Why are there Some Illnesses that We Cannot Get Twice?
When people have had smallpox or measles or scarlet fever once, they seldom suffer from them again, even if there should be an epidemic all around them. In some way or other the tissues of the body have become changed as the result of the first attack, and are better able to resist that particular kind of infection. We say then that the body is immune to that disease. Doctors make use of this immunizing power of the body, in a very wonderful way. They can give us a very mild form of a disease, or of a similar disease, by vaccination, and after that we are safe for years from any danger of getting the disease itself. For example, vaccination protects us from smallpox. People are often vaccinated against typhoid and other illnesses; they are inoculated with the actual disease germs, which have been killed or weakened to the point where they are no longer dangerous. To inoculate simply means to introduce into the body disease germs or their products.
Sometimes, instead of actual disease germs, a person is inoculated with blood serum of an animal that has been given a disease and has created in its blood “antibodies” that fought and killed the disease bacteria, or anti-toxins that rendered harmless the poisons produced by the bacteria.
Diphtheria anti-toxin belongs to this class of medicine that prevents disease or, if given in the early stages, cures it.
What is an Aptitude Test?
An aptitude test is a test of a person’s natural, untrained fitness to do some special thing, like learn foreign languages, play the violin or operate a complicated machine. Aptitude tests should not be confused with intelligence tests or with tests of skill given to people who have had training along the lines of the test. Aptitude tests aim to discover fitness along certain lines rather than to measure progress or intelligence.
Suppose you plan to study the violin but have no idea as to your fitness to do so. There is one aptitude test you can take which may warn you that you face a handicap. This is a test of musical pitch. It will discover your ability, or lack of ability, to tell when a given musical tone is higher or lower than some other given tone. If you do very badly with the test, it would hardly be wise for you to study the, violin since your ear might never permit you to play in tune, no matter how much you practiced. Or suppose you wanted to prepare yourself for an occupation requiring a reliable sense of color. An aptitude test might show that you were color-blind, unable to distinguish red from brown and yellow from orange. The test would save you from a costly mistake. If it showed you to have a fine color sense, you would then have a solid basis of encouragement.
Most aptitude tests are not so simple as the two mentioned because a person’s fitness to do some one thing often depends on more than one ability. This holds true even in the case of playing the violin. Violin playing does not depend on pitch sense alone. There is the question of the player’s sense of melody, for which a test exists. Also there is his sense of harmony to be considered for which various different tests have been developed. Rhythmic sense is also very important to musical performance and it can be tested, Tests are frequently given in groups, called batteries, and in that way tell about a person’s aptitudes somewhat more clearly than single tests could do.
There is a way to test the tests themselves. That is to compare the results of aptitude tests with the later success or failure of the people who took them. These comparisons, known as correlations, are not always favorable to the accuracy of the tests. People have done well at things for which the tests showed them unfitted and the other way around. The problem of discovering talents and predicting success is still far from solved.
Why is So Much of Nature Green?
The green coloring of plants is not just something pretty, to attract birds and insects (like some bright flowers). The green coloring is as necessary to the plant’s life as our red blood is to our own life. The green color is in certain cells, chlorophyl cells, in leaves and sometimes in stems of plants. The chlorophyl cells are little factories. They take in water and minerals from the ground, through the roots of the plant, and they take in carbon dioxide from the air. Now they have the materials for their work. They need energy. They get this from the sunlight, and now they set to work, breaking down the carbon dioxide. The oxygen they set free, and it escapes into the air. The carbon, water and minerals are mixed and changed into starch and sugar and other materials which the plant needs. These manufactured foods then go from the factory (the green cells) to the other parts of the plant and build it up. The green cells go on making more materials, as long as they have water and minerals, air, sunlight and enough heat.
With very few exceptions the whole plant kingdom depends for its life on the green cells or chlorophyl.
Why Does Glue Hold Wood Together?
Glue is one of the substances which, when heated, change their form gradually from the solid to the liquid, and not instantly, as ice melting to water. It is what men of science call viscous; that is to say, it is, for a time, only imperfectly fluid, and does not flow very quickly.
When we smear the surface of a piece of wood with glue in the semi-liquid state, and place another piece of wood on top, the glue is forced into the hollow air spaces of the wood, and as soon as the glue cools and hardens it is firmly anchored in each piece of wood, and the adhesion of the molecules of glue holds the whole structure together. The glue does not penetrate the cell walls of the wood, but merely enters the exposed openings of the cell cavities.
When a section is cut through two pieces of wood glued together, and examined under the microscope, it will be seen that the glue has filled the exposed pores of both pieces of timber. The more cavities in a piece of wood, and the larger the cavities are, the more easily it can be glued. Thus basswood can be glued more easily than oak. Woods having a fine texture and a large proportion of cells with very small cavities are the most difficult to glue.
The force that attracts like molecules is called cohesion. The force that attracts unlike molecules (molecules of different materials) is adhesion. Usually cohesion is a stronger force than adhesion. But in the case of glue when it is spread upon some other materials, such as wood, adhesion is stronger than cohesion. The glue adheres (sticks) to the wood.
Build a Birdhouse for Your Favorite Bird
The most important things for you to observe from the outset is the kinds of birds which frequent the yards where you are planning to locate your houses. Many people have failed to house their favorite birds because they did not know that the entrance openings vary with the size of the bird.
Use the following sizes for entrance openings which have been found highly satisfactory by several people who have studied the habits and haunts of birds. The diameters of the openings are as follows (in each case the measurement given represents the minimum diameter):
- For blue birds, tree-swallows and hairy and downy woodpeckers - 2 inches.
- For chickadees and Carolina wrens - 1 1/8 inches.
- For house wrens - 7/8 inch.
- For house-finches, crested fly-catchers and red-headed woodpeckers - 2 inches.
- For tufted titmouses, white-breasted nut- hatches and downy woodpeckers - 1 1/4 inches.
In the case of other species, experience alone will enable you to determine which is the diameter appropriate for each species.
Robins, barn-swallows and phoebes need one or more sides open.
Wood is found to be the most satisfactory material from which to make a bird-house. To avoid having it warp out of shape choose stock which is 3/8 inch, or even 1/2 inch in thickness. Then, too, you should construct your bird-house so that water will not run inside when it rains.
If possible, first decide upon the kind of bird for which you wish to make a house. The few suggestions which are given for birdhouse designs may help you to determine the type of construction you will use. It is best to allow for cleaning through the side, the top or the bottom. Keep in mind that the size of your bird-house will depend upon whether you are making it for one family or for several families. In the latter case, partitions, and sometimes an additional floor, will be needed to separate them.
After you have decided these matters it is more satisfactory to make a working sketch on which you should place the exact dimensions of each piece of wood to be used. Next, select your wood, which should be over 1/4 inch in thickness, for the reason mentioned, and shape each part to the desired size. Before making the entrance hole as shown, note, from the suggestions already given, the exact size of the opening which has been found most satisfactory for your favorite bird. For example, the opening for a house wren should be inch in diameter. This opening should be placed quite near the roof so that the bird may have ample room to make its nest. To give proper ventilation you will need to make even smaller holes above the entrance and near the roof. Do not forget these ventilation holes. If they are not provided your feathered guests may not desire to remain in their othei-wise perfectly comfortable quarters.
Regardless of whether you use brads, nails or screws, it is more satisfactory to set or countersink these deep enough that the heads may be covered with putty. Now you are ready to preserve your bird-house and improve its appearance with paint. Having decided about where you will locate it, you will be able to decide if green, brown or another color will harmonize best with the surroundings. When the paint is dry, securely fasten the bird-house to a tree or building, being careful not to have the entrance face the prevailing winds. The house should be placed out of the cat’s reach.
The designs that we show in the above illustration are very simple. Any bright boy may make these bird-houses out of odds and ends of wood, using a few simple tools like those which we describe in our article on carpentry. Boys who have had experience in carpentry may be able to work out more elaborate designs of their own.
How to Make Jump-Peg Game
This fascinating game is good fun at all times, but it is especially suited for dull afternoons. It is easily made and is played by only one person at a time. The object of the game is to get all of the pegs off the board except one, which should be left in the centre hole. The pegs are removed by jumping with one over another, removing each peg jumped, as is done in playing checkers.
How to Make
A peg game may be made from heavy cardboard, the following directions are given for constructing it from poplar or other soft wood because of the lasting qualities of wood. If you prefer the more permanent game secure one piece of poplar or other soft wood either or 34 of an inch by 93/2 inches by 934 inches, and one piece dowel rod 34 of an inch by 48 inches.
After squaring and planing the board as desired, mark off spacing for holes, making them x inch apart on centres, as shown in the accompanying drawing. Then bore holes to depth of 3/2 inch, using 34 inch bit. Next cut 32 pins from the rod, making each pin x3/ inch long and slightly pointed on both ends. Now sand the board with No. 34 or any fine sandpaper. If desired, the board may be made more attractive by marking off x inch squares, as shown in the accompanying drawing. These squares can then be painted with two bright colors, as shown. Either water-colors or paints may be used. Shellac with thin shellac (two coats must be applied) or wax with good furniture or floor wax.
How to Play
The jump-peg game is played by filling with pegs all of the holes except the centre one. The player begins to play by jumping any peg over another into the hole and removing from the board the peg thus jumped. All jumps should be made in straight lines, either in a horizontal or a vertical direction.
As explained at the outset, the purpose of this game is to remove all pegs except one from the board. This last peg should be left in the centre hole. This may look very easy, but it is much more difficult than it seems at first sight. The peg sometimes will not jump to the centre hole. If you wish, you may compare your skill in playing jump-peg with that of your friends by seeing who can remove all the pegs but one in the least possible number of moves.
Do We See Something as Soon as We Look at it?
Certainly not. Seeing, like every other kind of sensation, takes time. From the instant that the light strikes the curtain at the back of our eyes to the instant that we see is quite a long time compared with some things - for instance, compared with the time it takes light to travel a mile.
We are likely to think a second the shortest part of time that is worth mentioning, but that is absurd. A second is a period of time so long that light, radiant heat and electricity could travel almost as far as the moon in such a time.
The wonderful thing about seeing is that it takes such a small fraction of a second for all the things to happen which are necessary before we can see. Complicated chemical changes have to take place in the living cells of the curtain at the back of the eye. These changes have to produce special nerve- currents, which in some amazing way correspond exactly to them; and these run along the eye-nerves, first to a group of cells in the lower part of the brain, and then from them, along another set of nerves, to the real eyes - a group of nerve-cells at the very back of the brain, which have developed, and live, in utter darkness. Something happens in them, and then we say we see. The marvel is that only a few hundredths of a second are needed for so much to happen.
Testing Car Tires
Tire manufacturers use three methods of testing their products. They are: first, by the use of their own fleets of cars which they call “test fleets”; second, by means of information supplied by taxicab companies and other companies making daily use of large numbers of automobiles; and third, by specially devised machines.
A manufacturer’s test fleet is very advantageous because it may be sent to different parts of the country to test out the actual wearing qualities of tires under varying conditions, and to determine the sizes and types of tires that are best adapted to different localities.
The manufacturers of tires often make contracts with taxicab companies to keep a careful record of the performance of the tires used, and in this way important information is secured as to the particular sizes and types of tires which are successful. Information secured by this method, tests out tires for heavy traffic.
The third way of testing the tires, by special testing machines, gives specific information that can not be secured by the first two methods. There are many problems relating to speed, liability to blowouts and tensile strength of the fabric and the like, which must be solved in this way.
Testing tires has become more important than ever since the Japanese conquests in the Malayan Peninsula and the East Indies cut off our supplies of rubber from the Far East. Various kinds of synthetic rubber and combinations of synthetic and natural rubber have been tried. We must test them to see exactly how they will serve.
What Happens When Your Foot Goes to Sleep?
Our muscles, as we call the bands of flesh which move the different parts of our bodies, can move only when directed to do so by our motor nerves, which may be roughly described as telegraph wires between our nerve centres and our muscles. Before the mysterious order is sent from the nerve centre along the motor nerve to the muscle directing it to move, the nerve centre has to receive a message from another and quite different nerve called the sensory nerve. Scientific men call this reflex action.
From the brain, or from the spinal cord (the big nerve which runs up the backbone) motor and sensory nerves are connected to every part of the body. If a motor nerve is cut, we lose all control of the part of the body it serves, but sensation remains. If a sensory nerve is cut, we lose sensation in the part which it serves, but retain the power of movement. If both are cut, we lose both sensation and the power to move.
Fortunately, serious damage to a nerve does not often occur, but we sometimes experience what undue pressure on a nerve can do. If we sit on a chair so that a sharp edge presses the nerves of our leg, we may easily find that our foot ‘goes to sleep.” What has happened is that pressure has affected the nerves serving the foot, and by compressing their fibres has made them incapable of transmitting impulses. On attempting to rise we can not feel our foot because the sensory nerve has been pressed, and we can not direct the foot to act because the motor nerve has been pressed. The foot is numb. Gradually the nerves recover as the pressure is removed, and we get the tingling we call “pins and needles.”
The Sun and the Moon
Will the World Become Like the Moon?
Our earth will probably become like the moon. There will be certain differences, because the earth is much larger than the moon. The moon has been too small to hold to itself the gases outside it. It has no air, or atmosphere. The earth is able to keep its atmosphere because it is bigger, and so the power of its attraction is much greater. Besides, the earth makes some heat of itself, though not very much, probably.
Another difference is that, in consequence of the rapid cooling of the moon, the changes on its surface have been more violent than those on the earth. The biggest volcano on the earth is as nothing compared with the moon’s. But all these points of difference do not affect the fact that our earth is likely some day, though after a long, long time, to become cold and lifeless.
Does the Sun Move or Does it Stand Still?
The sun has two movements. Like the earth, the sun spins, or rotates, upon itself, and in the same direction as the earth. Thus. we can notice a sun-spot appear at one side of the sun, travel across its face, disappear for scveral days, and then reappear where we saw it first.
Besides this movement of rotation, the sun has a movement of translation, as it is called - that is, an actual bodily movement from place to place. We do not doubt that all other stars are in motion too. It used to be impossible to see any sort or arrangement or order in the movements of the sun and the other stars; but a Dutch astronomer, Professor Kapteyn, and others following him seem to have shown that the stars consist of two great hosts which are streaming through or past each other in opposite directions and at different speeds; and they think that our sun belongs to one of these groups of stars. There may be more than two groups, or galaxies. Of course, where the sun moves he carries all his family with him - planets, moons, comets, and so on, together with everything that is born and borne upon them; but neither astronomer nor other scientist can say where the sun is carrying us nor what the result will some day be.
