EARTH AND SKY
THE BROOK
_Teacher’s Story_
“_Little brook, sing a song of a leaf that sailed along,
Down the golden braided center of your current swift and
strong._”
--J. W. RILEY.
A brook is undoubtedly the most fascinating bit of geography which
the child encounters; and yet how few children who happily play in
the brook--wading, making dams, drawing out the crayfish by his own
grip from his lurking place under the log, or watching schools of
tiny minnows--ever dream that they are dealing with real geography.
The geography lesson on the brook should not be given for the purpose
of making work out of play, but to conserve all of the natural
interest in the brook, and add to it by revealing other and more
interesting facts concerning it. A child who thus studies the brook
will master some of the fundamental facts of physical geography, so
that ever after he will know and understand all streams, whether they
are brooks or rivers. An interesting time to study a brook is after a
rain; and May or October give attractive surroundings for the study.
However, the work should be continued now and then during the entire
year, for each season gives it some new features of interest.
Each brook has its own history, which can be revealed only to the
eyes of those that follow it from its beginning to where it empties
its water into a larger stream or pond. At its source the brook
usually is a small stream with narrow banks; not until it receives
water from surrounding hills does it gain enough power to cut its bed
deeper into the earth, thus making its banks higher. Where it flows
with swift current down a hillside, it cuts its bed deeper, because
swift-moving water has more power for cutting and carrying away the
soil. However, if the hillside happens to be in the woods, the roots
of trees or bushes will help to keep the soil from being washed away.
Unless there are obstacles, the course of the brook is likely to
be more direct in flowing down a hillside than when crossing level
fields. The delightful way in which brooks meander crookedly across
the level areas is due to the inequalities of the surface, which
interfere more with water on a plain than on a hillside, since the
gravity which pulls it forever down has less chance to act upon it
forcibly in these situations. After a stream has thus started its
crooked course, in time of flood the current strikes with more force
against the curves, and cutting them deeper, makes the course still
more crooked. The places on the banks where the soil is bare and
exposed to the force of the current, are the points where the banks
are cut most deeply at flood time.
But the brook is not simply an object to look at and admire; it
is a very busy worker, its chief labor being that of a digger and
carrier. When it is not carrying anything--that is, when its waters
are perfectly clear--the stream is doing the least work. The poets,
as well as common people, speak of the playing of the brook when its
limpid waters catch the sunbeams on their dimpling surface; but when
the waters are roily the brook is working very hard. This usually
occurs after a rain, which adds much more water to the volume of the
brook; the action of gravity upon this larger and heavier body forces
it to flow more swiftly and every drop in the stream that touches
the bank or bottom, snatches up a tiny load of earth and carries it
along. And every drop thus laden, when it strikes against a corner of
the bank, tears more soil loose through the impact, and other drops
snatch it up and carry it on down the stream. And after a time there
are so many drops carrying loads and bumping along, knocking loose
more earth, that the whole brook, which is made up of drops, looks
muddy. In its work as a digger, every drop of water that touches the
soil at the bottom or on the banks of the brook uses its own little
load of earth or gravel as a crowbar or pickaxe to pry up other bits
of dirt and gravel; and all of the drops hastening on, working hard
together, dig the channel of the brook wider and deeper. In some
steep places, so many of the drops are working together that they
are able to pick up pebbles or stones, with which they batter and
tear down larger pieces of the bank and scrape out greater holes
in the bottom of the stream. On and on the brook flows, a gang of
workmen each of which is using its own load as a tool, all in close
procession and working double quick. But as soon as the brook reaches
a plain or level, its activity ceases; the drops act tired and seem
to have no ambition to pick up more soil, and each lets fall its own
load as soon as possible, dropping the larger pieces of gravel and
rock first, carrying the finer soil farther, but finally letting that
down also. If we examine the sediment of a flooded brook, we find
that the gravel is always dropped first, and that the fine mud is
carried farthest before it is deposited.
The roar of a flooded stream is very different from the murmur of
its waters when they are low. It is not to be wondered at, when we
once think of all that is going on in the brook during periods of
flood. There are some simple experiments to show what the force of
water can do when turned against the soil. Pour water from a pitcher
into a bed of soft soil, and note how quickly a hole will be made;
if the pitcher is held near the soil, less of a hole will be formed
than if the pitcher is held high up, which shows that the farther the
water falls, the greater is its force. This explains why the banks of
streams are undermined when a strong current is driven against them.
The swift current, of course, breaks away more earth at bends and
curves than when it is flowing in a straight line; for ordinarily,
when flowing straight, the current is swiftest in the bed of the
stream, and is therefore only digging at the bottom; but when it
flows around curves, it is directed against the banks, and therefore
has much more surface to work upon. Thus it is that bends are cut
deeper and deeper. If the bare arm is thrust into a flooded brook, we
find that many pieces of gravel strike against it; and if we reach
the bottom, we can feel the pebbles being moved along over the brook
bed.
LESSON CCIX
THE BROOK
_Leading thought_--The water from the little brook near our
schoolhouse is flowing toward the ocean, and is meanwhile digging out
and carrying along with it the soil through which it flows.
[Illustration: _What the children find living in the brook._]
_Method_--The best time to study a brook is after a rain, and October
or May is an interesting time for beginning this lesson. The work
should be continued during the entire year. It may be done at noon
or recess, if the brook is near at hand; or there may be excursions
after school, if the brook is at some distance. The observations
should be made by the class as a whole.
_Observations_--1. Does the brook have its source in a spring or a
swamp, or does it receive its water as drainage from surrounding
hills? Follow it back to its very beginning. Do you find this in open
fields or woods? Is the land about it level or hilly?
2. Are its banks deeper at the beginning, or is the brook at first
almost on a level with the surrounding fields? Do the banks become
deeper farther from the source? Are the banks higher where the brook
flows down hill, or where it is on a level?
3. Is the course of the brook more crooked on a hillside or when
flowing through a level area? Are the banks more worn away and steep
where the brook flows through woods or bushes than through the open
fields?
4. Can you find the places where the water is cutting the banks
most, when the brook is flooded? Why does it cut the banks at these
particular points?
5. Into what stream, pond or lake does the brook flow? If you should
launch a toy boat upon the waters of this brook, and it should keep
afloat, through what streams would it pass to reach the ocean?
Through what townships, counties, states or countries would it pass?
6. When is the brook working and when is it playing? What is the
difference between the color of the water ordinarily and when the
brook is flooded? What causes this difference?
7. Make the following experiment to show what the brook is carrying
after a storm when the water is roily. Dip from the swift portion of
the stream a glass fruit jar full of water. Place it on a window-sill
and do not disturb it until the water is clear. How much sediment has
settled at the bottom of the jar? Where was this sediment when you
dipped up the water? If this quart of water could carry so much soil
or sediment, how much, do you think, would the whole brook carry?
8. Where did the brook get the soil to make the water roily? Study
its banks in order to answer this question. Do you think the soil in
the water came from the banks that are covered by vegetation or from
those which are bare?
9. How did the brook pick up the soil that it carried when it was
flooded? Do you think that one of the tools that the brook digs with
is the current? Try to find a place where the swift current strikes
the bank, and note if the latter is being worn away.
10. Does the swift current take more soil where it is flowing
straight, or where there are sharp bends? How are the bends in the
brook or creek made?
11. Thrust your bare hand or arm into the swift current of the brook
when it is flooded. Do you feel the gravel strike against your arm or
hand? Wade in the water. Do you feel the pebbles strike against the
feet or legs, as they are being rolled along the bed of the stream?
12. Does the water, loaded with soil and pebbles, dig into the banks
more vigorously than just the water alone could do? Which washes away
more earth and carries it down stream--a fast or a slow current?
13. Does the water of the brook flow fastest when its waters are low
or high? When the brook is at its highest flood, do you think it
is working the hardest? If so, explain why? When it is working the
hardest and carrying most soil and gravel, does it make a different
sound than when it is flowing slowly and its waters are clear?
14. How does the brook look when it is doing the least amount of work
possible?
15. Make a map of your brook showing every pool, indicating the
places where the current is swiftest and showing the bends in its
course. To test the rapidity of the current, put something afloat on
it and measure how far it will go in a minute.
16. How many kinds of trees, bushes and plants grow along the banks
of your brook? How many kinds of fish and insects do you find living
in it? How many kinds of birds do you see frequently near it?
_A brook puzzle for pupils to solve_--When we have a load to carry we
go slowly because we are obliged to; and the heavier the load, the
slower we go. On the other hand, when we wish to run very swiftly we
drop the load so as not to be weighted down; when college or high
school boys run races in athletic games, they do not wear even their
ordinary clothing, but dress as lightly as possible in trunks and
tights; they also train severely so that they do not have to carry
any more flesh on their bones than is necessary. How is it that in
the case of a brook just opposite is true? The faster the brook runs,
the more it can carry; and the heavier it becomes the faster it runs;
and the faster it runs the more work it can do.
[Illustration: _Where the stream drops its load._]
HOW A BROOK DROPS ITS LOAD
_Teacher’s Story_
The brook is most discriminating in the way it takes up its burdens,
and also in the way it lays them down. It, with quite superhuman
wisdom, selects the lightest material first, leaving the heaviest
to the last; and when depositing the load, it promptly drops the
heaviest part first. And thus the flowing waters of the earth are
eternally lifting, selecting, and sifting the soils on its surface.
The action of rain upon the surface of the ground is in itself an
excellent lesson in erosion. If there is on a hillside a bit of bare
ground which has been recently cultivated or graded, we can plainly
see, after a heavy rain, where the finer material has been sorted out
and carried away, leaving the larger gravel and stones. And if we
examine the pools in the brook, we shall find deltas as well as many
examples of the way the soil is sifted as it is dropped. The water of
a rill flowing through pasture and meadow is clear, even after a hard
rain. This is owing, not so much to the fact that the roots hold the
banks of the brook firmly, as that the grass on the surface of the
ground acts as a mulch and protects the soil from the erosive impact
of the raindrops. On the other hand, and for a reverse reason, a rill
through plowed ground is muddy. On a hillside, therefore, contour
plowing is practiced--that is, plowing crosswise the hillside instead
of up and down. When the furrow is carried crosswise, the water after
showers can not dash away, carrying off in it all the finer and
more fertile portions of the soil. There are many instances in our
Southern States where this difference in the direction of plowing has
saved or destroyed the fertility of hillside farms.
The little experiment suggested at the beginning of the following
lesson, should show the pupils clearly the following points: It is
through motion that water takes up soil and holds it in suspension.
The tendency of still water is to drop all the load which it is
carrying and it drops the heaviest part first. We find the pebbles at
the bottom of the jar, the sand and gravel next, and the fine mud on
top. The water may become perfectly clear in the jar and yet, when
stirred a little, it will become roily again because of the movement.
Every child who wades in a brook, knows that the edges and the still
pools are more comfortable for the feet than is the center of the
stream under the swift current. This is because, where the water
is less swift at the sides, it deposits its mud and makes a soft
bottom; while under the swifter part of the current, mud is washed
away leaving the larger stones bare. For the same reason, the bottom
of a stream crossing a level field is soft, because the silt, washed
down from the hills by the swift current, is dropped when the waters
come to a more quiet place. If, across a stony brook, the pupils can
build a dam that will hold for two or three months in the fall or
spring when the brook is flooded, they will be able to note that the
stones will soon be more or less covered with soft mud; for the dam,
stopping the current, causes the water to drop its load of silt. It
would have to be a very recently made pool in a stream, which would
not have a soft mud bottom. The water at times of flood is forced to
the side of the streams in eddies, and its current is thus checked,
and its load of mud dropped.
It should be noted that at points where the brook is narrowest the
current is swiftest, and where the current is swiftest the bottom
is more stony. Also, where there is a bend in the stream the brook
digs deeper into the bank where it strikes the curve, and much of the
soil thus washed out is removed to the other side of the stream where
the current is very slow, and there is dropped. (See Introduction
to Physical Geography, Gilbert and Brigham, pp. 51 and 52.) If
possible, note that where a muddy stream empties into a pond or lake,
the waters of the latter are made roily for some distance out, but
beyond this the water remains clear. The pupils should be made to
see that the swift current of the brook is checked when its waters
empty into a pond or lake, and because of this they drop their load.
This happens year after year, and a point extending out into the lake
or pond is thus built up. In this manner the great river deltas are
formed.
_References_--The Brook Book, Mary Rogers Miller; Brooks and Brook
Basins, Frye; Up and Down the Brooks, Bamford; Physical Geography,
Tarr; Introduction to Physical Geography, Gilbert and Brigham.
LESSON CCX
HOW A BROOK DROPS ITS LOAD
_Leading thought_--The brook carries its load only when it is flowing
rapidly. As soon as the current is checked, it drops the larger
stones and gravel first and then the finer sediment. It is thus that
deltas are built up where streams empty into lakes and ponds.
_Method_--Study the rills made in freshly graded soil directly after
a heavy rain. Ask the pupils individually to make observations on the
flooded brook.
_Experiment_--Take a glass fruit jar nearly full of water from the
brook, add gravel and small stones from the bed of the brook, sand
from its borders and mud from its quiet pools. Have it brought into
the schoolroom, and shake it thoroughly. Then place in a window and
ask the pupils to observe the following things:
(a) Does the mud begin to settle while the water is in motion; that
is, while it is being shaken?
(b) As soon as it is quiet, does the settling process begin?
(c) Which settles first--the pebbles, the sand or the mud? Which
settles on top--that is, which settles last?
(d) Notice that as long as the water is in the least roily, it means
that the soil in it has not all settled; if the water is disturbed
even a little it becomes roily again, which means that as soon as the
water is in motion it takes up its load.
_Observations_--1. Where is the current swiftest, in the middle or at
the side of the stream?
2. What is the difference, in the bottom of the brook, between the
place below the swift current and the edges? That is, if you were
wading in the brook, where would it be more comfortable for your
feet--at the sides or in the swiftest part of the current? Why?
3. Does the brook have a more stony bed where it flows down a
hillside than where flowing through a level place?
4. Place a dam across your brook where the bottom is stony, and note
how soon it will have a soft mud bottom. Why is this?
5. Can you find a still pool in your brook that has not a soft, muddy
bottom? Why is this?
6. Does the brook flow more swiftly in the steep and narrow places
than in the wide portions and where it is dammed?
7. Do you think if water, flowing swiftly and carrying a load of mud,
were to come to a wider or more level place, like a pool or millpond
dam, that it would drop some of its load? Why?
8. If the water flows less swiftly along the edges than in the
middle, would this make the bottom below softer and more comfortable
to the feet than where the current is swiftest? If so, why?
9. If you can see the place where a brook empties into a pond or
lake, how does it make the waters of the latter look after a storm?
What is the water of the brook doing to give this appearance, and why?
10. What becomes of the soil dropped by the brook as it enters a pond
or lake? Do you know of any points of land extending out into a lake
or pond where the stream enters it? What is a stream delta?
* * * * *
“_In the bottom of the valley is a brook that saunters between
oozing banks. It falls over stones and dips under fences. It
marks an open place on the face of the earth, and the trees
and soft herbs bend their branches into the sunlight. The
hangbird swings her nest over it. Mossy logs are crumbling
into it. There are still pools where the minnows play. The
brook runs away and away into the forest. As a boy I explored
it but never found its source. It came somewhere from the
Beyond and its name was Mystery._
_The mystery of this brook was its changing moods. It had its
own way of recording the passing of the weeks and months. I
remember never to have seen it twice in the same mood, nor
to have got the same lesson from it on two successive days:
yet, with all its variety, it always left that same feeling of
mystery and that same vague longing to follow to its source
and to know the great world that I was sure must lie beyond.
I felt that the brook was greater and wiser than I. It became
my teacher. I wondered how it knew when March came, and why
its round of life recurred so regularly with the returning
seasons. I remember that I was anxious for the spring to come,
that I might see it again. I longed for the earthy smell when
the snow settled away and left bare brown margins along its
banks. I watched for the suckers that came up from the river
to spawn. I made a note when the first frog peeped. I waited
for the unfolding spray to soften the bare trunks. I watched
the greening of the banks and looked eagerly for the bluebird
when I heard his curling note somewhere high in the air._”
--“The Nature-Study Idea,” L. H. BAILEY.
CRYSTAL GROWTH
_Teacher’s Story_
To watch the growth of a crystal is to witness a miracle;
involuntarily we stand in awe before it, as a proof that of all
truths mathematics is the most divine and inherent in the universe.
The teacher will fail to make the best use of this lesson if she does
not reveal to the child through it something of the marvel of crystal
growth.
[Illustration: _A snow crystal._
Photomicrograph by W. A. Bentley.]
That a substance which has been dissolved in water should, when the
water evaporates, assemble its particles in solid form of a certain
shape, with its plane surfaces set exactly at certain angles one to
another, always the same whether the crystal be large or small, is
quite beyond our understanding. Perhaps it is no more miraculous
than the growth of living beings, but it seems so. The fact that
when an imperfect crystal, unfinished or broken, is placed in water
which is saturated with the same substance, it will be built out and
made perfect, shows a law of growth so exquisitely exemplified as to
again make us glad to be a part of a universe so perfectly governed.
Moreover, when crystals show a variation in numbers of angles and
planes it is merely a matter of division or multiplication. A snow
crystal is a six-rayed star, yet sometimes it has three rays.
The window-sill of a schoolroom may be a place for the working of
greater wonders than those claimed by the astrologists of old,
when they transmuted baser metals to gold and worthless stones to
diamonds. It may be a place where strings of gems are made before the
wondering eyes of the children; gems fit to make necklaces for any
naiad of the brook or oread of the caves.
It adds much to the interest of this lesson if different colored
substances are used for the forming of the crystals. Blue vitriol,
potassium bichromate, and alum give beautiful crystals, contrasting
in shape as well as in colors.
Copper sulphate and blue vitriol are two names for one substance;
it is a poison when taken internally and, therefore, it is best for
the teacher to carry on the experiment before the pupils instead of
trusting the substance to them indiscriminately. Blue vitriol forms
an exquisitely beautiful blue crystal, which is lozenge-shaped with
oblique edges. Often, as purchased from the drug store, we find it
in the form of rather large, broken, or imperfect crystals. One of
the pretty experiments is to place some of these broken crystals in a
saucer containing a saturated solution of the vitriol, and note that
they straightway assert crystal nature by building out the broken
places, and growing into perfect crystals. Blue vitriol is used much
in the dyeing and in the printing of cotton and linen cloths. It has
quite wonderful preservative qualities; if either animal or vegetable
tissues are permeated by it they will remain dry and unchanged.
Potassium bichromate is also a poison and, therefore, the teacher
should make the solution in the presence of the class. It forms
orange-red crystals, more or less needle-shaped. It crystallizes so
readily that if one drop of the solution be placed on a saucer the
pupils may see the formation of the crystals by watching it for a few
moments through a lens.
The common alum we buy in crystal form, however, it is very much
broken. Its crystals are eight-sided and pretty. Alum is widely used
in dyes, in medicines, and in many other ways. It is very astringent,
as every child knows who has tried to eat it, and has found the lips
and tongue much puckered thereby.
Although we are more familiar with crystals formed from substances
dissolved in water, yet there are some minerals, like iron which
crystallize only when they are melted by heat; and there are
other crystals, like the snow, which are formed from vapor. Thus,
substances must be molten hot, or dissolved in a liquid, or in form
of gas, in order to grow into crystals.
LESSON CCXI
CRYSTAL GROWTH
_Leading thought_--Different substances when dissolved in water
will re-form as crystals; each substance forms crystals of its own
peculiar color and shape.
_Method_--Take three test tubes, long vials or clear bottles. Fill
one with a solution made by dissolving one part of blue vitriol
in three parts of water; fill another by dissolving one part of
bichromate of potash with twenty-five parts of water; fill another
with one part of alum in three parts of water. Suspend from the mouth
of each test tube or vial, a piece of white twine, the upper end
tied to a tooth pick, which is placed across the mouth of the vial;
the other end should reach the bottom of the vial. If necessary, tie
a pebble to the lower end so that it will hang straight. Place the
bottles on the window sill of the schoolroom, where the children may
observe what is happening. Allow them to stand for a time, until
the string in each case is encrusted with crystals; then pull out
the string and the crystals. Dry them with a blotter, and let the
children observe them closely. Care should be taken to prevent the
children from trying to eat these beautiful crystals, by telling them
that the red and blue crystals are poisonous.
_Observations_--1. In which bottle did the crystals form first? Which
string is the heaviest with the crystals?
2. What was the color of the water in which the blue vitriol was
dissolved? Is it as brilliant in color now as it was when it was
first made? Do you think that the growth of the crystals took away
from the blue material of the water? Look at the blue vitriol
crystals with a lens, and describe their shape. Are the shapes of the
large crystals of the vitriol the same as those of the small ones?
3. What is the shape of the crystals of the potassium bichromate?
What is the color? Are these crystals as large as those of the blue
vitriol or of the alum?
4. What shapes do you find among the crystals of alum?
5. Do you think that vitriol and potassium bichromate and alum will,
under favorable circumstances, always form each its own shape of
crystal wherever it occurs in the world? Do you think crystals could
be formed without the aid of water?
6. How many kinds of crystals do you know? What is rock candy? Do you
think you could make a string of rock candy if you dissolved sugar in
water and placed a string in it?
SALT
_Teacher’s Story_
A “saturated solution” is an uninspiring term to one not chemically
trained; and yet it merely means water which holds as much as it can
take of the dissolved substance; if the water is hot, it dissolves
more of most substances. To make a saturated solution of salt we need
two parts of salt or a little more, for good measure, to five parts
of water; the water should be stirred until it will take up no more
salt.
[Illustration: _Form of a salt crystal._]
A slip of paper placed in a saucer of this solution will prove a
resting place for the crystals as they form. In about two days the
miracle will be working, and the pupils should now and then observe
its progress. Those saucers set in a draft or in a warm place will
show crystals sooner than others, but the crystals will be smaller;
for the faster a crystal grows, the smaller is its stature. If
the water evaporates rapidly, the crystals are smaller, because
so many crystals are started which do not have material for large
growth. When the water is evaporated, to appreciate the beauty of
the crystals we should look at them with a lens or microscope. Each
crystal is a beautiful little cube, often with a pyramid-shaped
depression in each face or side. After the pupils have seen these
crystals, the story of where salt is found should be told them.
Salt is obtained by two methods: by mining large deposits of rock
salt, and by evaporating water containing a strong solution of salt.
The oldest salt works in this country are in Syracuse, New York,
where the salt comes from salt springs which were famous among the
American Indians. At Ithaca, N. Y., the salt deposits are about
2000 feet below the surface of the earth. Water is forced down into
the stratum of rock, which was evidently once the bottom of a briny
sea; the water dissolves the salt, and it is then pumped up to the
surface and evaporated, leaving the salt in crystals. In Michigan and
Louisiana there are other large salt works of a similar character.
The largest salt mines in the world are those in Poland, which have
been used for hundreds of years. In these mines there are fifty miles
of corridors, and the salt has been carved into beautiful chambers
with statues and other decorations, all cut from the solid salt. One
of these chambers represents a chapel beautifully ornamented.
When the United States was first settled, salt was brought over from
England; but this was so expensive that people could not afford it
and they soon began to make their own salt by evaporating sea water
in kettles on the beach. In those countries where it is scarce, salt
is said to be literally worth its weight in gold. The necessity for
salt to preserve the health of both people and animals has tempted
the governments of some countries to place a special tax upon it; in
Italy, especially, the poor people suffer greatly on account of the
high price of salt from this cause.
Salt lakes are found in natural basins of arid lands, and are always
without outlets. The water which runs in escapes by evaporation, but
the salt it brings cannot escape, and accumulates. A salt lick is a
place where salt is found on the surface of the earth, usually near a
salt spring. Animals will travel a long distance to visit a salt lick
which gained its name through their attentions.
LESSON CCXII
SALT
_Leading thought_--Salt dissolves in water, and as the water
evaporates the salt appears in beautiful crystals.
_Method_--Let each pupil, if possible, have a cup and saucer, a
square of paper small enough to go into the saucer, some salt and
water. Let each pupil take five teaspoonfuls of water and add to this
two spoonfuls of salt, stirring the mixture until dissolved. When the
water will take no more salt let each pupil write his name and the
date on the square of paper, lay it in the saucer, pressing it down
beneath the surface. Let some place their saucers in a warm place,
others where they may be kept cool, and others in a draft. If it is
impossible for each pupil to have a saucer, two or three pupils may
be selected to perform the experiments.
_Observations_--1. When you pour the salt into the water, what
becomes of it? How do you know when the water will hold no more salt?
2. After a saucer, filled with the salt water, stands exposed to the
air for several days, what becomes of the water? From which saucers
did the water evaporate fastest--those in the warm places, or those
in the cold? In which did the crystals form first?
3. Which saucers contained the largest crystals--those from which the
water evaporated first, or those from which it evaporated more slowly?
4. Could you see how the crystals began? What is the shape of the
perfect salt crystal? Do the smallest crystals have the same shape as
the largest ones?
5. What happens to people who cannot get salt to eat?
6. How is dairy salt and table salt obtained? What is rock salt? What
are salt licks? Where are the salt mines found? Why is the ocean
called “the briny deep?”
7. Name and locate the salt lakes. Why are lakes salt?
LESSON CCXIII
HOW TO STUDY MINERALS
Many children are naturally interested in stones. I once knew two
children, aged seven and five, who could invariably select the
boulders and pebbles of metamorphic rock in the region about Ithaca.
They also could tell, when the pebbles were broken, which parts were
quartz and which mica. They had incidentally asked about one of these
stones, and I had told them the story of the glacial period and how
these stones were torn away from the mountains in Canada and brought
down by ice and dropped in Ithaca. It was a story they liked, and
their interest in these granite voyagers was always a delightful
element of our walks in the field.
[Illustration: _A snow crystal._
Photomicrograph by W. A. Bentley.]
For the pupils in the elementary grades it seems best to limit
the study of minerals to those which make up our granite and
common rocks. In order to teach about these minerals well, the
teacher should have at least one set of labelled specimens. Such
a collection may be obtained from Edward E. Howell, 612 17th St.,
N. W., Washington, D. C., and also from Ward’s Natural Science
Establishment, College Avenue Rochester, N. Y. These collections vary
in number of specimens and price from one to two dollars and are
excellent. The teacher should have one or two perfect crystals of
quartz, feldspar and calcite. An excellent practice for a boy is to
copy these crystals in wood for the use of the teacher.
The physical characteristics used in identifying minerals are briefly
as follows:
1. _Form._ This may be crystalline, which shows the shape of the
crystals definitely; granular, like marble, the grains having the
internal structure, but not the external form, of crystals; compact,
which is without crystalline form, as limestone or flint.
2. _Color._
3. _Luster or shine_, which may be glassy like quartz, pearly like
the inside of a shell; silky like asbestos; dull; or metallic like
gold.
4. _Hardness_ or resistance to scratching, thus: Easily scratched
with the finger nail; cannot be scratched by the finger nail; easily
scratched with steel; with difficulty scratched with steel; not to be
scratched by steel. A pocket knife is usually the implement used for
scratching.
[Illustration: _Forms of quartz crystals._]
QUARTZ
_Teacher’s Story_
There is in the Cornell University Museum a great quartz crystal,
a six-sided prism several inches in thickness. One-half of it is
muddy and the other half clear, transparent and beautiful. The
professor in charge, who has the imagination necessary to the
expert crystallographer, said to his class: “This crystal was begun
under conditions which made it cloudy; then something happened,
perhaps some cataclysm that changed all the conditions around the
half-grown crystal, and it may have lain a hundred or a thousand
years unfinished, when, some other change occurring, there came about
conditions which permitted it to resume growth, and the work began
again exactly where it was left off, the shaft being perfected even
to its six-sided pyramidal tip.” And ever afterwards that crystal,
half clouded and half clear, remained in the minds of his pupils as a
witness of the eternal endurance of the laws which govern the growth
of crystals.
Quartz is the least destructible and is one of the most abundant
materials in the crust of the earth as we know it. It is made up of
two elements chemically united--the solid silicon and the gas oxygen.
It is the chief material of sand and sandstones, and it occurs, mixed
with grains of other minerals, in granite, gneiss, and many lavas;
it also occurs in thick masses or sheets, and sometimes in crystals
ornamenting the walls of cavities in the rocks. Subterranean waters
often contain a small amount of silica, the substance of quartz,
in solution; from such solutions it may be deposited in fissures
or cracks in the rock, thus forming bodies called “veins.” Other
materials are often deposited at the same time, and in this way
the ores of the precious metals came to be associated with quartz.
Sometimes quartz is deposited from hot springs or geysers, forming a
spongy substance called sinter. In this case, some of the water is
combined with the quartz, making what is called opal. Quartz crystal
will cut glass.
Quartz occurs in many varieties: (a) In crystals like glass. If
colorless and transparent it is called rock crystal; if smoky brown,
it is called smoky quartz; if purple, amethyst. (b) In crystals,
glassy but not transparent. If white, it is milky quartz; if pink,
rose quartz. (c) As a compact crystalline structure without luster,
waxy or dull, opaque or translucent, when polished. If bright red, it
is carnelian; if brownish red, sard; if in various colors in bands,
agate; if in horizontal layers, onyx; if dull red or brown, jasper;
if green with red spots, bloodstone; if smoky or gray, breaking with
small, shell-like or conchoidal fractures, flint.
Rock crystals are used in jewelry and especially are made to imitate
diamonds. The amethyst is much prized as a semi-precious stone.
Carnelian, bloodstone and agate are also used in jewelry; agate is
used also in making many ornamental objects, and to make little
mortars and pestles for grinding hard substances.
One of the marvels of the world is the petrified forest of Arizona,
now set aside by the government as a national reserve. Great trees
have been changed to agate and flint, the silica being substituted
for the tissues of the wood so that the texture is preserved though
the material is changed.
When our country was first settled, flint was used to start fires
by striking it with steel and letting the sparks fly into dry,
fine material, called tinder. It was also used in guns before the
invention of cartridges, and the guns were called flintlocks. The
Indians used flint to make hatchets and for tips to their arrows. The
making of flint implements dates far back into prehistoric times; it
was probably one of the first steps upward which man achieved in his
long, hard climb from a level with the brute creation to the heights
attained by our present civilization.
Quartz sand is used in making glass. It is melted with soda or potash
or lead, and the glass varies in hardness according to the minerals
added. Quartz is also used for sandpaper and glass paper; and ground
to a fine powder, it is combined with Japans and oils and used as
a finish for wood surfaces. Mineral wool is made from the slag
refuse of furnaces where glass is made, and is used for rat-proof
and fireproof padding for the walls of houses. Quartz combined with
sodium or potassium and water, forms a liquid called water-glass,
which is used for waterproof surfaces; it is also fireproof to
a certain degree. Water-glass is the best substance in which to
preserve eggs; one part of commercial water-glass to ten parts of
water makes a proper solution for this purpose.
LESSON CCXIV
QUARTZ
_Leading thought_--Quartz is one of the most common of minerals. It
occurs in many forms. As a crystal it is six-sided, and the ends
terminate in a six-sided pyramid. It is very hard and will scratch
and cut glass. When broken, it has a glassy luster and it does not
break smoothly but shows an uneven surface.
_Method_--The pupils should have before them as many varieties of
quartz as possible; at least they should have rock crystal, amethyst,
rose and smoky quartz and flint.
_Observations_--1. What is the shape of quartz crystals? Are the
sides all of the same size? Has the pyramid-shaped end the same
number of plane surfaces as the sides?
2. What is the luster of quartz? Is this luster the same in all the
different colored kinds of quartz?
3. Can you scratch quartz with the point of a knife? Can you scratch
glass with a corner or piece of the quartz? Can you cut glass with
quartz?
4. Describe the following kinds of quartz and their uses: amethyst,
agate, flint.
5. How many varieties of quartz do you know? What has quartz to do
with the petrified forests of Arizona?
FELDSPAR
_Teacher’s Story_
We most commonly see feldspar as the pinkish portion of granite.
This does not mean that feldspar is always pink, for it may be the
lime-soda form known as labradorite, which is dark gray, brown or
greenish brown, or white; or it may be the soda-lime feldspar called
oligoclase, which is grayish green, grayish white, or white; but the
most common feldspar of all is the potash feldspar--orthoclase--which
may be white, nearly transparent, or pinkish. Orthoclase is different
from other feldspars in that, when it splits, its plane surfaces form
right angles. Feldspar is next in the scale of hardness to quartz,
and will with effort and perseverance scratch glass but will not cut
it; it can be scratched with a steel point. Its luster is glassy and
often somewhat pearly.
[Illustration: _Forms of feldspar crystals._]
Maine leads all other states in the production of feldspar. It is
quarried and crushed and ground to powder, as fine as flour, to
make the clay from which china and all kinds of pottery are made.
Our clayey soils are made chiefly from the potash feldspar which is
weathered to fine dust. Kaolin, which has been used so extensively
in making the finest porcelain, is the purest of all clays, and is
formed of weathered feldspar; floor tiling and sewer pipes are also
made from ground feldspar. Moonstone is clean, soda-lime feldspar,
whitish in color and with a reflection something like an opal.
LESSON CCXV
FELDSPAR
_Leading thought_--Feldspar is about five times as common as quartz.
The crystal is obliquely brick-shaped, and when broken splits in two
directions at right angles to each other. It is next in hardness to
quartz, and will scratch glass but will not cut it.
_Method_--If possible, have the common feldspar (orthoclase),
the soda-lime feldspar (oligoclase) and the lime-soda feldspar
(labradorite).
_Observations_--1. What is the shape of the feldspar crystal?
2. What colors are your specimens of feldspar? How many kinds have
you?
3. What is the luster of feldspar?
4. Can you scratch feldspar with the point of a knife? Can you
scratch it with quartz? Can you scratch glass with it?
5. When you scratch feldspar with steel what is the color of the
streak left upon it?
6. If feldspar is broken, does it break along certain lines, leaving
smooth faces? At what angles do these smooth faces stand to each
other?
7. How can you tell feldspar from quartz? Write a comparison of
feldspar and quartz, giving clearly the characteristics of both.
8. Hunt over the pebbles found in a sand-bank. Which ones are quartz?
Do you find any of feldspar?
9. When there is so much more feldspar than quartz in the earth’s
crust, why is there so much more quartz than feldspar in sand?
MICA
_Teacher’s Story_
The mica crystal when perfect is a flat crystal with six straight
edges. These crystals separate in thin layers parallel with the base.
In color mica varies, through shades of brown, from a pale smoked
pearl to black. Its luster is pearly, and it can be scratched with
the thumb nail. Its distinguishing characteristic is that the thin
layers into which it splits bend without breaking and endure great
heat.
Mica was used in antiquity for windows. Because it is transparent
and not affected by heat, it is used in the doors of stoves and
furnaces and for lamp chimneys. Its strength makes it of use for
automobile goggles. Diamond dust is powdered mica, as is also the
artificial snow scattered over cotton batting for the decoration of
Christmas trees. When ground finely, it is used as an absorbent for
nitroglycerine in the manufacture of dynamite.
Mica mines are scarce in this country. There is an interesting one in
North Carolina which had evidently been worked centuries before the
advent of the white man in America. There are other mica mines in New
Hampshire and Canada. The entire production of this mineral in the
United States for the year 1908, was valued at a little more than a
quarter of a million dollars. Nearly all of this output was used in
the electrical industries, since mica is one of the best insulating
materials known.
LESSON CCXVI
MICA
_Leading thought_--Mica is a crystal which flakes off in thin scales
parallel with the base of the crystal. We rarely see a complete mica
crystal but simply the thin plates which have split off. The ordinary
mica is light colored, but there is a black form.
_Method_--If it is not possible to obtain a mica crystal, get a thick
piece of mica which the pupils may split off into layers.
_Observations_--1. Describe your piece of mica. Pull off a layer with
the point of your knife. See if you can separate this layer into two
layers or more.
2. Can you see through mica? Can you bend it? Does it break easily?
What is the color of your specimen? What is its luster? Can you cut
it with a knife? Can you scratch it with the thumb nail? What color
is the streak left by scratching it with steel?
3. What are some of the uses of mica? How is it especially fitted for
some uses?
4. Write a theme on how and where mica is obtained.
GRANITE
_Teacher’s Story_
In granite, the quartz may be detected by its fracture, which is
always conchoidal and never flat; that is, it has no cleavage
planes. It is usually white or smoky, and is glassy in luster. It
cannot be scratched with a knife. The feldspar is usually whitish or
flesh-colored and the smooth surface of its cleavage planes shines
brilliantly as the light strikes upon it; it can be scratched with
a knife but this requires effort. The mica is in pearly scales,
sometimes whitish and sometimes black. The scales of these mica
particles may be lifted off with a knife, and it may thus be
distinguished. If there are black particles in the granite which do
not separate, like the mica, into thin layers, they probably consist
of hornblende.
[Illustration: _The granite obelisk still standing on the site of the
ancient city of On._
Photo by J. H. Comstock.]
Granite is used extensively for building purposes and for monuments.
It is a very durable stone but in the northeastern United States
where there is much rain and cold weather, the stone decays. Mica
is the weakest, hornblende next, and feldspar is next to quartz,
the strongest constituent of granite. Water permeates the mica,
hornblende, feldspar and sometimes the quartz, and by its expansion
in freezing causes the stone to crumble. The reason why polished
granite endures better than the rough finished, is that the smooth
surface gives less opportunity for the water to lodge and freeze.
When the weathered granite is cut up into small particles by the
waters of streams, they are sifted and all the parts which are
soluble are carried off, leaving a sand composed of quartz and mica,
which are insoluble. This sand is washed by streams into lakes, and
then is dropped to the bottom; if enough is thus carried and dropped,
it forms sandstone rock. All of our sandstones used for building
purposes were thus laid down.
Cleopatra’s Needle, which stood for thousands of years in the dry
climate of Egypt, soon commenced to weather and crumble when placed
in Central Park, N. Y. This shaft has a most interesting history. It
was quarried near Assuan, in the most famous of all granite quarries
of ancient Egypt. It was cut as a solid shaft in the quarry and
carried down the Nile River for 500 miles--an engineering feat which
would be hard to accomplish to-day, with all our modern appliances.
It was one of the obelisks that graced the ancient city of On, later
called Heliopolis, situated on a plateau near the present city of
Cairo; On was the city where Moses was born and reared. There is
still standing where it was first placed as a part of a magnificent
temple, the temple a part of a magnificent city, one of these
obelisks. It now stands alone in the middle of a great fertile plain,
which is vividly green with growing crops; a road shaded by tamarisk
and lebbakh trees leads to it; nearby is a sakiah, creaking as the
blindfolded bullock walks around and around, turning the wheel that
lifts the chain of buckets from the well to irrigate the crops; and
a hooded crow, whose ancestors were contemporaries of its erection,
caws hoarsely as it alights on the beautiful apex of this ancient
shaft, which has stood there nearly four thousand years and has seen
a great city go down to dust to fertilize a grassy plain.
LESSON CCXVII
GRANITE
_Leading thought_--Granite is composed of feldspar, quartz and mica,
and often contains hornblende.
_Method_--Specimens of coarse granite and a pocket knife are needed.
_Observations_--1. What minerals do you find in granite? How can you
tell what these minerals are? Look at the granite with a lens. How
can you tell the quartz from feldspar? Take a knife and scratch the
two. Can you tell them apart in that way? How can you tell the mica?
How can you tell the hornblende?
2. What buildings have you seen made of granite? What monuments have
you seen made from it?
3. Which mineral in granite is especially affected by water? Which
remains unharmed the longest?
4. What is weathering? Mention some of the characteristics of
weathering. Why does the rough-finished granite weather sooner than
that which is polished?
5. Examine some sand with a lens. What mineral do you find present in
it in the greatest quantity?
6. Write the story of the Cleopatra’s Needle in Central Park, New
York City.
CALCITE, MARBLE AND LIMESTONE
_Teacher’s Story_
Calc spar, or calcium carbonate, is a mineral and is the material of
which marble, limestone and chalk are made. The faces of the calcite
crystal are always arranged in groups of three or multiples of
three--a three-sided pyramid or two pyramids joined base to base. The
pyramids may be obtuse or acute. When acute and formed of three pairs
of faces, the crystals are called dog-tooth spar. The crystals appear
in a great variety of forms, but they all have the common quality
of splitting readily in three directions, the fragments resembling
cubes which are oblique instead of rectangular. When these cleaved,
or split pieces, are transparent, they are called Iceland spar. When
an object is viewed through Iceland spar at least one-quarter inch
thick, it appears double. The calcite crystal is often transparent
with a slight yellowish tinge, but it also shows other colors; and it
has a slightly cloudy or slightly pearly or almost glassy luster like
feldspar. It is easily scratched with a knife and will not scratch
glass. If a drop of strong vinegar or weak hydrochloric acid falls
upon it, it will effervesce.
[Illustration: _Forms of calcite crystals._]
Limestone--so called because it is burned to make quicklime--was
formed on the bottoms of oceans; its substance came chiefly from the
skeletons of corals and the shells of other sea creatures, since
sea-shells and coral stems are pure calcium carbonate in composition.
In the water, the shells and corals were broken down, and then
deposited in layers on the bottom of the sea. So wherever we find
limestone, we know that there was once the bottom of a great sea.
Such layers of limestone are now being deposited off the shores of
Florida, where corals grow in great abundance. Limestone is used
extensively for building purposes, and in most climates is very
durable. The great pyramids of Egypt are of limestone. It is not a
good material for making roads, since it is so soft that it wears out
readily, making a fine easily-blown dust. It is slowly dissolved in
water, especially if the water be acid; thus, in limestone regions,
there are caves where the water has dissolved out the rock; and
attached to their roofs and piled upon their floors may be large
icicle-shaped stalactites and stalagmites, which were made by the
lime-bearing water dripping down and evaporating, leaving its burden
in crystals behind it. When the roof of a cave falls in, the cavity
thus made is called a sink hole and is often dangerous. The famous
Natural Bridge in Virginia is all that is left of what was once the
roof of such a cavern. The water in limestone regions is always hard,
because of the lime which it holds in solution; and in such regions
the streams usually have no silt, but have clean bottoms; moreover,
the springs are likely to become contaminated because the water has
run through long caves instead of filtering through sand.
Chalk is similar in origin to limestone; it is made up of the shells
of minute sea creatures, so small that we can only see them with
the aid of a microscope. Try and think how many years it must have
required for the shells of such tiny beings to build up the beds
which make the great chalk cliffs of England!
Marble is formed inside of the earth from limestone, under the
influence of heat and pressure; it differs from limestone chiefly
in that the grains are of crystalline structure, and are larger;
it is usually white or gray in color, and sometimes is found in
differing colors. At Cadiz in California, marble is found showing
twenty or more quite different colors. The most famous marbles are
the Carrara of Italy, the Parian from the Island of Paros, and the
Pentelican from the mountain of that name near Athens. The reason
why these marbles are so famous is that in ancient times sculptors
carved beautiful statues from them, and the architects used them for
building magnificent temples. The principal marble deposits in the
United States are in Vermont, Georgia, Tennessee and California.
Marble deteriorates when it is exposed to air which is filled with
smoke and gases. It is also used to make lime. When either marble or
limestone is heated very hot, it separates into two parts, one of
which is lime, and the other carbonic acid gas--the same that is used
for charging soda-water fountains.
LESSON CCXVIII
CALCITE, MARBLE AND LIMESTONE
_Leading thought_--Calcite or calc spar is formed more than half of
lime. The best known forms of its crystals are cubelike, but instead
of having twelve right-angled edges, the sides are lozenge-shaped,
and are set together with six obtuse angles and six acute. Dog-tooth
spar is one form of calcite crystal. Limestone is a solid form of
calcite. Marble is granular limestone which shows the broken crystals
of calcite. Chalk is very fine, pulverized calcite.
_Method_--Specimens of dog-tooth spar, limestone, marble, shells of
oysters or other sea creatures and coral should be provided for this
lesson; also a bottle of dilute hydrochloric acid, and a piece of
glass tubing about six inches long with which to drop the acid on the
stones. Some strong vinegar will do instead of the acid.
_Observations_--1. What is the form of the calcite crystal? What
is the luster of the crystal? Is it the same as the inside of
sea-shells? Will calcite scratch glass? Can you scratch it with a
knife? What happens to calcite if you put a drop of weak hydrochloric
acid upon it?
2. Is marble made up of crystals? Examine it with a lens to see. What
is its color? Have you seen marble of other colors than white? Do you
know the reason why marble is sometimes clouded and streaked?
3. Put a drop of weak hydrochloric acid on the marble. What happens?
4. What are the uses of marble? What have you ever seen made from
marble? Why is it used for sculpture? What famous statues have you
seen which were made of marble? Name some of the famous ancient
marble buildings.
5. Test a piece of limestone for hardness. Can you scratch it with
a knife? Is it as soft as marble? Put on it a drop of acid. Does it
effervesce? If there are any fossils in your piece of limestone, test
them with acid and see if they will effervesce. Any other mineral
that you have which will effervesce when touched with acid, is
probably some form of calcite.
6. Are there any buildings in your town made of limestone? How do you
know the stone is limestone? Where was it obtained? Is it affected by
the weather?
7. Is limestone a good material for making or mending roads? Give a
reason.
8. Why is water in limestone regions hard? Why are limestone regions
likely to have caves within the rocks? How are stalactites and
stalagmites formed in caves? What are sink holes? How are they
formed? In what county of your state is limestone found?
9. How is the lime which is used for plastering houses made?
10. Write a theme on how the chalk rocks are made?
11. Test a shell with acid; test a piece of coral with acid. How does
it happen that these, which were once a part of living creatures, are
now limestone? Of what are our own bones made?
* * * * *
“_A great chapter in the history of the world is written in
the chalk. Few passages in the history of man can be supported
by such an overwhelming mass of direct and indirect evidence
as that which testifies to the truth of the fragment of the
history of the globe, which I hope to enable you to read,
with your own eyes, to-night. Let me add, that few chapters
of human history have a more profound significance for
ourselves. I weigh my words well when I assert, that the man
who should know the true history of the bit of chalk which
every carpenter carries about in his breeches-pocket, though
ignorant of all other history, is likely, if he will think his
knowledge out to its ultimate results, to have a truer, and
therefore a better, conception of this wonderful universe, and
of man’s relation to it, than the most learned student who is
deep-read in the records of humanity and ignorant of those of
Nature._”
“_During the chalk period, or ‘cretaceous epoch,’ not one
of the present great physical features of the globe was
in existence. Our great mountain ranges, Pyrenees, Alps,
Himalayas, Andes, have all been upheaved since the chalk
was deposited, and the cretaceous sea flowed over the sites
of Sinai and Ararat. All this is certain, because rocks of
cretaceous or still later date, have shared in the elevatory
movements which give rise to these mountain chains; and may be
found perched up, in some cases, many thousand feet high upon
their flanks._”
--THOMAS HUXLEY.
THE MAGNET
_Teacher’s Story_
[Illustration: U]
Until comparatively recent times, the power of the magnet was so
inexplicable that it was regarded as the working of magic. The tale
of the Great Black Mountain Island magnet described in the “Arabian
Nights Entertainments”--the story of the island that pulled the
nails from passing ships and thus wrecked them--was believed by the
mariners of the Middle Ages. Professor George L. Burr assures me that
this mountain of lodestone and the fear which it inspired were potent
factors in the development of Medieval navigation. Even yet, with
all our scientific knowledge, the magnet is a mystery. We know what
it does, but we do not know what it is. That a force unseen by us
is flowing off the ends of a bar magnet, the force flowing from one
end attracted to the force flowing from the other and repellent to a
force similar to itself, we perceive clearly. We also know that there
is less of this force at a point in the magnet half-way between the
poles; and we know that the force of the magnet acts more strongly if
we offer it more surface to act upon, as is shown in the experiment
in drawing a needle to a magnet by trying to attract it first at its
point and then along its length. That this force extends out beyond
the ends of the magnet, the child likes to demonstrate by seeing
across how wide a space the magnet, without touching the objects, can
draw to it iron filings or tacks. That the magnet can impart this
force to iron objects is demonstrated with curious interest, as the
child takes up a chain of tacks at the end of the magnet; and yet the
tacks when removed from the magnet have no such power of cohesion.
That some magnets are stronger than others is shown in the favorite
game of “stealing tacks,” the stronger magnet taking them away from
the weaker; it can also be demonstrated by a competition between
magnets, noting how many tacks each will hold.
One of the most interesting things about a magnet is that like poles
repel and opposite poles attract each other. How hard must we pull
to separate two magnets that have the south pole of one against the
north pole of the other! Even more interesting is the repellent power
of two similar poles, which is shown by approaching a suspended
magnetized needle with a magnet. These attractive and repellent
forces are most interestingly demonstrated by the experiment in
question 13 of the lesson. These needles floating on cork join the
magnet or flee from it, according to which pole is presented to them.
Not only does this power reside in the magnet, but it can be imparted
to other objects of iron and steel. By rubbing one pole of the magnet
over a needle several times, always in the same direction, the needle
becomes a magnet. If we suspend such a needle by a bit of thread
from its center, and the needle is not affected by the nearness
of a magnet, it will soon arrange itself nearly north and south.
It is well to thrust the needle through a cork, so it will hang
horizontally, and then suspend the cork by a thread. The magnetized
needle will not point exactly north, for the magnet poles of the
earth do not quite coincide with the poles of the earth’s axis.
The direction assumed by the magnetized needle may be explained by
the fact that the earth is a great magnet, but the south pole of the
great earth magnet lies near the North Pole of the earth. Thus, a
magnet on the earth’s surface, if allowed to move freely, will turn
its north pole toward the south pole of the great earth magnet.
Then, we might ask, why not call the earth’s magnetic pole that lies
nearest our North Pole its north magnetic pole? That is merely a
matter of convenience for us. We see that the compass needle points
north and south, and the arm of the needle which points north we
conveniently call its north pole.
The above experiment with a suspended needle shows how the mariner’s
compass is made. This most useful instrument is said to have been
invented by the Chinese, at least 1400 B. C., and perhaps even longer
ago. It was used by them to guide armies over the great plains, and
the needle was made of lodestone. The compass was introduced into
Europe about 1300 A. D., and has been used by mariners ever since. To
“box the compass” is to tell all the points on the compass dial, and
is an exercise which the children will enjoy.
We are able to tell the direction of the lines of force flowing from
a magnet, by placing fine iron filings on a pane of glass or a sheet
of paper and holding close beneath one or both poles of a magnet;
instantly the filings assume certain lines. If the two ends of a
horseshoe magnet are used, we can see the direction of the lines of
force that flow from one pole to the other. It is supposed that these
lines of magnetic force streaming from the ends of the great earth
magnet cause the Northern Lights, or _Aurora Borealis_.
Lodestone is a form of iron with a special chemical composition, and
it is a natural magnet. Most interesting stories are told of the way
the ancients discovered this apparently bewitched material, because
it clung to the iron ends of their staffs or pulled the iron nails
from their shoes. In the Ward’s collection of minerals sent out to
schools, which costs only one dollar, there is included a piece of
lodestone, which is of perennial interest to the children.
Magnets made from lodestone are called natural magnets. A bar magnet
or a horseshoe magnet has received its magnetism from some other
magnet or from electrical sources. An electro magnet is of soft iron,
and is only a magnet when under the influence of a coil of wire
charged with electricity. As soon as the current is shut off the iron
immediately ceases to be a magnet.
LESSON CCXIX
THE MAGNET
_Leading thought_--Any substance that will attract iron is called
a magnet, and the force which enables it to attract iron is called
magnetism. This force resides chiefly at the ends of magnets, called
the poles. The forces residing at the opposite ends of a magnet act
in opposite directions; in two magnets the like poles repel and the
unlike poles attract each other. The needle of the mariner’s compass
points north and south, because the earth is a great magnet which has
its south pole as a magnet at the North Pole of the world.
_Method_--Cheap toy horseshoe magnets are sufficiently good for this
lesson, but the teacher should have a bar magnet, also a cheap toy
compass, and a specimen of lodestone, which can be procured from any
dealer in minerals. In addition, there should be nails, iron filings
and tacks of both iron and brass, pins, darning needles or knitting
needles, pens, etc. Each child, during play time, should have a
chance to test the action of the magnets on these objects, and thus
be able to answer for himself the questions which should be given a
few at a time.
_Observations_--1. How do we know that an object is a magnet? How
many kinds of magnets do you know? Of what substance are the objects
made which the magnets can pick up? Does a magnet pick up as many
iron filings at its middle as at its ends? What does this show?
2. How far away from a needle must one end of the magnet be before
the needle leaps toward it? Does it make any difference in this
respect, if the magnet approaches the needle toward the point or
along its length? Does this show that the magnetic force extends out
beyond the magnet? Does it show that the magnetic force works more
strongly where it has more surface to act upon?
3. Take a tack and see if it will pick up iron filings or another
tack. Place a tack on one end of the magnet, does it pick up iron
filings now? What do you think is the reason for this difference in
the powers of the tack?
4. Are some magnets stronger than others? Will some magnets pull the
iron filings off from others? In the game of “stealing tacks,” which
can be played with two magnets, does each end of the magnet work
equally well in pulling the tacks away from the other magnet?
5. Pick up a tack with a magnet. Hang another tack to this one end
to end. How many tacks will it thus hold? Can you hang more tacks to
some magnets than to others? Will the last tack picked up attract
iron filings as strongly as the first next to the magnet? Why? Pull
off the tack which is next to the magnet. Do the other tacks continue
to hold together? Why? Instead of placing the tacks end to end, pick
up one tack with the magnet and place others around it. Will it hold
more tacks in this way? Why? If a magnet is covered with iron filings
will it hold as many tacks without dropping the filings?
6. Take two horseshoe magnets and bring their ends together. Then
turn one over and again bring the ends together. Will they cling to
each other more or less strongly than before? Bring two ends of two
bar magnets together; do they hold fast to each other? Change ends
with one, now do the two magnets cling more or less closely than
before? Does this show that the force in the two ends of a magnet is
different in character?
7. Magnetize a knitting needle or a long sewing needle by rubbing one
end of a magnet along its length twelve times, always in the same
direction, _and not back and forth_. Does a needle thus treated pick
up iron filings? Why?
8. Suspend this magnetized needle by a thread from some object where
it can swing clear. When it finally rests does it point north and
south or east and west?
9. Bring one end of a bar magnet or of a horseshoe magnet near to the
north end of the suspended needle; what happens? Bring the other end
of the magnet near the north end of the needle; what happens?
10. Magnetize two needles so that their eyes point in the same
direction when they are suspended. Then bring the point of one of
these needles toward the eye of the other, what happens? Bring the
eye of one toward the eye of the other, what happens? When a needle
is thus magnetized the end which turns toward the north is called the
north pole, and the end pointing south is called the south pole.
11. Try this same experiment by thrusting the needles through the
top of a cork and float them on a pan of water. Do the north poles
of these needles attract or repel each other? Do the south poles of
these needles attract or repel each other? If you place the north
pole of one needle at the south pole of the other do they join and
make one long magnet pointing north and south?
12. Take a pocket compass; place the north end of one of the
magnetized needles near the north arm of the compass needle; what
happens? Place the south pole of the needle near the north arm of
the compass needle, what happens? Can you tell by the action of your
magnet upon the compass needle which end of your magnet is the north
pole and which the south pole?
13. Magnetize several long sewing needles by rubbing some of them
toward the eye with the magnet and some from the eye toward the
point. Take some small corks, cut them in cross sections about
one-fourth inch thick, thrust a needle down through the center of
each leaving only the eye above the cork. Then set them afloat on a
pan of water. How do they act toward each other? Try them with a bar
magnet first with one end and then with the other, how do they act?
14. Describe how the needle in the mariner’s compass is used in
navigation.
15. Place fine iron filings on a pane of glass or on a stiff paper.
Pass a magnet underneath; what forms do the filings assume? Do they
make a picture of the direction of the lines of force which come from
the magnet? Describe or sketch the direction of these lines of force,
when the poles of a horseshoe magnet are placed below the filings.
Place two similar poles of a bar magnet beneath the filings; what
form do they take now?
16. What is lodestone? Why is it so called?
17. What is the difference between lodestone and a bar magnet? What
is an electro magnet?
18. Write an English theme on “The Discovery and Early Use of the
Mariner’s Compass.”
_Supplementary reading_--Electrical Experiments, Bonney; The Wonder
Book of Magnetism, Houston; “The Third Royal Calendar” from Arabian
Nights Entertainments.
* * * * *
“_Now, chief of all, the magnet’s power I sing,
And from what laws the attractive functions spring;
The magnet’s name the observing Grecians drew
From the magnetic regions where it grew;
Its viewless potent virtues men surprise,
Its strange effects they view with wondering eyes,
When, without aid of hinges, links, or springs,
A pendant chain we hold of steely rings
Dropt from the stone--the stone the binding source,--
Ring cleaves to ring, and owes magnetic force:
Those held superior, those below maintain,
Circle ’neath circle downward draws in vain,
Whilst free in air disports the oscillating chain._”
--“De Rerum Naturæ,” LUCRETIUS, 93–52 B. C.
[Illustration: _A tiller of the soil._]
THE SOIL
_Teacher’s Story_
_The soil is the sepulcher and the resurrection of all life
in the past. The greater the sepulcher the greater the
resurrection. The greater the resurrection the greater the
growth. The life of yesterday seeks the earth to-day that new
life may come from it tomorrow. The soil is composed of stone
flour and organic matter (humus) mixed; the greater the store
of organic matter the greater the fertility._
--JOHN WALTON SPENCER.
Because the child, after making mud pies, is told that his face is
dirty, he naturally concludes that all soil is dirt. But it is only
when out of place that it is dirt; for, in place, it is the home of
miracles--the matrix from which comes that wonderful force which we
call life. After the study of the brook, the crystals, the minerals
and the rocks, the pupils are ready for a more careful study of the
soil. However, most of the study in soils belongs to agriculture
rather than to nature-study.
_The Soil Makers_
If we could go back to the very beginning, we should find that the
soil consisted solely of broken off particles of rock--particles so
finely ground by nature’s forces that we might properly call them
“rock flour.” In our study of the brook, we noted that those stones
with sharp corners were just beginning their experience in the brook
mill, and those that were rounded out, forming pebbles, had their
corners ground off in the making of the soil grist. And in the work
of the brook we saw how this grinding was done, and how the soil
grist is sifted, sorted, carried and dropped.
[Illustration: _One of Uncle John’s nieces making stone flour._]
But there are other agencies besides water that help in grinding the
stone flour. If we visit some rocky cliff, we are sure to find at its
base a heap of stones, gravel and soil, which the geologists call
_talus_. In our eastern country we know that these pebbles and soil
were pried loose by Jack Frost with his ice wedges. The water filters
into all the cracks and crevices of the rock, and since water, when
freezing, is obliged to expand, the particles of rock were thereby
torn loose and forced off and fell to the bottom of the cliff.
Moreover, rocks expand when hot, and are often thus broken without
the aid of water and frost. In the rocks of the desert, the changes
in temperature pry off the rock particles, which the winds carry
away to make up the sands of the desert. The winds hurl these sands
against other rocks which are still standing, and hurl them with
such force that more particles are torn off, making more sand. In
fact, the wind, in some regions, grinds the rocks into stone flour as
effectually as does the water in other places. Then, too, the gases
of the air also cause rocks to decay. We know how iron rusts and
falls to pieces through contact with the gases of the air. Some rocks
decompose in a similar way. We often see that the inscriptions on old
headstones have been almost obliterated, because the gases in the air
have so decomposed the marble.
[Illustration: _Lichens growing on rocks._
Photo by Verne Morton.]
In addition to the other soil makers, there are the little plants
which we call lichens. The spores of these plants are so minute
that we cannot see them, and they drift about in the air until they
find resting place upon some rock. Here they begin to grow, and as
they grow they become strongly acid; they are thus enabled to eat a
foothold into the rock, softening its surface and powdering it into
stone flour. And in these situations other plants grow later, sending
their roots down into every crack and crevice and thus prying off
more of the rock.
_The Soil Carriers_
In the study of the brook we have seen how the water lifts, carries
and deposits the soils; and since, at one time or another, the entire
surface of the earth has been under water, we can see that water
has been the most important of the soil carriers and has done the
greatest work. The wind carries much soil, especially in the arid
regions; the movements of the sand dunes in the deserts and on the
seashores bear witness to what the wind can do as a soil carrier.
But in the northern United States, from New England to the Dakotas,
much of our soil has been carried by a great ice river that once
upon a time flowed down upon our lands from the North. This great,
slow-moving river, perhaps a mile or more high, plowed up the soil
and stones, and freezing them fast carried and shoved them along
under its great weight. After a time the ice melted and dropped its
burden. Many of the stones were of granite taken up from the old
mountains of northern Canada and ground off and rounded during their
journey. We call these stones which were brought down to us from the
North, “boulders;” and the soils which were brought along on the
bottoms of glaciers and dropped and pressed down by the tremendous
ice weight and thus made compact although unsorted, we call “hardpan.”
_The Kinds of Soil_
By the work of these soil makers and soil carriers, the rock flour
was made. But if we should take some of it and plant our seeds in it,
we should find that they would not grow thriftily, even though we
watered them and gave them every care. The reason for this is that
most rock flour does not have in it the substances which the plants
most need for their growth. But if we should go to the woods and get
some of the black woods-earth and mix it with rock flour, we should
find that our plants would thrive. This rich, earth mold in the
forest is almost wholly made up of matter once alive, but which is
now decayed, and which we call “humus.” The more humus that we have
in the rock flour, the richer it is in plant food, and the more plant
growth it will support.
In general, soils may be divided into clay, sand, gravel, loam and
humus.
Clay in its purest state is kaolinite, the result of weathering of
feldspar, or mica. It is finely powdered and is used for pottery,
while the less pure clays are used for brickmaking. Clayey soil is
sticky and slippery when wet, and bakes hard and cracks when dry. It
is hard to cultivate, but it absorbs moisture from the air and holds
fast to its fertility, and is especially good for permanent pastures
and meadows.
Sand, in a pure state, is made up mostly of finely broken particles
of quartz and feldspar, and is used for the making of glass. A sandy
soil is light and open and easy to work. It absorbs little water from
the air and has little power for holding plant food, since the water
washes it out. It is especially valuable for truck gardening, because
it is a warm soil. It is warm because water does not evaporate from
its surface rapidly.
Humus is composed of decayed animal and vegetable matter. It is very
rich in plant food. Wherever there is humus in the soil it is likely
to be darker in color than the stone flour.
Loam is a mixture of clay, sand and humus. For many crops it is the
most desirable soil.
LESSON CCXX
THE SOIL
_Leading thought_--The soil is composed of rock flour and humus.
Soil, to support life, must be porous, so that the roots of the
plants may receive through it both water and air.
_Method_--The children should bring in specimens of soils from
various localities near the school. Parts of each specimen should be
wet to see if they are clayey, that quality showing quickly in the
puttylike adhesiveness when rubbed between the fingers. It would be
well to get some pure blue clay, and let the children make marbles
of it to impress upon them this quality of clay. They should try and
make marbles of other soils to show the lack of adhesiveness in them.
They should examine sand through a lens and should examine humus in a
similar way. After they are familiar with these three kinds of soils,
they are ready for the lesson.
_Observations_--1. Look at any kind of soil with a lens, and tell why
you think it is made up of small pieces of stone and rock.
2. Take a piece of rock and pound it fine. What does it look like? Do
you think that your plants will grow well if you plant them in the
rock flour which you have just made? Try the experiment and describe
the results.
3. How does the water grind off the stones and make soil? How does
the wind do it?
4. How do water and frost pry off pieces of rock? Is there a cliff
in your neighborhood that has at its foot a heap of soil and stones?
Where did these come from?
5. How do the lichens and other plants pry off the outside of rocks?
Have you ever found lichens growing on stones?
6. Have you ever noticed old headstones in the cemetery that were
falling to pieces? What causes them to decay?
7. Write an English theme on the great glacier that formerly covered
the northeastern portion of the United States.
8. Go to the woods, scrape off the leaves and get some of the black
earth beneath them. Of what is this soil composed? Is it rock flour?
What makes it so black? Why do you call this soil rich? What does
it do if you add it to the soil in the pots where your flowers are
growing?
9. Find a railroad cut or some other place where the earth is exposed
for some distance up and down. Is there solid rock at the bottom? How
deep is the soil above the rock? Is the soil the same color at the
surface as it is below? Why is this?
10. _Experiment 1: To show which kinds of soil hold most water_--Take
three lamp chimneys, or bottles from which the bottoms have been
broken. Place in one loam, in another clay, in another fine-grained
sand, using in each case the same amount. Tie cheesecloth over the
bottom, so that the soil will not fall out; make the soil compact by
jarring down. Place each over a tumbler. From a cup of water, held
as near as possible to the soil, pour water into one of the bottles
slowly, so as to keep the surface of the soil covered. Consult a
watch and note how long before the water begins dripping below. Do
the same with the other two. Compare the results. Which soil takes
the water most rapidly? Which lets it through first? Which lets
through the most? How would rain affect fields of clayey soil? Of
sandy soil? Of loam?
_Hints for teacher on Experiment No. 1_--Through sand the water
passes very rapidly--in less than a minute if the sand is coarse. It
takes several minutes (14 min.) to go through loam, but requires some
hours to appear below the clay. It requires more water to saturate
clay. Care should be taken to use the same amount of water on the
three kinds of soil. More than one application will be required for
clay, since the amount of water accommodated in the chimney above the
soil will not be sufficient to saturate clay.
[Illustration:
_Loam._ _Sand._ _Clay._
Note that the sand has allowed the most water to drip through it, the
loam next, while no water has passed through the clay.]
More water will be found to have percolated through sand than through
loam or clay. The latter are more retentive of moisture than is
sand, although absorbing rain less readily than sand. The mixture of
sand and clay in loam is most ideal for cultivated fields, absorbing
moisture more readily than clay and retaining it better than sand.
_Experiment 2_--Fill a glass tumbler with very small marbles or
buckshot. Pour water over them to fill the glass. Placing cheesecloth
over the top of the tumbler pour off all the water that easily drains
away. Remove the cheesecloth, and immediately examine the marbles for
the film of water which surrounds each one and can clearly be seen
where one marble comes in contact with another marble or the side of
the glass.
_Hints for teacher on Experiment 2_--It is such a film of water as
remains on the marbles that on each particle of soil supplies the
plant with water and food. The water between the marbles has been
drained off. This water corresponds to that carried out of the soil
by drainage; it is injurious to the plant, keeping “its feet too
wet,” and should be removed.
_Experiment 3--To show that soil lifts water up from below_--Use the
same soils arranged in the same way as for Experiment 1, but instead
of pouring water in at the top, place the three lamp chimneys in a
pan which has water in it about an inch deep. In which soil does the
water rise most rapidly? In which does it rise the highest? After
the water has been taken up, let the soil stand in the lamp chimneys
for several days. Which soil dries out the soonest? If we had three
fields, one of loam, one of clay, and one of sand, in which would
the most water be lifted from below for the use of the plants? Which
would retain the water longest?
[Illustration:
_Sand._ _Clay._ _Loam._
The water has nearly reached the upper surface of the sand and is
halfway up the loam; in the clay it has climbed but a short distance.
]
_Hints for teacher on Experiment 3_--Water rises through the sand
in a short time; if rather fine sand is used it requires less than
half an hour. To rise through loam it will require three or four
times as long, and may not reach the top of the clay for several
days. If the glass tubes were three or four feet long and allowed to
stand for several days, we would find that although the water climbs
very slowly through the clay it will climb to a greater height in
clay than in loam or sand. Under field conditions clay will retain
moisture for a longer time than sand or loam.
_Experiment 4--To show that mulch keeps the water from evaporating
from soils_--Take two of the lamp chimneys filled half full with
loam. Pour in the same amount of water in each until the soil is
thoroughly wet. Cover the top of one with an inch deep of dry, loose
earth. Which dries out first? What does the loosening and pulverizing
of the soil in our fields by harrowing do for our planted crops? What
is a mulch?
[Illustration:
_The unmulched loam in the chimney at the left dried out in four
days. The loam covered with a dust mulch in the other chimney
retained moisture for a month._ ]
_Hints for teacher on Experiment 4_--The soil covered with a layer
of dry soil--a dust mulch--will retain moisture much longer than the
unmulched soil. Hence, the farmer or gardener loosens and pulverizes
the top soil by harrowing, hoeing or raking in order to retain
moisture for plant roots. A mulch may also be of straw, boards,
leaves or stones. Fallen leaves form a natural mulch in the woods.
There, at any time, under this covering, may be found moist earth. A
mulch is a soil cover which breaks the capillary connection, so that
water will not rise to the surface to be evaporated. To be efficient
a mulch must be _dry_. After rain the “dust blanket” on the garden
bed should be renewed by cultivation.
_Experiment 5_--Fill several vials with different soils from fields
in the neighborhood. If the soil in any of the vials is dry, moisten
it. Take a piece of blue litmus paper and press down into the soil in
each vial. Does the litmus paper turn red as it becomes dampened by
the soil in any of the vials? If so, this soil is acid. Add a little
lime and mix it in thoroughly with the soil in the vial that shows
the acid soil. Test it again with the litmus paper. Does the paper
remain blue or turn red? Does alfalfa and clover grow on acid soils?
Why should we add lime to such soils?
[Illustration: _Experiment to show the proper treatment of clay
soil._]
_Hints for teacher on Experiment 5_--A slightly acid soil may show no
reaction with litmus paper. It may be well to have a prepared soil
with a few drops of vinegar or other acid added, which will show the
reaction. The addition of lime will correct the acid condition. Soils
for alfalfa or clover should never be acid. They are usually well
limed before an attempt is made to grow these legumes.
_Experiment 6, which indicates the proper treatment of clay
soils_--Fill four pie tins with clay which has been wet and smoothly
puddled. In one mix with the clay a small portion of lime; in another
add a larger portion of muck; leave two with pure clay, and place one
of these out-of-doors where it will freeze hard. Then place the four
tins on a shelf and allow to dry. In which of these is the clay most
friable? In which is it the hardest?
_Hints to the teacher on Experiment 6_--This experiment shows that
freezing the clay rendered it finer, so that it may be broken easily
into particles small enough to set closely about the plant’s roots.
The clay mixed with lime is much more friable than the one mixed with
muck, showing that clay needs lime more than organic matter to make
it of greatest use. The pure clay which is dried without freezing
hardens into large, flat pieces, each being almost as hard as stone.
_Supplementary reading_--Ch. I, II, III in The Great World’s Farm,
Gaye: Ch. IV. in Practical Forestry, Gifford.
* * * * *
_Beside the moist clods the slender flags arise filled with
the sweetness of the earth. Out of the darkness--under that
darkness which knows no day save when the ploughshare opens
its chinks--they have come to the light. To the light they
have brought a colour which will attract the sunbeams from now
till harvest._
--RICHARD JEFFERIES.
* * * * *
“_Here is a problem, a wonder for all to see.
Look at this marvelous thing I hold in my hand!
This is a magic surprising, a mystery
Strange as a miracle, harder to understand._
_What is it? Only a handful of dust: to your touch
A dry, rough powder you trample beneath your feet,
Dark and lifeless; but think for a moment, how much
It hides and holds that is beautiful, bitter, or sweet._
_Think of the glory of color! The red of the rose,
Green of the myriad leaves and the fields of grass,
Yellow as bright as the sun where the daffodil blows,
Purple where violets nod as the breezes pass._
_Strange, that this lifeless thing gives vine, flower, tree,
Color and shape and character, fragrance too;
That the timber that builds the house, the ship for the sea,
Out of this powder its strength and its toughness drew!”_
--From “Dust,” CELIA THAXTER.
* * * * *
_Some years ago there was received at Cornell University a
letter from a boy working upon a farm in Canada. In this
letter he said_:
“_I have read your leaflet entitled, ‘The Soil, What It Is,’
and as I trudged up and down the furrows every stone, every
lump of earth, every shady knoll, every sod hollow had for me
a new interest. The day passed, the work was done, and I at
least had had a rich experience._”
[Illustration: _Fog on Mount Tamalpais, California._
Photo by G. K. Gilbert.]
WATER FORMS
_Teacher’s Story_
Water, in its various changing forms, is an example of another
overworked miracle--so common that we fail to see the miraculous in
it. We cultivate the imagination of our children by tales of the
Prince who became invisible when he put on his cap of darkness, and
who made far journeys through the air on his magic carpet. And yet
no cap of darkness ever wrought more astonishing disappearances than
occur when this most common of our earth’s elements disappears from
under our very eyes, dissolving into thin air. We cloak the miracle
by saying “water evaporates,” but think once of the travels of one of
these drops of water in its invisible cap! It may be a drop caught
and clogged in a towel hung on the line after washing, but as soon
as it dons its magic cap, it flies off in the atmosphere invisible
to our eyes; and the next time any of its parts are evident to our
senses, they may occur as a portion of the white masses of cloud
sailing across the blue sky, the cloud which Shelley impersonates:
“I am the daughter of Earth and Water,
And the nursling of the Sky;
I pass through the pores of the ocean and shores;
I change, but I cannot die.”
We have, however, learned the mysterious key-word which brings back
the vapor spirit to our sight and touch. This word is “cold.” For if
our drop of water, in its cap of darkness, meets in its travels an
object which is cold, straightway the cap falls off and it becomes
visible. If it be a stratum of cold air that meets the invisible
wanderer, it becomes visible as a cloud, or as mist, or as rain. If
the cold object be an ice pitcher, then it appears as drops on its
surface, captured from the air and chained as “flowing tears” upon
its cold surface. And again, if it be the cooling surface of the
earth at night that captures the wanderer, it appears as dew.
But the story of the water magic is only half told. The cold brings
back the invisible water vapor, forming it into visible drops; but if
it is cold enough to freeze, then we behold another miracle, for the
drops are changed to crystals. The cool window-pane at evening may
be dimmed with mist caught from the air of the room; if we examine
the mist with a lens we find it composed of tiny drops of water. But
if the night be very cold, we find next morning upon the window-pane
exquisite ferns, or stars, or trees, all formed of the crystals grown
from the mist which was there the night before. Moreover, the drops
of mist have been drawn together by crystal magic, leaving portions
of the glass dry and clear.
If we examine the grass during a cool evening of October we find
it pearled with dew, wrung from the atmosphere by the permeating
coolness of the surface of the ground. If the following night be
freezing cold, the next morning we find the grass blades covered with
the beautiful crystals of hoar frost.
[Illustration: _Composite snow crystal formed in high and medium
clouds._
Photomicrograph by W. A. Bentley.]
If a raincloud encounters a stratum of air cold enough to freeze,
then what would have been rain or mist comes down to us as sleet,
hail or snowflakes, and of all the forms of water crystals, that of
snow in its perfection is the most beautiful; it is, indeed, the most
beautiful of all crystals that we know. Why should water freezing
freely in the air so demonstrate geometry by forming, as it does, a
star with six rays, each set to another, at an angle of 60 degrees?
And as if to prove geometry divine beyond cavil, sometimes the rays
are only three in number--a factor of six--and include angles of
twice 60 degrees. Moreover, the rays are decorated, making thousands
of intricate and beautiful forms; but if one ray of the six is
ornamented with additional crystals the other five are decorated
likewise. Those snow crystals formed in the higher clouds and,
therefore, in cooler regions may be more solid in form, the spaces
in the angles being built out to the tips of the rays including air
spaces set in symmetrical patterns: and some of the crystals may
be columnar in form, the column being six-sided. While those snow
crystals formed in the lower currents of air, and therefore in warmer
regions, show their six rays marvellously ornamented. The reason why
the snow crystals are so much more beautiful and perfect than the
crystals of hoar frost or ice, is because they are formed from water
vapor, and grow freely in the regions of the upper air. Mr. W. A.
Bentley, who has spent many years photographing the snow crystals,
has found more than 1300 distinct types.
[Illustration: _Snow crystal formed in high clouds._
Photomicrograph by W. A. Bentley.]
The high clouds are composed of ice crystals formed from the cloud
mists; such ice clouds form a halo when veiling the sun or the moon.
When the water changes to vapor and is absorbed into the atmosphere,
we call the process evaporation. The water left in an open saucer
will evaporate more rapidly than that in a covered saucer, because
it comes in contact with more air. The clothes which are hung on the
line wet, dry more rapidly if the air is dry and not damp; for if the
air is damp, it means that it already has almost as much water in it
as it can hold. The clothes will dry more rapidly when the air is
hot, because hot air takes up moisture more readily and holds more of
it than does cold air. The clothes will dry more rapidly on a windy
day, because more air moves over them and comes in contact with them
than on a still day.
[Illustration: _Blizzard type of snow crystal formed in low cloud._
Photomicrograph by W. A. Bentley.]
If we observe a boiling teakettle, we can see a clear space of
perhaps an inch or less in front of the spout. This space is filled
with steam, which is hot air saturated with hot water vapor. But what
we call “steam” from a kettle, is this same water vapor condensed
back into thin drops of water or mist by coming into contact with the
cooler air of the room. When the atmosphere is dry, water will boil
away much more rapidly than when the air is damp.
The breath of a horse, or our own breath, is invisible during a warm
day; but during a cold day, it is condensed to mist as soon as it is
expelled from the nostrils and comes in contact with the cold air.
The one who wears spectacles finds them unclouded during warm days;
but in winter the glasses become cold out of doors, and as soon as
they are brought into contact with the warmer, damp atmosphere of a
room, they are covered with a mist. In a like manner, the window-pane
in winter, cooled by the outside temperature, condenses on its inner
surface the mist from the damp air of the room.
The water vapor in the atmosphere is invisible, and it moves with
the air currents until it is wrung out by coming into contact with
the cold. The air thus filled with water vapor may be entirely
clear near the surface of the earth; but, as it rises, it comes in
contact with cooler air and discharges its vapor in the form of mist,
which we call clouds; and if there is enough vapor in the air when
it meets a cold current, it is discharged as rain and falls back
to the earth. Thus, when it is very cloudy, we think it will rain,
because clouds consist of mist or fog; and if they are subjected to
a colder temperature, the mist is condensed to rain. Thus, often in
mountainous regions, the fog may be seen streaming and boiling over
a mountain peak, and yet always disappears at a certain distance
below it. This is because the temperature around the peak is cold and
condenses the water vapor as fast as the wind brings it along, but
the mist passes over and soon meets a warm current below and, presto,
it disappears! It is then taken back into the atmosphere. The level
base of a cumulus cloud has a stratum of warmer air below it, and
marks the level of condensation.
[Illustration: _Dew on spider’s web; Dewdrops on strawberry leaf;
Hoar frost on strawberry leaf._
Photographs by W. A. Bentley.]
At the end of the day, the surface of the ground cools more quickly
than the air above it. If it becomes sufficiently cold and the air
is damp, then the water from it is condensed and dew is formed
during the night. However, all dew is not always condensed from the
atmosphere, since some of it is moisture pumped up by the plants,
which could not evaporate in the cold night air. On windy nights,
the stratum of air cooled by the surface of the earth is moved along
and more air takes its place, and it therefore does not become
cold enough to be obliged to yield up its water vapor as dew. If
the weather during a dewy night becomes very cold, the dew becomes
crystallized into hoar frost. The crystals of hoar frost are often
very beautiful and are well worth our study.
The ice on the surface of a still pond begins to form usually around
the edges first, and fine, lancelike needles of ice are sent out
across the surface. It is a very interesting experience to watch the
ice crystals form on a shallow pond of water. This may easily be
seen during cold winter weather. It is equally interesting to watch
the formation of the ice crystals in a glass bottle or jar. Water,
in crystallizing, expands, and requires more room than it does as
a fluid; therefore, as the water changes to ice it must have more
room, and often presses so hard against the sides of the bottle as
to break it. The ice in the surface soil of the wheat fields expands
and buckles, holding fast in its grip the leaves of the young wheat
and tearing them loose from their roots; this “heaving” is one cause
for the winter-killing of wheat. Sleet consists of rain crystallized
in the form of sharp needles. Hail consists of ice and snow compacted
together, making the hard, more or less globular hailstones.
LESSON CCXXI
WATER FORMS
_Leading thought_--Water occurs as an invisible vapor in the air
and also as mist and rain; and when subjected to freezing, it
crystallizes into ice and frost and snow.
[Illustration: _Frost crystals on window-pane._
Photo by W. A. Bentley.]
_Method_--The answers to the questions of this lesson should, as far
as possible, be given in the form of a demonstration. All of the
experiments suggested should be tried, and the pupils should think
the matter out for themselves. In the study of the snow crystals a
compound microscope is a great help, but a hand lens will do. This
part of the work must be done out of doors. The most advantageous
time for studying the perfect snow crystals is when the snow is
falling in small, hard flakes; since, when the snow is soft, there
are many crystals massed together into great fleecy flakes, and they
have lost their original form. The lessons on frost or dew may be
given best in the autumn or spring.
_Observations_--1. Place a saucer filled with water near a stove or
radiator; do not cover it nor disturb it. Place another saucer filled
with water near this but cover it with a tight box. From which saucer
does the water evaporate most rapidly? Why?
2. We hang the clothes, after they are washed, out of doors to dry;
what becomes of the water that was in them? Will they dry more
rapidly during a clear or during a damp day? Why? Will they dry more
rapidly during a still or during a windy day? Why? Will they dry more
rapidly during hot or cold weather? Why?
3. Watch a teakettle of water as it is boiling. Notice that near its
spout there is no mist, but what we call steam is formed beyond this.
Why is this so? What is steam? Why does water boil away? Do kettles
boil dry sooner on some days than on others? Why?
4. If the water disappears in the atmosphere where does it go? Why do
we say “the weather is damp”? What force is it that wrings the water
out of the atmosphere?
[Illustration: _Forms of hoar frost._
Photo by W. A. Bentley.]
5. Why does the breath of a horse show as a mist on a cold day? Why
do persons who wear spectacles find their glasses covered with mist
as soon as they enter a warm room after having been out in the cold?
Why do the window-panes become covered with mist during cold weather?
Is the mist on the outside or on the inside? Why does steam show as a
white mist? Why does the ice pitcher, on a warm day, become covered
on the outside with drops of water? Would this happen on a cold day?
Why not?
6. Why, when the water is invisible in the atmosphere, does it become
visible as clouds? What causes the lower edges of cumulus clouds to
be so level? What is fog? Why do clouds occur on mountain peaks? What
causes rain?
7. What causes dew to form? When the grass is covered with dew, are
the leaves of the higher trees likewise covered? Why not? What kind
of weather must we have in order to have dewy nights? What must be
the atmosphere of the air in relation to that of the ground in order
to condense the dew? Does dew form on windy nights? Why not? Does
all dew come from the air, or does some of it come from the ground
through the plants? Why is not this water, pumped up by the plants,
evaporated?
8. What happens to the dew if the weather becomes freezing during the
night? What is hoar frost? Why should water change form when it is
frozen? How many forms of frost crystals can you find on the grass on
a frosty morning?
9. When a pond begins freezing over, what part of it freezes first?
Describe how the first layer of ice is formed over the surface.
10. Place a bottle of water out of doors in freezing weather. How
does the ice appear in it at first? What happens later? Why does the
bottle break? How is it that water which has filled the crevices of
rocks scales off pieces of the rock in cold weather? Why does winter
wheat “winter-kill” on wet soil?
11. Why does frost form on a window-pane? How many different figures
can you trace on a frosted pane? Are there any long, needlelike
forms? Are there star forms? Can you find forms that resemble ferns
and trees? Do you sometimes see, on boards or on the pavement, frost
in forms like those on the window-pane?
[Illustration: _High cloud snow crystal._
Photomicrograph by W. A. Bentley.]
12. When there is a fine, dry snow falling, take a piece of dark
flannel and catch some flakes upon it. Examine them with a lens,
being careful not to breathe upon them. How many forms of snow
crystals can you find? How many rays are there in the star-shaped
snow crystals? Do you find any solid crystals? Can you find any
crystals that are triangular? When the snow is falling in large,
feathery flakes, can you find the crystals? Why not?
13. What is the difference between a hailstone and a snow crystal?
What is sleet?
_Supplementary reading_--Water Wonders, Thompson; Forms of Water,
Tyndall.
* * * * *
“_When in the night we wake and hear the rain
Which on the white bloom of the orchard falls,
And on the young, green wheat-blades, where thought recalls
How in the furrow stands the rusting plow,
Then fancy pictures what the day will see--
The ducklings paddling in the puddled lane,
Sheep grazing slowly up the emerald slope,
Clear bird-notes ringing, and the droning bee
Among the lilac’s bloom--enchanting hope--
How fair the fading dreams we entertain,
When in the night we wake and hear the rain!_”
--ROBERT BURNS WILSON.
* * * * *
“_The thin snow now driving from the north and lodging on my coat
consists of those beautiful star crystals, not cottony and chubby
spokes, but thin and partly transparent crystals. They are about
a tenth of an inch in diameter, perfect little wheels with six
spokes without a tire, or rather with six perfect little leaflets,
fern-like, with a distinct straight and slender midrib, raying from
the center. On each side of each midrib there is a transparent thin
blade with a crenate edge. How full of creative genius is the air
in which these are generated! I should hardly admire more if real
stars fell and lodged on my coat. Nature is full of genius, full
of divinity. Nothing is cheap and coarse, neither dewdrops nor
snowflakes._”
“_A divinity must have stirred within them before the crystals did
thus shoot and set. Wheels of storm-chariots. The same law that
shapes the earth-star shapes the snow-stars. As surely as the petals
of a flower are fixed, each of these countless snow-stars comes
whirling to earth, pronouncing thus, with emphasis, the number six._”
--THOREAU’S JOURNAL.
[Illustration: _Temple of the winds at Athens._
Photo by J. H. Comstock.]
THE WEATHER
BY WILFORD M. WILSON
Section Director, U. S. Weather Bureau, and Professor of Meteorology
in Cornell University.
[Illustration:
_Composite snow crystal; the center formed in a high cloud and
the margins in a lower cloud._
Photomicrograph by W. A. Bentley.]
The atmosphere, at the bottom of which we live, may be compared to
a great ocean of air, about two hundred miles deep, resting upon
the earth. The changes and movements that take place in this ocean
of air, the storms that invade it, the clouds that float in it, the
sunshine, the rain, the dew, the sleet, the frost, the snow, and the
hail are termed “weather.” We live in it; we partake of its moods;
we reflect its sunshine and shadows; it invades the everyday affairs
of life, influences every business and social activity, and molds
the character of nations; and yet nearly everything we know about
the weather has been learned within the lifetime of the present
generation. Not that the weather did not interest men of early times,
but the problem appeared to be so complicated and so complex that it
baffled their utmost endeavors.
THE TEMPLE OF THE WINDS AT ATHENS
The Temple of the Winds, erected probably about five hundred years B.
C., indicates the knowledge of the weather possessed by the ancient
Greeks. This temple is a little octagon tower, the eight sides of
which face the eight principal winds. On each of its eight sides is a
human figure cut in the marble, symbolizing the kind of weather the
wind from that particular direction brought to Athens.
[Illustration: _Snow crystal._
Photomicrograph by W. A. Bentley.]
Boreas, the cold north wind, is represented by the figure of an old
man wearing a thick mantle, high buskins (boots) and blowing on a
“weathered horn.” The northeast wind, which brought, and still brings
to Athens, cold, snow, sleet and hail, is symbolized by a man with
a severe countenance who is rattling slingstones in a shield, thus
expressing the noise made by the falling hail and sleet.
The east wind, which brought weather favorable to the growth of
vegetation, is shown by the figure of a beautiful youth bearing fruit
and flowers in his tucked-up mantle.
Natos, the warm south wind, brought rain, and he is about to pour the
water over the earth from the jar which he carries.
Lips, the southwest wind, beloved of the Greek sailors, drives a ship
before him, while Zephros, the gentle west wind, is represented by a
youth lightly clad, scattering flowers as he goes.
The northwest wind, which brought dry and sometimes hot weather to
Athens, is symbolized in the figure of a man holding a vessel of
charcoal in his hands. Thus, the character of the weather brought by
each separate wind is fixed in stone, and from this record we learn
that, even with the lapse of twenty centuries, there has come no
material change.
HISTORICAL
[Illustration: _Snow crystal._
Photomicrograph by W. A. Bentley.]
There is no record of any rational progress having been made in
the study of the weather until about the middle of the seventeenth
century, when Torricelli discovered the principles of the barometer.
This was a most important discovery and marks the beginning of the
modern science of meteorology. Soon after Torricelli’s discovery
of the barometer his great teacher, Galileo, discovered the
thermometer, and thus made possible the collection of data upon
which all meteorological investigations are based. About one hundred
years after the discovery of the barometer, Benjamin Franklin made
a discovery of equal importance. He demonstrated that storms were
eddies in the atmosphere, and that they progressed or moved as a
whole, along the surface of the earth.
It might be interesting to learn how Franklin made this discovery.
Franklin, being interested at that time in astronomy, had arranged
with a friend in Boston to take observations of a lunar eclipse
at the same time that he, himself, was to take observations at
Philadelphia. On the night of the eclipse a terrific northeast wind
and rain storm set in at Philadelphia, and Franklin was unable to
make any observations. He reasoned, that as the wind blew from the
northeast, the storm must have been experienced in Boston before it
reached Philadelphia. But imagine his surprise, when he heard from
his friend in Boston that the night had been clear and favorable
for observation, but that a fierce wind and rain storm set in on
the following morning. Franklin determined to investigate. He sent
out letters of inquiry to all surrounding mail stations, asking for
the time of the beginning and ending of the storm, the direction
and strength of the wind, etc. When the information contained in
the replies was charted on a map it showed that, at all places
to the southwest of Philadelphia, the beginning of the storm was
earlier than at Philadelphia, while at all places to the northeast
of Philadelphia the beginning of the storm was later than at
Philadelphia. Likewise, the ending was earlier to the southwest and
later to the northeast of Philadelphia than at Philadelphia. He also
found that the winds in every instance passed through a regular
sequence, setting in from some easterly point and veering to the
south as the storm progressed, then to the southeast and finally
to the west or northwest as the storm passed away and the weather
cleared.
A further study of these facts convinced Franklin that the storm was
an eddy in the atmosphere, and that the eddy moved as a whole from
the southwest toward the northeast, and that the winds blew from all
directions toward the center of the eddy, impelled by what he termed
suction.
Franklin was so far in advance of his time that his ideas about
storms made little impression on his contemporaries, and so it
remained for Redfield, Espy, Loomis, Henry and Maury and other
American meteorologists, a hundred years later, to show that Franklin
had gained the first essentially correct and adequate conception of
the structure and movement of storms.
During the first half of the nineteenth century, considerable
progress was made in the study of storms, principally by American
meteorologists, among whom was William Redfield of New York, who
first demonstrated that storms had both a rotary and progressive
movement. James Espy followed Redfield in the construction of weather
maps, although he had already published much on meteorological
subjects before the latter entered the field.
Professor Joseph Henry, Secretary of the Smithsonian Institution
at Washington, was the first to prepare a daily weather map from
observations collected by telegraph. He made no attempt to make
forecasts, but used his weather map to demonstrate to members of
Congress the feasibility of a national weather service.
An incident occurred during the Crimean War that gave meteorology a
great impetus, especially in Europe. On November 10th, 1854, while
the French fleet was at anchor in the Black Sea, a storm of great
intensity occurred which practically destroyed its effectiveness
against the enemy. The investigation that followed showed that the
storm came from western Europe, and had there been adequate means of
communication and its character and direction of progress been known,
it would have been possible to have warned the fleet of its approach
and thus afforded an opportunity for its protection.
This report created a profound impression among scientific men and
active measures were taken at once that resulted in the organization
of weather services in the principal countries of Europe between 1855
and 1860.
The work of Professor Henry Abbe, and others in this country would,
doubtless, have resulted in such an organization in the United States
in the early 60’s, had not the Civil War intervened, absorbing
public attention to the exclusion of other matters. It was not until
1870, that Dr. Increase A. Lapham of Milwaukee, in conjunction with
Representative Paine of that city, was able so to present the claims
for a national weather service that the act was finally passed that
gave birth to the present meteorological bureau in the United States.
Dr. Lapham issued from Chicago on November 10, 1871, the first
official forecast of the weather made in this country.
THE ATMOSPHERE
[Illustration: _Snow crystal._
Photomicrograph by W. A. Bentley.]
What is known about the atmosphere of our earth has been learned from
the exploration of a comparatively thin layer at the bottom. There
is reason to believe that the atmosphere extends upwards about two
hundred miles from the surface of the earth. We have a great mass
of observations made at the surface, some on mountains, but few in
the free air more than a few miles above the surface. Our knowledge
of the upper atmosphere is, therefore, in the nature of conclusions
drawn from such observations as are at hand, and is subject to
changes and modifications as the facts become known by actual
observation.
During the past few years a concerted effort has been made in
various parts of the world to explore the upper atmosphere by means
of kites and balloons, carrying meteorological instruments that
automatically record the temperature, pressure, humidity, velocity
and direction of the wind, etc. In this country this work has been
carried on principally at the Mount Weather Observatory, which is
located in Loudon County, Virginia, and is under the direction of
the United States Weather Bureau and at Blue Hill Observatory, a
private institution located near Boston and supported by Professor
Lawrence Rotch. From observations thus obtained much has been learned
about the upper atmosphere that was not even suspected before. Some
theories have been confirmed and some destroyed, but this line of
research is gradually bringing us nearer the truth.
_Air as a Gas_
Air is not a simple substance, as was once supposed, but is composed
of a number of gases, each one of which tends to form an atmosphere
of its own, just as it would if none of the other gases were present.
The different gases of the atmosphere are not chemically combined but
are very thoroughly mixed, as one might mix sugar and salt. Samples
of air collected from all parts of the world show that the relative
proportion of the gases forming the atmosphere is practically uniform.
_The Composition of Air_
Dry air is composed chiefly of oxygen and nitrogen. There are,
however, small quantities of carbon dioxide, argon, helium, krepton,
neon, hydrogen and xenon, and probably other gases yet to be
discovered.
The approximate proportion, by volume is as follows: Nitrogen 78
parts, oxygen 21 parts, argon 1 part, carbon-dioxide .03 parts, and
krepton helium and xenon a trace. Pure dry air does not exist in
nature, so there is always present in natural air a variable amount
of water vapor, depending upon the temperature and the source of
supply. Besides these, which may be termed the permanent constituents
of the atmosphere, many other substances are occasionally met with.
Lightning produces minute quantities of ammonia, nitrous acid
and ozone. Dust comes from the earth, salt from the sea, while
innumerable micro-organisms, most of which are harmless, besides the
pollen and spores of plants, are frequently found floating in the
atmosphere. Recent investigations in atmospheric electricity lead
to the conclusion that electric ions are also present, and perform
important functions, especially with respect to precipitation.
_Oxygen_
Oxygen is one of the most common substances. It exists in the
atmosphere as a transparent, odorless, tasteless gas. It combines
with hydrogen to form the water of the oceans, and with various other
substances to form much of the solid crust of the earth. Chemically,
it is a very active gas, and because of its tendency to unite with
other substances to form chemical compounds, it is believed that the
volume of oxygen now in the atmosphere, is less than during the early
history of the earth. It supports combustion by combining with carbon
and other substances, producing light and heat. It combines with
some of the organic constituents of the blood, through the function
of respiration, which is in itself a slow process of combustion, and
thus supports life and maintains the bodily heat.
_Nitrogen_
Nitrogen forms the largest proportion of the atmosphere, but unlike
oxygen it is a very inert substance, uniting with no element at
ordinary temperatures, and at high temperatures with only a few;
and when so united the bonds that hold it are easily broken and the
gas set free. For this reason, it is utilized in the manufacture of
explosives, such as gunpowder, guncotton, nitroglycerine, dynamite,
etc. Its office in the atmosphere appears to be to give the air
greater weight and to dilute the oxygen, for in an atmosphere of
pure oxygen a fire once started could not be controlled. Although
nitrogen does not contribute directly to animal life, in that it
is not absorbed and assimilated from the air direct as oxygen is,
nevertheless, it is a very important element of food both for animals
and plants, and in combination with other substances forms a large
proportion of animal and vegetable tissues.
_Carbon Dioxide_
Carbonic acid gas, known chemically as CO₂ is a product of
combustion. It results from the burning of fuel and is exhaled by
the breathing of animals. It also results from certain chemical
reactions. The amount in the atmosphere varies slightly, being
somewhat greater at night than by day and during cloudy weather
than during clear weather. Air containing more than 0.06% of carbon
dioxide is not fit to breathe, not because air loaded with carbon
dioxide is poisonous, but because it excludes the oxygen and thus
produces death by suffocation. It is considerably heavier than air,
and in certain localities, where it is emitted from the ground,
accumulates in low places in such quantities as to suffocate
animals. Death’s Gulch, a deep ravine in Yellowstone Park, and Dog’s
Grotto near Naples, are examples. At the latter place, the gas,
on account of being heavier than air, lies so close to the ground
that a man, standing erect, will have no difficulty in breathing,
while a dog will die of suffocation. It also accumulates in unused
wells, cisterns and mines, and can usually be detected by lowering a
lighted candle. If carbon dioxide is present in large quantities, the
candle will be extinguished because of the lack of oxygen to support
combustion.
Although carbon dioxide forms but a small proportion of the
atmosphere, it is a very important element in plant life. Animals
consume oxygen and exhale carbon dioxide, while plants take in
carbon dioxide and give off oxygen; thus, the amount of these gases
in the atmosphere is maintained at an equilibrium. Plants, through
their leaves, absorb the carbon dioxide, which is decomposed by the
sunlight, returning the oxygen free into the air, while the carbon is
used to build up plant tissue.
_Other Gases_
Argon, on account of its resemblance to nitrogen, was not discovered
until 1894, having been included with the nitrogen in all previous
analyses of air. It constitutes about 1% of air by volume. Krepton,
neon and xenon exist in minute quantities and have some interest
chemically, but little for the meteorologists. Helium and hydrogen
probably exist at great elevations in the atmosphere.
_Water Vapor_
The vapor of water in the atmosphere varies from about one per cent.
for arid regions to about five per cent. of the weight of the air for
warm, humid regions. It is a little over one-half as heavy as air
and moist air is, therefore, lighter than dry air; but the increase
of moisture near the center of cyclones has only a slight effect in
reducing the pressure. The amount of vapor decreases very rapidly
with elevation, and probably disappears at an elevation of five or
six miles above the surface. The amount of water in the form of vapor
that can exist in the atmosphere increases with the temperature,
being .54 grains Troy per cubic foot at zero temperature and 14.81 at
90°. When the air has taken up all the moisture it can contain at a
given temperature it is said to be saturated.
The dewpoint is the temperature at which saturation occurs. If the
air is saturated, the temperature of the air and the dewpoint will be
the same, but if the air is not saturated the dewpoint will be below
that of the air.
Relative humidity is expressed in percentages of the amount necessary
to saturate. If the air contains one-half enough vapor to saturate
it, the relative humidity will be 50%; if one-fourth, enough to
saturate, 25%; if saturated 100% etc.
The absolute humidity is the actual amount of water in the form
of vapor in the air, and is usually expressed by weight in grains
per cubic foot or in inches of mercury, the weight of which would
counterbalance the weight of the vapor in the air. The conditions
present in a volume of saturated air at a temperature of 32° may be
expressed as follows: Relative humidity 100%; dewpoint 32°; absolute
humidity 2.11 grains per cu. ft. or .18 inch.
_Pressure of Atmosphere_
Although the atmosphere is composed of these various gases, it acts
in all respects like a simple, single gas. It is very elastic, easily
compressed, expands when heated and contracts when cooled. It is
acted upon by gravity and, therefore, has weight and exerts pressure,
which at sea level amount to about 14.7 pounds on each square inch of
the surface. Because it is compressible and has weight, it is more
dense at the surface than at any elevation above the surface, and
as we ascend in the atmosphere the weight or pressure decreases in
proportion to the weight of that part of the atmosphere left below.
The weight or pressure of the atmosphere is measured by means of a
barometer and is expressed in terms of inches of mercury. The normal
atmosphere at sea level will sustain a column of mercury about thirty
inches high, and we therefore say that the normal pressure of the
atmosphere is thirty inches. (See Lessons on air pressure and the
barometer.)
_The Height of the Atmosphere_
[Illustration: _Snow crystal._
Photomicrograph by W. A. Bentley.]
The air that surrounds the earth is called its atmosphere, but it is
a rather curious fact that the earth has really ten atmospheres and
may have others not yet discovered.
The air near the surface is a mixture of eight different gases, and
each individual gas arranges itself so as to form an atmosphere
just as it would if no other gases were present. Thus, the earth is
surrounded by an atmosphere of oxygen, an atmosphere of nitrogen, one
of carbon dioxide, one of water vapor, one each of argon, krypton,
neon, and xenon, while hydrogen and helium are believed to exist at
great elevations above the earth’s surface.
These gases are kept from flying off into space by the force of
gravity, just as a piece of iron, stone, or a building is held fast
to the earth by the same force. Gravity acts with greater force on
some things than on others. For example, a piece of iron is pulled
down by gravity with greater force than is a piece of wood of the
same size; likewise, a piece of lead is pulled down with greater
force than a piece of iron. We, therefore, say that iron is heavier
than wood and that lead is heavier than iron, simply because gravity
acts with greater force on the one than on the other. The weight of
gases differ just as the weight of different solids, such as lead,
wood or iron differ. For instance, nitrogen is 14 and oxygen 16 times
heavier than hydrogen.
Gases having the least weight extend upward the farthest, because the
lighter the gas the greater its expansive force. Every boy who rides
a bicycle takes advantage of the expansive force of air when he pumps
his tires. The air is compressed by the pump into the tube and the
expansive force exerted by the air in trying to expand makes the tire
“stand up.” If it requires 10 pounds pressure to compress the gas
into the tube, the expansive force will be just ten pounds.
There are two forces in constant operation on each gas that surrounds
the earth, viz., expansive force and gravity. Expansive force pushes
the gas up and gravity pulls it down, but the force of gravity
decreases as the distance from the center of the earth increases,
so there is a point at a certain distance above the earth where the
two forces just balance each other, and each gas will expand upward
to that point but will not rise beyond it. Therefore, if we know the
expansive force of a gas and the rate at which gravity decreases, it
is possible to calculate the height to which the different gases that
compose the air will rise.
In this way it has been determined that carbon dioxide, which is one
of the heavier gases, extends upward about ten miles, water vapor
about 12 miles, oxygen about 30 miles and nitrogen about 35 miles
while hydrogen and helium, the lightest gases known, do not appear at
the surface at all, but probably exist at a height of from 30 miles
to possibly 200 miles.
[Illustration: _The zone of twilight in midwinter._
From Todd’s New Astronomy.]
There are other ways in which we are able to gain some idea of the
approximate height at which there is an appreciable atmosphere. When
the rays of light from the sun enter our atmosphere they are broken
up or scattered--diffracted--so that the atmosphere is partially
lighted for some time before sunrise and after sunset. This is called
twilight. If there were no atmosphere, there would be no twilight,
and darkness would fall the instant the sun passed below the horizon.
Twilight, which is caused by the sun shining on the upper atmosphere,
is perceptible until the sun is about 16° below the horizon. From
this it is calculated that the atmosphere has sufficient density at a
height of 40 miles to scatter, or diffract, sunlight.
Observations of meteors, commonly called shooting stars, indicate
that there is an appreciable atmosphere at a height of nearly 200
miles. Meteors are solid bodies flying with great velocity through
space. Occasionally they enter our atmosphere. Their velocity is so
great that the slight resistance offered by the air generates enough
heat by friction, or by the compression of the air in the path of the
meteor, to make it red hot or to burn it up before it reaches the
bottom of the atmosphere. Only the largest meteors reach the earth.
When a meteor is observed by two or more persons at a known distance
from each other, and the angle which the line of vision makes with
the horizon is noted by each, it is a simple matter to calculate
the distance from the earth where the lines of vision intersect,
and thus determine the height of the meteor. In this way, reliable
observations have given the height at which there is sufficient
density in the atmosphere to render meteors luminous as 188 miles.
_Temperature of the Atmosphere_
The condition of the atmosphere with respect to its temperature is
determined by means of the thermometer. This instrument is in such
common use that a detailed description is not necessary. It might be
interesting to note that the instrument invented by Galileo was very
different from those now in use. Galileo’s original thermometer was
what is known as an air thermometer, and its operation when subjected
to different degrees of heat or cold depended upon the expansion and
contraction of air instead of mercury or alcohol. It had one serious
defect, viz., the length of a column of air is affected by pressure
as well as by temperature and it was, therefore, necessary, when
using this thermometer, to obtain the pressure of the atmosphere by
means of the barometer before the temperature could be determined.
This is obviated in the modern thermometer by the use of mercury or
alcohol in a vacuum tube. Mercury is not used when it is expected
to register very low temperatures, because it congeals at about 45
degrees below zero Fahr.
_Thermometer Scales in Use_
There are three systems in common use for marking the degrees on the
scale, viz., Fahrenheit, Centigrade and Reaumur.
The Fahrenheit scale was the invention of a German by that name, but
it is worthy of note that this scale is used principally by English
speaking nations and is not in common use in Germany. Fahrenheit
found that by mixing snow and salt he was able to obtain a very low
temperature, and believing that the temperature thus obtained was the
lowest possible he started his scale at that point, which he called
zero. He then fixed the freezing temperature of water 32 degrees
above this zero, and the boiling point of water at 212 degrees. There
are, therefore, 180 divisions or degrees between the freezing and
boiling point of water on the Fahrenheit scale.
The Centigrade scale starts with zero at the freezing point of water
and makes the boiling point 100. Thus 180 degrees on the Fahrenheit
scale equal 100 degrees on the Centigrade. The Fahrenheit degree
is, therefore, only a little more than half as large, to be exact
five-ninths of a degree, as a degree on the Centigrade scale. The
Centigrade scale is in common use in France and is used almost
exclusively in all scientific work throughout the world.
The Reaumur scale is used generally in Russia and quite commonly in
some parts of Europe, especially in Germany. On this scale the zero
is placed at the freezing point of water and the boiling point at
80 degrees. The divisions are, therefore, larger than those of the
Centigrade scale and more than twice as large as the Fahrenheit. The
general use of these different scales has led to endless confusion
and made the comparison of records difficult, so that even at the
present time when making a temperature record it is necessary to
indicate the scale in use.
_Distribution of the Temperature and Pressure_
The heat received on the earth from the sun is the controlling factor
in all weather conditions. If the earth were composed of all land or
all water, and the amount of heat received were everywhere the same
throughout the year, there would be no winds, no storms and probably
no clouds and no rain, because the force of gravity, which acts on
everything on the earth’s surface and on the air as well, would soon
settle all differences and the atmosphere would become perfectly
still. But the earth is composed of land and water and the land
heats up more rapidly under sunshine than the water and also gives
off--“radiates” its heat more rapidly than water. As a result, the
air over the land is warmer in summer than the air over the water.
During the winter this is reversed, and the air over the oceans is
warmer than the air over the land. The great ocean currents, by
carrying the heat from the equatorial regions toward the poles, and
by bringing the cold from the polar regions toward the equator,
assist in maintaining a constant difference in temperature between
the continents and the adjacent oceans.
Furthermore, the fact that the path of the earth about the sun is
not a circle but an ellipse, and that the axis of the earth is
not perpendicular to the plane of its orbit, result in an unequal
distribution of heat over the surface. It is always warmer near the
equator than at the poles, and warmer in summer than in winter. All
these differences in temperature cause corresponding differences in
density, which, in turn, cause differences in weight or pressure over
various parts of the earth’s surface. These changes are, in no way,
the result of chance but are determined by the operation of fixed
natural laws, and with this in mind we may now take up the study of
the winds of the world.
THE WINDS OF THE WORLD
[Illustration: _Snow crystal._
Photomicrograph by W. A. Bentley.]