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.
[Illustration: HOW THE SHIELD IS PUSHED FORWARD
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.]
[Illustration: This shows one side of an air lock bulkhead wall with
the air lock in place. The boiler-like appearance of the lock is
clearly visible, as well as the door and the pressure gauge to tell the
air pressure inside the lock.]
[Illustration: This is a rear view of one of the Pennsylvania Tunnel
shields, taken after a length of tunnel had been completed. All the
details of construction are shown, but in this case the erector is
clearly seen also. The valves which control the erector and the rams
which push the shield forward are seen near the top of the shield. The
rods across the tunnel are turn-buckles used to keep the iron lining
from getting out of shape in the soft mud. These are removed later. The
floor and tracks in the bottom are temporary and are used for bringing
materials to and from the shield.]
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.
[Illustration: This is a photograph of a model of the Pennsylvania
Tunnels to New York City, made for the Jamestown Tercentenary
Exposition of 1907. It is given because it illustrates, as no
photograph of actual work could do, the relationship between the
shield, the tunnel itself and the air lock. This view shows the rear
part of the shield on the extreme left, with the erector picking up an
iron plate. It shows a man bringing a car with two of the iron plates
up to the shield. Behind this man comes the bulkhead wall with the
emergency air lock in the top and the ordinary air lock for passing
in and out at the bottom. It also shows the upper platform to the
emergency lock along which the men can get to the emergency lock in
case of an accident.]
[Illustration: This is another view of the same model, but showing the
front view of the shield. The doors on the air locks are clearly shown.]
[Illustration: This is a photograph taken in one of the Pennsylvania
tunnels under the Hudson River. It shows the soft mud, through which
the tunnel is being built, flowing in a thick stream through one of
the doors of the shield. The mud under the Hudson, where these tunnels
are, is so soft that often the shield was pushed through the mud with
all the doors shut, so that no mud came into the tunnel and no digging
had to be done, but the shield pushed its way bodily through the mud,
the rings of iron lining being built up behind as usual. Generally,
however, a certain amount of mud was brought in and had to be removed.
This photograph shows how it looked.]
~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.
[Illustration: MAKING THE JOINTS WATER TIGHT
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.]
[Illustration: The last, or closing, plate of each iron ring is called
the “key,” and is much shorter than the others. This photograph shows
the shield erector on one of the Pennsylvania tunnels picking up and
putting into place a key plate. This picture gives an idea of the mud
and dirt and wet in which the men who work in tunnels have to do their
work.]
[Illustration: Wherever possible, every space and crevice outside the
iron lining is filled with cement forced, in a liquid state, through
the iron lining by compressed air. This photograph shows the operation
of “grouting,” as it is called. The man at the left is in control of
the grouting. He has the hose, through which the grout is forced,
screwed to a pipe which passes through a hole made for the purpose in
the iron lining plates and called a “grout hole.” The two men in the
middle of the picture are attending to the “grouting machine” by which
the work is done. Water and cement are fed into the small boiler-like
tank, the tank closed and compressed air admitted thus blowing the
liquid cement through the hose and behind the iron lining. When no
more grout can be forced behind the iron lining all the space has been
filled. The man on the right is the engineers’ inspector taking note of
how much grouting is done, and seeing that the work is properly carried
out.]
[Illustration: This shows the process by which the iron lining is made
perfectly water-tight, so that, when the compressed air is taken off,
no water at all can get into the tunnel. Two operations are shown here.
One is called “grommetting the bolts,” the other is called “caulking
the joints.” The two men on the left, hanging on to the wrench, are
tightening up the bolts as tight as they can after having put on,
underneath the washers at the head and nut of each bolt, a ring of
spun yarn dipped in red lead and oil or tar or some such water-proof
material. A few of these “grommets” may be seen at the feet of the
third man from the left. The other four men are caulking the joints
between the iron plates by driving into the joints a mixture of sal
ammoniac and iron borings. This sets as hard as iron and if properly
done makes a perfectly water-tight joint.]
[Illustration: THE REMARKABLE ACCURACY OF ENGINEERING
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.
[Illustration: SHIELD AT END OF JOURNEY
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.]
[Illustration: This shows a piece of curved tunnel near Morton Street,
on the Hudson and Manhattan Railroad, and is given because of the clear
showing it gives of the iron lining. The track and floor are only the
temporary roads for use during construction.]
[Illustration: Sometimes it is necessary to make borings of the ground
below the tunnels. In some of these bore holes vast quantities of water
are found at a much higher pressure than the tunnel compressed air.
This picture shows a spouting bore hole in one of the Pennsylvania
tunnels during construction.]
[Illustration: The last thing to do before laying the track is to put
the concrete inside the iron lining. This picture shows this work going
on and the wooden forms or ribs for holding up the concrete while it is
setting.]
[Illustration: THE LAND END OF A GREAT TUNNEL UNDER THE HUDSON
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?
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.
[Illustration: WHERE PAINT COMES FROM