced by the pressure of the water behind to a considerable height.

FORMATION, CHARACTERISTICS AND
PECULIARITIES OF RIVERS

Rivers have their sources from springs or from the melting of accumulations of snow. They do not, however, receive their largest supplies from the actual summits of mountains, for copious springs are rarely met with in such situations, nor are glaciers formed on the highest points of mountains, but more usually on slopes of the upper mountain valleys. It is, accordingly, in the latter localities that many of the largest rivers take their rise.

Watershed. It not unfrequently happens that several rivers take their rise in one mountain ridge, some flowing in one direction, and others taking an opposite course. Such a ridge is termed a watershed. Thus the Rhine, the Rhone, and the Danube all take their rise in the Alps, the first discharging itself into the North Sea, the second into the Mediterranean Sea, and the last into the Black Sea.

Basin. The portion of country drained by a river and its tributary streams is called its basin, from its catching the rains which fall within its circuit, and which the river carries to the sea. The largest river-basin in Europe is that of the Volga, in Asia, that of the Ganges, in Africa that of the Nile, in North America that of the Mississippi, and in South America that of the Amazon.

THE GREAT RIVERS OF THE WORLD

Deltas and Estuaries. Owing to local peculiarities at the mouths of rivers, accumulations of sedimentary matter take place in the middle of the stream, dividing it into two or more branches. By these depositions deltas (so called from the Greek letter (?) delta) are formed—many of them, as those of the Mississippi and Orinoco and of the Rhine and the Ganges, being of great extent. Some rivers fall into the ocean through estuaries or wide channels, and are subject to a great swell or sudden rise of the waters when the tide enters.

PICTURE DIAGRAM GIVING A COMPARATIVE VIEW OF
THE WORLD’S FAMOUS RIVERS AND MOUNTAINS

Large illustrations:
Rivers (left-hand side) (480 kB)
Rivers (right-hand side) (137 kB)
Moutains (left-hand side) (187 kB)
Moutains (right-hand side) (554 kB)

Most rivers are subject to an occasional, and in some instances to a periodical increase of volume. These seasons of flood are by no means regular, being partly dependent on the melting of the snows, and partly on occasional heavy falls of rain; and hence depend on the climatic variations of the country in which rivers originate.

FAMOUS MOUNTAIN AND OTHER ELEVATIONS OF THE WORLD

Note: The numbers refer back to the Picture Diagrams on the preceding page.

FRESHWATER AND
SALT LAKES

Lakes are of different kinds. Some are mere tanks which receive the first outpourings of springs, others consist of basins or reservoirs which occur in the line of a river’s course; some consist of basins or cavities, into which rivers flow, but which, on account of their depression or their mountainous cincture have no outlets; lakes are also formed in the craters of extinct volcanoes; and some lakes are periodic, or subject to have their basins alternately empty and full of water.

Mountain Lakes, which are valleys or chasms filled by streams, are long and narrow, rarely of extensive area, but often of great depth. Examples of this class are found in Lakes Champlain and George, among the Appalachian Mountains; Lakes Constance and Geneva, on the northern side of the Alps; and Lake Maggiore and Lake Como, on the south side; all of which are renowned for the loveliness of their shores, or the grandeur of the surrounding mountain scenery.

Lake Maggiore, which is hardly three miles wide, is, according to Italian engineers, 2,623 feet deep—more than double the depth of Lake Superior—its basin reaching 1,936 feet below the sea level.

The forms of mountain lakes are very irregular, for the water often covers several contiguous and connected valleys. This is the case in Lake Como, which has two long arms; and Lakes Lucerne and Lugano, each of which fills four distinct valleys, meeting one another nearly at right angles.

Lakes in Plains. The lake basins in plains and plateaus are, usually, simple depressions in a comparatively uniform surface. The lakes are, therefore, often of great size, broad in proportion to their length, but of little depth compared with their area.

The largest lakes of the globe—the Caspian and Aral seas, and the great North American and African lakes—and the largest in Europe and South America, all belong to this class. Their vast expanse, together with the tameness of their shores, deprives them of the picturesque beauty of mountain lakes.

Characteristics of Salt Lakes. Numerous lakes in the interior of the continents, though receiving affluents, have no outlet. Their waters are chiefly lost by evaporation, though some portion may be absorbed by the sandy soil.

The surfaces of the continents having been the beds of the primeval oceans, the presence of salt in the soil is a natural consequence.

Famous Salt Lakes. The Great Salt Lake of Utah, in the Great American Basin, is one of the finest examples of its class. The Caspian and Aral seas, at the bottom of the vast depression between Europe and Asia, are the most extensive salt lakes. The former has about four times the area of Lake Superior; and the latter is a little larger than Lake Michigan.

The Caspian, though receiving the Volga, the largest river of Europe, evaporates so much water that its surface is about 83 feet lower than that of the Mediterranean, varying with the seasons. Many lakes in its neighborhood disappear entirely in the heat and drought of summer, leaving their beds covered with a crust of pure white crystalline salt.

The Remarkable Dead Sea, in Syria, is a lake in which the salt has accumulated until the water is converted into a heavy brine. It may be the remnant of an ancient sea of much greater extent, which has been gradually reduced in size by the excess of evaporation over the supply of water in its basin.

This celebrated body of water lies in the deepest part of a long chasm or valley, which is sunk not less than 4,000 feet below the level of the surrounding country. The surface of the lake is 1,286 feet, and its bottom 2,500 feet, below the level of the Mediterranean.

Its feeder, the river Jordan, flows almost throughout its entire course below the level of the sea, the only known instance of the kind. The beautiful lake of Tiberias, the scene of so many of the miracles of Jesus, which is but an expansion of the Jordan in its upper course, is about 650 feet below the surface of the Mediterranean.

HOW THE LAKES ARE DISTRIBUTED
OVER THE GLOBE

Lakes are most numerous in the central and northern portions of Asia, Europe and North America. The southern continents, except Africa, have comparatively few.

Asia is pre-eminently the continent of salt lakes. They occur in countless numbers, both in the steppes north of the Caspian and Aral, and in all the interior plateaus. Lakes of fresh water are also found among the Altai Mountains and adjacent chains. Lake Baikal, one of these, is the largest mountain lake known, being nearly 500 miles long.

Europe. The most characteristic and celebrated lakes are those which adorn the Alps of Switzerland and Scandinavia, and the less lofty mountain chains of the British Isles. But the largest lakes are found in the low lands and slight swells which surround the Baltic Sea, in western Russia and Sweden. Lakes Ladoga and Onega in Russia, and Wener and Wetter in Sweden, are the largest in Europe.

North America is peculiarly rich in great lakes. No continent presents a more remarkable series than that which stretches from northwest to southeast, through the central plains, along the line of contact of the oldest geological formations of the continent. This series includes Great Bear and Great Slave lakes, Athabasca and Winnipeg, and the five great lakes of the St. Lawrence, with many of less area.

Innumerable small lakes are scattered throughout the middle portions of the central plain, and the northern and less regular part of the Appalachian mountain region; but south of the parallel of Lake Erie there is an almost entire absence of lakes, whether large or small.

Large illustration (397 kB)

PRINCIPAL SALT-WATER LAKES OF THE WORLD

Large illustration (371 kB)

PRINCIPAL FRESH-WATER LAKES OF THE WORLD

Africa. The great plateau lakes are typical of the continent. The Victoria Nyanza and Albert Nyanza, feeding the White Nile; Tanganyika, whose outlet is unknown; Tzana, at the head of the Blue Nile; and Lake Nyassa, in the Zambezi basin, all rest on the high plateaus of Central Africa. Lake Tchad alone, among large African lakes, is surrounded by low plains.

Waterfalls and Rapids. The variations in the slope of a river-bed, arising from unequal erosion, or from the original irregularities in the surface, give rise to rapids and falls.

The first occur where an increased slope causes the stream to flow with more than its average velocity. The second are caused by nearly perpendicular rocky walls, down which the foaming water descends in picturesque cascades, or imposing cataracts.

The famous “Cataracts of the Nile” are merely rapids which impede but do not entirely obstruct, the navigation as cataracts must. The so-called Falls of St. Anthony, in the upper Mississippi, and the rapids of the St. Lawrence, above Montreal, are among the finest rapids in American rivers.

The highest falls are in the upper course of rivers, in mountainous regions; the greatest and most imposing, in their middle course.

The Niagara Falls exhibit a most important industrial utilization of water power. The Falls of St. Anthony in the Mississippi, the Falls of Foyers in Scotland, the Rhine falls, the Rhone falls of Bellegarde, and the innumerable waterfalls of Scandinavia, Switzerland, and similar mountainous lands, are all utilized in this way. It has been proposed to convey power generated at the Victoria falls of the Zambezi to the Rand goldfield of the Transvaal, and a scheme for this is now being prepared.

FAMOUS WATERFALLS OF THE WORLD

FAMOUS WATER PICTURES OF THE NEW AND OLD WORLD

CAUSE OF THE TIDES, WAVES AND
CURRENTS OF THE OCEANS

The waters of the ocean are retained in their bed by the attraction of gravitation. This power is great in proportion to the mass; and as the earth is of much greater mass than the particles of water on its surface, it attracts them and keeps them in their assigned places. But the sun and moon also possess this power of attraction, and notwithstanding their distance, attract and draw them up to a certain elevation. The vast mass of the waters being drawn up by the moon into a mountain or curve of water forms what is called the “great primary or tidal wave.”

Ebb-tide and Flood-tide. This drawing up of the waters of mid-ocean causes a recession from the shores, thus giving rise to ebb-tide, or low water. But when the temporary attraction ceases the waters flow back to their natural level, returning to shore and forming flood-tide, or high water. This culmination or rising of the waters in the great tidal wave takes place twice in twenty-four hours and fifty minutes. The combined influence of the sun and moon at new and full moon augments the size of this wave, and causes the “spring-tides” at those periods.

Height of Tides. High water at the various points along the coast is dependent on the return of this great wave, though some variations are caused by local peculiarities; and the height of the tide also varies greatly in different parts of the earth.

On the eastern coast of North America, the average rise of the tide is from nine to twelve feet. At the entrance to the Bay of Fundy, however, it rises eighteen feet, while at the head of that bay it reaches sixty, and in the highest spring tides, even seventy feet. At Bristol, in England, the spring tides rise to forty feet; and at St. Malo, on the south coast of the English Channel, they reach fifty feet.

THE MAELSTROM, CHARYBDIS
AND HELL GATE

Differences in level, produced by high tides, cause currents which vary in force and direction with the condition of the tide, producing, in some cases, dangerous whirlpools. The famous Maelstrom, off the coast of Norway, is but a tidal current, which rushes with great violence between two of the Lofoden Islands, causing a whirling motion in the water which is reversed at each ebb and flow of the tide.

Such is, also, the famous whirlpool of Charybdis, in the Straight of Messina, and many others of less note. The powerful currents of Hell Gate, in the passage from Long Island Sound to New York Bay, are due to a similar cause, high water occurring at different hours in the bay and in the west end of the sound.

WHAT CAUSES THE WAVES
OF THE OCEAN

The waves of the ocean, which are caused by the action of the wind, and which are called secondary or wind waves are of a totally different character from the tidal wave. The influence of the wind is supposed not to extend to a greater depth than forty or fifty feet, the deep sea, though raised in a great mass by the grand tidal movement, being free from agitation. Wind waves at a distance from the shore are comparatively low and long, but in shoal water they assume a greater curvature, and fall on the beach either in gentle ripples or in mighty breakers, according to the depth of the water and the force of the wind. The heavy swell which occasionally takes place, called the “ground sea,” is supposed to originate in distant storms of wind.

THE RIVERS IN
THE SEA

Currents in the ocean arise from various causes. They may be produced by long-continued gales of wind, by the melting of polar ice, or by any cause that may give rise to onward movements of limited portions of the great mass of waters. Other currents, and of these only is it necessary to speak in this connection, are permanent. The most remarkable of these are the polar currents and the equatorial currents.

Polar Currents are produced by the perpetual movement of the waters from the poles to the equator. In accordance with the laws of mechanics, an accumulation of the waters takes place on that part of the globe which has the greatest velocity of motion; and as the earth in turning on its axis moves with far greater velocity at the equator, the waters continually flow toward that line from the poles.

Equatorial Currents. This accumulation of the waters at the equator tends to produce the equatorial currents, which consist of the continuous progression of the tropical seas in a westerly direction. When the wave brought by the polar currents arrives—coming as it does from regions where it naturally has less velocity—it does not at once acquire the velocity of the earth’s motion at the equator; and since the rotation of the earth is from west to east, this portion of the water lagging behind forms a stream or current which has an apparent motion from east to west, that is to say, apparent as regards the earth, but real in relation to the adjacent land and water. The trade winds, which in this zone blow constantly in the same direction, lend their aid in maintaining the equatorial current.

THE GREAT SYSTEMS OF
OCEAN CURRENTS

An extensive system of currents appears to arise in the Antarctic Ocean. A current of cold water flowing northward joins the equatorial current in the Pacific. Entering the Indian Ocean, it maintains its westerly course until it approaches the shores of Africa; then bending southward it rushes through the Mozambique Channel, and doubling the Cape of Good Hope travels northward until it arrives at the Bight of Benin. This current then joins the equatorial current, and crossing the Atlantic from the coast of Guinea to that of Brazil, it is divided into two branches by the projecting headland of Cape San Roque, one flowing southward and the other northward.

The Gulf Stream. After passing the Island of Trinidad, this great oceanic current enters the Gulf of Mexico, and there acquires a high temperature, and sweeping round that sea it again pours forth into the Atlantic, forming the most powerful of known currents, called the Gulf Stream. Issuing from the Gulf of Mexico, this current of warm water rushes with considerable force through the Bahama Channel; then taking a northerly course it travels along the eastern shores of North America, and at Newfoundland is turned to the eastward by an opposing cold current which sets in from Baffin’s Bay. It now maintains an easterly direction, and crossing the Atlantic arrives at the Azores in about twenty-eight days, and divides its waters on the coast of France and Spain: one portion goes southward and at length joins the grand current which sets from the coast of Guinea; and another portion travels northward and skirts the western coasts of Europe. These currents are seldom more than 500 feet deep.

ATMOSPHERE, CLIMATE AND WEATHER

The atmosphere is the vast ocean of air that envelops the earth and makes life possible on our globe. It absorbs the heat and vapors caused by the action of the sun upon the surface of both land and water, and is the medium through which the ever-changing phenomena of climate and weather are produced. The two great forces of nature acting in connection with it are gravitation and heat, or solar radiation; and the results of their ceaseless action may be summed up as follows: (1) Temperature, or heat, which we soon learn to know by our senses, and to measure by the thermometer. (2) Evaporation, which changes the weight of the air by carrying invisible moisture through it. This change of weight is indicated by the barometer. (3) Condensation, producing fog, dew, rain, hail, and snow; all estimated accurately by the rain gauge or pluviometer. (4) Motions, as in the winds, varying from the gentle breeze to the awful cyclone, the force and velocity of which are indicated by the anemometer. (5) Electricity, producing lightning, thunder, magnetic and chemical changes in the atmosphere. (6) Optical Phenomena, such as rainbows, haloes, coronas, mirage, and the auroras.

THE ATMOSPHERE: ITS EXTENT, CHARACTER, USE AND EFFECT

The Earth is enveloped in its own atmosphere, which like a transparent covering surrounds it, and revolves with it. This atmosphere does not extend to more than forty or fifty miles above the earth’s surface, and is higher at the equator than at the poles.

Large illustration (278 kB)

WHAT THE ATMOSPHERE
IS COMPOSED OF

The atmosphere is an elastic fluid consisting of a mixture (not a compound) of oxygen and nitrogen, in the proportions of about twenty-one of the first to seventy-nine parts of the last named. It also contains a small quantity of carbonic acid gas, and a yet smaller quantity of ammonia; and water in the form of invisible vapor is always present in it, though the quantity is subject to great variations. All these substances move freely among each other, and are continually changing places: the oxygen being ever ready to perform the office assigned to it of sustaining life and combustion; the carbonic acid to promote the growth of vegetation; the nitrogen to perfect the fruits of the earth, and the vapor to descend to the thirsty ground, in the form of showers and dew.

The atmosphere is elastic, and therefore capable of expansion and compression; and is also a ponderable body. The consequence of these properties is, that it is much lighter and thinner in the upper regions than nearer the earth’s surface; for at the sea-level its whole weight presses on its lower strata and gives it greater density. Ascending from the earth’s surface it becomes gradually lighter and thinner, and at great elevations is so rarefied as to be unsusceptible of sustaining life.

HOW THE ATMOSPHERE IS
WEIGHED AND MEASURED

The weight of the atmosphere at the level of the sea is equal to about fourteen and one-half pounds on every square inch of surface. This weight is balanced by a column of mercury thirty inches in height; but at an elevation of 18,000 feet it would be balanced by a column of only fifteen inches in height, and at 36,000 by one only seven and one-half inches in height. It is on this principle that the mercurial barometer has been constructed; and since the mercury in the barometer stands at the same point at all places at the sea-level, and falls in a regular ratio on ascending therefrom, this instrument forms a most useful standard for measuring altitudes.

As we ascend from the sea the atmosphere becomes colder; but, as with the density, the temperature does not appear to pass through regular gradations of change. From experiment, however, it has been assumed that the atmosphere loses one degree of heat by Fahrenheit’s thermometer for every 350 feet of ascent; and hence even in the hotter regions very lofty mountains are covered with perpetual ice and snow.

DISTRIBUTION OF TEMPERATURE
OVER THE EARTH

The amount of heat produced by the sun upon the Earth’s surface, is greatest near the Equator, and diminishes gradually towards the Poles. Three general causes, each referable to the spherical form of the Earth, combine to produce the gradual diminution of temperature from the Equator to the Poles.

1. The angle at which the Sun’s rays strike the surface. In the Equatorial regions they are perpendicular to the surface of the sphere, and there produce their maximum effect; but, on account of the curved outline of the globe, they fall more and more obliquely with increasing latitude, and the intensity of action diminishes proportionately. At the Poles their effect is practically nothing.

2. The area on which a given amount of heating power is expended, is least at the Equator, consequently the resulting heat is greatest. The area covered increases, and the effect diminishes, with the increasing obliquity of the Sun’s rays in higher latitudes, which, as we have seen above, results from the spherical form of the Earth.

3. The absorption of heat by the atmosphere, as the Sun’s rays pass through it, is least where they fall perpendicularly,—that is, in the Equatorial regions,—and increases, with their increasing obliquity, towards the Poles.

EFFECT OF THE MOTIONS
OF THE EARTH

The Earth revolves constantly around the Sun, and at the same time rotates upon an axis inclined twenty-three and one-half degrees towards the plane of its orbit. In consequence of the inclination of the axis, the declination of the Sun, or its angular distance from the Equator, varies with the advance of the Earth in its orbit, causing periodical variations in the length of day and night, and, consequently, in temperature.

Vernal Equinox. On the twentieth of March, at mid-day, the Sun is vertical at the Equator. Rising directly in the east it ascends the heavens to the zenith, and, descending, sets directly in the west.

The illuminated hemisphere extends from pole to pole, and embraces half of every parallel of latitude; hence every point on the Earth’s surface is under the rays of the Sun during half of the diurnal rotation; the days and nights are equal all over the globe; and the heating power of the Sun is the same in both the northern and the southern hemisphere.

Summer Solstice. As the Earth advances in its orbit the vertical Sun declines northward; and on the twenty-first of June, at the Summer Solstice, it is over the northern Tropic, twenty-three and one-half degrees from the Equator.

The illuminated hemisphere, extending ninety degrees on each side of the parallel of the vertical Sun, reaches twenty-three and one-half degrees beyond the North Pole; but, at the south, it barely touches the Antarctic circle. It embraces more than half of each parallel north of the Equator, hence throughout the northern hemisphere the day is longer than the night, the difference in their duration increasing with the latitude; and all points within the Arctic circle are in the light during the entire rotation.

In the southern hemisphere, less than half of each parallel being illuminated, the night is longer than the day, and within the Antarctic circle there is constant night. The heating power of the Sun is now at the maximum in the northern hemisphere, while in the southern it is at the minimum.

Autumnal Equinox. On the twenty-second of September, the distribution of light and heat upon the two hemispheres is the same as at the Vernal, and at the Winter Solstice, on the twenty-second of December, it is the reverse of that at the Summer Solstice.

WHAT CAUSES THE SEASONS AND DAY AND NIGHT

EFFECT OF UNEQUAL DAYS AND
NIGHTS ON TEMPERATURE

The inequality in the length of the days in different parts of the year, occasioned by the inclination of the Earth’s axis, is of itself sufficient to produce a marked variation in temperature.

During the day the Earth receives from the Sun more heat than it radiates into space; while during the night it radiates more than it receives. Hence a succession of long days and short nights results in an accumulation of heat, raising the average temperature and producing summer; while long nights and short days result in a temperature below the average, producing winter.

Again, the heating power of the Sun in each hemisphere is greatest at the period of the longest days, because of its greater altitude in the heavens; and least at the period of shortest days. Thus long days and a high sun operate together to produce the high temperature of summer; while long nights and a low sun cause the low temperature of winter.

The following table gives the length of the longest day, excluding the time of twilight, and of the shortest night, in the different latitudes, with the difference of duration in hours and minutes, thus exhibiting more clearly the above law.

TABLE OF UNEQUAL DAYS AND NIGHTS

LAW OF VARIATION OF
DAY AND NIGHT

The inequality of day and night increases slowly in the tropical regions, but more and more rapidly towards the polar circles. Beyond these circles the Sun, in the hemisphere in which it is vertical, makes the entire circuit of the heavens, without sinking below the horizon, for a period varying from twenty-four hours to six months; while in the opposite hemisphere there is a corresponding period of continuous night.

RESULT OF THIS LAW IN
DIFFERENT ZONES

In the tropical regions, where the days and nights vary little in length, the temperature is nearly uniform throughout the year; while the increasing inequality of day and night towards the Poles, causes an increasing difference between the summer and the winter temperature.

Again, the length of the day, in the summer of high latitudes, compensates for the diminished intensity of the Sun’s influence; so that the temperature, in the hottest part of the day, may equal, or even exceed, that within the tropics. A summer day in Labrador or Petrograd may be as warm as one under the Equator; but in the former latitudes there are only a few days of extreme heat in the year, while with increasing nearness to the Equator the number of warm days constantly increases.

HOW THE SEASONS VARY IN
DIFFERENT LATITUDES

The high latitudes have short, hot summers, and long, severe winters. The transition seasons, spring and autumn, on account of the very rapid change in the length of the days, are short and scarcely perceptible.

In the middle latitudes the summer and winter are more nearly equal in length, with less difference in the extreme temperatures; and the transition seasons are distinctly marked. Farther towards the Equator the summer increases in length, and the winter diminishes, while the tropical latitudes have constant summer.

WINDS AND OTHER AIR CURRENTS

The winds appear to be caused by partial changes in the density of the atmosphere in a great measure arising from a diverse distribution of heat. When air is warmed it becomes less dense, or, in other words, it occupies a greater space. If an adjacent stratum of air be cooler, it will on coming in contact with the warmer air expand and pour into space occupied by the latter, thus forming a current. The greater the difference between the temperature of the one or other portion, the greater will be the force which the cold portion will rush into the space occupied by the warm portion, or, in other terms, the more violent will be the wind. In temperate climates the winds are variable; but in some parts of the world they blow with great regularity, and in others are subject to periodical changes.

WHAT CAUSES THE
TRADE-WINDS

The most remarkable of the regular winds are the trade-winds. The atmosphere at the surface between the tropics is much warmer than in the higher latitudes; and since air expands when heated, the light warm air of intertropical regions perpetually rises, and its place is as perpetually supplied by the colder air from the north and the south. If it were not for the Earth’s rotation, these would be merely north and south winds; but like the equinoctial water-currents, these cool currents of air coming from regions which have not an equal velocity of rotation with the air at the equator, pause and hang back, and thus these aerial currents acquire a westerly direction, forming north-easterly constant winds in the northern hemisphere, and south-easterly in the southern hemisphere.

MONSOONS AND THEIR
LOCATION

The monsoons or periodical winds of the Indian Ocean owe their origin to the same cause which gives rise to the trade-winds, though they acquire a different character in consequence of the proximity of the land. In the southern portions of the ocean which are remote from this cause of disturbance, the trade-wind blows with its wonted regularity; but in the seas occupying the region between the eastern coast of Africa on the one side, and the Malay peninsula and the island of Sumatra on the other, the course of the trade-wind is reversed for half the year. This change occurs from April to October; the sun at that period being vertical north of the equator, and the land in the adjacent regions acquiring in consequence a high temperature, and the air over the sea being cooler than that over the land, a south-west wind prevails. This wind, called the “south-west monsoon,” commences at about three degrees south of the equator, and passing over the ocean arrives charged with moisture, and accordingly usually deposits copious supplies of rain in India and some of the adjoining territories. In the remaining half of the year, or from October to April, the wind assumes the ordinary north-easterly direction of the trade-wind.

Sea-breezes, which occur in regions bordering on the ocean in hot climates, are produced by causes similar to those which give rise to the south-west monsoon, but on a more limited scale of action, and changing their direction daily.

THE WHIRL OF THE
HURRICANE

Hurricanes are storms of wind which sweep or whirl round a regular course, and are at the same time carried onward along the surface of the Earth. In the northern hemisphere the whirling motion follows the course of east, north, west, and south to east again, and in the southern hemisphere it takes the opposite course. In the Atlantic Ocean, the principal region of hurricanes lies to the eastward of the West India Islands. They are also frequent in the Indian Ocean, at no great distance from the island of Madagascar. The “typhoons” of the China seas, and the “ox-eye” of the Cape of Good Hope, are also revolving storms.

TORNADOES AND OTHER
CHARACTERISTIC STORMS

The tornadoes of the western coast of Africa, the pamperos of South America, and the northers of North America appear to be of a different character, and not to possess a revolving motion. The sirocco of Italy and Sicily, and the solano of Spain, as also the simoon of Arabia, and the harmattan of western Africa, are all winds which owe their origin to the heated surfaces of Africa and Arabia. The principal difference between these winds appears to be, that the sirocco and the solano acquire some moisture in their passage across the Mediterranean, and therefore do not possess that extreme degree of aridity which forms the distinguishing character of the simoon and the harmattan.

CLOUDS—THEIR FORM AND
CLASSIFICATION

Clouds are continually varying in their form and appearance, but may be classed under the four principal heads of the cirrus, the cumulus, the stratus, and the nimbus.

The cirrus is a light, fleecy cloud resembling a lock of hair or a feather.

The cumulus or summer cloud is generally massive and of a round form; sometimes of small size, and sometimes covering nearly the whole sky, and occasionally appearing in the horizon like mountains capped with snow.

The stratus is a horizontal, misty cloud sometimes observed on fine summer evenings comparatively near the ground, and often crossing the middle regions of mountainous or hilly districts.

The nimbus or rain cloud has a uniform gray tint; it is fringed at the edges when these are displayed, but usually covers the whole sky. The region of clouds is a zone extending in the atmosphere from about one to four miles above the Earth. The most elevated clouds, which are light and fleecy, are those comprehended under the name of cirrus, and the lowest are those which are called stratus.

The cirro-cumulus, cirro-stratus, and cumulo-stratus are only modifications and combinations of the above-named principal classes.

FORMS OF ATMOSPHERIC VAPOR

Warm air is capable of holding suspended a larger quantity of moisture than cold air, and therefore the amount of vapor present in the atmosphere is subject to great variations.

WHAT CAUSES
DEW

These facts also account for the formation of dew, which is caused by the reduction of the temperature and the deposition of the moisture which the warmer atmosphere of the day had held in suspension. Dews will hence be usually most abundant when cool nights succeed warm days, and on a clear night than when the skies are obscured by clouds, because a cloudless sky is usually much colder than a beclouded one. It is also essential for the copious formation of dew, that the ground or other substance on which it is deposited should be much cooler than the superincumbent air; for if the ground be warm it will impart its temperature to the air near its surface and dew will not be formed.

FORMATION OF MISTS
AND FOGS

When the ground or water is warmer than the air, mists and fogs are frequently formed; and since water and marshy surfaces cool less rapidly than dry land, mists and fogs are of more common occurrence in low, damp situations than in dry, elevated districts. They are formed by the condensation of the vapor, or, in other terms, its transformation into the minute globules of water, which instead of descending to the earth in the form of dew, remain suspended above the land or the water.

RAIN, HAIL AND
SNOW

Clouds are formed by the condensation of vapor at considerable but various elevations in the atmosphere. Vapor is always invisible, clouds, therefore, are not vapor but water, and consist of a fine watery powder, the size of each particle being exceedingly minute; and consequently they are so light that clouds formed of an accumulation of such particles are readily borne forward by the winds. Clouds are sometimes suddenly formed and as suddenly disappear, probably owing to sudden and partial changes of temperature. When a considerable difference of temperature prevails in the aerial currents which may come in contact with the local atmosphere, a further condensation takes place, and the particles of this fine watery powder unite into drops, and, becoming heavier, fall to the earth in the form of rain, hail or snow.

SNOW AND SNOW-
CRYSTALS

Vapor condensed in air having a temperature below thirty-two degrees Fahrenheit freezes, or passes to a crystalline form, producing snow. Snowflakes occur in a great variety of forms, which usually present the outline of either a regular hexagon or a six-pointed star.

Their size depends upon the temperature and the relative humidity of the air through which they fall, for, like raindrops, they increase by successive additions from the vapors with which they come in contact in descending. Thus in mild weather they are much larger than in very cold weather.

PICTORIAL CHART OF THE CLOUDS, SHOWING THEIR FORMS AND POSITION

When the lower air is warm enough partially to melt the crystals, they form minute balls. When raindrops, formed in the upper air, fall through a cold current, they are often frozen, producing sleet instead of snow.

WHERE PERMANENT
SNOW EXISTS

Though the winter snows upon the plains, and the slopes of mountains of medium height, disappear during the warm season; yet, in all latitudes, the tops of high mountains are covered with a layer of permanent snow, which the summer heat of these great altitudes is not sufficient to melt.

The lower limit of perpetual snow, called the snow line, is found, within the tropics, about three miles above the level of the sea. In temperate latitudes it occurs at the height of a little less than two miles; and at the northern limit of the continents, it is about half a mile above the level of the sea, or, perhaps, even less than this.

On the Arctic Islands, vast fields of snow remain permanently, at a few hundred feet above the sea level.

The winter snows, falling into the icy waters of the polar oceans, are but partially dissolved; and, remaining upon the freezing surface, they help to form those vast ice floes which encumber the polar seas at all times.

The following table gives the observed height of the snow line in the different latitudes:—

HEIGHT OF THE SNOW LINE

HOW SNOW AND ICE FORM
GLACIERS AND ICEBERGS

Glaciers (from the French glace, ice) are vast streams of ice which descend from the lower edge of the perpetual snows, like long icicles from a snow-covered roof. They follow the windings of the Alpine valleys, and terminate abruptly in a massive wall of ice, from beneath which the waters of the melting glacier escape, through a large icy vault.

MOST FAMOUS GLACIER
REGION

The mountain systems in the middle latitudes, with abundant snows and alternate warm and cold seasons, are most favorable to the formation of glaciers. The best known, and probably the most remarkable glaciers are those of the high Alps, in the heart of which are Mont Blanc, Monte Rosa, and the Bernese Alps. Late explorers have found large glaciers in the Caucasus and in the Himalayas, the last being of the grandest proportions. In the Scandinavia are many which descend, in the deep western fiords, nearly to the sea level.

In the New World glaciers are less frequent. On Mount Shasta and Mount Rainier fine examples are in evidence.

By far the most extensive glaciers however, are found on the snow-covered islands of the polar oceans.

Vast masses of ice, broken from the ends of these glaciers, form the enormous icebergs (mountains of ice) which are so numerous in the polar seas, and are transported by the currents even to middle latitudes.

CLIMATE AND WEATHER

The term climate is used to express the combination of temperature and moisture which prevails at any particular place, or, in more familiar terms, the prevailing weather.

The most prominent causes of diversity of climate are the heat of the sun, the respective position of land and water, and the elevation of land above the level of the sea. To these may be added, as producing considerable though less marked effects, the nature of the soil, the prevailing winds, the position of mountain ranges, and the currents of the ocean.

THE SUPREME INFLUENCE
OF THE SUN

The sun is the grand agent in diffusing heat over the earth’s surface. While the sun is above the horizon of any place, that place is receiving heat; and when the sun is below the horizon, it is parting with it by the process called “radiation.” Whenever therefore the sun remains more than twelve hours out of the twenty-four above the horizon of any place, and consequently less than twelve hours below, the general temperature of that place will be above average; and when the reverse occurs, it will be below average. If the temperature depended solely on the heat of the sun, then indeed a tolerably accurate view of the respective climates of the zones of the globe might easily be assumed; but it is so greatly modified by other circumstances, that considerable differences prevail in countries situated in the same parallels of latitude.

HOW AFFECTED BY POSITION OF
LAND AND WATER

The relative position of the land and water is an essential cause of this diversity. The waters of the ocean are of very equal temperature, and have a tendency to moderate both heat and cold, wherever their influence extends. Thus when a cold wind passes over the sea, it becomes warmed, while a hot wind becomes cooled; and thus islands generally experience milder winters and more temperate summers than continents. Such countries are said to possess an insular climate. But when any region experiences great severity of cold in winter and a high degree of heat in summer, it is said to possess an extreme or excessive climate. The most striking instances of an extreme climate are drawn from places like Yakutsk, situated in the depths of Siberia, where the difference between the average temperature of winter and summer amounts to the astonishing sum of 101 degrees Fahrenheit.

THE LIFE-GIVING SUN SENDING HEAT AND LIGHT

The sun is the great life-giver of our earth. Its waves of light and heat and electricity come to the earth through a measureless ocean of ether and make it a living rather than a dead world. The above illustration shows how these waves are constantly bombarding the earth, and not only giving it life but contributing to it the glory of the seasons, the wonders of color, and the brilliant effects of light which we see in the skies and call Auroras, or Northern and Southern Lights.