The earth and its natural elements

The Universe.The universe includes all material things; the earth is a part of the universe. Large bodies of the universe are stars. The sun is a star, but it is not the largest star. It appears large because it is the star nearest the earth. The distance from the earth to the sun is 93 000 000 miles. Light travels swiftly – 186 000 miles per second – and passes from the sun to the earth in 300 seconds.

Beyond the sun, the star nearest the earth is so far away that light from it takes 4 years to reach the earth. The light that we receive from the North Star left the North Star 47 years ago. Light from very distant stars requires more than 100 000 00 years to reach the earth.

The Solar System. The sun is the central body of that part of the universe called the solar system. The solar system consists of the sun, planets, moons or satellites, comets, meteors.

The sun has a diameter of 860 000 miles. It is many times greater in size, or volume, than all its planets combined.

Planets are bodies that revolve around the sun. The nine planets in order from the sun outward are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Jupiter is the largest planet. Its diameter is 86 000 miles, more than ten times the diameter of the earth. Saturn, Uranus and Neptune are also larger than the earth. Mercury, Venus, Mars and Pluto are smaller than the earth.

Satellites, or moons are smaller than their planets and revolve around them. The earth has one satellite, our moon, the largest satellite of the solar system. The moon is 2 000 miles in diameter and 240 000 miles distant from the earth. Mars has two moons, Jupiter – nine, Saturn – nine, Uranus –four and Neptune – two.

Comets are luminous heavenly bodies, often having a long tail. Comets follow an orbit about the sun.

Meteors, or shooting stars, are fragmentary solid bodies that become heated and visible as they pass through the earth's atmosphere.

The Earth.The earth forms an extremely small portion of the universe. It rotates on its axis from west to east, riving a succession of day and night every 24 hours. The earth revolves around the sun once in 365 1/4 days, or one year. The path followed by the earth in its annual movement around the sun is the earth's orbit. The plane that includes the path of the earth around the sun and that also passes through the center of the earth and the center of the sun is the plane of the earth's orbit.

The earth's axis is inclined 23 1/2° from the perpendicular to the plane of the earth's orbit. The axis always points to the same part of the heaven, the north end toward the North Star.

The related position of the earth and sun, and the movements of the earth, cause the change of seasons. The vertical ray of the sun annually migrates north and south between the Tropic of Cancer and the Tropic of Capricorn. Long days and the steep rays of .summer bring warm weather; short days and the slanting rays of winter bring cold weather.

Shape and Size of the Earth. The earth is spherical in shape, but not a perfect sphere. The polar diameter of the earth is 26 miles shorter than the equatorial diameter. Because the earth is so nearly a sphere in shape, all parts of the earth's surface are about the same distance from the earth's center, and a body on the earth's surface weighs approximately the same in different parts of the earth.

Latitude and Longitude.Latitude and longitude make it possible to locate a place with absolute accuracy.

Latitude is distance north or south of the Equator, measured in degrees, minutes and seconds of a circle. The highest latitude that any place on the earth may have is 90°. The North Pole and South Pole have the highest latitude, 90° from the Equator. Latitude, or distance from the Equator, is a chief factor determining climate. Regions within 30° of the Equator lie in low latitudes, regions between 30° and 60° north or south of theEquator are in middle latitudes, while regions between 60° and the poles are in high latitudes.

Longitude is distance east or west of the prime meridian, measured in degrees, minutes and seconds of a circle. The prime meridian passes from pole to pole through Greenwich, London, and its longitude is 0°. The greatest longitude a place on the earth may have is 180°, on the meridian opposite the prime meridian. If one knows the latitude and longitude of a place, he can readily locate the place on a globe or on a map.

Change in time on the earth corresponds to change of longitude, a change of 15° in longitude corresponding to a change of 1 hour in time.

The Earth and Parts.The earth consists of the lithosphere, the hydrosphere and the atmosphere.

The lithosphere or rock sphere, one-fourth of which forms land and three-fourths ocean bottom.

The hydrosphere, or water sphere, covers three-fourths of the outer surface of the lithosphere.

The atmosphere, or air sphere, several hundred miles thick, completely envelops the lithosphere and hydrosphere.

Distribution of Land and Water. Of the 197 000 000 square miles of the earth's surface, 29 per cent consists of land, while oceans, and seas cover the remaining 71 per cent. The land areas consist of seven continents and numerous islands. The water area consists of five oceans and many gulfs, bays and seas.

The Oceans.Five large oceans and their gulfs, bays and seas make up the world ocean, or hydrosphere. Although the ocean averages more than 2 miles in depth, in proportion to the diameter of the earth it forms only a thin film of water on the outside of the lithosphere. The ocean is one continuous body of water broken into irregular parts by continents and islands.

The portion of the earth's crust, or lithosphere, covered by waters consists of the shallow continental shelf, bordering continental shores to a depth of 600 feet; the continental slope, extending from the continental shelf to the true ocean basin at a depth of 10 000 feet or more; the deep ocean basin, occupying three-fourths of the ocean bed, or more than one-half the surface of the earth.

Land Forms.The land area consists of plains, plateaus, hill lands and mountains.

Plains are lowlands of the earth. Plains bordering the ocean are coastal plains. Plains lying far from the ocean are interior plains. Plains may slope upward from the ocean far into the continent and gradually reach an elevation of several thousand feet above sea level.

Plateaus are highlands whose broad summit areas stand above adjoining plaint, or extend among mountains.

Mountains are highlands having small summit areas. Mountains rise higher than adjacent plains or plateaus. Mountains may be classified as young mountains and old mountains.

Young mountains have great heights, steep slope and barren surface, especially at high altitudes. The Rocky Mountains, the Alps are young mountains. Old mountains have less altitude and more gentle slopes, and are often covered to the top with vegetation. The Urals are old mountains.

Hill lands resemble low mountains because of their steep slopes. The
foothills of mountains are hill lands.

The Atmosphere.The atmosphere is a mixture of gases, enveloping land and water areas and extending outward from the earth for several hundred miles. The temperature of the lower atmosphere, or troposphere, grows colder with an increase of altitude to a height of about 7 miles. In the upper part of the atmosphere called stratosphere, temperature changes slightly. The zone between the troposphere and the stratosphere is the tropopause.

Pure, dry air in the lower atmosphere consists of 21 per cent oxygen, 78 per cent nitrogen, nearly one per cent argon and three-hundredths of 1 per cent carbon dioxide. Dust particles and water vapor are also present in the lower atmosphere. Water vapor may form 1 to 5 per cent of the volume of the atmosphere. The life of people and animals depends on oxygen which they obtain when they breathe. Plants need carbon, which they obtain through green leaves from the carbon dioxide in the air. Nitrogen and argon are not so important to life as oxygen and carbon dioxide.

The force of gravity holds the atmosphere to the earth. At sea level, air exerts an average pressure of 15 pounds per square inch. This is equal to the weight of a column of mercury 30 inches high or a column of water 34 feet high. Differences in air pressure are the chief cause of winds.

Weather.Weather is the condition of atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness, during a short period of time.

In middle latitudes, as in the United States, Central Europe and Central Asia, changes of weather are frequent, with a wide range between summer and winter. In low latitudes as in the Amazon Basin of South America, changes of weather are slight, with a small range between summer and winter. In high latitudes, changes of weather are frequent, with colder winters and cooler summers than in middle latitudes.

Climate.Climate is the aggregate, or sum total, of weather during a long period of years.

Climatic conditions vary with latitude, distance from the sea, altitude above sea level, direction of wind and other factors. A climatic region includes those areas of the earth that possess similar climatic conditions.

Elements of Weather and Climate.Elements of climate and weather are: temperature, pressure, winds, precipitation, humidity, cloudiness and sunshine. Temperature is condition of the atmosphere with reference to heat. Pressure is weight of atmosphere, and at sea level is 15 pounds per square inch.

Winds are moving air, varying from gentle breezes to terrific winds that uproot trees and destroy buildings.

Precipitation is moisture condensed from the air as rain, snow, sleet or hail.

Humidity is the condition of atmosphere with reference to its water-vapor content. Humidity is low in the desert of Sahara, high in the tropical forests of the Congo Basin.

Cloudiness and sunshine denote the percentage of daytime that the sun shines.

The various climatic regions of the world possess different combinationsof the six elements of weather and climate, and different parts of one region may vary considerably in the combination of the six elements.

The six elements of weather and climate operate everywhere, but we are most aware of temperature, precipitation and winds. Temperature and precipitation largely determine native and cultivated vegetation. In regions of warm temperature and abundant rainfall, tropical forests thrive; in regions of cool temperature and abundant rainfall, temperate forests; in regions of warm temperature with distinct wet and dry seasons, tropical grasslands or savannas; in regions of cool temperature and moderate rainfall, steppes and prairies.

An annual rainfall of less than 10 inches results in deserts, whether in warm or cold regions.

Insolation.Heat or energy radiated from the sun is solar radiation. Solar radiation that reaches the earth is insolation. The sun radiates energy in all directions, but only 1/2 000 000 000 th part reaches the earth, 93 000 000 miles away. Yet this small fraction of the sun's energy is the significant factor controlling the earth's weather and climate. The sun determines temperature; temperature, in turn, influences pressure, winds, humidity and precipitation.

Insolation is not evenly received over the earth’s surface. The vertical rays of the sun strike the earth's surface at an angle of 90° and concentrate on a small area. Oblique rays are slanting rays, spreading over a greater surface as they become more oblique. The tangent rays of sunrise and sunset merely graze the earth's surface. The vertical rays and very steep rays pass a shorter distance trough atmosphere than the more oblique rays and therefore lose less of the sun's energy.

Insolation varies with the length of day. At the equinox when the noonday sun shines vertically on the Equator and the days and nights are everywhere twelve hours in length, insolation is greatest at the Equator and gradually decreases poleward as the sun's rays become more oblique and extend for a longer distance through the atmosphere. In northern summer, insolation increases in latitudes north of the Equator, because of the longer days and steeper rays; at the same time, in southern winter, in latitudes south of the Equator, insolation decreases, because of the shorter days and more oblique rays.

In equatorial and tropical regions where the noonday sun is always high in the sky and days vary slightly in length, temperatures remain high and uniform throughout the year. In middle latitudes, however, where, in summer, the noonday sun is high and the day long, and, in winter, the noonday sun is low and the day short, annual changes of temperature are great. At 40° latitude, north or south of the Equator, the length of day varies from 9 hours in winter to 15 hours in summer, and the noonday sun varies in altitude from 26 1/2° in winter to 73 1/2° in summer.

Air is heated by the processes of: absorption, radiation, conduction, compression and mixing.

As the sun's rays pass through the atmosphere, the air is warmed somewhat by absorbing the sun's energy directly.

Land and water radiate their heat, and air is warmed by radiation. Air in contact with the earth heats by conduction, and the heat is transferred from one body to another.

As cold upper air settles toward the earth, it is under increased pressure, which produces heat by compression.

Warm south winds mingling with cooler north winds warm the air by mixing.

Warm light air pushed upward by cooler, heavier air is a convection current, which also transfers heat by mixing.

Air is cooled by the processes of radiation, conduction, mixing and expansion.

Air cools if it radiates heat more rapidly than it receives heat. At night air cools because insolation ceases and radiation continues.

By conduction, heat passes from the air to the cooler earth below and to the cooler air above.

Mixing warmer air with colder air both cools the warmer air and warms the cooler air. Convection encourages mixing.

When warm, light air is pushed upward by cooler, heavier air, the upward-moving air, under less pressure, expands and cools.

Effect of Land and Water on Temperature.Continents and islands of various sizes break the water surface of the earth. Land surfaces differ in size, altitude, nature of soil and kind of vegetation. Water heats and cools more slowly than land, because:

1) water has higher specific heat than land; 2) water reflects the sun's rays more readily than land; 3) the evaporation of water uses much heat; 4) movement brings cooler water to the surface to be heated; 5) the sun's rays penetrate water to greater depths than they penetrate land.

Four times as much heat is required to raise the temperature of a given volume of water 1°, as a like amount of land 1°. The smooth water surface reflects a larger proportion of the sun's rays than does the rugged land surface. There is evaporation from moist land, but the amount of water evaporated from the land is slight compared with the amount evaporated from the ocean: Ocean waters move as tides, waves and currents; land surfaces are stationary and receive the full effect of the sun's rays. The sun's heat may penetrate ocean waters for more than 60 feet; it penetrates land not much more than 4 feet.

Differences in the heating and cooling of land and water directly influence climate. In summer, when land is hotter than the ocean, see breezes bring coolness to the land. In winter, when land is cooler than the ocean, winds from the sea bring milder temperatures. Far from the sea summers are hotter and winters colder than near the sea. Regions near the ocean have marine climate; those far inland continental climate.

Isotherms usually extend east-west, but temperature differences of land and ocean often cause isotherms to curve northward and southward.

Range of temperature is the difference between maximum, of highest, and minimum, or lowest, temperatures. We have daily, monthly and annual ranges of temperature.

Temperature determines the length of the growing season. In equatorial regions, where frost never occurs, the growing season continues throughout the year. In intermediate climates the growing season varies from 275 days near the tropics to less than 90 days near the Arctic Circle, Cotton requires a growing season of 200 days or more; corn 150 days or more; wheat 90 days or more.

TROPICAL LANDS

The hottest parts of the earth lie between the Tropics. On the map of the world, the Equator is marked as a line lying midway between the two Poles. At a certain distance on each side of the Equator are two lines, known as the Tropic of Cancer and the Tropic of Capricon. Every place between these two lines has the sun directly overhead twice during the year. The heat is intense during the daytime, but the air becomes much cooler late in the evening, and during early morning.

These hot countries have no winter though snow can be se on the peaks of the highest mountain ranges.

At certain places within the Tropics rain never falls. The sky is always blue, and the wind hot and dry. Plants cannot grow, because there is no water, and plants need water as much as sunshine and good soil.

Places which have no vegetation are called "deserts" and are often miles of country covered with hot, dry sand. Only a few miles from such a desert, if there is a good rainfall, all kinds of plants may flourish.

In some places the damp, dark forests are so thick that one can scarcely pass through them.

Africa, the Dark Continent. – It is sometimes called the "Dark Continent", not because there is more darkness than in other places, but because, when the name was first given, so little was known about the country. Like South America, Africa is wide in the north and narrower in the south, and, also as in South America, a mighty river flows through dense forests near the Equator. This river is the Congo.

The north of Africa lies only a few miles from Europe, and the Isthmus of Suez formerly joined Africa to Asia. In former times all ships from England to India, China or Japan had to sail thousands of miles round the south of Africa.

Between the northern shore and the rest of Africa stretches the Sahara desert, the largest in the world. For hundreds of miles the ground is nothing but hot, dry sand. Very rarely you can see a plant. Water is scarce and is found only at certain wells. Such a district is known as an "oasis", or fertile area in the desert.

The river Nile flows across the desert lands of Africa. It is a large river, which has made the land near it extremely fertile. In the Nile Valley are found large groves, pretty flowers, rich orchards and fields of golden grain.

The River Nile rises in a big lake called Victoria Nyanza. When the rains are heavy, several rivers flow from the mountains to this lake.

At this season the Nile spreads itself over a large area. Mud and sand are carried from the mountains to the river, and the dark, muddy waters flow for hundreds of miles. On the flat land the river crawls along slowly and there the mud settles down.

Lakes have been made to store up water for the land and small canals have been cut to carry this water to the fields.

The Nile as it flows over the marshy land to the sea, divides into several streams and thus has various mouths. The land of the delta is mostly marshy, and in hot countries is suitable for the growth of rice. Where the land is less marshy, the cotton plant grows, and Egypt is famous for cotton and tobacco. The Suez Canal, through which ships from Europe pass to Asia is only a few miles from the Nile delta. It is nearly a hundred miles long, and on each side stretches dry, sandy desert. The canal is so wide and deep that the largest liners can go through, and at special places two ships can pass each other. It took years to cut the Suez Canal and even now boats called "dredgers" have to keep it clean, because a great deal of sand is blown into it from the desert lands around.

The map of Africa shows a line known as the Equator drawn across the middle of the continent from east to west. Twice every year the sun stands directly overhead, at all places through which this line passes. At other times of the year other places have the sun overhead at noon.

When it is summer time in England, places north of the Equator have the sun overhead, but during our winter the sun is highest and hottest at places south of the Equator.

The sun is always high in the sky in the middle of the day, in Central Africa, and this hot region has no winter. At certain times heavy rains fall, and during the "rainy season" the rivers become flooded.

Four of the largest rivers in the world flow through the hot region of Africa. These are the Nile, Congo, Niger and Zambesi. During the rainy season, the floods of the Congo spread over miles of a country, and form wide swamps. Nearly all the rivers have falls and rapids, or a delta and mud flats. Victoria Falls on the Zambesi are the highest in the world.

The climate in South Africa is cooler than in Central Africa, and white men can live here almost as well as in the hotter lands in the south of Europe. The rainfall is not so heavy as in the areas drained by the Congo and Niger, and nowhere do we find such thick, dark forests as those near the Equator.

The seasons in Africa are the opposite of ours. It is summer during our winter, and the cool season is in our hottest months. Snow is very rare, but rains are heavy in December. Before the rains fall the grasses are scorched and the ground is cracked. Even the rivers run dry at such a time, and it is possible to walk along the bed of the streams. When the rains come all this is changed. Soon the rivers are full, and the wells and springs contain plenty of water. The ground is quickly soaked and green grass covers a large part of the veldt (grassy plain).

THE OCEAN WORLD

The sea is the largest unknown part of the world. It covers 71 percent of the earth. But we know very little about it.

The first oceanographic voyage was made in 1872 when an English ship, the Challenger, sailed from England on a three-and-one-half-year journey through the world's oceans. From this voyage came information that formed the principles on which oceanographic studies were based for the next half century. In her travels, the Challenger sailed in every important part of the sea, except the arctic regions.

Six scientists on the ship conducted studies of the ocean conditions. They established 362 observing "stations" during their voyage. During a station, some of which lasted more than two days, the ship stopped while measurements and examples were taken. The information obtained included water temperature, conditions of the climate, direction, and speed of the surface and deep-water currents. Examples were collected of water and ocean bottom, and of animal and plant life, including 4717 kinds never before known. Although the instruments used by the Challenger were simple compared with today's, some very useful information was gathered during the voyage. Fifty large books were written about the Challenger.

Other voyages soon followed. One was made by Fridtjof Nansen and his brave men to the arctic in the small ship Fram. During the last part of the 19th century more studies and maps were made than ever before in the Pacific, the Indian Ocean and the Caribbean Sea.

The Climate "Machine". – The sea is a part of a great climate "machine" that cools and waters the earth when it needs it, warms the colder regions, and keeps the air we breathe fresh and pure.

Because water can hold 3 000 times more heat than air, the ocean has become the vast container for the earth's supply of heat energy, most of which comes from the sun. Although the air surrounding the earth is usually calm, the heat energy from the sun keeps it moving. Winds blow across the surface of the earth and spread this energy.

When the energy from the sun is taken in by the ocean, more mixing begins as currents carry it from one part of the sea to another. Here, too, the principle that heat will rise is proved as warm waters stay on the surface and colder waters go down.

On the top level of the ocean and on the land, steam from water returns to theair and rises in the heat to form rain clouds in the cool upper air. When the water in these clouds becomes heavy, it changes into rain. In very cold air the water stays together closely and forms snow.

Temperature.– Temperature is one of the three main qualities of sea water thatvaries considerably all through the world ocean. The other qualities are salinity and pressure.

The temperature of the sea changes only a little along our coast. But in some parts of the ocean the surface temperature can vary from 28 degrees Fahrenheit in thearctic regions, to 86 degrees F. in the hot climates of the world.

How can water remain at 28 degrees without freezing? The secret is that the water contains salt, which gives it a lower freezing point than the 32 degrees F. at which "pure" water freezes.

The Red Sea has the warmest water in the world – 90 degrees F. – almost hot to the touch. The Indian Ocean is also very warm with average temperatures of 80 degrees F. But 90 percent of the world ocean is cold, always below 40 degrees F. The ocean is deep, most of it two miles deep. Two miles is a long way from the surface sunshine.

Salinity. – Everyone knows that sea water is salty, but not many people know why. The reason is that salt and minerals are washed from the rocks on land and brought to the ocean by rivers, streams and rain.

After the three-and-a-half-year voyage of the Challenger it was discovered what most of these salts were. The total amount of salts in sea water varies in different parts of the ocean, depending on how warm the water is and how much surface water is changing into steam. The Red Sea which has the most salt, contains about 40 parts of salt to 1 000 parts of water. The arctic and antarctic waters are freshest, for very little of their water changes to steam. Also, water that is not salty is added from melting ice.

The most salty water is usually found near the surface. But recently an area of warm and salty water was discovered at the bottom of the Red Sea, 6 400 feet below the surface.

Scientists believe the ocean contains every known element, although only about 52 (including hydrogen and oxygen) have been discovered. The ocean even contains gold, silver, iron and tin.

THE MOVING WATERS

Long ago 13 brave men nearly died in the arctic proving what oceanographers now know is a fact: that a broad current of water and ice moves slowly across the top of the world from Siberia to Greenland.

In 1893, led by a young Norwegian, Fridtjof Nansen this group of men sailed from Norway in a small boat, the Pram. More than three years later, the men returned, after a dangerous 1 028-mile voyage through the arctic ice where they could travel only a mile or less each day.

But Nansen proved the existence of a westward-moving current. He found also that this current was caused by the wind, and not by the temperature or salinity differences, as had been thought.

Nansen and another young man, Frederik Johansen had left the ship 226 miles from the North Pole in an unsuccessful attempt to be first to reach the North Pole. But they had to change direction and travel to a deserted island 200 miles north of Siberia. They were rescued by an Englishman named Jackson who with a group of men was returning to England after an unsuccessful attempt to reach the North Pole by land.

On the same day that Nansen arrived in Norway the Fram was moving out of the ice between Spitsbergen and Greenland. Nansen therefore was able to prove that his little ship had been pulled by a current all the way across the frozen Arctic Ocean from Siberia to Spitsbergen.

Currents.– The world's oceans move because of the winds near the equator. These winds blow from the north-east in the northern part of the world, and from the south-east in the southern part of the world. They create huge "across" currents. As the currents quicken, they begin to move as though in a circle. This is because the earth itself turns in a circle, making the northern currents turn to the right and the southern currents turn to the left.

The Gulf Stream is a mixture of many small and large currents. Sometimes these currents change their course or go around and around, or perhaps down deep in the ocean. But the large mass of water that forms the Gulf Stream always moves in the same manner: toward Europe.

Where the Gulf Stream meets the colder waters of the Greenland Current, the Atlantic is covered by low clouds. Most of the Gulf Stream continues down the coast of northern Europe where it becomes the North Atlantic Current. Here it is joined by currents caused by winds near the shore. When it returns to its place of origin, the circle is completed.

In the calm and quiet regions around the equator, winds and currents are weak and sometimes do not exist at all.

In the South Atlantic Ocean the current system moves in a circle, from left to right. As the water hits Brazil, part of the current moves toward the north. Then it turns around and goes down the coast of Africa to complete the circle.

Around the Antarctic Continent, the west wind creates another circle. Here the winds from the South Pole blow the water away from the continent, where it is caught by the westerly winds to form a current. Part of this current joins with others, such as the Brazil Current, as it moves north.

From the deep ocean near Peru and Chile, the winds blow in such a manner that the surface water is moved away and deep, cold waters take their place. This movement brings sea life to the top for fish and birds to feast on. The fish are used as food by man and animals, and to produce certain oils.

Other currents, such as those in the North Расific Ocean and the Indian Ocean, are different and move in a different manner.

Deep Water Movement. – Where does the deep water come from? Where does it go? Oceanographers have found that the great water masses of the world's ocean are produced in two main regions: the icy South Pole and the North Atlantic seas. Here, as surface waters cool and become heavy, they sink toward the bottom. Their action starts currents moving that carry deep, cold water away from the North and South poles toward the equator. This water then slowly sinks down into the deepest part of the world's oceans.

The greatest flow of bottom water starts near the Antarctic: when the surface water begins to freeze. As the water changes to ice, salts are pushed out of the ice, causing the remaining water to become saltier and therefore heavier. It then sinks to the bottom, moving slowly toward the equator.

During the northern winter deep and bottom waters are being produced in the North Atlantic, and they flow toward the south. Salty waters from the Gulf Stream and the Mediterranean add weight to these deep waters as they move south. The North Atlantic deep water becomes caught between the heavier water of the Antarctic bottom and the less salty water that has flowed up from the southern regions. In some sections both the antarctic levels are colder than the North Atlantic level, although usually the deeper the water, the colder it is. Another mixing process occurs in the South Atlantic where some of the antarctic waters combine with the south-flowing deep waters and return to the antarctic.

In the Indian Ocean deep water does not travel south, except for a small amount of salty water pouring in from the Red Sea.

We do not yet know all the facts concerning the deep-water movement of the ocean. In 1955, Dr. Henry Stommel suggested we reexamine some of our accepted theories. He thought it could be possible that the deep water flows south, away from the Pole, not in a wide, slow-moving band, but in quick, narrow streams located along the western sides of the ocean, due to the earth's turning. If Stommel's theory is true, a strong current must exist under the Gulf Stream, bringing cold water toward the equator.

In 1957-1959, oceanographers clearly proved the presence of this strong current under the Gulf Stream. Floats were carried at a rate of five to eight miles a day by the current flowing 6 000 to 10 500 feet beneath the surface. The deep-sea movement of water is being studied by various new methods. One way to find the age of the water (that is to say, when it moved away from the surface) is to measure the oxygen content. "Old" water from the deep ocean has very little oxygen. This is because by the time surface water reached this level, its oxygen has been almost completely used by sea life. The oldest waters are those in the deep part of the ocean near Peru. "Young" waters are found on the surface of the Antarctic and North Atlantic oceans. Here new water is formed each year as the ice melts.

Waves. – The ocean's surface is broken by waves different kinds of waves, from short, little white waves of a windstorm to big waves that beat on sandy beaches with a thunder-like noise. There are also waves we can't see because they move beneath the surface. And there are waves we hope never to see, such as those caused by earthquakes or when undersea volcanoes become active.

Surface waves caused by the wind are the most frequent. Waves are rarely higher than 40 feet, but the highest wave ever recorded was seen during a storm in the Pacific in 1933 and measured more than 100 feet.

THE GREAT AMERICAN DESERT

The land that is known as the "Great American Desert" is an area containing 900 000 square miles of land in western United States. It is an area divided into three parts: on the east are the Great Plains, in the center the Rocky Mountains, and farther west the part that is known as the Great Basin. It is difficult to determine exactly where the desert begins and where it ends because it establishes its own borders. How far the desert extends varies on the eastern side according to the amount of rain that falls. On this side one sees the neverending struggle between the desert plants and the grasses that grow on the plains, each tries to occupy as much land as possible. Where there is little rain the desert may extend as far as states of Texas, Kansas North and South Dakota.

Many people think that a desert consists of nothing but sand. But most desert land also contains rocky places, basins surrounded by mountains, and canyons. All of these kinds of land are found within the borders of the Great American Desert.

One fifth of the world's surface is desert, which supports no more than four percent of the total population. The world's deserts vary in appearance, but they all have features that are similar. They are very dry, hot during the day and cold at night. The wind blows and few animals and plants can live there.

In 1824 school books in the United States indicated that the Great American Desert extended from the Mississippi Valley to the eastern side of the Rocky Mountains. As more travelers went west, the maps changed and included more desert land. But when settlers learned that animals and crops could be grown on this land, they changed their ideas. The land first known as the Great American Desert became known as the Great Plains. By 1870 the idea that all the land west of the Mississippi River was a vast desert had disappeared entirely from maps and school books.

Few books agree on the borders of the modern American Desert. Perhaps this lack of agreement results from the fact that there are mountains in the center of this region. However, mountains are the principal cause of deserts. They prevent the passage of the cold winds over them to let the rain fall on the desert land. This is true in most dry regions of the world where cold ocean currents force the clouds to lose their moisture on the sea instead of on the land.

Many scientists think that land receiving only ten inches of rain a year should be known as a desert. However, land on the American Desert receives an average of about 12 inches a year. Some places receive as little as 3 inches a year and others as much as 16 inches a year.

Other scientists believe that deserts cannot be determined by the amount of rain that falls on them because so many things combine to determine what effect the rain has on plant and animal life. Often on the high plains and in the Great Basin the rains are sudden and heavy and do the land little good. Great amounts of water run off the dry hard land not sinking into it. Floods are caused which do considerable harm.

The rate of evaporation is as important as the amount of rain. This process is very rapid on the Great American Desert because of the small amount of moisture in the air and the strong winds that blow.

The air above the warm parts of the ocean can easily pick up large quantities of water. When this warm wet air moves over the land, heavy rains result. This does not happen along the coast of the southern part of the state of California where the desert meets the ocean. Here the ocean currents are cold and the winds along the coast prevent the passage of moisture inland.

In northern California the winds blow off warmer waters. But as they continue east, a series of mountain ranges causes them to lose much of their moisture. As a result cities on the western side of the mountains receive large amounts of rain.

Most of the world's deserts possess a supply of underground water. But it often remains so deep under the surface that it can be taken out only with powerful machines. And in a few places there are wells where the water comes out of the earth by itself. A few such wells exist in the state of New Mexico.

On almost all of the world's deserts there are rivers that have their source in the rainy highlands and flow across dry land: the Nile River in Egypt, the Indus in Pakistan and the Colorado in the United States. The Colorado River has its source in the state of Wyoming and flows south into the Gulf of California. It is the fifth largest river in the United States. Like other desert rivers the Colorado lacks branch rivers and, as a result, the amount of water becomes smaller as it flows downstream.

The Great American Desert is a land of great differences. The lowest land is 270 below sea level – Death Valley in California. The highest mountains are 14 000 feet above sea level. Parts of the desert in summer may be as hot as 130 degrees. And in the northern part of the desert the temperature may be 10 degrees below zero in the winter.

In many places rain water that flows from the highlands passes through the rock. Sometimes lakes are created that are not very deep. Rapid evaporation removes the water from them leaving great areas of shiny white salt and minerals. The Great Salt Lake in Utah is one of these lakes, but it receives enough rain to allow a high level of water to exist all the time.

Among the important features of the southern part of the Great American Desert are the great piles of sand and vast areas of land where desert plants grow. In the distance, pointed mountain tops and large formations of sharp rocks may be seen.

Not many plants and animals can live on the very dry part of the desert. The cactus is the most common of the plants that grow on the desert. These cactus plants can live unharmed by great changes of climate. They can hold water in reserve and thus continue to grow during the hottest months. They develop long roots near the surface of the ground. These roots can easily use the rain water. Cactus plants also develop long roots that extend deep into the ground to use the sources of water far below the surface. Sometimes these roots are so long that they extend thirty feet down into the ground. They may also serve to keep the plants secure in a strong wind storm. Plants which do not have long roots are usually not very tall and grow close to the ground. Thus they cannot blow away in a strong wind.

Many desert plants are able to turn leaves to avoid the direct rays of the sun. Others have hard or shiny surfaces that prevent loss of moisture.

Like the native plants certain animals also adapt to life on the desert. They depend on the few scattered streams for water, and many go out of their shelter only in the cool of the evening. Some animals are able to manufacture water within their own bodies from their dry food. Therefore they never need to drink water.

The colour of almost all desert animals is similar to the surroundings and thus it serves as a protection to them. Smaller animals dig into ground where the rays of the sun cannot touch them. During the cold winter months they remain asleep.

EUROPE

Position and size. – With an area of about 3 750 000 square miles, Europe is the smallest of the continents excepting Australia. It is characterized by the comparatively great length of the coastline, broken up everywhere by peninsulas, gulfs, bays and fringed by islands with the result that only the heart of Russia is more than 500 miles from the sea. Europe lies almost entirely in middle latitudes – the North Temperate Zone; only a small fragment in the north is within the Arctic Circle.

Climate.– In general, it may be said that a number of factors have a determining influence on the character of European weather and climate. The factors may be grouped as follows:

a) The western coasts of the continent are bathed by warm current – the North Atlantic Drift, which is a continuation of the Gulf Stream. The existence of this warm current, especially round the British Isles, has undoubtedly an important effect in ameliorating winter conditions. But the effect of the warm waters themselves is enormously enhanced by the prevalent south-westerly wind; for the warmth is communicated through winds and not by the actual warm current.

b) According to the modern concepts of air-mass meteorology, a very important difference is found between cold Polar air and warm air coming from tropical regions. The position and amount of the cold Polar air varies with the season. The margin, the Polar front, in winter may be regarded as following roughly the 32° F isotherm. The currents of warm tropical air which reach Europe is the Westerlies exert their influences on the remainder of Europe.

c) The configuration of Europe, particularly the existence of the Mediterranean Sea and its continuation in the Black Sea, as well as, to a less extent, the existence of the Baltic Sea, permits the penetration eastwards of oceanic conditions.

We may now consider climatic conditions in Europe by contrasting the winter with the summer.

Winter conditions. –Owing to the warm currents of the sea and the wind system, in the winter months the whole of Europe, with the possible exception of Iceland, lies in the belt of the westerly winds, thus enjoying thewarm, moisture-laden winds from the Atlantic Ocean. The extra-tropical, high-pressure belt at this season lies to the south of Europe, over the Sahara and its continuation to the Atlantic that is, to the south of the Azores. But the eastern part of the continent at this season is very near the great land mass of Central Asia, which gets extremely cold. One may picture a great mass of cold heavy air centred over Asia and eastern Europe, giving rise to a permanent high-pressure system in the winter. The warm moisture-laden air masses from the Atlantic move up against this as against a wail, and either find their way away to the north-east past the coast of Norway, or they escape to the south along the Mediterranean. At times this great high-pressure system of eastern Europe, with its cold, out-blowing winds, extends its influence even as far as the eastern shores of the British Isles and gives rise there to spells of cold and frosty, though often sunny, weather. Indeed, it may be said that the winter weather of the whole of Europe is determined by the relative strength or importance of the great pressure system: the semi-permanent low-pressure system over Iceland, the permanent high-pressure system over Europe and the high-pressure system south of the Azores. Bearing these facts in mind, it is not difficult to understand why in winter months it gets steadily, colder as one travels eastwards in Europe, and that the isotherm of 32°, or freezing-point, divides the continent roughly into two halves. Nor is it difficult to understand why the whole of the western and Mediterranean margins receive a considerable proportion of their rain in the winter (more than half the total in the west of the British Isles, western France, and the Mediterranean), whereas as the result of the cold, out-blowing winds, precipitation of any sort is less in eastern Europe in winter than in summer.

In western and southern Europe most rain falls where there are mountain ranges which intercept the winds.

Summer Conditions. – At this season the wind system of the world have moved to the north so that only the northern part of Europe is under the influence of the Westerlies. The southern part of Europe, namely, the countries surrounding the Mediterranean Sea – lies within the influence of the high-pressure belt which surrounds the globe just outside the Tropics. The high pressure reigning in the summer months over the Mediterranean prevents the penetration of the cooling or rain-bearing winds from the Atlantic Ocean; consequently the Mediterranean lands suffer from considerable heat, and comparative or even complete rainlessness – the typical Mediterranean climate. In the Atlantic the high pressure center of the Azores which forms pert of this belt of high pressure, is north of its winter position, and frequently extends its influence as far as the British Isles. On the other hand, the Polar front is farther to the north, and the belt of cyclones which is associated with it tends to lie to the north of Iceland, and to affect such regions as the British Isles far less than in the winter.

In eastern Europe the conditions of winter are reversed. The great continental land mass becomes greatly heated and a large low-pressure area is the result. There is a tendency for the low pressure to be particularly marked over southern Russia, and towards this area the rain-bearing winds from the Atlantic blow, and result in the light spring rain of the steppe lands of south-eastern Europe. Most of central and eastern Europe thus have the greater part of their rain, that is to say, more than half their annual amount, in the summer half of the year rather than in the winter.

The south-westerly winds do not blow steadily but travel across the continent rather as a succession of cyclones or depressions with intervening wedges of high pressure which characterize the whole of the westerly wind belt. Thus the rainfall of Europe brought by these winds is partly orographical, partly cyclonic in its origin; with the result that although the rainfall on the mountains is nearly always heavier than on the lowlands even as far from the Atlantic as south-eastern Europe, the rainfall is sufficient for agriculture, only in the south of the Soviet Union surrounding the Caspian does it drop below the minimum amount for agriculture.

Climatic Regions.– The continent of Europe includes five of the great climatic regions of the world:

a) The Mediterranean.

b) The Cool Temperate Oceanic.

c) The mid-Latitude Continental or mid-Latitude Grasslands.

d) The Cold Temperate or Coniferous Forest:

e) The Tundra.

Natural Vegetation and Soils. – When we study the natural vegetation of Europe we must bear in mind that so much of the continent is densely populated and has been populated for many generations, that but little of the natural vegetation remains. It is possible, however to distinguish the following broad major divisions,

1) The Mediterranean Region. – Mediterranean climate is characterized essentially by its cool moist winters, and its hot, dry summers. In such a climate, where the hot seasons and wet seasons do not coincide, soil forms but slowly, and it is only in local patches of alluvium that one finds rich soil in Mediterranean lands. Moreover, the young fold mountains which play such a prominent part in the formation of the relief of Mediterranean lands are composed to a very considerable extent of hard limestones, which tend to give only poor, thin, red soil. The natural vegetation of Mediterranean lands therefore has to overcome not only the difficulties of the climate when the moisture is available in the winter and spring but not in the summer, but also has to overcome the difficulties of thin, poor soil. The resulting vegetation is essentially woody rather than herbaceous; and may vary from the low scrubland through Mediterranean woodland, in which the trees are characterized by protective devices against the loss of moisture, to, in the wetter and more favoured parts, forests of such trees as cork oak and pines.

2) The Cool Temperate Oceanic Climatic Region, including the sub­division of north-western and central Europe, and central USSR. With the cooler, and by no means rainless, summer, when vegetation processes can go on, the principal vegetation of this region is the Deciduous Forest: forest, because the rain throughout the year maintains a deep-seated water supply well fitted to trees; deciduous because the cold of the winter provides a good resting season. Wherever increasing severity of conditions is apparent, the deciduous forest tends to be replaced by coniferous forest; hence the large patches of coniferous forest associated with all the major mountain areas and with such areas of poor soil as the sandy tracts of the Lands in France, or of the heathlands of southern England. It should be noted that deciduous forest tracts are associated with fine rich brown soil, the colour of which is due to the humus, or decayed vegetable matter resulting from the leaf fall taking place every autumn. The actual formation of soil from the weathering of rocks is considerably more rapid than in Mediterranean climates and is hastened by the never-ceasing work of the earth-worm, which is particularly active in these humus-rich soils of temperate lands. Where cleared, the deciduous forest tends naturally to be replaced by the rich green grassy meadows so characteristic of the British Isles. The climate is sufficiently moist throughout the year to permit the grass to remain green. It must be remembered, however, that large tracts of the Deciduous Forest belt in Europe have probably never been forested e. g. some sandy tracts which evidently were covered with heathlandand moorland.

3) The Northern Coniferous Forest Belt. – This region stretches as a broad belt across the north of Europe, through Norway and Sweden, nearly the whole of Finland, and across the north of Russia. In these northern latitudes soil forms slowly, and, in addition, the complex of ancient rocks which underlies much of this tract does not furnish material readily available for soil formation; the whole area was swept bare of loose deposits and soil by the great ice sheets – and these three factors combined have resulted in large areas of poor sterile soil. Nor is it enriched by leaf mould in the same way as it is in the deciduous forests, so that even where mineral debris has accumulated the light siliceous soils of the Northern Coniferous Forest form a well-marked type known as podsols. Except in favoured hollows where glacial debris forms better soils, there is comparatively little agriculture possible. Moreover the climatic conditions do not favour arable farming, and in particular, very little cereal farming is possible. These northern regions, therefore remain comparatively thinly populated and forests cover very large areas. In these forests two trees are predominant – the Norway spruce and the Scot pine. The latter occurs especially in the sandier and lighter soils, the former especially in damper regions. As we go northwards into Polar regions, so the growth of forest trees is slower. It takes about 50 or 60 years for a forest that has been cut over to grow again to trees large enough for lumber in the southern tracts; towards the northern parts of this belt, the growth is so much slower that it takes about 200 years for a tree to grow sufficiently to be cut for lumber.

4) The Tundra Region. – In the extreme north of Russia and of Norway one comes into those regions which are beyond the Polar limit of tree growth. Similar conditions are reached in the higher parts of the mountain divide between Norway and Sweden. A low scrub or woodland usually occurs beyond the limit of the Northern Coniferous Forest but in the Tundra proper the ground is simply covered by a thick growth of lichens, of which the so-called "reindeer moss" is the most important constituent together with a large proportion of mosses, and usually a number of low bushes six inches to a foot high, of dwarf birch. In places however, the Tundra regions are occupied by surprisingly rich meadow lands.

5) The Steppe lands of South-eastern Russia. In the south-east of European part of the Soviet Union, with its late spring and early summer rainfall and its cold winters, the conditions are ideal for the growth of grass, but inimical to the growth of trees; so here existed the low, treeless, rolling grassy plains known as the steppes now replaced by ploughlands. They occupy the whole of the southern portion of Russia in Europe, with the exception of the Mediterranean fringe of the Crimea, the desert surrounding the Caspian and the mountainous belt of the Caucasus. They stretch some distance to the south-west into the plains of Rumania, whilst the Great Hungarian Plain may be regarded as an outlier of the main belt. The wide, open, rolling plains of the Anatolian plateau in Asia Minor are intermediate in character between the Russian sleppelands and the Mediterranean vegetation. They are for the most part treeless except near the water-courses, but instead of a luxuriant growth of grass one finds a number of the smaller plants and flowers (especially in the spring) that one associates more particularly with Mediterranean regions.

6) The Mid-Latitude Desert. – Very, very dry stepplands fading into desert country occur in Russia round the northern fringe of the Caspian Sea. The desert is a partly climatic, with a low rainfall, partly edaphic (or, due to soil) owing to the large proportion of salt in the soil where one approaches the great salt lake of the Caspian Sea itself.

СОДЕРЖАНИЕ

PART I

Text 1. The Earth............................................................................. 3

Text 2. The Solar System................................................................. 7

Text 3. The Atmosphere................................................................. 10

Text 4. Relief of the Earth............................................................. 13

Text 5. Changes in the earth's crust.............................................. 18

Text 6. Materials of the Lithosphere. Rocks................................. 22

PART II

Supplementary reading…………………………………………..…26

TNE EARTH AND ITS NATURAL ELEMENTS................................. 26

TROPICAL LANDS........................................................................... 32

TNE OCEAN WORLD....................................................................... 34

TNE MOVING WATERS .................................................................. 36

THE GREAT AMERICAN DESERT................................................. ..39

EUROPE………................................................................................ 42