Why do we have different Seasons?

Since earliest times, man has been curious about the changing of the seasons. Why is it warm in summer and cold in winter? Why do the days gradually grow longer in the spring? Why are the nights so long in winter?

We all know the earth revolves around the sun, and at the same time it revolves on its own axis. As it moves around the sun, it's also spinning like a top. Now if the axis of the earth (the line from the North Pole through the South Pole) were at right angles to the path of the earth around the sun, we would have no such thing as different seasons, and all the days of the year would be of equal length.

But the axis of the earth is tilted. The reason for this is that a combination of forces is at work on the earth. One is the pull of the sun, the other is pull of the moon, the third is the spinning action of the earth itself. The result is that the earth goes around the sun in a tilted position. It keeps that same position all year, so that the earth's axis always points in the same direction, towards the North Star.

This means that during part of the year the North Pole tilts towards the sun and part of the year away from it. Because of this tilt, the direct rays of the sun sometimes fall on the earth north of the Equator, sometimes directly on the Equator, and sometimes south of the equator. These differences in the way the direct rays of the sun strike the earth cause the different seasons in different parts of the world.

When the Northern Hemisphere is turned towards the sun, the countries north of the Equator have their summer season, and the countries south of the Equator have their winter season. When the direct rays of the sun fall on the Southern Hemisphere, it is their summer and it is winter in the Northern Hemisphere. The longest and shortest days of each year are called "the summer solstice" and "winter solstice".

There are two days in the year when night and day are equal all over the world. They come in the spring and fall, just halfway between the solstices. One is the autumnal equinox, which occurs about September 23, and the other is the spring equinox, which occurs about March 21.

How does the Weatherman know what to say?

All the conditions of the atmosphere are "weather". Whether it's hot or col, dry or wet, sunny or cloudy, windy or still, its "weather". The weather is changeable from day to day, and the total effect during a year is called "the climate".

There are many complicated reasons for changes in the weather, but the most important influence is the sun. The sun's heat evaporates water and warms the air, so that rising currents of warm air carry water vapor into the sky. There the air cools and the vapor condense into rain. These things happen gently or violently. When they take place violently, we have storms.

In Great Britain there are approximately 200 weather reporting stations and roughly the same number spread over the rest of Europe. In addition, "weather ships" stationed in the Atlantic and special aircraft on regular patrol send back systematic reports on weather conditions. From this and other meteorological data weather forecasts are produced.

The maps which the weather experts study show them many things: places where the air pressure is equal, places of equal temperature, directions of local winds, cloudy or clear skies, rain or snow, the amount of rainfall, and regions where the air pressure is higher or lower than normal.

The weatherman can also tell from looking at the map what is likely to happen under the conditions that exist. He knows that low pressures indicates storms, because cold air is moving in to replace warm rising air that is laden with moisture. High pressures indicates fair weather.

In the Northern Hemisphere, the winds around a high-pressure area blow outward in a direction similar to that followed by the hands of a clock. In a low pressure area they blow inward in a counterclockwise direction. Thus the directions the winds will take can be predicted. The weatherman also knows how fast these pressure areas are moving across the country.

Knowing all these things, and having reports of the weather in most parts of the country before him, the weatherman begins to get a pretty good idea of what to say about the weather that's coming to your area!

Why do Volcanoes appear only in certain places?

There are no volcanoes near New York City or London or Paris- nor are there likely to be any in the future. Yet there are parts of the world where there are several volcanoes quite near each other.

Central America, bordering the Pacific Ocean, is one of the most active volcano areas of the world. In fact, more than two-thrids of the active volcanoes and a lrage numer of those which have been extinct for only a short time are found around the borders of the Pacific Ocean!

The reason is simply this: The earth's crust in these areas must be "weak" or have weak spots in realtion to the earth's crust in other parts of the world. For without a weak spot in the crust of the earth, a volcano couldn't come into being.

Here is how a volcano is born: As you know, the center of the earth is hot. The deeper you go under the surface of the earth, the higher the temperature. At a depth of about 20 miles, it is so hot (1,000 to 1,100 degrees centigrade) that most rocks found there simply melts.

When rock melts, it expands and needs more space. In certain areas of the world, new mountain ranges have recently been formed (new in terms of thousands of years). Under and near these new mountain ranges, the pressure in less than elsewhere. It is a kind of "weak spot" in the earth's solid crust.

So the molten rock, which is called "magma", expands into these parts and a local reservoir of the molten rock is formed. This material rises along the cracks formed by the uplift. When the pressure in the reservoir of molten rock becomes greater than the strengthof the roof over it, it burst forth as a volcano. The eruption lasts until the gas is gone.

The material that comes out of volcano is mainly gaseous, but large quantities of molten rock (which we call "lava") and solid particles that look like cinders and ash are also thrown out. The eruption is really a gas explosion, but some of the lava becomes finely p[owdered and makes the eruption look like black smoke.

Why do we still have Glaciers today?

The great ice mass that began the Ice Age in North America has been called "a continental glacier" it may have been about 4,500 metres thick in its center. This great glacier probably formed and then melted away at least four times during the Ice Age.

The Ice Age or glacial that took place in other parts of the world still had not had a chance to melt away! For instance, the big island of Greenland is still covered with a continental glacier, expect for a narrow fringe around its edge. In the interior, this glacier often reaches heights of more than 3,000 metres. Antarctica is also covered by a vast contiental glacier which is nearby 4,000 metres high in places!

So the reason we still have glaciers in certain parts of the world is that they have not had a chance a melt away since the Ice Age. But most of the glacier that exist today have been formed in recent times. These glaciers are usually the valley type of glacier.

It starts in a broad, steep-walled valley shaped like a great amphitheatre. Snow is blown into this area or slides in from avalanches from the slopes above. This snow doesn't melt during the summer but gets deeper year by year. Eventually, the increasing pressure from above, together with some melting and refreezing, forces the air out of the lower part of the mass and changes it into solid ice. Further pressure from the weight of ice and snow above eventually squeezes this mass of ice is the valley glacier.

There are more than 1,200 such glaciers in the Alps of Europe! Galciers are also found in the Pyrenees, Carpathian, and Caucasus Mountains of Europe, and in southern Asia. In Southern Alaska, there are tens of thousands of such glaciers, some from 25 to 50 miles long!

When did the Ice Age end?

Most people think of the Ice Age as something that happened so long ago that not a sign of it remains. But did you know that geologists say we are just now reaching the end of the Ice Age? And people who live in Greenland are actually still in the Ice Age as far as they're concerned.

About 25,000 years ago, any people who may have been living in central North America saw ice and snow the year around. There was a great wall of ice that stretched from coast to coast, and the ice extended northward without an end. This was the latest Ice Age, and all of Canada, much of the United States and most of northwestern Europe were covered by a sheet of ice over 1000 metres thick.

This didn't mean taht it is was always icy cold. The temperature was only about 5 degrees lower than it is now in Northern United States. What caused the Ice Age was that the summers were very cool. SO there wasn't enough heat during the summer months to melt away the winter's ice and snow. It just continued to pile up until it covered all the northern area.

But the Ice Age consisted of really four periods. During each period the ice formed and advanced, then melted back towards the North Pole. It is believed this happened four times. The cold periods are called "glaciations", and the warm periods are called "interglacial" periods.

It is believed that in North America the first period of ice came about 2,000,000 years ago, the second about 1,250,000 years ago, the third about 500,000 years ago, and the ,last about 100,000 years ago.

The last Ice Age didn't melt at the same rate everywhere. For example, cie that reached what is now Wisconsin began to melt about 40,000 years ago. But ice that had covered New England melted about 28,000 years ago. And there was ice covering what is now Minnesota until about 15,000 years ago!In Europe, Germany got from under the ice 17,000 years ago and Sweden remanined covered with ice until about 13,000 years ago!

How are Icebergs formed?

We may think of icebergs as exiting and interesting things to see, but they are a great danger to ocean vessels when they drift into shipping routes. One of the greatest disasters at sea took place when the Titanic struck an iceberg on the night of April 14, 1912, and 1,513 people lost their lives.

An iceberg is a piece of glacier that has broken off. This happenes where the glacier( which are like rivers of ice) push down valleys until they reach the sea. The end of the glacier breaks off there and forms a floating iceberg.

Some glaciers do not reach the open sea but end in deep, steep-sided valleys called "fiords". The icebergs float down to the ocean from these fiords. In case of some galciers, the ends are worn or melted back by the waves. This leaves a big "foot" of ice submerged below the surface of the water. Such ice feet break off from time to time and rise suddenly to surface as icebergs.

Icebergs vary greatly in size. Small ones up to 5 or 10 metres across are often called "growlers" by seamen. But icebergs which measure over 100 metres are very common, and there have been some giant bergs seen that measured as much as 1000 metres across.

The ice in icebergs is only eight-ninths as heavy as sea water, so that only one-ninth of the iceberg sticks out above sea level and eight-ninths is below where it cant be seen. So a berg which rises 45 metres above the sea may extend 300 metres below! The amount of ice in an iceberg is almost unbelievable. Did you know that many of them weigh as much as 180,000,000 tonnes?

Because icebergs extend so far below the surface of the ocean, they dont drift with the winds but instead follow the ocean currents. Eventualy most icebergs are carried to lower warmer latituted where they melt. Very few of them last longer after they meet the warm Gulf Stream east of Newfoundland, Canada. But those that do become a menace to ships. That's why the United States Coast Guard maintains an iceberg patrol there to warn ships of the location of icebergs.

What are fossils?

The study of fossils is so important in helping man learn about his own past and that of animals who lived millions of years ago that it has developed into a separate science called "paleontology".

Fossils are not, as some people think, the remains of bodies buried ages ago. Actually, there are three different kinds of fossils. The first is the part of actual body of the organism, which has been preserved from decay, and which appears just as it was originally. But fossils may also be just the cast or mold of shape of body, which remains after the body of the plant or animal has been removed. And fossils may merely be the foorprints or trails that animal have left as they moved over the soft muds or clays.

When a fossil is found that consists of part of the organism itself, it is usually only the hard parts, such as shells or skeletons, that are preserved. The softer parts are destroyed by decay. Yet, in some cases, even such soft-bodies animals as jellyfish, which are 99 per cent water, have left perfect fossils of themselves in rock! And certain fossils found in ice not only have the skeleton preserved but also the flesh and skin on the bones.

Fossils have nothing to do with size. For instance, the fossils of tiny ants which lived millions of years agocan be found perfectly preserved in amber. The chances for animals being preserved as fossils depends mostly on where they lived. The most numerous of all fossils are water animals because when they die their bodies are quickly covered over by mud and so kept from decaying. Land animals and plants are exposed to the destroying action of the air and weather.

It is chiefly through the study of fossils that we know about animal life as it existed millions and hundereds of millions of years ago. For example, fossils taken from certain rocks tell us that millions of years ago there was an Age of Reptiles, with monsters so huge that they were 24 metres long and wieghed 36 tonnes. These were the dinosaurs. And out entire knowledge about the earliest bird, called "the archaeopteryx", is based on just two fossils of it that have been found!

How are caves formed?

Caves have long been linked with the history of man in many interesting ways. We know that late in the Old Stone Age, caves were the winter dwelling place of people who had no other shelter.But long after man stopped using caves as homes, ancient people believed many strange things about caves. The Greeks believed caves were the temples of their Gods, zeus, Pan, Dionysus and Pluto. The Romans thought that caves were the homes of nymphs and sibyls. The ancient Persians and others associated caves with the worship of Mithras, chief of the earth spirits.

Today , huge and beautiful caves all over the world are tourist attractions. Caves are dep hollow places in the rocky sides of hills or cliffs. Large caves are called "caverns".Caves are formed in many different ways. Many caves have been hollowed out by the constant beating of the sea waves against the rocks. Some caves appear under the surface of the earth. These are usually the old courses of underground streams which have worn away layers of soft rock such as limestone. Others are formed by the volcanic shifting rocks, or by the eruption of hot lava.

The most common type of cave in the United States is that made by the wearing away of thick layers of limestone. This is done by the action of water containing carbon dioxide. In indiana, Kentucky, and Tenessee, where there are great beds of limetsone with an average thickness of 53 metres, such caves are numerous.

Some caves have openings through their roofs, called "sink holes". These formed where the surface water first gathered and seeped down. Some caves have galleries in tiers or rows, one above another. underground streams wind through some caves, though in many cases after a cave has been formed, the streams that once flowed through it may find a lower level and leave the cave dry.

In many cases, each drop of water that drips from a cave roof contains a bit of lime or other mineral matter. As part of the water evaporates, some of this matter is left behind. It gradually forms a stalactite, shaped like an icicle hanging from the roof. Water dripping from the stalactite to the floor builds up a column called "a stalagmite".

Why are there so many different kinds of rocks?

Is there any boy anywhere in the world who hasn't filled up his pockets at some time with all kinds of strange rocks? The different sizes and shapes, some rough and smooth, the different colors, some almost like precious gems, just make us want to collect them!

What makes rock look so fascinating is the fact that they are composed of one or more minerals. It is these minerals which often give different rocks their wonderful colors, or make them sparkle like gems.

All rocks aren't formed in the same way. One kind of rock, called "sedimentary rock", has been formed by sediment. This means that substances are laid down long ago by water, wind, ice, or by the work of plants and animals. Since they are laid down in layers which are called "strata", these rocks are called "stratified" rocks. The tiny particles of which such rocks are made are usually rounded, since the sharp edges and corners have been worn off while the particles were being rolled along stream beds, washed by the waves, or blown by winds. Sandstones and limestones are typical sedimentary rocks.

Other rocks were once molten matter deep in the earth. They have been poured out on the surface or forced into cracks in the other rocks. Such rocks are called "igneous" rocks, and examples of this type are granite and basalt.

aThe third kind of rocks was once in some other form (igneous or sedimentary), but has been changed into its present form by heat or pressure. This type is called "metamorphic" rock, and marble and quartzite are examples of this type.

In some rocks, different metal-bearing minerals are found mixed with the other rock substances. If there is enough of the metal to make it worth while separating it, the rock containing it is called an "ore".

How were the mountains made?

Because mountains are so big and grand, amn thinks of them as unchanging and everlasting. But geologists, the scientist who study mountains, can prove that mountains do change, and that they are not everlasting.

Certain changes in the earth's surface produced the mountains, and they are constantly being destroyed and changed. Boulders are broken from mountainsides by freezing the water; soil and rock particles are carried away by rainwash and streams. In time, even the highest mountains are changed to rolling hills or plains.

Geologists divide mountains into four classifications, according to how they were formed. All mountains, however, are the result of violent changes in the earth's surface, most of which happened millions of years ago.

Folded mountains were made of rock layers, squeezed by great pressure into large folds. In many places in such mountains, you can see the rock layers curving up and down in arches and dips, caused by the squeezing and pressure on the earth's surface. The Appalachian Mountains and the Alps of Europe are examples of folded mountains.

In dome mountains, the rock layers were forced up to make great blisterlike domes. In many such cases, molten lava, coming with great pressure from below the earth's surface, lifted these rock layers. The Black Hills of South Dakota are examples of dome mountains.

Block mountains are the result of breaks, or faults, in the earth's crust. Huge parts of the earth's surface, entire "blocks" of rock, were raised up or tilted at one time. The Sierra Nevada Range of Clifornia is a block that is 400 miles long and 80 miles wide!

Volcanic mountains are built of lava, ash, and cinders poured out from within the earth. The usual volcano is cone-shaped with large hole, or carter, at the top. Among the famous volcanic mountains were Mount Ranier, Shasta, and Hood in the United States, Fujiyama in Japan, and Vesuvius in Italy.

Many mountains ranges have been formed by more than one of the ways described. In the Rockies are mountains made by folding, faulting, doming, and even erosion of lava!

Why does water flow out of a Spring?

All the water that flows out of every spring once fell as rain. The rain water soaks into the soil and enters into rocks through cracks. Of course, much of the rain water remains near the surface and evaporates into the air, or plants absorb it through their roots.

The rest of the rain water is drawn downward by gravity, and it goes as far down as the openings in the rocks will allow. Below the surface of the land, but at a different depth in each place, there is a zone where all the openings int he rocks are completely filled with water. This is called "the ground water zone". The upper surface of this water is called "the water table".

A spring occurs when water finds a natural opening in the ground that is below the water table. That's why most springs are in valley or low places. The ground water escapes as spring water through the cracks in the rocks along the sides or bottoms of these low places. A spring doesn't defy gravity; it is always flowing down from some water level above it.

Some spring recieve water from deep within the water zone. These usually flow all year and are called "permanent" springs. Other springs have their openings near the water table. They usually flow only in the rainy season when the water table is at its highest. These are called "intermittent" springs.

Since all spring water passes through rocks during its underground travels, all spring water carries some mineral matter, such as sulphur or lime. Springs that have water containing an unusual amount of mineral matter are called "mineral" springs.

In some places, espicially in areas where there have been volcanoes, the water in spring has been in contact with hot, underground rocks. This type of spring is called a "thermal", or "hot" spring.

An artesian well is quite different from a spring. In an artesian well, the rain water sinks down into the ground until it reaches a layer of porous rock or sand that is buried between two layers of solid rock. Pressure is built up around this water , and when a hole is bored down to reach it, the water escapes with a gush. The well must be driven at a point lower than that at which the water enters the ground.

What is the Gulf Stream?

The Gulf Stream is an ocean current, the most famous ocean current of all. It is like a river that flows through the sea instead of on land. But the Gulf Stream is so vast that it is larger than all the rivers in the world put together!

The Gulf Stream moves northward along the coast of Eastern United States, across the North Atlantic Ocean, and then to northwest Europe. The Gulf Stream has a clear indigo-blue color and it can be seen clearly where it contrasts with the green and gray waters that it flows through.

The water of the Gulf Stream comes from the movement of the surface water near the Equator in the Atlantic. This movement or "drift" is westward. So the Gulf Stream starts by moving north of South America and into the Carribian Sea. It actually becomes what we call the Gulf Stream when it starts moving northward along the east coast of the United States.

Since the Gulf Stream starts in the warm part of the world, it is a current of warm water. And the presence of this huge current of warm water makes amazing differences in the climate of many places!

Here are some curious examples of this: Winds passing over this current in northern Europe (where it is called "the North Atlantic Drift") carry warm air to parts of Norway, Sweden, Denmark, The Netherlands, and Belgium. Result- they get milder
winter temperatures than other places just as far north! It also means that ports along the Norwegian coast are ice-free the year round.

Thanks to the Gulf Stream, London and Paris enjoy mild winter climates, though they lie just as far north as southern Labrador, for example, which has bitterly cold winters. The winds that pass over the Gulf Stream are made warm moist. When these winds become chilled, as they do near Newfoundland, dense fog results. And so we have the famous dangerous fogs of the Grand Banks of Newfoundland.

The Gulf Stream doesn't have as great an effect on the winter climate of North America as on Europe, because the winter winds don't blow over it and then inland, as they do in Europe.

What is a Whirlpool?

When most of us think of a whirlpool, we imagine a huge spinning body of water into which people and ships can be sucked and pulled down to their death. While whirlpool are often dangerous, the fact is that there is no suction or downward-draft in them in the open ocean.

Let's see what a whirlpool is. You've probably noticed smaller whirlpool in a brook. They take place where a bank juts out into the current and gives a circular twist to the water.

As the water spins around in the narrow space behind the bank, it tends to pile up on the outside of the circle and to hollow out into a funnel-shaped hole in the center. This is the result of the action of centrifugal force. That is the same force which holds the water in the bottom of a bucket when the bucket is swung around in circles.

What causes the larger whirlpools, those we all know as dangerous to ships and men? When the tide comes sweeping in and it meets the ebb current of the preceding tide head-on, the ocean currents start to move in rotary fashion. This happens quite often in the narrow passages between groups of islands and the shores of the mainland.

When the narrow passage through which the tide flow is very deep, the rotary turning of the water sometimes changes to a
spiraling flow and then a downward-draft to a deep center does take place. But as mentioned before, it doesn't take place in the open ocean. A whirlpool in the open ocean is nothing more than an eddy on a large scale, which simply means a flow of water in rotary fashion.

Whirlpools occur in many parts of the world. The three most famous ones are the Maelstorm, the Charybdis, and the whirlpool downstream from the Niagara Falls. The Maelstorm is located off the Norwegian coast, and the Charybdis lies in the narrow channel between Italy and Sicily.

What are tidal waves?

On August 27, 1883, the island of Krakatoa in the Dutch East Indies pratically blew up in a tremendous volcanic explosion. As a result of this explosion, huge waves rose more than a hundered feet in the air. They wiped out hundereds of villages. And they rushed across the ocean at speed up to 700 miles an hour and made themselves felt on the coasts of Australia and California, thousands of miles away!

In 1946, there was an earthquake on the ocean floor near Aleutian Islands. A gigantic wave was set up and in less than five hours it traveled 2,000 miles and struck Hawaii. It lifted houses and bridges and threw them hundereds of metres. More than 170 people were drwoned.

Both of these great waves are what we call "tidal waves". They are completely unlike the normal waves at sea or those along the shore, and have nothing to do with winds or tides.Scientists have a special name for tidal waves. They call them by the Japanese name tsunami. A tidal wave, ot tsunami, is caused by some disturbance in the bottom of the sea. Usually, this is an earthquake that takes place on the ocean floor.

An earthquake on the ocean bottom produces a shock wave taht travels through the water, just as a loud sound travels through the air. In fact, this shock travels thorugh the water with the speed of sound.

If a ship happens to be in the region, it will actually be rocked by the shock, and it feels just as if the ship has struck a rock!

When an earthquake takes place on the ocean bottom, the ocean floor shifts and slides. It is the motion and the shock of the disturbance that produce tidal waves. Sometimes a great depression is suddenly created in wave is formed and begins at once to move across the sea at great speed.

When a tidal wave approaches land, the first sign, oddly enough, is a swell or rise that is just like an ordinary wave. Then the sea level falls for a number of minutes, as if it were very low tide. A big area of the ocean floor near the coast may be exposed. Then the great tidal wave comes crashing in!

What causes the waves?

If you've ever spent some time near a body of water, then you noticed that on a calm day there are very few waves in the water, and on a windy or stormy day there are many waves.

This, of course, explains what causes waves in the water. It is the wind. A wave is a way in which some form of energy is moved from one place to another. Some sort of force or energy must start a wave, and the wind provides that energy in the water.

When you watch the waves move, one after the other, the water seems to move forward. But if there is a piece of floating wood in the water, it will not move forward as the waves seem to do. It will only bob up and down with the waves. It moves only when the wind or tide moves it.

Then what kind of motion is taking place in a wave? A water wave is mostly the up and down movement of water particles. The movement passes on towards the shore, but not the particles of water. For example, if you have a rope you can send a kind of wave along the rope. The up-and-down movement passes along the rope, but not the particles of the rope.

As the bottom of a water wave strikes the ground a short distance from the beach, it slows up because of friction. The top keeps going, and then topples over, and thus forms a "breaker".The energy that formed the waves loses itself against the shoreline. All you have to do is stand among the waves along a beach and you'll soon find out that they have energy!

In a water wave, the water particles move in a circular path, up and forward, as they are pushed by the wind. Then they move down and back as gravity draws the headed up water back to common level. These up-and-down movements carry the wave along.The distance from the crest to crest of a wave is the wave length, and the low point is called "the trough".

Which is the deepest ocean?

In many ways, the oceans still remain a great mystery to us. We don't even know how old the oceans are. It seems certain that in the first stages of the earth's growth no oceans existed.

Today, man is exploring the bottoms of the oceans to learn more about them. Covering the floor of the ocean to a depth of 3,660 metres is a soft, oozy mud. It is made up of the limy skeletons of tiny sea animals. The floor of the deep , dark regions of the sea, where the water is more than four miles deep, is covered by a fine, rusty-colored ooze called "red clay". It is made up of tiny parts of skeletons of animals, the coverings of tiny plants, and volcanic ash.

The way the depth of oceans is measured today is by sending down sound waves which are reflected back from the bottom. The depth is found by measuring the time it takes for the sound wave to make the round trip and dividing this time in half.

Based on these measurements, we have a pretty good idea of the average depth of various oceans, and also the deepest point in each one. The ocean which has the greatest average depth is the Pacific Ocean. This is 4,281 metres. Next in average depth is the Indian Ocean which has an average of 3,963. The Atlantic is the third with an average depth of 3,926 metres. The Baltic Sea is at the other extreme, with an average depth of only 55 metres!

The single deepest spot so far known is in the Pacific near Guam, with a depth of 10,790 metres. The next deepest spot is in the Atlantic off Puerto Rico where it measures 9,219. Hudson Bay, which is larger than many seas, has its deepest point at only 183 metres!

Why is the Ocean salty?

Every now and then, we come across a fact about our earth which mystifies us and for which no answer has yet been found. Such a fact is the existence of salt in the oceans. How did it get there?

The answer is we simply don't know how the salt got into the ocean! We do know, of course, that salt is water-soluble, and so passes into the oceans with rain water. The salt of the earth's surface s constantly being dissolved and is passing into the ocean.

But we don't know whether this can account for the huge quantity of salt that is found in oceans. If all the oceans were dried up, enough salt would be left to build a wall 180 miles high and a mile thick. Such a wall would reach once around the world at the Equator! Or put another way, the rock salt obtained if all the oceans dried up would have a bulk about 15 times as much as the entire continent of Europe!

The common salt which we all use is produced from the sea water or the water of salt lakes, from salt springs, and from deposits of rock salt. The concentration of salt in sea water ranges from about three per cent to three-and-one-half per cent. Enclosed seas, such as the Mediterranean and the Red Sea, contain more salt in the water than open seas. The Dead Sea, which covers an area of about 340 square miles, contains about 10,523,000,000 tonnes of salt.

On the average, a litre of sea water contains approximately 30 grams of salt. The bed rock salt that are formed in various parts of the world were all originally formed by the evaporation of sea water millions of years ago. Since it is necessary for about nine-tenths of the volume of sea water to evaporate for rock salt to be formed, it is believed that the thick rock salt beds that are found were deposited in what used to be partly enclosed seas. These evaporated faster than fresh water entered them, and the rock salt deposits were thus formed.

Most commercial salt is obtained from raock salt. The usual method is to drill wells down to the salt beds. Pure water is pumped down through a pipe. The water dissolves the salt and it is forced through another pipe up to the surface.

What is a Nebula?

If you have seen pictures of nebulae in books, great spirals and whirlpools and clouds-don't expect to find anything like that in the sky. Most of the nebulae are so faint that they cannot be seen without a telescope. The word "nebulae", by the way, is really the Latin word for "mists" because they looked like mists when first observed through small telescopes.

There are two chief classes of nebulae, the galactic and the extra-galactic. The galactic nebulae are found in our own galaxy (the Milky Way) and are composed of dust and gas. Since "extra" means "outside", the extra-galactic nebulae are nebulae outside our own galaxy. They are made up largely of stars.

The galactic nebulae number less than 2,000. This means that most of the nebulae known to man are outside our own galaxy. How many are there? For all we know, there may be millions of them out there in the vast space beyond the Milky Way.

The extragalactic nebulae are sometimes called "island universes" or "galaxies". This means that if someone were looking at our own galaxy from out there he might well see it as a nebula.

The extragalactic nebulae have various forms. Some are irregular or elliptical. The most numerous are the spirals. The spirals, like our own galaxy, are made up of large number of stars, big gaseous clouds, and vast tracts of dust. These nebulae usually have a nucleus in the center and from this, arms extend in spiral fashion. The spiral nebula Andromeda is the nearest to earth and the largest and brighest nebula known. It gives out about 1,500,000,000 times as much light as our sun!

Why are there different kinds of clouds?

Here is how clouds are formed: Warm air, laden with moisture, rises into the sky. When it gets to certain height, the warm air cools. At the cooler temperature it can no longer hold all its moisture in the form of water vapour. So the extra moisture changes into small drops of water, or bits of ice, and this forms clouds.No two clouds are exactly alike, and they are always changing their shape. The reason we have different types of clouds is that cloud formation takes place at different hights and temperature. And clouds will be composed of different particles, depending on their height and temperature.

The highest clouds are called "noctilucent" clouds. They may be up as high as 30 to 50 miles! The next highest are called "nacreous", or "mother-of-pearl" clouds. They're 12 to 18 miles high. They are very thin, beautifully colored clouds composed of dust or waterdrops, and they are seen only after sunset, at night, or before sunrise.

The next highest clouds, which are five or more miles above the earth, are called "cirrus" clouds, "cirrostratus" clouds, and "cirrocumulus" clouds. The cirrus are feathery and threadlike, the cirrostratus are thin, whitish sheets, and the cirrocumulus are small, round clouds which form "mackerel" pattern in the sky. All these clouds are made up of tiny bits of ice.

Lower clouds are made of little drops of water. The highest of these, the altocumulus clouds, are about two to four miles above the earth, and are made of larger, rounded masses than the cirrocumulus clouds. At the same levels are the altostratus clouds, which often cover the whole sky with a greyish veil through which the sun and moo shine as spots of pale light.

Lower still, about a mile high, are the stratocumulus clouds, large and lumpy. At the same level are the rain clouds, the nimbostratus, thick, dark, and shapeless. Very low, less than 610 metres above the ground, are the stratus clouds, which are sheets of high fog. Two other kinds of clouds, the cumulus, and the cumulonimbus, are the big, fat, "cauliflower" clouds that bring thunder and storms.

Why did Astronomers think there might be life on Mars?

As you know scientists are now conducting all llinds of experiments to see if life can be found anywhere else in the universe. Naturally, it is easier to explore our own solar system for signs of life than it is to probe outer space. And one of the places where some scientist thought a form of light might have been found is the planet Mars.

Why did they pick Mars? Well, Mars is considered to be a sort of twin of our own planet earth. It is the next planet the earth in distance from the sun. Mars is about half the diameter of the earth and it rotates around the sun in just under two years. But Mars has a day that ia almost equal in length to our day here on earth.

In observing Mars, astronomers have noticed certain things that indicate a form of life may be possible there. First of all, Mars has seasons like the earth. In fact, as the seasons change on Mars, there seem to be changes on its surface. The dark areas get stronger in the spring and summer, and the color changes from bluish-green to yellow. Could this be vegetation?Atronomers also believe that tehre is at least a samll amount of water vapour in the atmosphere of Mars, and this would be very helpful in supporting life. Then, too, in 1887 an Italian astronomers, Giovanni Schiaparelli, reported seeing markings on the surface of Mars that resembled canals. "Could these have been built by Martians in order to carry water from the polar regions to the desert areas?" scientists wondered.

In 1976 two American Viking space probes landed on the surface of Mars. The instruments in the space probes searched the surroundings soil for signs of life and radioed their results back to earth. These results showed that either there are germs in the soil or that the soil is very unusual and not like that on earth. If life does exits on Mars it would be a very simple form of life.

What is a Comet?

At one time, the appearences of a comet caused people to tremble with fear. They believed that comet s were evil omens fortelling plagues, wars, and death.Today, we have a pretty good idea of what comets are, though we still don't have all the answers about them. When a comet first appears, it is seen as a tiny point of light, though it may be thousands of miles in diameter. This point of light is "the head", or nucleus, of the comet. Scientists thick it is probably made of a great swarm of bits of solid matter, combined with gases. Where this matter originally came from is what is still a mystery.

As the comet approaches the sun, a tail usually appears behind it. The tail consists of very thin gases and fine particles of matter that are shot off from the comet's nucleus when it comes under the influence of the sun. Surrounding the nuleus of the comet is a third portion, known as "the coma". It is glowing cloud of matter that sometimes reaches a diameter of 150,000 miles, or even more.

Comet tails are very different in shape and size. Some are short and stubby. Others are long and slender. They are usually at least 5,000,000 miles in length. Sometimes they are almost 100,000,000 miles long! Some comets have no tail at all.

As the tail grows, the comet gains in speed beacuse it is nearing the sun, moving towards it head first. Then a curious thing happens. When the comet goes away from the sun, it goes tail first with head following. This is because the pressure of light from the sun drives off the very small particles from the comet's head to form its tail, always in a direction away from the sun.

As a result, when the comet goes away from the sun, its tail must go first. During its journey away from the sun, the comet gradually slows down and then disappears from sight. Comets may remain out of sight for many years, but most of them reappear eventually. Comets make trip after trip around the sun, but they may require a long time to make a single revolution. Halley's Comet, for example, takes about 75 years to make iits trip around the sun.

At present, astronomers have listed almost 1,000 comets, but there must be several hundered thousand comets in our solar system which remain unseen!

What are falling stars?

For thousands of years men have looked up at "falling stars" and wondered what they were and where they come from. At one time it was believed that they came from other worlds.Today we know that they are not "stars" at all. We call them "meteors". They are small, solid bodies which travel through space, and which may also pass through the earth's atmosphere.When meteors coem within our atmosphere, we can see them because they leave a fiery train of light. This is caused by the heat made by the friction, or rubbing, of air upon their surfaces.Strangely enough, most individuals meteors particles are quite small, about the size of a pinhead. Occasional meteors weigh many tons. Most meteors are destroyed entirely by heat as they pass through the earth's atmosphere. Only the larger meteors fragments ever reach the earth. Scientists believe that thousands of meteors fall each day and night, but since most of the earth's surface is covered by water, they usually fall into oceans and lakes. Meteors may appear in the sky singly and travel in partially any direction. But meteors usually occur in swarms of thousands.
As the earth travels in its path around the sun, it may come close to such swarms of meteors, they become fiery hot upon contact with upper layers of the atmosphere, and we see a "meteoris shower".Where do meteors come from? Astronomers now believe that the periodic swarms of meteors are broken fragments of comets. When
comets break up, the millions of fragments continue to move through space as a meteor swarm or stream. The swarms move in regular orbits, or path, through space. One such swarm crosses the earth's path every 33 years.Whena piece of meteor reaches the earth, it is called "meteorite". It has fallen to the earth because gravity has pulled it down. Far back in Roman times, in 467 B.C., a meteorite fell to the earth and its fall was considered such an important event that it was recorded by Roman historians!

What is the brightest star?

Have you ever looked up at the sky and tried to find the brightest star?You may imagine that the number of stars you can see is countless. But the most that can be seen without a telescope are about 6,000 stars, and one-quarter of them are too far south to be seen in North America.Ever since the days of the Greek astronomers, 2,000 years ago, the stars have been divided into classes according to their magnitude or brightness. Untill the invention of telescope, only six magnitudes, or degrees of brightness, were recognized.
Stars of the first magnitude are the brightest, and stars of the sixth magnitude the fintest. Stars fainter than the sixth magnitude cannot be seen without a telescope. Today, stars can be photographed with modern telescopes down to the 21st magnitude. A star of any given magnitude is about two and a half times fainter than a star of the magnitude above it. There are 22 stars of the first magnitude, brightest star, and the brightest star of all is Sirius, which has a magnitude of- 1.6.This makes
Sirius over 1,000 times brighter than the faintest star that can be seen with the naked eye.The lower we go down in magnitude, the more stars there are in that class. Thus, there are 22 stars of the 1 st magnitude and about 1,000,000,000 stars of the 20th magnitude.

What is the Milky Way?

There is probably nothing more mysterious and wonderful-looking in the sky than the Milky Way, streching like a band of jewels from one end of the sky to the other. In ancient times, when people gazed at this spectacle, they were filled with the wonder and beauty of it just as you are. But scince they didn't really know what it was, they made up all sorts of strange and beautiful explanations of the Milky Way.For example, in early Christian times, people thought it was a pathway for the angels, so they could go up to heaven on it.
Or they imagined it was an opening in the heavens, so that we here on earth could have a glimpse of the glory that existed beyond.Knowing the facts about the milky way, as we do today, dosen't remove any of the wonder of it. The facts are just as amazing as any "made-up" idea!Our galaxy is shaped roughly like a watch, round and flat. If you could get above it and look down on it, it would look like an immense watch. But we are inside the galaxy, and when we loook up we are looking towards the edge from inside the "watch".
So we see that edge curving around us. And since there are millions of Stars in it, we see it as the Milky Way.Did you know that there are at least 3,000,000,000 stars in the galaxy? And here is an idea of its size. It takes eight mintues for light from the sun to reach the earth. For light from the center of the galaxy to reach the sun, it takes about 27,000 years!The galaxy rotates about its center like awheel. From our position in it, it takes about 200,000,000 years just to make mone revolution!

What is a constellation?

Have you ever looked at the stars and traced out squares, letters, and other familiar figures? In nearly all parts of the world, people of long ago did this and gave names to the group of stars they observed. Such a group is called "a constellation" , from the Latin terms meaning "star"(stella) and "together".The names of the constellations in use today have come down to us from times of the Romans and from the even more ancient Greeks. Wjhat the Greeks knew about the stars came partly from the Babylonians.The Babylonians named some of their star figures after animals and other after kings, queens, and heroes of their myths. Later, the Greeks changed many these Babylonian names to the names of their own heroes such as Hercules, Orian and Perseus. The Romans made further changes. The same ancient names are still used, but it is not always easy to make out in the sky the figures that suggested them. For example, Aquila is the eagle, Canis Major and Canis Minor are big and little dogs, and Libra is the scales, but the constellations don't look much like these figures to us.About the year A.D.150, the famous astronomer Ptolemy listed 48 contellations that were known to him. This list did not cover the entire sky; there were many blank spots. So, in later times, astronomers added constellations to Ptolemy's list. Some of these later constellations are named for scientifis instruments, such as Sextant, the Compasses, and the Microscpoe. Today, astronomers recognize 88 constellations in the sky.A constellation is really an area in the sky. This means that every star lies in one constellation or other, just as any city in the United States is in some one state. The boundaries of the constellations used to be very irregular. Many of them had curved lines. But in 1928, astronomers decided to straighten them out so that the outline of any constellation includes only straight lines.

Why are eclipses so rare?

When the moon in its journey around the earth passes directly between the earth and the sun, it casts its shadow on the surface of the earth, and an eclipse of the sun takes place.An eclipse of the sun occurs only when the moon is new, for then the moon is on that side of the earth facing towards the sun. Then why isn't there an eclipse of the sun every time there's a new moon? The reason is that the path of the moon around the earth does not lie directly in line with the orbit of the earth about the sun. In its 29-day trip around the earth, the moon passes sometimes above and sometimes below the path of the earth.An eclipse of the sun can be total, anuular, or partial. If the moon hides the sun completely, the eclipse is total. But the moon is not always the same distance from the earth. Often, it is too far from the earth to hide the sun completely. Then , when an eclipse takes place, the moon is seen as a dark disk which covers the whole sun except a narrow ring around its edge. This thin circle of light is called " the annulus", meaning "ring". This is an annular eclipse. An eclipse is partial whwnever only a part of the disk of the moon comes between the sun and the earth.An eclipse of the moon occurs only when the moon is full, for then it is at the opposite side of the earth from the sun. When the moon comes directly behind the earth, as seen from the sun, it passes gradually into the great shadow-cone cast by the earth and disappears from view. A total eclipse of the moon then occurs. A partial eclipse takes place when the moon enters only partly into the shadow.In some years, no eclipse of the moon occur. In other years, there are from one to three. Every year, there must be at least two solar eclipses, and there may be as many as five. At any one place on the earth's surface, a total solar eclipse will be visible only once in about 360 years.

What is the earth made up of?

Man is now exploring the moon and the other planests- and yet he still dosnt know exactly what his own earth is made up of!A sort of rough answer to This question would be: The earth is a big ball, or sphere, made mostly of rock. Inside the earth the rock is melted, but the outside cover is hard rock. The central core of the earth is also solid because the rock is under tremendous pressure.Now let's consider this in a little more detail. The outside of the earth is a crust of rock about 10 to 30 miles thick. This crust is sometimes called "the lithosphere." The high parts of this crust are the continents, and the low parts of it holds the waters of the oceans and the great inland seas and lakes. All the water on the surface, including the ocean, lakes, rivers, and all the smaller streams, is called " the hydrosphere."Men have been able to examine only the outermost part of the crust of rock that forms the outside of the earth, which is why it's so hard to know what the earth is like on the inside. In drilling wells and digging mines, it has been found that the deeper the hole is made, the higher the temperature becomes. At two miles below the surface of the earth, the temperature is hight enough to boil water.But scientists have also been able to find out about the inside of the earth from studies of earthquakes. They believe that the temperature does not increase as rapidly deep down as it does in the crust. So they think that at the core or center of the earth the temperature may not be more than 5,500 degrees centigrade. Of course, that's very hot- since a temperature of 1,200 degrees would melt rocks!The crust of the earth has two layers. The upper layer, which makes the continents, is of granite. Under the layer of granite is a thick layer of very hard rock called "basalt". Scientist believe that at the center of the earth is a huge ball of molten iron, with a diameter of about 4,000 miles. Between the central ball and the rocky crust is a shell about 2,000 miles thick called "the mantle". The mantle is probably made of a kind of rock called "olivine".

Why does the sunset look red?

A beautiful red sunset, the colors warm and glowing, is one of the loveliest sights we can imagine. And sometimes, when we look at it we might say, "See how red the sun is!"

But, ofcourse, we know that the sun itself hasnt become red or changed in any way. It merely looks that way to us at that particular time of day. In fact, at that very moment people are looking at that same sun thousands of miles to the west and it doesn't look red to them at all.

What produces the color of the sunset is the distance that the sunlight must travel through our atmosphere. The lower it is, the more of our earth's atmosphere does that light travel through.

But first, let's remind ourselves that sunlight is a mixture of light of all colors. Normally, this mixture of light appears as white to out eyes. But the atmosphere has molecules of air, dust, water vapour, and other impurities present in it. As the light passes through them, different colors are scattered by these particles. Now, it so happens our atmosphere scatters out violet, blue, and green light more than it does the reds and yellows. So when the sun is low, this scattering leaves more reds and yellows for us to see and we have a reddish sunset.

By the way, this scattering of light also explains why the sky looks blue. Violet and blue light have short waves and are scattered about 10 times more than red light waves by our atmosphere. This means that the red rays go straight through our atmosphere, while the blue waves don't come through directly but are scattered by the air, water and dust paricles. It is this scattered light that we see as the blue sky when we look up.

What keeps the sun shinning?

It may be hard for you to believe, but when you look at the stars that shine at night and the sun that shines by day, you are looking at the same kinds of objects!
The sun is really a star. In fact, it's nearest star to the earth. Life as we know it depends on the sun. Without the sun's heat , life could not have started on earth. Without sunlight, there would be no green plants, no animals, no humjan beings.
The sun is 93,000,000 miles from the earth.The volume , or bulk, of the sun is about 1,300,000 times that of the earth! Yet an interesting thing about the sun is that it is not a solid body like the earth.
Here is how we know this: The teperature on the surface of the sun is about 6,000 degrees centigrade. This is hot enough to change any metal or rock into a gas, so the sun must be a globe of gas!
Years ago, scientists believed that the reason the sun shone, or gave off light and heat, was that it was burning. But the sun has been hot for hundreds of millions of years, and nothing could remain burning for that long.
Today scientist believe that the heat of the sun is the result of a process similar to what takes place in an atom bomb. The sun changes matter into energy.
This is different from burning. Burning changes matter from one form to another. But when matter is changed into energy, very little matter is needed to produce a great deal of energy. Twenty eight grams of matter could produce enough energy to melt more than a million tonnes of rock!
So if science is right, the keeps shining because it is constantly changing matter into energy. And just one percent of the sun's mass would provide enough energy to keep it hot for 150 thousand million years!

Why is the solar system the way it is?

As far as we know, ther is no reason why the solar system is arranged exactly as it is. It might have been arranged differently, just as there are other solar systems in the universe arranged differently. This has to do with the way it originated. But man has discovered certain laws of nature that seem to keep the solar system in its present pattern.
Earth, like the other planets, follows its path, or orbit, around the sun. The period of time that the earth takes to go around the sun is called a year. The other planets have orbits larger or smaller than the earth's.
How this solar system came to be and how the planets came to have the size, location, and orbits they have, astronomers cannot fully explain. But they have two main types of theories. One type of theory suggests that the formation of the planets was a part of the gradual change of the sun from a whirling mass of hot gas to its present size and brilliance.The planets formed as small whirling masses in the giant gas and dust cloud as it turned.
Another group of theories is based on the idea that at some time there was a near-collision between the sun and another star passing nearby. Large pieces of the sun were pulled away and began to revolve around the sun at different distances. These are now planets.
No matter which theory is right, the solar system cam to be as it now is more or less by chance. Why does it stay this away? Kepler's Laws of Planetory Motion states that all planets travel about the sun in an elliptical (oval) path; that a planet moves faster in its orbit as it nears the sun; and that tehre is realtion between its distance from the sun and the time it takes to make an orbit. Newton's Law of Gravitation, of which Kepler's three laws were an indispensable part, explained how two objects attract each other. So the solar system remains as it is because certain laws of nature maintain the relationship of the sun and the planets.

How big is the universe?

It is impossible for the human mind to conceive a true picture of the size of the universe. We not only dont know how big it is, but it is hard for us even to imagine how big it might be.
If we start from the earth and move out, we"ll see why this is so. The earth is part of the solar system, but a very tiny part of it. The solar system consists oof the sun, the planets that revolve around it, the asteroids, which are tiny planets, and the meteors.
Now, this whole solar system of ours is only a tiny part of another, much bigger system called "a galaxy". A galaxy is made up of many millions of stars, many of which may be much lagger than our sun, and they may have solar systems of their own.
So the stars we see in our galaxy, which we call "the milky way", are all suns. they are all so far away that distances are measured in light years instead of in miles. Light travels about 6,000,000,000,000 miles in a year. The brighest star is nearest to the earth is alpha centauri. Do you know how far away is it? 25,000,000,000,000 miles!
But we are still talking only about our own galaxy. This is believed to be about 100,00 light years in width. This means 100,000 times 6,000,000,000,000 miles! And our galaxy isonly a tiny part of still larger system.
There are probably millions of galaxies out beyond the Milky Way. And perhaps all these galaxies put together are still only a part of some larger system.
So you see why it is impossible for us to have an idea of the size of the universe. Incidentally, it is believed by scientists that the universe is expanding. This means that every feww billion years two galaxies will find themselves twice as far apart as they were before!