How a Tunnel Is Dug Under Water.

Fig. 1. On the left is a cross section showing, in diagram, the back view of a shield. The heavy black circle is the “tail” or “skin.” The small circles within the tail are the hydraulic rams which at a pressure of 5,000 pounds to the square inch force the shield forward. The square compartments within the shield are the openings through which the men pass to dig away the ground. In the middle of the shield is shown the swinging “erector” which picks up the iron lining plates and puts them in position.

The view on the right is a longitudinal section of the tunnel showing the shield and the bulkhead wall across the tunnel with the air locks built into it. The front of the shield ahead of the doors is made with a sharp edge called the “cutting edge” and this makes it easier for the shield to advance in case all the ground in front has not been removed. This view shows how the tail overlaps the last portion of the iron lining.

Some distance behind the shield comes the concrete bulkhead wall with the air locks contained in it. There are two shown in the view. The upper one is the emergency air lock, always kept ready so that in case of an accident the men have a means of escape even though the lower part of the tunnel is filled with rushing water or mud. The lower air lock is for the passage of men and materials during ordinary working. This view also shows that all the tunnel ahead of the bulkhead wall is under compressed air while the finished tunnel behind the bulkhead wall is under the ordinary or normal air pressure. When the tunnel is finished the air locks and bulkhead walls are removed.

HOW TUNNELS ARE BUILT.

These notes describe very generally the way in which tunnels are built through mud and gravel under parts of the sea or large rivers in such a way that the men who build them are protected and as safe as the carpenter who is building a house.

The way these tunnels are built is called the “shield” way because the machine used is called a shield. It is given this name because it shields the tunnel builders from the water and the mud which are ready at every moment to overwhelm them and kill them.

The shield was invented in 1818 by a great Engineer, Marc Isambard Brunel, who was a Frenchman living in England. The idea of the shield came to him as he saw how the sea worm which attacks the wooden piles of docks along the shore bores the holes it makes in the wood. The head of this worm is very hard and can bite its way through the hardest woods. As it goes through the wood its body makes a hard shelly coating which lines the holes which its head has made and prevents the hole from getting filled up. This is the general idea of a tunnel built by a shield.

The first shield was used by Mr. Brunel to make a tunnel across the Thames River at London, England. This is still the biggest tunnel ever built by a shield, although not the longest, and is still used by railroad trains. This tunnel was begun in 1825 and was finished in 1843, and provides a history of almost unexampled and not-to-be-excelled courage in attacking difficulties and skill in defeating them.

Since the days of Brunel many great improvements have been made in the shield and in the way of working it but the same idea is still there.

After the days of Brunel’s shield another great help was given to tunnel builders by the invention of the use of compressed air to hold back the water which saturates the ground in which the tunnel is being built.

The first real invention of compressed air for this purpose was made by Admiral Sir Thomas Cochrane who, in 1830, took out a patent for the use of compressed air to expel the water from the ground in shafts and tunnels and, by this means, to convert the ground from a condition of quicksand to one of firmness. This patent covers all the essential features of compressed air working.

As suggested above, the thing which compressed air does in a tunnel is to push the water out from all the spaces which it fills in the ground, so that the men who are digging away the ground for the tunnel are working in firm dry ground instead of a mixture of earth and water which will run into and fill the hole they dig as soon as it is dug.

Whenever a tunnel is being built below a body of water through ground which is porous, or in other words through any ground except solid rock or dense clay, the water fills every crevice and space in the ground and is exerting a pressure of about half a pound per square inch above the ordinary pressure of the air, (which is 15 pounds to the square inch) for every foot of depth below the surface of the water; so that supposing the tunnel is 40 feet below the water the water has a pressure of nearly 20 pounds per square inch on every square inch of the surface of the tunnel. This pressure causes the water to flow violently into any hole or opening that is made in the ground, and, unless the water is prevented from moving by some means or other, the opening made would be very quickly filled with water and also with ground as the rush of water will carry the sand, gravel or mud with it.

By Cochrane’s invention the whole tunnel is filled with air under a pressure equal to the pressure of the water. This compressed air therefore balances the pressure of the water and holds it back from moving, and if the pressure of the air is made slightly greater than that of the water the water is driven back from the tunnels for a short distance so that when the tunnel is being dug the ground instead of being wet is quite dry.

This explains the principles of the shield and compressed air way of making a tunnel.

The following describes very shortly how these principles are put to actual use.

Most tunnels which are built by shield and compressed air under rivers or arms of the sea are lined with cast iron plates to protect the railway or roadway which is in the tunnel.

The tunnel is a circular tube, or shell, and the plates have flanges on all sides which are bolted together. This shell is put into place, plate by plate, by means of the shield which not only protects the workmen and the work under construction, but which helps to build the iron shell. In fact it corresponds to the sea worm which bores through the wood and lines the hole with a shell. In the case of the tunnel the shell is made of iron. The shield itself consists of a steel tube or cylinder slightly bigger in diameter than the tube or tunnel it is intended to build. The front edge of this shield is made up of a ring of sharp edged castings which form what is called the “cutting edge.” Just behind the cutting edge is a bulkhead or wall of steel, in which are openings which may be opened or closed at will. Behind this bulkhead are placed a number of hydraulic jacks or presses arranged around the shield and within it, so that by thrusting against the last erected ring of iron lining the whole shield is pushed forward. The rear end of the shield is a continuation of the cylinder which forms the front end, and this part, called the “tail,” always overlaps the last few feet of the built up iron shell.

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.

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.

What Do We Mean by Fahrenheit?

The name Fahrenheit is used to distinguish the kind of scale most commonly used on thermometers in Great Britain and the United States. Gabriel Daniel Fahrenheit, a native of Dantzic, made the first thermometer on which this scale was used, and it is named after him. In this scale for thermometers the space between the freezing point and the boiling point is divided into 180 degrees—the point for freezing being marked 32 degrees and the boiling point 212 degrees.

Why Can’t We Swim as Easily in Fresh Water as in Salt Water?

Our bodies are heavier than fresh water, i. e., a bulk of fresh water equal to the size of our body would weigh less than our body, so that the first tendency is to sink to the bottom if we find ourselves in fresh water. If man had not learned to swim that is what he would always do, sink to the bottom; but having learned how to keep from sinking, he is able to swim in fresh water. However, we find that an amount of salt water equal to the bulk of a man in size is heavier than an equal amount of fresh water, although such a bulk of ordinary salt sea water will still weigh less than the man. A man will sink in salt water also if he has not learned to swim or float, but he can keep up with less effort in salt water, and also swim in it more easily. In a nutshell, then, the answer to this question is that salt water is heavier than fresh water. You can make salt water so full of salt that it becomes heavier than a man. Great Salt Lake in Utah is so salty that one cannot sink in it for this reason. You could drown yourself in it, of course, by keeping your head under water, but whether in shallow water or deep water you would not sink in Great Salt Lake.

Why Do We Say Some Water Is Hard and Other Water Soft?

What we call hard water contains certain salts which soft water does not contain. This salt in hard water is lime or some other salts which the water has picked up out of the ground as it passed through either coming up or going down. On the other hand, we can guess after having been told this much that if we can find any water that has not passed through the ground, and, therefore, not had a chance to pick up any salts, we will have soft water. From that point it is easy to guess, then, that rain water must be soft water, and so it is. The water in the cisterns, which is rain water, is soft water, and the kind we get out of the wells is hard water.

We do not like to wash either our faces or our clothes in hard water, especially when it is necessary to use soap, because when we use soap with hard water the soap undergoes chemical change which prevents its dissolving in the water. Therefore, you cannot easily do a good job of washing in hard water. On the other hand it is easy to dissolve the soap in pure rain water or soft water and that is the kind we, therefore, prefer for washing.

How Does Water Put a Fire Out?

This is at first a puzzling question, because back in your mind is the thought that since hydrogen and oxygen are necessary to make a fire burn, it seems strange that water, which is composed of oxygen and hydrogen, will also put it out.

A burning fire throws off heat, but if too much of the heat is taken from the fire suddenly the temperature of the fire is sent down so far below the point at which the oxygen of the air will combine with it that the fire cannot burn. We speak commonly as though water thrown on a fire drowns it. That is practically what happens. Scientifically what happens is that the water thrown upon the fire absorbs so much of the heat to itself that the temperature of the fire is reduced below the point where oxygen will combine with the carbon in the burning material and the fire goes out.

To answer the unasked part of your question at the same time I will say that hydrogen and oxygen when combined as water will put the fire out rather than make it burn, more because when these gases take the form of water they are already once burned, and you know that anything, substance or gas, which has already been burned cannot be burned again. It required great heat to make oxygen and hydrogen combine and form water, and it also takes great heat to separate them again. So they are really burned once before they become water.

Where Does the Rain Go?

Eventually almost all of the rain that falls runs into the rivers and lakes and later finds its way into the ocean, where it is again taken up into the air by the sun’s rays. But many other things happen to parts of the rain which do not find their way into the ocean. In the paved street, of course, where the water cannot sink in, it flows into the gutter and thence into the sewer and on down to the river or wherever it is that the sewers are emptied. You see, it depends very much on what the earth’s surface is covered with at the place where the rain falls. When it strikes where there is vegetation a great deal of it stays in the soil at a depth of comparatively few feet. If it is soil where trees and other plants grow a great deal of it is sucked up from the ground by this vegetation and given back into the air through the leaves and flowers. Some of the rain keeps sinking on down into the earth until it strikes some substance like rock or clay, through which it cannot sink, and then it follows along this until it finds something it can get through and collects in a pool and forms an underground lake, and may cause a spring to flow. Then there are also worms and other forms of animal life in the earth which use up some of the water. But it all gets back into the air eventually to come down some time again in the form of rain.

Why Does Rain Make the Air Fresh?

The main answer to this question must be that the rain in coming down through the air drives the dust and other impurities which are in the air before it, and so cleans the air and makes it absolutely clean. In addition to this it is now stated that since very often rain is produced by electrical changes in the air, and that these electrical changes produce a gas called ozone, which has a delightfully fresh smell, it is this ozone that makes us say the air has become fresh.

The air above our cities is almost constantly filled with smoke, containing various poisonous gases, and these are driven away by the falling rain.

Then, too, there is always a greater or less accumulation of dirt, garbage and other things in the cities which give off offensive smells constantly, but which we do not notice always because we become used to them. When the rain comes down it washes the streets and destroys these smells, and that makes the air fresh and delightful to take into the lungs.

In the country the air is more nearly pure all the time, because the things which spoil the air in the city are not present.

Is a Train Harder to Stop Than to Start?

The answer is yes. It is harder to stop a train than to start it, or rather it takes more power. The speed of a train depends upon the motive power. When a train is stopped and you wish to start it, you must apply enough motive power to start it going. There must be enough power to move the weight of the train and overcome the friction of the wheels on the track. It is, of course, easier to move a thing that weighs less than a heavier one. If you throw a ball ten feet into the air, it will perhaps not sting your hand when you catch it on its return; but, if you throw it one hundred feet into the air, it will sting your hands when you catch it. Besides, it will come down faster the last ten feet of the way than the ball which you threw only ten feet into the air. This is because when movement is applied to anything you add power to it. The ball which comes down from one hundred feet in the air acquires more power in falling and it takes more power to stop it. A train in motion has not only the power of the weight of the train behind it, but also the additional weight which the movement of the train has given it. Therefore, it takes more power to stop it than to start it. To stop a train you must apply the same amount of power as is in the moving train because the power to stop any moving thing must always be at least as great as the power which is moving it.

What Makes the Knots In Boards?

We find knots in the boards which we notice in a lumber pile or in any other place where boards happen to be, because the smaller limbs which grow away from the larger limbs of trees grow from the inside as well as the outside of the tree.

When you see a knot in a board it means that before the tree was cut down and the log sawed up into boards, a limb was growing out from the inside of the tree at the spot where the knot occurs.

You will also find that the wood in the knot is harder generally than the rest of the board. This is because more strength is required at the base of a limb and in the part of the limb which grew inside the tree than in other parts, for the limb must be strong enough to support not only the limb itself, but also the smaller limbs which grow out of it.

How Many Stars Are There?

Man may never know how many stars there are. The best we can do is to figure on the number that can be seen with the largest telescopes which have been invented, for, of course, you know there must be many millions of them which to us are invisible. We have counted the stars so far as we can see them; or, rather, so far as we can photograph them. Astronomers have found that a photographic plate exposed to the stars will show more of them than can be seen by the naked eye. This is because the materials on a photographic plate are more sensitive to the light of the stars than the human eye. By this method man has been able in a way to count the stars he can see. It adds up to more than a hundred million of them. Astronomers found this out by taking photographs of the heavens at night, devoting one picture to each section, until the entire heavens had been covered, and then counting them.