The grinding table is a large flat revolving platform made of iron,
twenty-five feet or more in diameter. The plate must be carried from
the annealing oven to the grinding machines, and thence to the racks,
by men skilled in the art. Twenty men are required to carry the larger
plates of glass, ten on each side, using leather straps and stepping
together in perfect time. The lock-step is absolutely essential to
prevent accident. The grinding table is prepared by being flooded with
plaster of Paris and water; then the glass is carefully lowered, and a
number of men mount upon the plate and tramp it into place until it is
set. After this, greater security is obtained by pegging with prepared
wooden pins; and then the table is set in motion. The grinding is done
by revolving runners. Sharp sand is fed upon the table, and a stream of
water constantly flows over it. After the first cutting by the sand,
emery is used in a similar manner.
The plates are inspected after leaving the grinding room, and if any
scratches or defects of any kind are found they are marked. Some of
these can be rubbed down by hand. There are also, not infrequently,
nicks and fractures found at this stage; and in such case the plate
must again be cut and squared. Afterward comes the polishing, which
is done on another special table. The polishing material is rouge, or
iron peroxide, applied with water, and the rubbing is done by blocks
of felt. Reciprocating machinery is so arranged that every part of the
plate is brought underneath the rubbing surface.
The grinding and polishing has taken away from the original plate half
of its thickness, sometimes more. There is no saving of the material;
it has all been washed away. When to this waste is added the fact that
fully half of the original weight of lime and soda has been released
by the heat of the furnace, escaping into the atmosphere in fumes and
acids, one may begin to understand something of the cost of converting
the rough materials of sand, limestone and soda into beautiful plate
glass.
~HOW MIRRORS ARE MADE~
In preparing plate glass for mirrors great care must be exercised in
the selection of the plates. This selection bears reference not only to
surface defects, but to the quality in general; defects which cannot
ordinarily be seen are magnified many fold after the glass has received
a covering of silver.
[Illustration: BEVELING PLATES]
In the process of beveling, the plate passes through the hands of
skilled workmen of five different divisions, namely: roughers,
emeriers, smoothers, white-wheelers and buffers; and different abrasive
materials are used in the order indicated by the titles. These
materials are sand, emery, natural sandstone imported from England,
pumice and rouge.
The roughing mill is a circular cast-iron disk about 28 inches in
diameter, constructed so that the face or top of the mill revolves upon
a horizontal plane at a speed of about 250 revolutions per minute. The
sand is conveyed to the mill from above through a hopper simultaneously
with a stream of water which is played upon the sand to carry it to
the mill. The rougher places the edge of the plate upon the rapidly
revolving mill, and the cutting of the bevel is done by the passage
of the sand between the mill and the plate of glass. A bevel of any
desired width may be produced. Pattern plates containing incurves,
mitres, etc., require a practiced eye and great skill upon the part of
the operator.
When the plate leaves the rougher’s hands the surface of the bevel has
been ground so deep by the coarse sand that polishing at this stage is
impossible. Consequently, in order to produce a surface fine enough
to render it susceptible of a high and brilliant polish it must go
through the various treatments we have mentioned. The emerier uses a
fine grade of emery on a mill similar in construction to a roughing
mill, which takes away considerable of the coarse surface given by the
first cutting. Then it goes to the smoother, who reduces the roughness
slowly by using a fine sandstone from England; then it goes to the
white-wheeler who operates an upright poplar-wood wheel using powdered
pumice stone as an abrasive; and then, as a last stage it reaches the
buffer, whose method of operation is shown in the illustration. The
buffer brings a high polish to the bevel by the use of rouge applied to
thick felt which covers his wheel.
[Illustration: SILVERING MIRROR PLATES.]
[Illustration: The two photographs here are of the same building taken
under contrasting conditions. The first picture was taken through a
window glazed with common window glass. It is an extreme example, to
be sure, but of a sort not infrequently seen. The second view shows
the same building taken through a window of polished, flawless plate
glass. An observing person can see this startling contrast any day as
he walks along a residence street. At intervals a front window will
be seen which gives a twisted, distorted reflection of the houses or
trees on the opposite side: this is window glass. The other kind--the
window that gives a sharp brilliant reflection--is _plate glass_. It
is practically impossible to obtain superior reflecting quality from
window glass. It can only be had from surfaces which have been ground
and polished.]
The plate, after leaving the beveling room, is again carefully examined
for surface defects. These defects may consist of scratches caused
inadvertently by permitting the surface of the plate to come into
contact with the abrasive material. These scratches are removed by hand
polishing, which must be skillfully done; otherwise the reflection will
become distorted through over-polishing in a given area or spot. The
plate is then taken to a wash table where the surface to be silvered
is thoroughly washed with distilled water; after which it is taken
to a table that is covered with blankets, and which is heated to a
temperature of from 90° to 110°. The blanketing is to protect the plate
from being scratched, and also to catch all of the silver waste. The
silvering solution is nitrate of silver liquefied by a certain formula,
and is poured over the plate; the fluid having an appearance which to
the ordinary observer looks like nothing other than pure distilled
water. Within a few minutes the silver, aided by a reactory, added
prior to pouring, begins to precipitate upon the glass; the liquids
remaining above, and thus preventing air and impurities from coming
into contact with the silver. Such contact would produce oxidation.
After the silver is precipitated the plate is thoroughly dried,
shellacked and painted; after which it is ready for commercial use.
Until about 25 years ago, practically all mirrors were silvered with
mercury. There have been two reasons for discouraging the use of
mercury for silvering; one being its injuriousness to the health of
the workmen. In some European countries stringent laws were enacted,
stipulating that men should work only a certain number of hours.
Other hygienic stipulations, added to the fact that the use of mercury
was already very expensive, have tended to replace that process by the
use of nitrate of silver.
Why Is the Sky Blue?
This question puzzled every one who thought of it for a long time.
Even astronomers, the men who make a business of studying the skies,
and other learned men, puzzled their brains about it and searched for
the answer long ago, until finally, as always happens when a lot of
people study a subject, Professor John Tyndall, a noted scientist of
the last century, discovered the answer. The explanation follows: All
the light we have is sunlight, which is pure white light. This white
light is made up of rays of light of different colors. These rays are
red, orange, yellow, green, blue, indigo and violet. It takes all of
these different rays of light to make our white sunlight, and when
you separate sunlight into its original rays you always produce the
rays of light in the above colors and in the same order. This is only
true, however, when the sunlight is passed through an object which does
not absorb any of its rays. This is the arrangement of the different
colors of light found in the rainbow. The rainbow is formed by sunlight
passing into raindrops or vapor in such a way as to divide the sunlight
into the different colored rays of light. When the rainbow is formed
none of the rays are absorbed by raindrops or vapor through which the
sunlight passes. Some of these rays of light are known as short rays
and others as long rays. But when sunlight meets other things besides
those which make a pure rainbow, these other objects have the ability
to absorb some of the rays of colored light, and they throw off the
remainder. When these rays have been thrown off those which have been
absorbed make many different combinations, and thus are produced all of
the different colors we know, the various tints and shades of color,
according to composition and size.
Now, then, to get back to the color of the sky, which is blue as we
know. The sky or air which surrounds the earth is filled with countless
tiny specks of what we may call dust--particles of solid things hanging
or floating in the air. These specks are of just the size and quality
that they catch and absorb part of the rays of light which form our
sunlight and throw off the rest of the rays, and the part which has
been absorbed forms the combination of color which makes our sky so
beautifully blue. Sometimes you notice, of course, that the sky is a
lighter or darker blue than at other times. This difference is due
to the kind and condition of tiny specks in the air at the time, and
to the direction or angle at which the sunlight strikes these tiny
particles. This fact brings up a question which you have not asked, but
which would come naturally as the result of your first.
What Makes the Colors of the Sunset?
The direction of the sun’s rays when they meet these large and small
particles in the air has a great deal to do with the combination of
colors that result as these objects absorb part of the rays and throw
off others. The sky is the most beautiful blue when the sun is high in
the sky. But when the sun is setting the light has a greater distance
to travel through the belt of air which surrounds the earth than
when it is high up over our heads. You know that if you stick a pin
straight down into an orange it won’t go in very far before it is clear
through the peel, but if you stick the pin into an orange along the
edge it will go through a great deal more of the peel than the other
way. That is the way it is with the sunset colors. The peel of the
orange is a good representation of the belt of air which surrounds the
earth. At sunset the light instead of coming straight down through the
belt of air, thus meeting the eye through the shortest possible amount
of air, strikes the air on a slant, and, therefore, travels through
a great deal more air and closer to the earth to reach it, with the
results that it meets a great many more of these little specks, besides
all the smoke and other things that hang in the air near the ground,
and we thus get many more colors, because some of the things in the air
absorb some of the rays and others absorb very different rays when the
light comes in this slanting way, and that is what makes the different
colors in the sunset. For this reason sunsets are often richer and more
beautiful in color when the air is not so pure, but has much dirt and
other matter floating about in it.
Are There Two Sides to the Rainbow?
No, there is only one side to the rainbow. The rainbow is made by
reflection of the rays of sunlight through drops of water in the air,
but you can never see a rainbow unless you are between it and the sun.
You could never see a rainbow if you were looking at the sun, and so
if you are looking at a rainbow you can be certain that anyone on
the other side of it could not see it, because they would have to be
looking right at the sun. The rainbow is always opposite to the sun and
there can never be two sides to it.
Do the Ends of the Rainbow Rest on Land?
The ends of the rainbow do not rest on anything. You see, the rainbow
is only the reflection of the sun’s rays thrown back to us by the
inside of the back of the raindrops, which are still in the sky after
the rain. Of course, if any of the drops of water touched the ground
they would cease to be raindrops and, therefore, could not reflect the
rays of the sunlight. So, what we think of as the ends of the rainbow
do not really exist at all. The rainbow is only a reflection of the
rays of sunlight from countless drops of water in the air, which the
sun’s rays must strike at a certain angle in order to reflect back the
light so we can see it. Where the sun’s rays do not strike the drops of
water at the right angle no light is reflected, and there is the end of
the rainbow.
What Causes the Different Colors of the Rainbow?
The colors of the rainbow, which are always the same, and are shown in
this order--red, orange, yellow, green, blue and violet--are sunlight
broken up into its original colors. It takes all of these colors in the
proportions in which they are mixed in the rainbow to make the pure
sunlight. These are known as the prismatic colors. As shown in another
answer to one of your puzzling questions, the rainbow is caused by the
rays of the sun passing into drops of water in the air and reflected
back to us with one part of the drop of water acting on it in such a
way as to break up the pure sunlight into these prismatic colors. When
a rainbow appears at a time when there is a great deal of sunlight, you
will generally see two rainbows. The inner rainbow is formed by the
rays of the sun that enter the upper part of the falling raindrops, and
the outer rainbow is formed by the rays that enter the under part of
the raindrops. In the inner or primary bow, as it is called, the colors
beginning at the outside ring of color are red, orange, yellow, green,
blue and violet, and being exactly reversed in the outer or secondary
bow. The secondary bow is also fainter. You may sometimes see smaller
rainbows, even if it has not been raining, when looking at a fountain
or waterfall. These are caused in exactly the same way.
What Makes the Hills Look Blue Sometimes?
This is due to the fact that when the hills look blue you are looking
at them at a distance, and there is a long stretch of air between you
and the hills. This air is filled with countless particles of dust
and other things, and what you see is not really blue hills, but the
reflection of the sun’s rays from the little particles in the air
striking your eye. The color is due to the angle at which the light
from the sun strikes these particles, and is reflected back to your eye
and partially due to the character of the particles in the air.
Do the Stars Really Shoot Down?
The answer is “No.” We have come to use the expression “shooting stars”
commonly, but we should probably be more correct if we said “shooting
rocks,” for the things we refer to commonly as “shooting stars” are
more like rocks than anything else. If any of the real stars were to
fall into the air surrounding the earth we should all be burned up by
the great heat developed long before it actually hit the earth, which
it would undoubtedly destroy.
The things that fall and leave a streak of light are really only
pebbles, stones, rocks or pieces of iron and other substances that fall
from some place into the earth’s air belt. When they strike the air
at the speed at which they are falling the friction of the air makes
a heat that causes them to become luminous, and by far the greater
part of them is burned up before they get very near the earth. We call
them meteorites. Sometimes, though rarely, one will manage to strike
the earth, coming at such great speed and being so large that the air
has not been able to burn it up completely, and it will strike the
earth and sink deep down into the soil. In most museums can be seen
such meteorites that have been dug up after striking the earth. These
are constantly falling into the air surrounding the earth, but in the
day-time their light is not strong enough to be seen while the sun is
shining.
Will the Sky Ever Fall Down?
No, the sky can never fall down, because it is not made of the kind of
things that fall. We have become used to thinking of it as the roof
of the earth, a great dome-shaped roof, because in our little way of
looking at things we compared the earth and what is above it with the
houses in which we live. The sky is just space in which the heavenly
bodies revolve in their orbits. We cannot really ever see sky. We see
only the sun’s light reflected by the air belt which surrounds the
earth. In this air belt are the clouds which do come closer to the
land at times than at others, and this is apt to aid in giving us an
incorrect impression of this.
What Is the Milky Way?
The “Galaxy,” or “Milky Way,” as it is popularly called, is a luminous
circle extending completely around the heavens. It is produced by
myriads of stars, as can be seen when you look at it through a
telescope. It divides into two great branches at one point, which
travel for some distance separately and then reunite. It has also
several branches. At one point it spreads out very widely into a
fanlike shape.
Why Do They Call It the Milky Way?
The stars in the group are so numerous that they present to the naked
eye a whiteness like a stream of milk. To produce this effect there are
not hundreds of stars, nor thousands of them, but actually millions of
them.
When you stop to think that each one of these stars in the Milky Way
is a sun like our own--some of them smaller, of course, but many of
them much larger--you begin to realize how impossible it is for man to
form any real idea of the magnitude and wonders of the earth. Here in
the Milky Way are so many suns like our own sun that they together as
we look at them form the particles of a path which makes the circle of
the heavens, and yet are so far away that to the naked eye each of them
looks to us like only one of countless drops of milk in a very large
stream of milk that goes around the whole sky.
Why Don’t the Stars Shine in the Day-time?
The stars do shine in the day-time. If you will go down into a deep
well or the open shaft of a deep mine and look up at the sky, of which
you can see a circular patch at the top of the well, you will be able
to see the stars in the day-time. The moon also shines in the day-time,
on some part of the earth. At certain times during the month you can
notice that the moon rises before the sun sets, and sometimes in the
morning you can still see the moon in the sky after the sun is up.
Usually you cannot see either the moon or the stars in the day-time,
because the light from the sun is so bright and strong that the light
of the stars and moon are lost in the brightness of the sun’s rays.
When the moon is visible before the sun sets or after the sun has risen
it is because the light of the sun is not so bright and strong at the
beginning or close of daylight. If you are fortunate enough some time
to witness a total eclipse of the sun you will be able to see the stars
in day-time without having to go down into a deep well or mine shaft.
How Far Does Space Reach?
Space surrounds all earths, planets, suns, and extends for an infinite
distance beyond each of them in all directions. It is impossible to
measure in terms of human knowledge how far space extends. It is one
of the things beyond the comprehension of the human mind, and for that
reason man can never know in miles or the number of millions of miles
how far it extends. Man has been able to measure the distance from
the earth of some of the stars, and some of the nearest of them are
millions of miles from the earth. Most of them are hundreds and even
thousands of million miles away, and when we stop to think that space
extends at least as far on the other sides of the stars as it does on
this side, and even beyond that, we can readily understand that it is
not only impossible to measure space, but also impossible to give in
words any conception of what its limits might be.
There is one word--infinite--which we are forced to use in speaking of
the extent of space. Infinite means “without end,” unbounded, and so
man has come to use the word “infinite” in describing the extent of
space, and that is as near as any one can describe it.
What Does Horse Power Mean?
The term “horse power” is used in describing the amount of power
produced by an engine or motor. When man made the first engines he
needed some term to use in describing the amount of power his engine
could develop. Up to that time man had used the horse for turning the
wheels of his machinery and the horse to him naturally represented the
most powerful animal working for man. When engines came into use they
replaced the horses because they were capable of developing many times
the power of the horse. In finding an expression which would accurately
convey to the mind of another the power of a particular engine, it
was natural to say that this engine would do the work of five, ten or
more horses, and as this described it accurately and in a way that was
entirely clear, it became customary to describe the power of an engine
as so many times the power of one horse.
To-day we still cling to the term “horse power” in describing the
strength of the engine, although the horse-power unit used to-day is
greater than the power of an average horse. To speak of an engine of
one horse power to-day means an engine that has the power to lift
30,000 pounds one foot in one minute.
[Illustration: WHERE OUR COAL COMES FROM