Part 9
We cannot see in the dark because there is no light to see by. To understand this we must first understand that when we see a thing, as we generally say, we do not actually see the thing itself, but only the light coming from it. But we have become so used to saying that we see the thing itself that for all practical purposes we can accept that as true, although it is not scientifically exact. Scientifically speaking, we see that part of the sunlight or other light which is shining upon it, which the object is able to reflect.
If there were no air about us we could not hear any sounds, no matter how much disturbance people or things created, because it requires air to cause the sound waves which produce sound, and air also to carry the sound waves to our ears. In the same way, if there is no light to produce light rays from any given object to our eyes, we can see nothing. It requires light waves to produce the reflections of objects to our eyes. Without light our eyes and their delicate organs are useless. You cannot see yourself in a mirror when the quicksilver which was once on the back of the glass has been removed, because there is then nothing to reflect the light. We can only see things when there is light enough about to reflect things to our eyes. When it is dark there is no light, and that is the reason we cannot see anything in the dark.
Why Can Cats and Some Other Animals See in the Dark?
They cannot see in the real dark any more than human beings. These animals can find their way in the dark and can see more than a human being, because of one distinct difference in their eyes, which may for them be considered an advantage. The pupils of their eyes can be made much larger, and they can, therefore, let more light into their eyes than people. The result is that when it is so dark that you cannot see a thing and you decide it is really dark, the cat can still see, because there is always a little more light left and she can open the pupils of her eyes and make them larger, thus letting in more light, and the little bit of light there is still left gets into her eyes and she is able to see. But in a really dark room a cat could see no more than you can. You see, our eyes open and shut more or less just like those of the cat, according to the intensity of the light. When you go out of the dark and shaded room into the bright sunlight and look at the sun, you naturally squint your eyes without deliberately intending to do so. This is nature’s way of preventing too much light getting into your eyes at one time. Gradually the pupils of your eyes contract and get smaller, until you can see, without squinting, anything in the sunlight. If, then, you were to go right back into a dark or shaded room, you would have to wait a moment or two before you could see things distinctly in the room--until the pupils of your eyes had dilated (become larger), so as to let in enough light to enable you to see normally. The eye automatically enlarges and contracts the pupil of the eye, to enable us to see distinctly in either light or less light places.
Why Is It Difficult to Walk Straight with My Eyes Closed?
The reason we cannot do this always is because when we walk naturally the steps taken by our right and left feet are not of equal length. This difference in the length of the steps is due to the fact that our legs are never exactly the same length. We think of them generally as of the same length, but they are not, and this will be proven if you measure them accurately. Now, then, the longer of the legs will always take a longer step than the shorter one, and so, if our eyes are shut, we walk in circles, unless we have something to guide us. When we walk with our eyes open, we are able to overcome the tendency to walk in circles, because our eyes help the brain to direct the legs on a straight course. Another reason which affects the matter is that our eyes are very necessary in keeping our bodies balanced on our feet, and it is very difficult to learn to keep the body balanced with the eyes closed. Now, when your eyes are closed and you attempt to walk in a straight line your body balances from one side to the other, and this fact, coupled with the first reason given, makes your course irregular. But, say you, the man on the tight-rope has his eyes bandaged and he walks a very straight line. Yes; but remember that he has a straight tight-rope to guide him, and all he needs is to maintain his balance. One can learn to walk in a straight line with the eyes closed, but it takes a good deal of practice, as you will learn if you try.
Why Can’t We Sleep with Our Eyes Open?
We cannot sleep with our eyes open, because to be asleep involves losing control of most of the functions of the body. When we sleep the brain sleeps also. Perhaps it would be stated more clearly to say that we cannot sleep while the part of the brain which controls our activities is awake. There is a part of the brain which has the power to open our eyes, i. e., lift the eyelids, and when that portion of the brain ceases to exercise its power to keep the eyes open, they go shut. Even when we are awake that part of our brain cannot keep our eyes from winking, because there is another part of the brain which sees to it that our eyes wink every so often. This is done for the purpose of washing the eye-ball, and is the answer to another of your questions which is given in another place in this book. When the engineer at the electric light plant shuts off the power all the lights go out, and when you go to sleep you automatically shut off the power that opens your eyes, and the eyes are shut. The brain is asleep also, and if it is not completely asleep, you are restless.
Why Do Our Eyes Sparkle When We Are Merry?
If you should watch very closely the eyes of a merry person when you see them sparkle you would probably notice that the eyelids move up and down more often under such conditions than ordinarily, and if you know what moving the eyelids up and down in front of the pupil of the eye does, you will have your answer.
Every time the eyelid comes down it releases a little tear, which spreads over the eyeball and washes it clean and bright. It does this every time the eyelid comes down. Now, there is something about being merry which has the effect of making the eyelids dance up and down, and thus, every time the lid comes down, the ball of the eye is washed clean and bright, and gives it the appearance of sparkling, as we say.
Why Do We Laugh When Glad?
We laugh when glad because the things which make us laugh combine together to rouse those parts of the body which are involved in a good laugh to act in a certain harmony, and when this combination is arranged in a certain way it produces a laugh. Certain things in the world, whether they are funny, ludicrous, or other things that produce the laughing effect, cause the brain to work certain muscles and nerves in a combination that produces a laugh. The impression which reaches the brain causes these muscles and nerves to act involuntarily and the laugh comes. It works just like the keys of the piano. Some combinations of notes produce sad sounds and other combinations produce glad sounds, but the combination when once touched will always produce the same sound. It is the impressions made on the brain which start the proper combination, and it does this instantly. Just as a pin prick in the arm will at once send a “hurt” message to the brain and cause the brain to jerk the arm away, so a laugh-producing combination of sounds, or things we see, or feel, sends an impression to the brain which at once sends out the “laugh” order. Some things make some people laugh while they do not affect others at all. That is because our brains are not always the same in regard to recording impressions. Some things impress some brains one way and others entirely in a different way or not at all. You do not laugh so heartily the second time you hear a funny story, because the impression the brain receives when the story is told the second time is not so vivid.
Why Do I Laugh When Tickled?
Practically the same things happen when we are tickled, and explains why you laugh when tickled. When some one tickles the bottom of your feet or your ribs or another part of your body it produces, in most cases, the same effect on the brain as the laugh-producing sound or sight, and arouses the same combination of muscles and nerves to activity. It is just like pushing the button of an electric bell. When you push the button the contact produces the spark which sets the machinery of the bell in motion and the bell rings and will continue to ring as long as you keep your finger on the button, or until the spark-producing power of the battery is gone. Then, as in the case of the bell, you cease to laugh, because the spark that produced the laugh combination is gone. That is why some things tickle some people very much and do not affect others. Some are not so sensitive to the laugh-producing combination as others. After the thing that tickles you has been going on for some time you are not tickled into laughter any more, because the impression on the brain ceases to be as strong.
Why Don’t I Laugh When I Tickle Myself?
Your mind tells you there is no need to laugh when you tickle yourself. Your mind will not respond to the tickling sensation when it is aware that the cause of the tickle is yourself. The reflex action of the mind which causes laughter and squirming when some one else tickles you only acts when it is not conscious of the cause.
The whole purpose of the sensitive organization of our skins is to give us information and cause action which will enable us to protect ourselves when any outside influence touches us. An injurious touch causes shock and pain, and the harmless tickle arouses the laughing and squirming sensation.
What Happens When We Laugh?
Laughter is what we call a reflex action. When something occurs to make us laugh, whether it is something we see, or feel, or hear, it is because certain sensory nerves receive an impression in one of three ways, carry it to the nerve centre and the nerve centre then sends the same impression along certain efferent nerves, which connect with certain muscles or glands, and excite them to activity. The action is practically the same as when you hold a light before a mirror. The rays from the light strike the surface of the mirror and are reflected back from the surface, lighting perhaps corners of the room, which the direct rays from the light could not reach, all depending upon the angle of reflection. Light will always reflect from a mirror that is exposed to it.
Now, then, when you see, hear or feel anything that makes you laugh, the sensory nerves have only to receive the impression to bring on the explosion of laughter. Something touched the laugh nerves or the laugh trigger that caused it to go off. You can prove that it is a matter of impression entirely by noting that some people can listen to a perfectly funny story, even when told by a clever performer, and never crack a smile, while others burst into uncontrollable laughter, and he who does not even smile may be listening even more intently than the other--he may even be looking for a laugh. It all depends upon the impression that is made upon the nerves. The muscles have the power to express the state of gladness which is indicated by laughter when certain impressions pass along the nerves which operate them, just as they can be made to do other things when the proper cause for action is shown them.
Why Do We Cry When Hurt?
We cry when we are hurt for the same reason that we laugh when we are glad. The muscles and nerves, under the direction of the brain, produce the cry just as the muscles and nerves produce laughter, although they are probably, but not necessarily, a different set of muscles and nerves.
When we are hurt in any part of our body or feelings the impression does not affect us until it reaches the brain. Then instantly, of course, the body and brain go to work to destroy the pain. The first thing, of course, is to give a warning to other parts of the body that there is a hurt, and our crying is a warning to other people that we are hurt. That is probably the only good that crying does. It does not remove the hurt--it only tells others of our troubles. We cry with the lower part of the brain--the only portion of the brain which is active in a little baby. This is why even a tiny baby can cry. Crying is the only thing a baby can do to give warning of its distress or discomfort. Later in life the upper part of the brain develops. This is the master of the lower part. Therefore, we do not always cry when hurt as we grow older, because the master brain sometimes tells the lower brain that to cry will not help matters in the least, even though we are inclined to cry. Sometimes the hurt or shock to older people is so great or sudden that we cry out before the controlling part of the brain has had time to get in its work of preventing the outcry, but we are able to stop crying when the master brain again secures control.
Where Do Tears Come From?
Tears are not made only when we cry. They seem to come only when you cry, because it is then that they spill over. A little part of you is making tears all the time, and your eyes are constantly washing themselves in them. You have often noticed how you wink every few seconds? You have often tried to keep from winking--to see how long you could keep from winking. Boys and girls often do that, and when you keep from winking what seems a long time, you notice how your eyes ache and feel very dry just before you have to let them wink, in spite of how hard you try not to, and just when you think you are not going to. I will tell you just what winking does for the eyes. All of the time your eyes are open the front, or the part you see things with, is exposed to the dust and dirt that fills the air at all times, although we cannot always see the dust. The wind, too, is constantly making them dry. But have you ever noticed that although you never wash the inside of the front of the eye, or pupil, it is always clean? Well, it is because your eye washes itself every time you wink. I will tell you how this is done. Up above each eye, inside, of course, there is a little gland called the tear-gland. This gland is busy all the time you are awake making tears. As soon as the front of your eye becomes dry, or if a particle of dust or anything else strikes it, the nerves you have there tell the brain, and almost at once the eyelid comes down with a tear inside of it, and so washes the front of your eye clean again. It does its work perfectly and as often as necessary. There is always a tear ready to be used in this way.
Where Do the Tears Go?
Let me show you. Look right down here at the inner corner of my eyelid, where you will see a little hole. That is where the tears get out of the eye, when they have washed your eyeball clean. Where do they go then? Did you ever notice how soon after you cry you have to blow your nose? The reason for that is that when the tears go through the little hole they run down into the nose. This making of tears and winking goes on all the time while you are awake, and after they wash your eye off they go on out through this little hole. But when you cry you make more tears come than you need, so many, in fact, that they cannot all get away through this little hole, and as there is no place else for them to go, and as there is no place to keep them inside the eye, they simply spill themselves right over the edge of your lower eyelid and run down your cheek.
Story in a Barrel of Cement
What Is Cement?
The dictionary tells us that cement is “any adhesive substance which makes two bodies cohere.” Thus any material performing this function may be called cement, such, for example, as the cement used in mending broken china. Glue also is a form of cement. This story has to do with Portland cement, which is a structural or building material used in countless ways.
Why Is Cement Called Portland Cement?
After being wet with water it hardens into stone, and it was given the name “Portland” because, when first manufactured in England, and mixed with sand and stone, it resembled a celebrated building stone called Portland, which was obtained from the Isle of Portland. Compared with other American industries, the manufacture of Portland cement is of recent origin. Formerly all Portland cement was brought from foreign countries. After successful manufacture became established in this country, however, the industry advanced with great rapidity. A few years ago the entire United States did not use as much cement as is now used in any one of our large cities. At the time these facts were written (1914) the manufacturers were making more than 90 millions of barrels a year.
What Is Cement Made Of?
Portland cement is composed chiefly of lime, alumina and silica. It is manufactured from rocks, marl, clay and shale containing these ingredients. If any one of them is lacking in the raw material as it is taken from the earth, it is supplied during process of manufacture. The greatest cement district in America is in Pennsylvania, and is known as the “Lehigh District.” A rock containing proper constituents for making Portland cement was found there in vast quantities, and for a number of years the Lehigh District was the center of the industry. In time it was found that certain clays, marls and shale could also be manufactured into Portland cement, and thus mills have been erected in all sections of the United States. One of the largest companies in the United States found that cement could be manufactured from a combination of blast-furnace slag and limestone, and this is now made by the company in large quantities, the product being a true Portland cement.
What Is Concrete?
Portland cement is the strongest and most lasting of all modern mortars or binding materials. When mixed with sand and stone the resulting mixture is called concrete. Being a plastic material when first mixed, it cannot be used as we use brick or stone, but must be poured into molds or forms, which hold it in place until it hardens into rock. It may be cast in any form or shape, and thus it is useful for a vast number of purposes. It will harden under water, and time and exposure to the elements merely increase its strength. The most common form in which it is used, one familiar to everybody, is in the construction of sidewalks. It is used in all great engineering projects, such as the building of dams, bridges, retaining walls, sewers, subways and tunnels. Being fireproof, large quantities of it are used in buildings and likewise on our farms, where it is extremely valuable as an enduring and sanitary material.
What Is Cement Used For?
It has been said that concrete is a plastic material, meaning that it is soft and pliable in the sense that clay or putty are plastic. For this reason it is cast in forms or molds. Sometimes it is used in the form of plain concrete, and on other occasions it is reinforced, meaning that iron rods, steel bars or woven wire mesh are imbedded in the concrete. When we speak of a “reinforced” concrete building, imagine a huge wire bird cage encrusted within and without with concrete. Place a block, beam or column of concrete upon the ground and it will bear a tremendous load, meaning that it has great strength in compression. On the other hand, if we were to place a long beam upon supports at either end, leaving the greater length of it suspended and without support, it would carry but a small load compared with concrete in compression. Therefore, in making concrete beams or girders in a building, strong steel bars are embedded in the concrete to take up what are termed the tensile strains.