Part 22

The diagram, Fig. 1, shows more clearly what is meant. From an inspection of Figure 1 it is clear that, when the openings in the shield bulkhead are closed, the tunnel is protected from an inrush of either water or earth; the openings in the bulkhead may be so regulated that control is maintained over the material passed through. After a ring of iron lining has been erected within the tail of the shield, the shield doors are opened and men go through them and dig out enough earth for the shield to go ahead. The rams are then thrust out thus pushing the shield ahead. Another ring of iron is built up within the tail for which purpose an hydraulic swinging arm, called the “erector,” is mounted on the shield face. This erector picks up the plates and puts them into position, one by one, while the men bolt them together. Excavation is then carried on again and the whole round of work repeated, gaining every time the jacks are rammed or thrust out a length equal to the length of one ring of iron lining. In carrying out this work in ground charged with water the shield is assisted by introducing compressed air as described before. To use the compressed air thick bulkhead walls of masonry are built across the tunnel behind the shield and into the space between the shield and the bulkhead wall air is pumped, compressed to the same pressure as that of the water in the ground, or in other words the pressure of the air in pounds per square inch is about half the number of feet the tunnel is below the water surface. This dries the ground and simplifies enormously the difficulty of working in it. The diagram, (Fig. 1) shows a bulkhead wall across the tunnel. In order to pass from the ordinary air outside the bulkhead into the compressed air inside it, all the men and the materials have to pass through the “air locks” which are built into the wall. They are called air locks because they are like the locks on a canal which raise the water from a lower to a higher level or lower it from a higher to a lower level as the case may be. The difference is that an air lock enables one to pass from air at a low pressure to one of a higher, or vice versa. An air lock is made like a large boiler with a door at each end. If we wish to enter the compressed air we enter the lock from the outside. The door at the end has been tightly closed to prevent the compressed air from rushing out. We close the door behind us and are now tightly shut in the boiler-like lock. We now open a valve and compressed air begins to flow quickly into the air lock and the air gets hotter and hotter, due to the compression of the air. Very likely an intense pain begins to make itself felt in the ears but by swallowing hard and blowing the nose it may be relieved. It is caused by the air pressure being greater on the outside of the ear drum than on the inside. If the delicate ear passages are choked, because of a cold or some such reason, it is unsafe to go further or the ear drum may burst. When the pressure in the air lock has reached that in the working chamber, the door leading to the shield may be opened and we can pass to the working space and note the work going on. There is no especial bodily sensation to be felt except a slight exhilaration and it is curious to find that one cannot whistle. On leaving the compressed air we enter the air lock by the door we left; a valve is turned and the air begins to escape and the pressure in the air lock begins to go down. As it does so the air becomes colder and colder and the whole lock is filled with a wet fog due to the chilling by expansion of the air. The air has to be allowed to escape very slowly, as bubbles of air and gas otherwise form in the blood vessels and tissues of the body giving rise to the very painful complaint known to tunnel builders as “the bends,” and in very serious cases to paralysis and even death. The higher the air pressure the more slowly must one come out into the ordinary air.

When the shield has been pushed across the entire length of the water way which has to be tunnelled, and the whole of the iron tube or shell is in place, a thick lining of concrete is placed inside the iron shell to protect it and make the tunnel stronger. As an added safeguard wherever the tunnel is in rock, gravel, strong clay or other ground which is not so soft that it does not close tightly in on the outside of the tube, liquid cement is forced by compressed air through holes made in the iron plates for this purpose. This liquid cement enters every pore or crevice in the surrounding ground and when it has set hard it still further protects the iron with a coating of cement. Pieces have been cut out of the iron lining of a tunnel built under the river Thames at London, England, in 1869, which showed that the iron at all places was as good as the day it was first put in forty years before, and iron put in the lining of the Hudson River Tunnel about 1878 when removed after thirty years was in perfect condition.

~DANGERS OF TUNNEL BUILDING~

This account of tunnelling by shield and compressed air is very short and gives no more than a bare statement of the principles and chief methods of such work. Nothing has been said of the engineering difficulties involved in the design of such work, nor of the delicate surveying work necessary if one should hope to start two shields a mile or two apart and have them meet as shown in Fig. 13 like two great glass tumblers placed rim to rim after having travelled through thousands of feet of every kind of ground. Nothing has been said of the men who work on this most arduous form of subterranean navigation, how they cheerfully face the dark and the water ever threatening above them and the unseen but not less deadly ally, and yet foe, the compressed air, with its dreaded result, the bends, or the men on the surface who keep the air compressors running without pause or stop day in and day out until the work is done so that their comrades below may work in safety. Nothing has been said of the curious accidents that are liable to occur as when the air pressure in the tunnel gets too high, overbalances the water pressure and blows a hole through the river-bed and forms a geyser in the river above. It gives no account of the special difficulties which arise when special conditions are found; for example, when the lower part of the tunnel is in rock and the upper part is in soft material. In fact it is nothing more than a bare outline but it hoped that some, who may not be clear in their minds as to how tunnels are built, may learn some of the first principles of this most romantic kind of work from this bald narrative.

Why Do My Teeth Chatter?

Your teeth chatter because when you are cold in a way that makes your teeth chatter the little muscles which close the jaw act in a series of quick little contractions which pull the jaw up, and then let it fall by its own weight. This is repeated many times and, as the action is quick, the chattering occurs. It is a peculiar thing that this occurs in spite of the will or brain, when, as a matter of fact, these muscles which operate the jaws are especially under the control of the brain. The chattering is really a spasm caused by the cold, and all spasms act independent of the will. Cold seems to act on the jaw muscles a good deal like some poisons which cause spasms.

Where Did All the Water in the Oceans Come From?

No, it did not come from the rivers which empty themselves into the oceans, because the oceans were there before the rivers existed. Part of it comes from the rivers now, but only a little in comparison to all the water there is in the ocean. I will try to tell you simply how all the water got into the ocean.

There was a time when there was no water on the earth at all. That was when the earth was red hot, just as it is to-day on the inside, and at that time all the water we have to-day was up in the air in the form of gases. Strange as it may seem to you, if you take two gases, one called hydrogen and the other oxygen, and mix them the right way, they will turn into water, and if you had the right kind of chemical apparatus you could take water and turn it into these gases again. When, then, the earth was still all red hot, all of our water was up in the air in the form of these two gases. Then, later on, when the amount of heat on the earth was just right to make these gases mix together, the water came down out of the air in great quantities, and there was so much of it that it completely covered the whole earth and no land was visible. Later on, for various reasons, mountains were thrown up on the earth’s surface by great earthquakes, and every time a mountain or a high place was formed there had to be a hole or low place some place else, and the water ran into these low places and stayed there, and that uncovered more of the land, because there wasn’t enough water to fill all the holes and cover the land too, and that is what makes our continents and islands and all of the land we see. There is now about three times as much earth covered with water as there is land. Of course, the sun is always picking up water through what is called evaporation, which means that it is taken into the air in the form of gases. Later it comes down again in the form of rain and falls into the oceans or on the land, where it sinks in, finally finding a stream or river, and sooner or later gets back into the ocean again.

Why Don’t the Water in the Ocean Sink In?

This is due to the fact that there is a kind of substance at the bottom of the ocean which the water cannot penetrate, in spite of the tremendous pressure which the great body of deep water exerts. In all places where the bottom of the ocean has a covering which water can sink into it does so, but there are such a few places where this is possible, by comparison, that the amount that gets out that way is not noticeable. This water, if it can keep on going, will eventually reach the inside of the earth, where it is red hot, and is turned into steam.

Where Does the Water in the Ocean Go at Low Tide?

To get to the answer of this you must know something about the tides. The tide is caused by the pull of the moon on the waters in the ocean. The moon revolves about the earth once each day and has the ability to draw up the waters in the ocean toward it, as we have seen in our study of the tides.

Now, when it is high tide in one place it is low tide in another. The moon does not make more water, but only pulls it toward it from side to side. When it is low tide where we are the water has simply moved as a body toward the place where it is high tide.

The tides act a good deal like a see-saw, except that they move from side to side instead of up and down. When one end of the see-saw goes up the other end goes down, and when the “down” end comes up the other end goes down. So the answer to your question really is that at low tide the water which made it high tide a few hours before has gone to some place where it is at that moment high tide.

Why Does the Ocean Look Blue at Times and at Other Times Green?

Sometimes when we look at the ocean from the pavilion or while on the sand of our favorite bathing beach the water in the ocean looks very beautifully blue, and on other days will look dark green from the same point. Why is it? If you will stop to think that at night when there is no moon or other light the water in the ocean looks black, I think you will soon be on the right track to answer the question yourself.

When the sky is blue--the kind of blue we like to see in the sky when we are at the beach--the water in the ocean is blue, because the sea reflects the color of the sky, and when the sky is overcast and gray the color reflected by the sea will be gray also.

But, say you, sometimes the water in the ocean is dark green, and yet the sky is never green. Quite true, and I will try to tell you what produces the green color. This happens sometimes where the water is shallow, either near the shore or out further where there is a sandbar or other shallow place. Sometimes at such points the sunlight strikes the water at such an angle that the rays go clear to the bottom and are reflected from that point--the bottom--to our eyes. In such a case the light will be changed through a combination of the color of the bottom at that point and the color of the sky itself at the time to make the color green as it is reflected to our eyes from the bottom.

Why Does Water Run?

Water runs because it has not enough of anything in it to make it stick together.

In school language we call this sticking-together-thing “cohesion.” The principle of cohesion makes all the difference there is, so to speak, between solids, liquids and gases. A brick, a stone, a stick of wood, or a piece of iron and all other solid substances have a certain amount of this property of cohesion, and the particles stick together, enabling us to build buildings and other things which become permanent structures. These solid substances are either naturally cohesive or else man, as in the case of the brick, has brought together certain things with little or no cohesion and made them stick together permanently. In the case of the brick, he takes a quantity of clay, which is cohesive only to a certain degree, bakes it in an oven and it becomes hard enough--more cohesive--so that he can pile one on top of the other and make a building. Then he puts sand, mixed with other things--lime and water--between the bricks to hold the bricks together, and makes a structure that will last. Two bricks have no natural cohesion for each other and, therefore, they can only be held together by something that has cohesion within itself and also for the bricks. The lime, sand and water make mortar which is cohesive when properly mixed, while in themselves neither lime nor sand have much cohesive property, and water has none at all.

Liquids have little or no cohesion. Water has none, or very little. Syrup has a good deal more, but will run over the edge of a piece of bread and butter if you are not careful.

Gases have no cohesive properties at all and, therefore, fly all over the place, through any opening they can find, either at the top of the room or under the crack of the door. They are always trying to get to some place else and will keep moving as long as not confined. Gases can move in any direction.

Liquids, however, while they are inclined to be constantly on the move, can only go in one direction--down hill, and they go down fast or slow if there is a chance, in proportion to the amount of stick-together properties they have. Liquids can never go up of their own accord, excepting in the process of evaporation, and then only when changed into gases. A lake of water will dry up completely by evaporation unless fed by streams of water constantly flowing in, because evaporation is constantly taking place wherever water is exposed to the air.

What Makes the Water Boil?

What we call boiling in the water we see when water is put over a hot fire long enough to make it boil, is the changing of the water from what we generally regard it--a liquid--into gases. Water consists of two gases--hydrogen and oxygen--in fact, two parts of hydrogen gas and one part of oxygen gas when mixed will always make pure water. Now, then, if liquid water is heated to a certain point or temperature it turns into the two gases, oxygen and hydrogen, and comes to the top of the water, which still remains in liquid form, in the form of a bubble and explodes into the air--not a very loud explosion, but still an explosion. The process of turning liquid water into gases is a gradual one, and that is why the water does not all turn into one large bubble at once and explode away. If you keep the fire going long enough, all the water in the vessel will explode away into the air, a few bubbles at a time. If you hold a cold plate over the vessel as the bubble explodes you can catch some of these gases in the form of bubbles on the under side of the plate, which are again liquid water. When the water becomes hot enough it turns into bubbles and as bubbles rise that is what makes the boiling you see. When the same gases then come together again in a certain proportion under proper temperature they turn into liquid water.

At What Point of Heat Does Water Boil?

The boiling point of water is the temperature at which it begins to pass into the form of gases. This varies in different altitudes. At the sea level the boiling point is at 212° Fahrenheit. On the top of mountains, for instance, water would boil at a much lower temperature. It would be possible to go high enough in a balloon so that the water would fly from the pan in the form of gas without making the water hot. Also, a mile below the level of the sea it would take many more degrees of heat to make the water boil. It is said that high up in a balloon you could not boil an egg hard in a pan of boiling water if you kept it in the boiling water for an hour or more, whereas we know that an egg will be hard-boiled if we keep it in boiling water down where we live for more than five minutes.

The degree of heat at which water passes away into the form of gases is regulated by the pressure of the air on the water and other things about us. At the average level in the United States where people live the pressure of the air on everything is fifteen pounds to the square inch, and at this pressure water boils only after it reaches a temperature of 212° Fahrenheit. As we go up the mountains the pressure becomes less and less as we go up. At the top of Mount Blanc, which is 15,781 feet high, water boils at 185° Fahrenheit. If we took a balloon from the top of the mountain we would come to a height where there was no air pressure at all.