A mirage is an optical illusion, or deception. It occurs especially in certain conditions of the air when the air is very hot. Sometimes in deserts there are spots called oases, where there is water, and, as there is water, there are also green trees and shade. We are told that sometimes travelers think they are coming to an oasis only a few miles away, where they can get water and shade; and then, as they travel on, it disappears. A great explorer once “discovered” and named a mountain which did not exist, but which he had seen as a mirage.

But a true mirage is not an appearance in the sky due to nothing at all, and it is not pure imagination on the part of those who see it. When the traveler sees an oasis in the desert, and it fades and deceives him, what he has seen is the image of a real oasis, much farther on, below the horizon. The light from the real oasis has been reflected from a layer of air, and the traveler sees it as if there were a huge mirror in the sky placed at such an angle as to give a view of the oasis to the traveler’s eyes. The oasis itself may be many weary miles distant.

The reason for this is that there are layers of air of different temperatures, and therefore of different density, and whenever light passes from one thing into another of different density, part of it does not go on, but is deflected back. Appearances due to a similar cause are often seen at sea. A ship near the horizon may seem to have another ship, exactly like itself, perched upside down upon it.

Resin is the name given to a gummy liquid that is found in many plants, but it is used especially to mean the crude turpentine that is exuded by various pine, fir and larch trees. The crude turpentine is distilled to separate the oil of turpentine from the solid matter. It is this solid matter which we call rosin. Rosin comes in hard, brittle lumps, ranging in color from a pale amber to a very dark brown.

Rosin is used for many purposes. Varnishes, sealing-wax, soap and cement are among the useful articles which contain it. Violinists rub rosin on their bows to make them grip the violin strings properly, and ballet dancers use it to prevent their shoes from slipping on the floor. Sometimes rosin is sold in solid form and sometimes as a powder.

A great deal of the world’s supply of resin and turpentine comes from the belt of long- leaf pine forest that extends from North Carolina to Florida and across the Gulf states as far as Texas. The resin, turpentine, pitch and tar obtained from these forests are called Naval Stores, because they were used in the building of ships in the days of wooden sailing vessels. Nowadays these products are not needed for shipbuilding, but they are widely used in a great many other ways, especially in the paint industry.

The sense of smell depends upon tiny particles of things coming in the air to the lining of our noses, especially certain parts of the lining of the nose. When we have a cold, this lining, or mucous membrane, of the nose gets swollen, and produces a much greater amount of mucus than usual, as we all can tell by the number of handkerchiefs we have to use in a day.

The chief reason why we can not smell so well when we have a cold is probably that this mucous, constantly pouring out of the lining of the nose and running over it, prevents the scent of things from getting to the sensitive part of the nose, and washes away any solid scented particles. Also, it may be that the poisons produced by the microbes that cause a cold poison the living cells of the mucous membrane and also poison the tiny ends of the nerves of smell that run to it, so that, even if scented things do reach the sensitive part of the mucous membrane, they can not be felt.

This applies alike to scents coming in from outside and also to the scents of food, which pass up at the back of the roof of the mouth into the nose, and which, when we have not a cold, help to give our food half its flavor.

As the earth spins round, the sun appears to rise sooner, the farther east we are, and to rise later, the farther west we are. So the apparent time, judged by the sun’s rising and setting, is different in different places, according as they are east or west of each other. Midday on one side of the earth is midnight on the other side.

So it is necessary to have some point from which to reckon our time, and the place on which many nations have agreed is Greenwich, near London, England. Countries have their own time for their own purposes; but for general purposes, as, for instance, events occurring in the sky, they refer to Greenwich time - that is to say, the time reckoned by what the sun seems to do at Greenwich. The lines on maps up and down the earth’s surface from north to south are called lines of longitude, or meridians. The spaces between them grow narrower as you go north and south of the Equator; and the lines meet at the poles. Places on the same line of longitude as Greenwich have Greenwich time exactly, and no other place can have it. That line is called the Greenwich meridian. Places east of that line are marked on maps so many degrees east longitude, and are sometimes spoken of as so many degrees east of Greenwich; and similarly for places west of the Greenwich meridian. Farthest east and farthest west of the Greenwich meridian is a single line that is both 180 degrees east and 180 degrees west of Greenwich. When it is noon at Greenwich, it is midnight along this line.

“The dark” is the absence of light. Now, what is the name for the absence of sound? What do we call the state of things when we hear no sound? The answer is silence. By darkness we mean absence of light, just as silence is absence of sound.

But there is more to say. There may be a wave motion in the ether, but it is hardly proper to call that light until someone sees it. Similarly, there may be a wave movement in the air, but it is hardly proper to call that sound unless someone actually hears it. Seeing and hearing, then, depend, ‘first of all, on there being something outside of us - a particular kind of wave; and secondly, on our being able to feel that something.

A blind man cannot see, even in the light. Our great poet Milton, in his poem on Samson, makes Samson say, when he had lost his sight: “Oh, dark, dark, dark amid the blaze of noon”. That famous line will help us to understand what darkness may depend on - either the absence of light or the absence of the power to see light.

It is often supposed that cats and tigers can “see in the dark,” but we must know that nobody at all can see if it is perfectly dark - that is to say, if there is no light at all. When we speak of being in the dark we usually mean that there is so little light at we see hardly anything.

That is because our eyes are so made that they cannot alter themselves to suit the conditions of very dim light; but some animals can make the pupil of the eye so wide as to get the benefit of whatever rays of light are about. This is the case with cats, and if we watch the cat’s eye when it is in the dark, we see that the pupil appears much enlarged. This allows all the light possible to enter the eye, and the cat, and lions and tigers and other night-prowling animals that have eyes like the cat, are able to see very much better in dim light than we can. But even among human beings there are some people, especially seamen, who can see farther in the dark than others.

It has been discovered that there is light which we cannot see. Our eyes are keyed to a certain scale or band of wave lengths. Beyond this band are shorter waves, which we call ultra-violet rays, still shorter ones, which we call X-rays, and unbelievably tiny ones, gamma rays. There may even be shorter ones than the gamma rays, called cosmic rays. Beyond the other end of the scale of light rays that are visible to us are invisible rays with longer waves. They are called infra-red rays. Instruments can detect these very short and very long rays, though our eyes cannot. Some insects are thought to see them. Some cameras can take pictures by infra-red light, which are the very long light rays; and you probably have seen X-ray photographs, which are pictures taken by extremely short waves.

There is no hard-and-fast line that can be drawn to mark the end of one geological period and the beginning of a new one. With a few exceptions, the various periods shade gradually into one another without any very abrupt change. However, the beginning of the Eocene period brought about such marked changes in the appearance of the world and its plant and animal life that it seems proper to regard this period as the beginning of a new era in the history of the earth.

Tremendous geological changes took place between the laying-down of the Cretaceous and the Eocene rocks. The Cretaceous rocks, during the greater part of the Cretaceous period, were beneath the water, some of them under the deep sea. In the Eocene, however, they, with all their lagoons, estuaries, lakes and seas, were generally forced up into dry land and mountain ranges. Not only were the chalky and clayey and sandy deposits of the Cretaceous raised above the level of the surrounding sea into such masses as the chalk cliffs of southern England and northern France, but even the floors of ancient seas were forced upwards, sometimes as high as 10,000 or 17,000 feet. It must have been a stupendous although gradual upheaval of the earth’s crust, for it took place all over the world.

When the Eocene deposits began to form, we find that not only was the face of the world completely changed, but its plant and animal life was unlike that of preceding periods. It is for this reason that the period has been called the Eocene, from the Greek word eos, meaning “dawn,” and it has been regarded as the beginning of a new system, which has been named the Tertiary. It was the dawn of the world as we know it to-day.

Eocene rocks are found all over the earth. In the Old World they are found chiefly in the Pyrenees, the Alps, the Carpathians, the Caucasus, Asia Minor, Northern Africa, Persia, Baluchistan, the Sulaiman Mountains, China and Japan. In eastern North America the Eocene rocks stretch along the Atlantic and all the way from New Jersey into Texas. In the interior of the continent there are large areas of Eocene rocks in Alberta, some in Saskatchewan, and in various parts of the central and western United States. There were shallow lakes and marshes and much volcanic activity in this region during the Eocene.

The vegetation of the period was very rich, and many of our common trees grew freely, as ashes, beeches, willows, poplars, elms and maples. Palms and bananas were also to be found. The climates of Alaska and Greenland were temperate, and luxuriant forests abounded in those regions.

The animal life of the period was varied. Mammals, birds, reptiles, fishes, insects and molluscs were well represented. Among the birds, snipes, seagulls, buzzards, hawks, ospreys, quails, pelicans, flamingoes and horn- bills were common. But the most striking feature of the Eocene animal life was the disappearance of the huge lizards of the Cretaceous period and the appearance of new and better developed mammals.

Just why the mighty race of dinosaurs died out is a mystery. The Eocene climate was cooler than that of the preceding period, and it may have been that these great reptiles could not stand the change in temperature. Perhaps the fact that the early mammals may have fed on dinosaur eggs had much to do with the reptiles’ rapid decrease in numbers and final and complete disappearance.

Two of the most extraordinary creatures living in the Eocene were the titanothere and the uintathere. The former appeared early in the Eocene as an animal no larger than a sheep, and gradually developed into a beast the size of a small elephant. The titanothere had a heavy body supported by thick legs. Two great bony lumps grew out of the front of the skull, and the brain was hardly as large as a man’s fist. For all their great size, the titanotheres must have been very stupid creatures.

The uintathere has been found only in America, where it was characteristic of the Eocene period. It also was as large as a small elephant, and was well armed with six stout horns, four above its nose and two above its ears. In the males the upper canines were developed into two long, sabre-like tusks. Like the titanothere, the uintathere had a ridiculously small brain within its bulky body.

Other mammals appearing in the Eocene were the ancestors of the horse, rhinoceros, tapir, pig, camel and monkey.

After the Eocene came the Oligocene, a period not very different from the Eocene. Oligocene rocks are not common east of the Mississippi except in the Gulf region. On the Pacific coast they are more common, however, and in the interior of the continent are large areas with many interesting fossils. In general the plants were like those of the Eocene, but the climate was cooler.

THE LARGEST MAMMAL THAT EVER WALKED THE EARTH

It was during the Oligocene that baluchitherium, the largest of land mammals, lived. The bones of this great beast were discovered in the hills of Baluchistan in southern Asia. It stood from twelve to thirteen feet high at the shoulder, and by lifting its head and stretching, it could eat leaves twenty feet above the ground. It was a distant relative of the modern rhinoceros with which we are all familiar.

The Miocene followed the Oligocene. Miocene rocks are found in France, Belgium, Switzerland and in many other countries, and show a gradual progression in all forms of life, together with the production of many new forms. In western North America Miocene rocks are widespread, but they are not very thick except on the Pacific coast.

The most remarkable of the new animals were the mastodon and the dinotherium, both huge beasts of the elephant family. The tusks of the mastodon were straight; those of the dinotherium turned downward. The first mastodon was discovered in 1613, and the history of its discovery is very curious, one of the rare examples of imposture in science.

THE SURGEON WHO DECEIVED THE WORLD OF SCIENCE

The skeleton was found by some workmen in a sand quarry at Dauphiny, in France. Such bones had never been found before, and a surgeon called Mazuyer saw a chance of making a great sensation. He purchased the bones and then pretended that he had found them in a tomb thirty feet long and fifteen feet broad, built of bricks, and bearing the inscription: “Teutobocchus Rex.” He also stated that he had found fifty medals of Marius in the tomb; and there seemed no reason to doubt his apparent good faith.

A barbarian king named Teutobocchus had actually invaded Gaul at the head of the Cimbri, had been defeated by the Roman Marius and led in triumph to Rome. According to tradition, Teutobocchus was so tall that he towered above all the trophies borne on the lances of the soldiers in the triumphal procession. Mazuyer’s story therefore fitted in perfectly with the legend and met with general acceptance. The giant formed out of the mastodon’s bones was twenty-five feet long, and the skeleton was exhibited throughout France and Germany. A great many people were rather skeptical, and a learned anatomist argued that the bones belonged to an elephant; but it was not until they were removed to the Museum of Natural History in Paris that the remains were actually identified as the skeleton of a creature like an elephant.

The dinotherium, found only in Europe and India, was larger than either the mastodon or the mammoth. It lived on herbage, and seems to have inhabited lakes.

In the Miocene, cats, and hogs and antelopes were also found. Wolves, panthers and sabre-tooth tigers were common.

The whole Tertiary period may be considered as the age of mammalian rise and domination, just as the Secondary period was the age of the rule of the great reptiles.

There is a very easy way to measure the height of a wall or tree - a method that anyone can use if he or she can do a problem in simple proportion. It is necessary that the sun should be shining at the time and that a shadow should be cast. That is all that is required to do this easy problem.

Suppose that you wish to measure a tree and that the sun is shining; then the shadow of the tree is cast on the ground. You must measure the distance from the tip of the shadow to the place right under the top of the tree. If the top point of the tree is right above the middle of the trunk, you must calculate half the diameter of the trunk in making your measurements.

Suppose that the distance from the point of the shadow to the trunk of the tree is 40 feet, and that the tree is 2 feet thick. Then the total distance is 41 feet (40 feet + half the diameter of the tree). Now take a stick of which you know the exact length. Suppose that it is 3 feet long.

Hold this upright with one end on the ground, and notice how far its shadow extends. You will find, perhaps, that it is 6 feet long. Multiply the length of the tree’s shadow (41 feet) by the length of the stick (3 feet); divide by the length of the stick’s shadow (6 feet). This gives 20.5; that is, the tree is 20.5 feet high.

You can also get the answer, though not quite so correctly, by seeing how many steps it takes to go from the edge of the shadow to the tree, being careful to make your steps as nearly equal as possible. Then, after measuring the length of one step, multiply its length by the number of steps. This is the distance from the shadow’s edge to the tree.

Be sure that you take the distance to a point right under the highest point. If it is a church spire, for example, make allowance for the distance between the wall up to which you measure and the center of the church tower topped by the spire.

The popularity of tropical fishes is due to various reasons. For one thing you will have a constant source of delight in their gorgeous coloring, which includes all the colors of the rainbow mixed together in profusion. Again, the remoteness of their origins adds a romantic interest to these beautiful little fish. Nor is it difficult to keep them, for they adapt themselves beautifully to home aquaria (an aquarium is a tank for marine animals; the plural is aquaria).

Tropical fishes may be divided into two groups, the live-bearers, which as the name implies bear their young alive, and the egg- layers, which reproduce through eggs. The first kind, the live-bearers, may be found in great numbers and many varieties in our own hemisphere. The other group of fish, the egg- layers, includes many more varieties and these are found in many parts of the world.

Tropical fish have been bred in such large numbers in this country that the number imported has been continually decreasing. Generally speaking, only the rarer species are now imported.
While almost all the small tropical fishes can be kept in home aquaria, there are some important rules that must be observed if the fish are to be kept in good condition.

For one thing, since fish have a soft mucous coat over their scales to protect them against bacteria, the water must be soft or this mucous coat will be destroyed with consequent harm to the fish. If the tap water is too hard, rain-water or water from a pond may be used. Furthermore the water should be kept sweet by healthy plants, which liberate oxygen and absorb gases that are harmful to the fishes.

An aquarium of five, ten or fifteen gallon capacity should have enough coarse sand to cover the bottom to a depth of at least an inch. It should be placed near a window, so that the plants will have sufficient light. Better results may be obtained by using artificial light from a reflector that fits over the top of the tank. A reflector of this sort concentrates the light on the fish and plants and brings out the best coloring of both. Besides, artificial light supplies a steady, dependable source of light. An ordinary 40-watt incandescent lamp will suffice for a io-gallon tank and a 50-watt lamp for a x-gallon tank. This artificial light should operate at least eight hours a day.

An important thing to remember is that most tropical fishes are jumpers and that many of them could easily leap from the average aquarium. To keep them from doing so, place a glass cover (window glass will do) over the entire aquarium. This glass will also keep out dust and other foreign matter. Be sure that the glass cover fits loosely, so that air can easily enter the aquarium.

The plants called Vallisneria, Sagittaria Sinensis, Sagittaria Subulata, hairgrass and spatterdock may be put in the aquarium. Remember that the front center of the aquarium should be left free of plants so that you may be able to observe the fish without difficulty.

When you have put the plants into the tank, a period of three days should elapse before you introduce the fish into the aquarium. This three-day interval will give the plants a chance to grow, and the chlorine that is often present in water for purifying purposes will have disappeared. As this chlorine is harmful to fishes, they should not be placed in fresh tap water.

When the water in the tank has been well conditioned, it is time to stock the aquarium with fish. In order to have enough oxygen for the fish, not more than two fish per gallon should be put into a tank. A good collection to start with would be a pair each of Angel Fish, Red Platies, Zebras, Black Mollies and Guppies.

The Angel Fish is noted for its haughty, dignified manners. The black vertical bars across the body contrast beautifully with the underlying silver-green. The Red Platy is a beautiful fish, vividly colored. The young in some strains are gold in color but become blood red as they reach maturity. The Zebra is the most peaceful of all aquarium fishes. Though it may chase other fishes, this is due only to its habit of swimming in schools. If there are no other Zebras in the tank, it will follow the fish of different species. The Black Molly is the only fish suitable for aquarium purposes that is entirely black.

Perhaps you may want to start a colony of Guppies. This exquisite little fish is very popular with fish fanciers. Its coloring has been described as “every color combined into one harmonious whole.”

Fish do not require much food and are not very particular eaters. Any prepared food, which you may buy at your dealer’s, will answer the purpose. It is well to use two or more foods alternately and occasionally to feed a little cooked spinach to the live-bearers. Live foods, such as mosquito larv and small worms, are good; these too may be bought at your dealer’s.

Do not overfeed the fishes. The food which is not eaten lies on the bottom of the tank and decomposes, causing the water to become cloudy. Harmful gases will be formed and the water will become deficient in oxygen. Overfeeding may be avoided by feeding no more than your fish will eat in ten or fifteen minutes. If food is left after that time, you have fed too much. It would be well not to feed the next day, and to feed less thereafter. Feeding once a day is usually enough.

Snails are useful scavengers in an aquarium; they consume surplus food and decaying vegetation. The Red Ramshorn Snail is a favorite with fish fanciers. Its red shell and body make an effective contrast with the green vegetation of the aquarium. Other species of snails that are used a scavengers are the Australian Red Snail, the Pond Snail and the Trumpet Snail.

Tropical fish require a water temperature of from 70 to 8o degrees. Since variation in temperature is one of the chief sources of trouble in an aquarium, everything possible should be done to avoid or minimize this variation. It would be well to have a heater that will supply heat along the bottom of the tank. If you cannot obtain a heater for the aquarium, cover the tanks on cold nights with a blanket or other cloth covering.

In spite of all your precautions, fish may sometimes become sick. The most common fish disease is White Spots or Icli. The word Ich is really the shortened form of Ichiyophthira, a tiny parasite that causes the disease. Ich is characterized by pinched fins and white spots on the body and fins. It is best to remove the affected fish from the aquarium and to place it in a separate container. Some common salt or patent fish remedy should then be added to the water. The tiny white spots should disappear in about a week.

Another common disease is Tailrot or Fungus. The fungus is a white scummy coating that may be found on many parts of the fish. The affected fish should be placed in clean water to which some salt has been added. Swabbing the affected parts with cotton soaked in mercurochrome or vinegar is effective.

When plants and food are placed in the aquarium, fish enemies may be introduced with them: such enemies as the diving beetle larva, the larv of various dragon flies and the water scorpion. It would be well to consider all unknown bugs and larv that are found in the aquarium as fish enemies.

There are kinds of wood that will not rot away, even though they are kept in water. The city of Venice is built on wooden piles buried in the shallow sea, and these have lasted for many centuries.

We shall guess what it is that makes wood rot when we learn what is done to protect wood that must be exposed to wet, for instance, the wood of which railway ties are made. These are often soaked with a chemical substance called creosote; and the particular property of creosote which makes it so valuable is that it is poisonous to microbes. So the answer to the question, in one word, is: Microbes. Wood will not rot if it is charged with something that kills microbes, or if, as in the case of Venice, it is protected from microbe enemies by being kept in salt water.

If you live in the southern United States, or have visited there, you have seen swamps with bald cypress trees growing in them. Cypress wood is remarkably hard and resists attacks of insects and microbes. The wood of the bald cypress, which grows in the swamps, is used for railway ties, fence posts, telegraph poles, piles for docks and other purposes for which a water-resistant wood is needed. From pine wood, which contains much creosote, we get wood tar that is applied to other woods to protect them.

Perhaps you have seen a hay fever sufferer, in the summer, sneezing violently and showing every sign of a serious cold. Hay fever is not a cold. It is not caused by a germ, but by the pollen of some plant, carried by the wind against the over-sensitive membranes of eye, nose and throat of the sufferer. It comes usually to people who, besides having sensitive mucous membranes, are tired or nervous; and every attack makes the sufferer more liable to another. Several plants such as goldenrod, dandelion and ragweed are chief offenders in causing hay fever.

Various cures have been attempted. The best cure is removal to the mountains or to the seashore, far away from the pollen. Various drugs, including cocaine, have proved helpful; but the effect of these is only temporary. Sometimes relief is provided by cauterizing the sensitive area of the mucous membrane. Some doctors inoculate the patient in order to build up resistance to the disease. Another method of warding off hay fever is to wear a mask provided with a filter in all areas where pollen is apt to be carried about by the wind.

Hay fever belongs to the family of allergic diseases, in which a person becomes ill because of special sensitivity to a certain food or drug or heat or cold or pollen or dust or even sunlight. Here is a remarkable illustration of the old adage that “one man’s meat is another man’s poison.” In no case is this truer than in the case of allergy.