What is lava?

Lava is red-hot liquid rock which comes from deep within the earth and breaks through the top of a volcano when it erupts. Millions of years ago the Earth had many volcanoes. As a result, much of the rock on the Earth today is hardened lava.

Lava may seep slowly out and cool quickly, or it may burst forth violently and pour at the rate of 40 miles an hour over fields, forests and villages.

The lava hardens in swirls and ripples like petrified molasses. It may pour over a cliff and harden into a solid waterfall. If the surface of lava cools and hardens while that beneath continues to flow, lava caves are created. Lava stalactites and stalagmites, twisted, corded and very fragile, may be found within the caves.

What is astrology?

Another name for astrology is "stargazing." Long ago it was the custom in some countries to call in an astrologer when a baby was born. The astrologer "read the stars" and from them told the baby's fortune. He told whether the child would grow up to be brave or timid, strong or weak, rich or poor. "Reading the stars" meant chiefly seeing where the planets were among the stars. Astrologers predicted such things as wars and famines, too. For centuries there was an astrologer in the court of almost every king.

Astrology is not a science. The stars, scientists agree, have nothing to do with the fortunes of people or nations. The astrologers of old predicted many things that never came true. But even now many people believe in astrology.

Astronomy is a science. It is the true study of the stars. It is an old science. But many of the early astronomers had to make their living by practicing astrology.

What is asbestos?

asbestos fibres

The name "asbestos" comes from a Greek word that means "will not burn." There is nothing surprising about a mineral that will not burn. Not many minerals will. But asbestos is different from most minerals in this way: it can be pulled apart into fine threads, or fibers. Sometimes it is called "mineral silk." Another name for it is "cotton stone."

Asbestos fibers can be twisted or woven or molded together. They can be made into rope or cloth or paper. They can be put into plaster or paint or cement.

Shingles on houses are often made of asbestos-cement. There is no danger that a roof covered with these shingles will catch on fire from a spark. Theaters have asbes­tos curtains. If a fire breaks out on stage, the curtain can be put down to keep the fire from spreading.

Neither heat nor electricity can travel easily through asbestos. Sheets of asbes­tos are wrapped around furnace pipes to keep the heat from escaping. Asbestos fi­bers may be put between walls to shut heat in. Asbestos may also be used around wires to keep electricity from going off the path it is supposed to follow.

Asbestos makes good automobile brake bands and clutch linings. These parts have to withstand great heat. Every year thousands of miles of asbestos bands are woven or molded for automobiles.

People have known about asbestos ever since the days of the ancient Greeks. In the Middle Ages the famous emperor Charlemagne had a tablecloth made of asbestos. To clean it he threw it into a fire. The fire burned off all the grease that had been spilled on it but did not damage the cloth itself. The Chínese were making cloth of asbestos in the days of Marco Polo. In those days people thought that salamanders could live in fire. Asbestos was sometimes called "salamander's wool."

What is a magnifying glass?

A magnifying glass is a convex lens, its makes objets look larger than they really are. The glass is thicker in the middle than it is at its edges. Its magnifying power is measured by the number of times larger an objet seen through it appears. For example, if the letter A on a poster is one inch tall but through a magnifying glass appears to be three inches tall, the magnifying power is three.

Convex lenses may magnify in another way, too, producing enlarged images on a movie screen. In this case the image is a real image which is always inverted. For this reason, the film is put into the projector upside down.

What are cradle books?

Gutenberg Bible

All the books that were printed when printing was in its babyhood -from the year 1455 to 1500- are called "cradle books." Some of the cradle books are among the most beautiful books ever made. Often they were decorated by hand after they were printed.

Probably the most famous of all the cradle books are the Gutenberg Bibles. Gutenberg is often called the inventor of printing with movable type. For a long time people thought the Gutenberg Bible was printed by Gutenberg. But very likely a printer named Schoeffer printed it. How surprised Schoeffer would have been if he had known that copies of this book would someday be worth their weight in gold.

The first book ever printed in English came from the press of the English printer William Caxton in 1474. The book was about the ancient city of Troy.