Almost all living things are made of cells. Some tiny froms of life consist of only one cell. Larger forms of life are built of many cells. That is why cells are called the basic unit of life.
The human body is made of billions upon billions of cells are probably the bacteria. These one-celled forms of life can barely be seen even with a microscope. Nerve cells are probably the largest cells. Some of them are more than 3 feet long.

The microscope has shown that all cells have three main parts: the cell membrane, the cytoplasm, and the nucleus. The cell membrane is the outside surface of the cell. Inside the membrane is the cytoplasm. This is a mass of colorless material, which hold many other tiny cell parts. The nucleus is usually near the center of the cell. It is somewhat round in shape and has an outer membrane of its own.

There are many kinds of cells. Usually each kind does one special job. For example, in animals some cells expand and contract; these are muscle cells. Other cells are sensitive to light; these cells form the retina of the eyes.

Modern microscopes show that cells are far from simple. They contain many complicated structures. And there is constant activity going on in cells.
Food and oxygen pass steadily into the cell through the membrane. The cell changes them into energy. It uses the energy to do various kind of work. The cell also makes new cell material moiré living matter. Some waste matter is left over from these processes. It passes out of the cell through the membrane. So do worn-out cell materials.

With few exception, a single cell does not live very long. But if it divides, the two new cells start life afresh. That is, intead of aging and dying, the old cell becomes two new cells.
Plants and animals grow because of cell division continues. New cells replace old or worn out cells. As cell division slows, the body ages.

Although plant and animals cells differ from each other in some ways, in a broad sense they are alike. And so we say that the cell is the unit of life.

When you blow out of candles on your birthday cake, ou following a very old custom. People used to believe that by blowing out birthday candles they were blowing away all the bad luck of past years. That is why there is a candle for every year.

How the first candle came about is not known. Perhaps, as primitive man was roasting animals, he poked the carcass with a stick to test its tenderness. The stick covered with animal fat was easy o light and burned brightly. Thus, the torch was created. An improvement was made by dipping twigs in resin, pich, or oil to make the light last longer. Later, bundles of fat-dipped reeds took the palce or twigs and were called rushlights.

The early Egyptians made candles. Tehir funeral service were lighted and possibly perfumed by cone-shaped lumps of wax or tallow (animal fat) stuck in holders. Bands of materials that would burn were wound around the candles to keep the hot tallow or wax from spreading into a shapeless mass. The Egyptoans may also have used wicks.

Candle making slowly became and established art. The early Roman Catholic Church made special beeswax candles for their services. By the 13th century candles were in general use. In Paris and other cities candle makers traveled from door to door restocking each household’s supply of candles.

Early in the 18th century Bartholemeu de Gusmao, a Portuguese priest, developed a samall hot-air balloon. But the first big balloon was built by two French brothers, Joseph Michel and Jacques Estenne Montgolfier, in 1783. They made a huge bage of paper and cloth, which they held over a fire. When they released the smoke-filled balloon, it soared into the sky.
Encouraged by their success, the Montgolfier brothers build a balloon with a basket underneath to carry passengers. In the basket they put a rooster, a duck, and a young sheep. The balloon was filled with hot air from a fire and was released. The basket, with iis animal passenger, floated upward. High above the earth the warm air inside the balloon cooled off, and the balloon slowly floated to earth. Live passengers had been carried into the air and returned safely for first time in history.
This feat inspired two men to become the first human beings to fly above the earth. They were Jean Francois Pilatre de Rozier, a French physician, and Francois Laurent, the Marquis d’Arlandes. A huge blue and gold hot air balloon carried them aloft over Paris on November 21, 1783.
Fire was always a danger at these early balloon launchings. One spark could send a balloon up in flame. But in the same year that the hot air balloon was developed, Jacques A.C Caharles. A French scientist, invented the gas balloon. Charles build a sphere of rubberized silk and filled it with hydrogen, a gas that is lighter than air without being heated. The balloon soared into the air and landed in a field near Paris, where it was destroyed by frightened farm workers.
With to help of others, Charles later developed a balloon thay could carry passengers. To rise higher, the balloonist released sand from bags tied to the side of the basket. To come down, the balloonist pulled a rope that opened a valve located at the top of the balloon. The open valve allowed hydrogen to escape, so that the balloon would slowly deflate and return to earth.

“Give a place to stand, and I can move the world.” The man who is supposed to have spoken those words was Archimedes, a Greek mathematician and inventor who lived some 2,200 years ago. Its was not and idle boast. Archimedes was one of the firs men to the develop the science mechnics. He understood that a man with a mechanical device such as alever could move many times his own weight. Challenged by the king to prove his point, Archimedes did so. He arranged a ship all by himself.

Archimedes was born about 287 B.C., in Syracuse, a Greek settlement on the island of sicily. Little is known about his personal lefe except that his father was an atronomer and may have been related to the king of Syracuse. Also, at some time in his life Archimedes studied in Alexandria, Egypt, a center of Greek culture.

Archimedes is best known for his many inventions. Among other things, he invented a compound pulley; a sphere that imitated the motion of heavenly bodies; and a water screw to raise water. He himself moast valued his work in mathematics and scientific theory. But his fame rest on inventions and the legends that grew up around him.
One legend tells how Archimedes made his most important discovery.
The king, it seems, had ordered a new crown. It was supposed to be made of solid gold, but the king suspected that the jeweler has cheated him. He asked Archimedes to tell him if the crown was solid gold.

At first Archimedes could not think how to do this. Then one day the solution came to him as he was getting into his bath. Legend has it that he ruhed naked into the streets shouting, “Eureka [I have found it]!”.

What had happened very simple. The bath was full, and it overflowed as Archimedes climbed into it. This started him thinking about the way objects displace water. And he studdenly saw how to solve his problem.

First he took a quantity of gold and silver were equal, but their volumes were not. The silver, being less dense, was bulker than the gold.
Next Archimedes two vessels filled to the brim with waqter. He palced the gold in one and the silver in the other. The silver, being bulkier, caused more water to overflow. Archimedes concluded that when a solid sinks in water, it displaces its own volume of water.

Finaly he tested the crown against the equal weight of gold. When placed in water, the crown caused more overflow. Therefore, the crown had to contain metal other than gold. This metal made it bulkier and caused the greater overflow.

Further experiments resulted in what is known as Achimedes’ principle ; An object in a fluid is buoyed up by a force equal to the weight of displaced fluid.
When Archimedes was an old man, the Roman attacked Syracuse. He turned his mind to defense, an inveted several engines of war that held off the enemy. It is claimed that he built a huge burning mirror. When it was used to focus the sun’s rays on the roman fleet, the ships were set on fire. Syracuse however, was defeated in 212 B.C., and Archimedes was killed. The story goes that he was drawing mathematical figure in the sand when Roman soldier struck him down. Archimedes was so higly respected that the Roman commander buried him with full honors.

Around A.D 1100 an unknown alchemist, probably in souther Italy, destiled some wine. He got a clear, colorless fluid with a strange odor. Through this strange new fluid looked like water, it did not behave new at all like water. It had a burning taste. When it wa drunk, it had powerful intoxicating effects. Strangest of all, it burned with a hot, blue flame. This mysterious “burning water,” which the alchemists believed was the spirit of wine, was ethyl alcohol.

Today hundreds of different alcohols are know. Some are clear, colorless fluid that mix freely with water. They evaporate quickly in open air. They have low freezing points. Ethyl alcohol freezes at about –1170C (-1790F). Other alcohols behave quite differently. They are thick, oily fluids that hardly mix with water at all. Some are even solid at room temperature.

Alcohols are made up of atoms of carbon, hudrogen, and oxygen. The carbon atoms from kind of framework around which the hydrogen and oxygen atoms are arranged.
In industry, alcohols are used as raw materials for making many other chemicals. They are also used to dissolve fats, oils, and may organic (carbon containing) substance, include some plastic. Another use is to extract flavoring oil, perfumes, and drugs from plant and animal materials. Some alcohols are burned as fuels in rockets. Scientists are testing the use of alcoholic fuels in automobiles and other land vehicles.
In the home, ethyl alcohol is found in antiseptics, flavorings, perfumes, and liquor. Colored red for visibility, it is used as the fluid in outdoor thermometers. Isopropyl alcohol is used as rubbing alcohol.

All the alcohols exept glycerol are more or less poisonous. Methyl alcohol (wood alcohol) is extremely poisonous, and even a small amount can couse blindness or death. Denatured alcohol is ethyl alcohol that has been made undrinkable by adding poisons or unpleasant-tasting chemicals to it.

Wherever you go, whatever you do inside, outside, on top of mountain, deep in a coalmine you are always surrounded by a sea of gases. This sea is called the air, or the atmosphere.
The gases of the atmosphere cannot be seen, and we are rarely aware of them. But they are of the greates importance. Without the atmosphere, people, animals, an plat could not live. Of almost equal importance is the quality of the atmosphere whether it is pure or polluted (meaning impure).

The atmosphere is made up mainly of the gases oxygen and nitrogen, together with water vapor and smaller amounts of carbon dioxide and other gases. But all air contains small amounts of impurities. In rural areas, far removed from factories and heavy traffic, the air my countain pollen from plants, dust from the soil, and even bacteris. These impurities are usually is such small amounts that they are not important.
Air is said to be polluted when it contains enough harmful impurities to effect the helath, safety, or comfort of living things. The impurities, or pollutans, could be tiny particle of matter or gases not normaly found in air.

When people breathe, pollutans in the air may be deposited in the lungs or absorbed into the body. And polluted air can harm animals and plants as well as people. For this reason, our air supply should be closely watched and managed to assure its good quality.

In 1900 there wa very little people could during a hot spell except grumble about the weather or – if they could afford it-go away to the mountains or the seashore. Today air conditioning has changed this. With air conditioning you can be comfortable any where indoors on oven the hottest, stickest day of the year. You will almost certainly find air conditioning in the movie theates you attend., in many of the stores where you family shop, an in the restaurant where you eat. It may even be ini your own house.


What is Air Conditioning?

You know that air conditioning make you feel cool. But air conditioning is more than just cooling. It means keeping the temperature and humidity (moisture content) of airin an enclosed space, weather it is one room or an entire building, at just the right level for the comfort of the people inside. It alse means circulating the air with fans and removing dust from it with filters. In winter it means heating the air and adding moisture if necessary. In summer it means removing moisture by passing the air over cold pipes that collect water from the air, much as drops of water condenses on a cold water glass on a hot, damp day. In fact, we might almost say that air conditioning means creating an artificial, comfortable climate. In this article we shall discuss only cooling.
In dry vlimates air can be cooled simply. The cooler may be no more than a large fan that draws hot, dry air trought a water soaked fiber mat. The air is cooled as it evaporates the water. It is so dry to begin with that the added moisture will not couse discomfort.

How does air conditioning work?


An air conditioning unit does not “add coolness” to the air. It removes heat. An air conditioner work on the same basic principle as a refrigrator-though it is not designed to produce such low temperatures. Heat is taken from the air by the rapid expantion of a refrigerant (cooling subtance) as it turns from a liquid at high pressure to a gas at low pressure. An accompanying illustration show the cycle of operation of a small home type air conditioner. Here the air is cooled directly by the macine. For the sake of simplicity, filters and motors are not shoen in this diagram. In large, central installations, such as those in office buildings and schools, a machine chills water that isi piped to a series of coils. Air from building is drawn over these coils and circulated through the building by blower.

To be partical, an air conditioning unit must be able to maintain a steady temperature. Otherwise people would have turn their units off and on continually as the temperature is maintained by a temperature-regulating device called a thermostat. The tremostat is set at the desired temperature. Itthen switches the cooling unit on off as needed.
The air conditioning system of the future may work by what is know thermoelectricity. A thermoelectric unit is very small and very quiet. It is made up of tiny “couples,” each couple consisting of a pair of semi conductor and connected in parallel to an electric current. These couples produce cooling at one end and heating at the other when direct current flows trough them. This is called the peltier effect. When a way is found to produce these tiny thermoelectric unit cheaply. They may cool abd head your whole house.

The abacus is a simple calculating machine that is used for doing arithmetic. A modern abacus is made of beads, rods, and a frame. The rods are fastened in the frame, and each stand for a place in the decimal system. One rod stans for units, one for 10’s, one for 100’s, and so on. The beads are counters. Numbers are added or subtracted by moving the beads on the rods.

Why the abacus was invented

The abacus wa invented thousands of years ago. It offered a way of doing arithmetic with using written numerals. And it was used in many ancient civilization.
The numeral of ancient times were very awkward for doing arithmetic. The trouble was that they did not express zero of place value.

For example, compare our numerals with the roman ones. Our numeral 5 can stand for 5, 50, or 500, depending on the place it is given. In the number 555 each 5 has a different value. The romans used letters as numerals and expressed place by using a different letter. 5, 50, and 500 were written as V, L, and D. Without the idea of zero and place, there was no way that the same nuimerals could be made so stand for more than one number. That is, V could not be made to stand for any number accept 5.

This system of numerals made written arithmetic very difficult. So the romans, like other ancient peoples, used the abacus. The idea of place was built into the abacus. One counter could be made to express 1, 10, or 100, according to its place on the abacus.
Thw abacus offered a quick and easy way of adding and subtracting. It could also be used for multiplication and division. Multiplication was done by adding equals groups. Division was done by substracting equal groups.

How to make and use abacus

The abacus is still widely used today. For example, it is used in china and japan. There numerals do not express place. People who are used to the abacus can do arithmetic on it very quicly. A Japanese abacus can be bought in almost any toy store or five-and-ten. The intruction that come with it explain how it works.

However, it is very simple to make an abacus like those of ancient times. You can draw four lines in sand, dust, or soil and use pebbles as counters. Or yopu can rule heavy lines on apiece of paper or cardboard. Use buttons, bottlecaps. Dried beans, or pebbles as counters.

The lines, from right to left, stand for 1’s (or units), 10’s, and 1,000’s. For example. If you want to express 2,222, you put two counters in each row. If you want to express 2,220, you put two counters in three of rows and none in the units row. Make up some numbers of your own and express them.

Now suppose you want to add 271 and 302. Set up one of the numbers; then set up the second number. A glance at the abacus give you the sum.
This is how to subtract 271 from 394. Set up the large number on the abacus and take away the counters that represent thw smaller number.