Chapter X: Part 10

It looks very much like the land on which we live. There are mountains and valleys, rocks and crags, trees and grass, just the same as we see on land, except, of course, that there are no human beings to be seen. Instead of birds flitting about the tree-tops, fish swim about them, and where the squirrel and rabbit bound through the woods on land, the great king crab and sea turtle drag their unwieldy forms on the ocean’s bottom. Some of the scenes at the bottom of the sea are like fairyland, and in tropical waters are often as beautiful and spectacular as those we see in theatrical pantomimes. Delicately tinted sea-shells, great trees of snow-white coral, sea foliage of every tint and shape, and deep dark caverns, in which lurk the devil-fish and other odd looking fish.

The Diver’s Outfit.

The armor of to-day consists of a rubber and canvas suit, socks, trousers and shirt in one, a copper breastplate or collar, a copper helmet, iron-soled shoes, and a belt of leaden weights to sink the diver.

This enables the diver to talk at all times to those above him.]

It is made of tinned copper, with three glass-covered openings, to enable the diver to look out.]

TESTING THE TELEPHONE.

Every precaution is taken to see that everything is in order before the diver goes down.]

The least error in the adjustment may mean death to the diver.]

The helmet is made of tinned copper, with three circular glasses, one in front and one on either side, with guards to protect them. The front eye-piece is made to unscrew and enable the diver to receive or give instructions without removing the helmet. One or more outlet valves are placed at the back or side of the helmet to allow the vitiated air to escape. These valves only open outwards by working against a spiral spring, so that no water can enter. The inlet valve is at the back of the helmet, and the air on entry is directed by three channels running along the top of the helmet to points above the eye-pieces, enabling the diver to always inhale fresh air. The helmet is secured to the breastplate below by a segmental screw-bayonet joint, securing attachment by one-eighth of a turn. The junction between the water-proof dress and the breastplate is made watertight by means of studs, brass plates and wing-nuts.

A life or signal-line and also a modern telephone enables the diver to communicate at all times with those above him.

The cost of a complete diving outfit ranges from $750.00 to $1,000.00. The weight of the armor and attachments worn by the diver is 256 pounds, divided as follows: Helmet and breastplate, 58 pounds; belt of lead weights, 122 pounds; rubber suit, 19 pounds; iron-soled shoes, 27 pounds each.

The air which sustains the diver’s life below the surface is pumped from above by a powerful pump, which must be kept constantly at work while the diver is down. A stoppage of the pump a single instant while the diver is in deep water would result almost in his instant death from the pressure of the water outside.

The greatest depth reached by any diver was 204 feet, at which depth there was a pressure of 88¹⁄₂ pounds per square inch on his body. The area exposed of the average diver in armor is 720 inches, which would have made the diver at that depth sustain a pressure of 66,960 pounds, or over 33 tons.

The water pressure on a diver is as follows:

20 feet 8¹⁄₂ lbs.
30 feet 12³⁄₄ lbs.
40 feet 17¹⁄₄ lbs.
50 feet 21³⁄₄ lbs.
60 feet 26¹⁄₄ lbs.
70 feet 30¹⁄₂ lbs.
80 feet 34³⁄₄ lbs.
90 feet 39 lbs.
100 feet 43¹⁄₂ lbs.
120 feet 52¹⁄₄ lbs.
130 feet 56¹⁄₂ lbs.
140 feet 60³⁄₄ lbs.
150 feet 65¹⁄₄ lbs.
160 feet 69³⁄₄ lbs.
170 feet 74 lbs.
180 feet 78 lbs.
190 feet 82¹⁄₄ lbs.
204 feet 88¹⁄₂ lbs.

The dangers of diving are manifold, and so risky is the calling that there are comparatively few divers in the United States. The cheapest of them command $10.00 a day for four or five hours’ work, and many of them get $50.00 and $60.00 for the same term of labor under water.

The greatest danger that besets the diver is the risk he runs every time he dives of rupturing a blood-vessel by the excessively compressed air he is compelled to breathe. He is also subject to attacks from sharks, sword-fish, devil-fish, and other voracious monsters of the ocean’s depths. To defend himself against them, he carries a double-edged knife as sharp as a razor. It is the diver’s sole weapon of defense.

Just how far back the art of submarine diving dates is a matter of conjecture, but until the invention of the present armor and helmet, in 1839, work and exploration under water was, at best, imperfect, and could only be pursued in a very limited degree.

Feats of Divers.

~THE GREATEST DIVING FEAT~

Millions of dollars’ worth of property has been recovered from the ocean’s depth by divers. One of the greatest achievements in this line was by the famous English diver, Lambert, who recovered vast treasure from the “Alfonso XII,” a Spanish mail steamer belonging to the Lopez Line, which sank off Point Gando, Grand Canary, in 26¹⁄₂ fathoms of water. The salvage party was dispatched by the underwriters in May, 1885, the vessel having £100,000 in specie on board. For nearly six months the operations were persevered in before the divers could reach the treasure-room beneath the three decks. Two divers lost their lives in the vain attempt, the pressure of water being fatal. The diver recovered £90,000 from the wreck, and got £4,500 for doing it.

One of the most difficult operations ever performed by a diver was the recovering of the treasure sunk in the steamship “Malabar,” off Galle. On this occasion the large iron plates, half an inch thick, had to be cut away from the mail-room, and then the diver had to work through nine feet of sand. The whole of the specie on board this vessel--upward of $1,500,000--was saved, as much as $80,000 having been gotten out in one day.

It is an interesting fact that from time to time expeditions have been fitted out, and companies formed, with the sole intention of searching for buried treasure beneath the sea. Again and again have expeditions left New York or San Francisco in the certainty of recovering tons of bullion sunk off the Brazilian coast, or lying undisturbed in the mud of the Rio de la Plata.

At the end of 1885, the large steamer Imbus, belonging to the P. & O. Co., sank off Trincomalee, having on board a very valuable East-India cargo, together with a large amount of specie. This was another case of a fortune found in the sea, for a very large amount of treasure was recovered.

Another wreck from which a large sum of gold coin and bullion was recovered by divers, was that of the French ship “L’Orient.” She is stated to have had on board specie to the value of no less than $3,000,000, besides other treasure.

A parallel case to “L’Orient” is that of the “Lutine,” a warship of thirty-two guns, wrecked off the coast of Holland. This vessel sailed from the Yarmouth Roads with an immense quantity of treasure for the Texel. In the course of the day it came on to blow a heavy gale; the vessel was lost and went to pieces. Salvage operations by divers, during eighteen months, resulted in the recovery of £400,000 in specie.

Humorous scenes do not play much of a part on the ocean’s bottom, and the sublime and awe-inspiring are far more in evidence there than the ludicrous, yet even beneath the waves there are laughable scenes at times. A diver had been engaged to inspect a sunken vessel off the coast of Cuba. Arriving on the scene he discovered a number of native sponge-divers, who descend to considerable depths, diving down from their canoes to the sunken vessel trying to pick up something of value. They paid little attention to the arrival of the wrecking outfit, and did not notice the diver descend, until suddenly what seemed to them to be a horrible human-shaped monster, with an immense head of glistening copper and three big, round, glassy eyes, came walking around the vessel’s bow and made a big salaam to them. That was enough. They shot surfaceward like sky-rockets, climbed frantically into their canoes and hurriedly rowed away.

What Happens When Anything Explodes?

By explosives are meant substances that can be made to give off a large quantity of gas in an exceedingly short time, and the shorter the time required for the production of the gas the greater will be the violence of the explosion. Many substances that ordinarily have no explosive qualities may be made to act as explosives under certain circumstances. Water, for example, has caused very destructive boiler explosions when a quantity of it has been allowed to enter an empty boiler that had become red hot. Particles of dust in the air have occasioned explosions in saw mills, where the air always contains large quantities of dust. A flame introduced into air that is heavily laden with dust may cause a sudden burning of the particles near it, and from these the fire may be conveyed so rapidly to the others that the heat will cause the air to expand suddenly, and this, together with the formation of gases from the burning, will cause an explosion.

It must not be thought, however, that fine sawdust or water would ordinarily be classed as explosives. The term is generally applied only to those substances that may be very easily caused to explode.

The oldest, and most widely known, explosive that we possess is gunpowder, the invention of which is generally credited to the Chinese. It is a mixture of potassium nitrate, or saltpeter, with powdered charcoal and sulphur. The proportions in which these substances are mixed vary in different kinds of powder, but they usually do not differ much from the following:

Sulphur 10 per cent.
Charcoal 16 per cent.
Saltpeter 74 per cent.

The explosive quality of gunpowder is due to the fact that it will burn with great rapidity without contact with the air, and that in burning it liberates large volumes of gas. When a spark is introduced into it, the carbon, charcoal, and sulphur combine with a portion of the oxygen contained in the saltpeter to form carbonic acid gas and sulphurous acid gas, and at the same time the nitrogen contained in the saltpeter is set free in the gaseous form. This action takes place very suddenly, and the volume of gas set free is so much greater than that of the powder that an explosion follows.

In the manufacture of gunpowder all that is absolutely necessary is to mix the three ingredients thoroughly and in the proper proportions. But to fit the powder for use in firing small arms and cannon it is made into grains of various sizes, the small sizes being used for the small arms with short barrels, and the large sizes for cannon. The reason for this is that if the powder is made in very small grains it all burns at once, and the explosion takes place so suddenly that an exceedingly strong gun is required to withstand the explosion, while if larger grains are employed the burning is slower and continues until the projectile has traveled to the muzzle of the gun. In this way the projectile is fired from the gun with as much force as if the explosion had taken place at once, but there is less strain on the gun.

What Causes the Smoke When a Gun Goes Off?

Powder of this latter kind always produces a considerable quantity of smoke when it is fired, because there is a quantity of fine particles formed from the breaking up of the saltpeter and from some of the charcoal which is not completely burned. This smoke forms a cloud that takes some time to clear away, which is a very objectionable feature. In order to get rid of it, efforts were made to produce a substance that would explode without leaving any solid residue, and that could be used in guns. These efforts were finally successful, and there are now several brands of smokeless powder in use.

What is Smokeless Powder Made Of?

The most satisfactory forms of smokeless powder are all made from guncotton or nitrocellulose. This substance, which is made by treating cotton with a mixture of nitric and sulphuric acids, is a chemical compound, not a mixture like gunpowder; and when it is exploded it is all converted into gases, of which the chief ones are carbonic acid gas, nitrogen, and water-vapor. To cause the explosion of guncotton it is not necessary to burn it, but a mere shock or jar will cause it to decompose with explosive violence. Of course, such a violent explosive as this could not be used either in small arms or in cannon, but guncotton can be converted into less explosive forms which are suitable for use in guns, and the majority, of smokeless powders are made in this way. The methods used in producing the smokeless powders are kept secret by the various countries that use them.

What is Nitroglycerine?

Another very powerful explosive, which is closely related to guncotton, is nitroglycerine. This compound is made by treating glycerine with the same sort of acid mixture that is used in making guncotton. It explodes in the same way that guncotton does and yields the same products. It is an oily liquid of yellow color, and on account of its liquid form it is difficult to handle and use. The difficulty in handling nitroglycerine led to the plan of mixing it with a quantity of very fine sand called infusorial earth. When mixed with this a solid mass called dynamite is formed, which is easier to handle and more difficult to explode, but which has almost as much explosive force as nitroglycerine.

A more powerful explosive than either nitroglycerine or guncotton is obtained by mixing them together. When this is done the guncotton swells up by absorbing the nitroglycerine and becomes a brownish, jelly-like substance that is known as blasting gelatin. This is generally considered the most powerful explosive obtainable.

What Makes Nitroglycerine and Guncotton Explode So Readily?

Let us now consider for the moment what it is that makes guncotton, nitroglycerine, and blasting gelatin explode so readily. The explanation is found in the presence in them of nitrogen. As you remember from what you learned about air, nitrogen is an extremely inactive element. It has no strong tendency to combine with other elements, and when it does enter into combination with them the compounds formed are almost always easily decomposed. In the compounds that have just been described a shock causes a loosening of the bonds that hold the nitrogen, and the whole compound goes to pieces just as an arch falls when the keystone is removed.

What Is Silver?

Since the earliest time recorded in history, silver has been the most used of the precious metals, both in the arts and as a medium of exchange. Even in the prehistoric times silver mines were worked and the metal was employed in the ornamental and useful arts. It was not so early used as money, and when it began to be adopted for this purpose, it was made into bars or rings and sold by weight. The first regular coinage of either gold or silver was in Phrygia, or Lydia, in Asia Minor. Silver was used in the arts by the Athenians, the Phœnicians, the Vikings, the Aztecs, the Peruvians, and in fact by all the civilized and semi-civilized nations of antiquity. It is found in almost every part of the globe, usually in combination with other metals. The mines in South America, Mexico, and the United States are especially rich. Silver is sometimes found in huge nuggets. A mass weighing 800 pounds was found in Peru, and it is claimed that one of 2,700 pounds was extracted in Mexico. The ratio of the value of silver and gold has varied greatly. At the Christian era it was 9 to 1; 500 A.D. it was 18 to 1; but in 1100 A.D. it was only 8 to 1. In 1893 it was as high as 2,577 to 1. The subject has entered largely into American politics as a disturbing element, and in 1896 the Democratic party, in its national convention, declared for the free coinage of the metals at 16 to 1. The Republican party adhered to the gold standard and declared against the free coinage of silver. Each party reaffirmed in 1900 this plank in its platform. In both years the Democrats were defeated.

What Is Worry?

Worry is a feeling of fear, but is never of the present. It is always about something that may happen or that has happened. It is generally in the future, sometimes in the past, but never in the present.

An animal that knows neither future nor past cannot worry. Babies, living only as they do in the present, cannot worry. All creatures, excepting human beings, live only in the present and therefore they do not worry, for such creatures cannot remember what happened in the past or guess what is going to happen.

A human being after arriving at a certain age is given such powers that his mind can go back to the past and cast itself forward into the future as he thinks it will be, because he has imagination. As a matter of fact we live less in the present than in the past or future.

Why Do We Worry?

We worry because we are able through a power called self-consciousness to place ourselves through our minds for the time being. Either--back somewhere in the past without carrying our physical bodies with us; for if we could take our bodies with us, we would be in the present again, and then worry is impossible; or, we use our imagination and project the future entirely apart from our bodies, for we cannot project our bodies into the future, and if we could we would again be in the present. We worry over going to have an operation performed which may or not be dangerous, but quite necessary. We may still think we worry when the operation begins, but as soon as that occurs the time becomes the present, and though we may fear, we cannot worry in the present.

_Back View of Shield_

_Longitudinal Section through Shield & Tunnel_

_Diagram showing method of tunnel construction by shield and
compressed air._

_Scale; ¹⁄₈ inch · 1 foot_

_Jacobs & Davies Inc. 30 Church St. N.Y._

_Oct. 15. 1910._

FIGURE 1.]

The Story in a Tunnel

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.

This shows the front of one of the shields used on the Pennsylvania Railroad tunnels crossing the North River at New York. The cutting edge is clearly seen and the various compartments, each with its door, which divide up the front of the shield. These shields weighed about 200 tons each.]

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.

This shows the rear end or tail end of one of the smaller shields, used on the Hudson and Manhattan Railroad tunnels under the North or Hudson River at New York. It shows the skin, the hydraulic jacks within the skin and the piping and valves for working them. It also shows the doors leading to the front or “face.” The erector is not shown, but the circular hole in the middle shows where it would be attached.]

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.

~WHO INVENTED THE COMPRESSED AIR METHOD~

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.

~HOW THE SHIELD CUTS THROUGH THE GROUND~

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.

This shows the erector building up the iron lining in one of the Pennsylvania tunnels at New York. It shows clearly how the iron plates are bolted together to make the rings of iron lining.]

Usually when crossing, with a tunnel, a wide river or estuary the tunnel is started from each shore and the shields are pushed through the ground until they meet somewhere about the middle of the river. This shows two of the Pennsylvania tunnel shields which have met far below the Hudson River. The white arrow shows where each shield ends. The platform of one shield on which the man stands corresponds exactly with the platform of the other shield. As may be imagined, it takes very careful and skillful engineering and surveying work, both before the work is begun and while it is being carried out, to enable tunnel shields to meet like this. This part of the art of tunnelling would take an article to itself.]

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.

Sometimes, however, shields are not driven to meet one another, but end their journey at some shaft or in some other tunnel previously built, after having gone through thousands of feet of all kinds of ground, from the hardest rock, which had to be blasted out foot by foot before the shield could advance, through hard pan, gravel, boulders, piles, rip-rap, made ground and mud so soft that it flows like melted butter. Naturally, after an experience like this a shield does not look as spick and span as when it started in life. This photograph shows one of the shields of the Hudson and Manhattan Railroad in New York just reaching the end of its journey, battered and bent but still in the ring.]

This view is given to show how complicated an underground structure may have to be made to take care of the requirements of traffic. This view shows the three great reinforced concrete caissons sunk through the earth at Jersey City in order to contain the switches and crossings required to form the New Jersey connections of the uptown and downtown tunnels of the Hudson and Manhattan Railroad.

These caissons were sunk under air pressure by excavating below them just as though they were tunnels turned up on end. In sinking these caissons the material passed through was water-logged made ground, and the hulls of two sunken canal boats were encountered and had to be cut into pieces small enough to be taken out through the locks.

The usual passenger rushing at high speed in the trains between Jersey City and Newark and New York has little idea of the very complicated structure necessary to allow of his doing so.

The information in this article was supplied by Jacobs & Davies, Inc., Consulting Engineers, 30 Church Street, New York, the Engineers for the Pennsylvania Railroad, Hudson River Tunnels, the Hudson and Manhattan Railroad, and many other tunnels in various parts of the world.

The illustrations were kindly supplied by the Pennsylvania Railroad and the Hudson and Manhattan Railroad.]

~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.

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?