Although air is continually evaporating water from the surface of the
earth, and continually restoring it again by condensation, yet, on the
whole and in the course of years, there seems to be no sensible gain or
loss of water in our seas, lakes, and rivers; so that the two processes
of evaporation and condensation balance each other.
It is evident, however, that the moisture precipitated at any moment
from the air is not at once evaporated again. The disappearance of the
water is due in part to evaporation, but only in part. A great deal of
it goes out of sight in other ways.
The rain which falls upon the sea is the largest part of the whole
rainfall of the globe, because the surface of the sea is about three
times greater than that of the land. All this rain gradually mingles
with the salt water, and can then be no longer recognized. It thus
helps to make up for the loss which the sea is always suffering by
evaporation. For the sea is the great evaporating surface whence most
of the vapor of the atmosphere is derived.
On the other hand, the total amount of rain which falls upon the land
of the globe must be enormous. It has been estimated, for example,
that about sixty-eight cubic miles of water annually descend as rain
even upon the surface of the British Isles, and there are many much
more rainy regions. If you inquire about this rain which falls upon
the land, you will find that it does not at once disappear, but begins
another kind of circulation. Watch what happens during a shower of
rain. If the shower is heavy, you will notice little runs of muddy
water coursing down the streets or roads, or flowing out of the ridges
of the fields. Follow one of the runs. It leads into some drain or
brook, that into some larger stream, the stream into a river; and the
river, if you follow it far enough, will bring you to the sea. Now
think of all the brooks and rivers of the world, where this kind of
transport of water is going on, and you will at once see how vast must
be the part of the rain which flows off the land into the ocean.
But does the whole of the rain flow off at once into the sea in this
way? A good deal of the rain which falls upon the land must sink
underground and gather there. You may think that surely the water which
disappears in that way must be finally withdrawn from the general
circulation which we have been tracing. When it sinks below the
surface, how can it ever get up to the surface again?
Yet, if you consider for a little, you will be convinced that whatever
becomes of it underneath, it can not be lost. If all the rain which
sinks into the ground be forever removed from the surface circulation,
you will at once see that the quantity of water upon the earth’s
surface must be constantly and visibly diminishing. But no such
changes, so far as can be seen, are really taking place. In spite of
the rain which disappears into the ground, the circulation of water
between the air, the land, and the sea continues without perceptible
diminution.
You are driven to conclude, therefore, that there must be some means
whereby the water underground is brought back to the surface. This
is done by springs, which gush out of the earth, and bring up water
to feed the brooks and rivers, whereby it is borne into the sea.
Here, then, are two distinct courses which the rainfall takes—one
below ground, and one above. It will be most convenient to follow the
underground portion first.
A little attention to the soils and rocks which form the surface of a
country is enough to show that they differ greatly from each other in
hardness, and in texture or grain. Some are quite loose and porous,
others are tough and close-grained. They consequently differ much
in the quantity of water they allow to pass through them. A bed of
sand, for example, is pervious; that is, will let water sink through
it freely, because the little grains of sand lie loosely together,
touching each other only at some points, so as to leave empty spaces
between. The water readily finds its way among these empty spaces. In
fact, the sand-bed may become a kind of sponge, quite saturated with
the water which has filtered down from the surface. A bed of clay, on
the other hand, is impervious; it is made up of very small particles
fitting closely to each other, and therefore offering resistance to
the passage of water. Wherever such a bed occurs, it hinders the free
passage of the water, which, unable to sink through it from above on
the way down, or from below on the way up to the surface again, is kept
in by the clay, and forced to find another line of escape.
Sandy soils are dry because the rain at once sinks through them; clay
soils are wet because they retain the water, and prevent it from freely
descending into the earth.
Now the rocks beneath us, besides being in many cases porous in their
texture, such as sandstone, are all more or less traversed with
cracks—sometimes mere lines, like those of a cracked window-pane, but
sometimes wide and open clefts and tunnels. These numerous channels
serve as passages for the underground water. Hence, although a rock
may be so hard and close-grained that water does not soak through it
at all, yet if that rock is plentifully supplied with these cracks, it
may allow a large quantity of water to pass through. Limestone, for
example, is a very hard rock, through the grains of which water can
make but little way; yet it is so full of cracks or “joints,” as they
are called, and these joints are often so wide, that they give passage
to a great deal of water.
In hilly districts, where the surface of the ground has not been
brought under the plow, you will notice that many places are marshy
and wet, even when the weather has long been dry. The soil everywhere
around has perhaps been baked quite hard by the sun; but these places
remain still wet, in spite of the heat. Whence do they get their
water? Plainly not directly from the air, for in that case the rest of
the ground would also be damp. They get it not from above, but from
below. It is oozing out of the ground; and it is this constant outcome
of water from below which keeps the ground wet and marshy. In other
places you will observe that the water does not merely soak through the
ground, but gives rise to a little run of clear water. If you follow
such a run up to its source, you will see that it comes gushing out of
the ground as a spring.
Springs are the natural outlets for the underground water. But, you
ask, why should this water have any outlets, and what makes it rise to
the surface?
Let us suppose that a flat layer of some impervious rock, like clay,
underlies another layer of a porous material, like sand. The rain which
falls on the surface of the ground, and sinks through the upper bed,
will be arrested by the lower one, and made either to gather there, or
find its escape along the surface of that lower bed. If a hollow or
valley should have its bottom below the level of the line along which
the water flows, springs will gush out along the sides of the valley.
The line of escape may be either the junction between two different
kinds of rock, or some of the numerous joints already referred to.
Whatever it be, the water can not help flowing onward and downward, as
long as there is any passage along which it can find its way; and the
rocks underneath are so full of cracks, that it has no difficulty in
doing so.
But it must happen that a great deal of the underground water descends
far below the level of the valleys, and even below the level of the
sea. And yet, though it should descend for several miles, it comes at
last to the surface again. To realize clearly how this takes place,
let us follow a particular drop of water from the time when it sinks
into the earth as rain, to the time when, after a long journey up and
down in the bowels of the earth, it once more reaches the surface.
It soaks through the soil together with other drops, and joins some
feeble trickle, or some more ample flow of water, which works its way
through crevices and tunnels of the rocks. It sinks in this way to
perhaps a depth of several thousand feet, until it reaches some rock
through which it can not readily make further way. Unable to work its
way downward, the pent-up water must try to find escape in some other
direction. By the pressure from above it is driven through other cracks
and passages, winding up and down until at last it comes to the surface
again. It breaks out there as a gushing spring.
Rain is water nearly in a state of purity. After journeying up and
down underground it comes out again in springs, always more or less
mingled with other materials, which it gets from the rocks through
which it travels. They are not visible to the eye, for they are held
in what is called chemical solution. When you put a few grains of salt
or sugar upon a plate, and pour water over them, they are dissolved in
the water and disappear. They enter into union with the water. You can
not see them, but you can still recognize their presence by the taste
which they give to the water which holds them in solution. So water,
sinking from the soil downward, dissolves a little of the substance
of the subterranean rocks, and carries this dissolved material up to
the surface of the ground. One of the important ingredients in the air
is carbonic acid gas, and this substance is both abstracted from and
supplied to the air by plants and animals. In descending through the
atmosphere rain absorbs a little air. As ingredients of the air, a
little carbonic acid gas, particles of dust and soot, noxious vapors,
minute organisms, and other substances floating in the air, are caught
up by the descending rain, which in this way washes the air, and tends
to keep it much more wholesome than it would otherwise be.
But rain not merely picks up impurities from the air, it gets a large
addition when it reaches the soil.
Armed with the carbonic acid which it gets from the air, and with the
larger quantity which it abstracts from the soil, rainwater is prepared
to attack rocks, and to eat into them in a way which pure water could
not do.
Water containing carbonic acid has a remarkable effect on many rocks,
even on some of the very hardest. It dissolves more or less of their
substance, and removes it. When it falls, for instance, on chalk or
limestone, it almost entirely dissolves and carries away the rock
in solution, though still remaining clear and limpid. In countries
where chalk or limestone is an abundant rock, this action of water
is sometimes singularly shown in the way in which the surface of the
ground is worn into hollows. In such districts, too, the springs are
always hard; that is, they contain much mineral matter in solution,
whereas rainwater and springs which contain little impurity are termed
soft.
When a stone building has stood for a few hundred years, the
smoothly-dressed face which its walls received from the mason is
usually gone. Again, in the burying-ground surrounding a venerable
church you see the tombstones more and more mouldered the older they
are. This crumbling away of hard stone with the lapse of time is a
common familiar fact to you. But have you ever wondered why it should
be so? What makes the stone decay, and what purpose is served by the
process?
If it seem strange to you to be told that the surface of the earth is
crumbling away, you should take every opportunity of verifying the
statement. Examine your own district. You will find proofs that, in
spite of their apparent steadfastness, even the hardest stones are
really crumbling down. In short, wherever rocks are exposed to the air
they are liable to decay. Now let us see how this change is brought
about.
First of all we must return for a moment to the action of carbonic
acid, which has been already described. You remember that rainwater
abstracts a little carbonic acid from the air, and that, when it sinks
under the earth, it is enabled by means of the acid to eat away some
parts of the rocks beneath. The same action takes place with the rain,
which rests upon or flows over the surface of the ground. The rainwater
dissolves out little by little such portions of the rocks as it can
remove. In the case of some rocks, such as limestone, the whole, or
almost the whole, of the substance of the rock is carried away in
solution. In other kinds, the portion dissolved is the cementing
material whereby the mass of the rock was bound together; so that when
it is taken away, the rock crumbles into mere earth or sand, which
is readily washed away by the rain. Hence one of the causes of the
mouldering of stone is the action of the carbonic acid taken up by the
rain.
In the second place, the oxygen of the portion of air contained in
rainwater helps to decompose rocks. When a piece of iron has been
exposed for a time to the weather, in a damp climate, it rusts. This
rust is a compound substance, formed by the union of oxygen with iron.
What happens to an iron railing or a steel knife, happens also, though
not so quickly nor so strongly, to many rocks. They, too, rust by
absorbing oxygen. A crust of corroded rock forms on their surface, and,
when it is knocked off by the rain, a fresh layer of rock is reached by
the ever-present and active oxygen.
In the third place, the surface of many parts of the world is made to
crumble down by means of frost. Sometimes during winter, when the cold
gets very keen, pipes full of water burst, and jugs filled with water
crack from top to bottom. The reason of this lies in the fact that
water expands in freezing. Ice requires more space than the water would
if it remained fluid. When ice forms within a confined space, it exerts
a great pressure on the sides of the vessel, or cavity, which contains
it. If these sides are not strong enough to bear the strain to which
they are put, they must yield, and therefore they crack.
You have learned how easily rain finds its way through soil. Even the
hardest rocks are more or less porous, and take in some water. Hence,
when winter comes the ground is full of moisture; not in the soil
merely, but in the rocks. And so, as frost sets in, this pervading
moisture freezes. Now, precisely the same kind of action takes place
with each particle of water, as in the case of the water in the burst
water-pipe or the cracked jar. It does not matter whether the water is
collected into some hole or crevice, or is diffused between the grains
of the rocks and the soil. When it freezes it expands, and in so doing
tries to push asunder the walls between which it is confined.
Water freezes not only between the component grains, but in the
numerous crevices or joints, as they are called, by which rocks are
traversed. You have, perhaps, noticed that on the face of a cliff, or
in a quarry, the rock is cut through by lines running more or less in
an upright direction, and that by means of these lines the rock is
split up by nature, and can be divided by the quarrymen into large
four-sided blocks or pillars. These lines, or joints, have been already
referred to as passages for water in descending from the surface. You
can understand that only a very little water may be admitted at a time
into a joint. But by degrees the joint widens a little, and allows more
water to enter. Every time the water freezes it tries hard to push
asunder the two sides of the joint. After many winters, it is at last
able to separate them a little; then more water enters, and more force
is exerted in freezing, until at last the block of rock traversed by
the joint is completely split up. When this takes place along the face
of a cliff, one of the loosened parts may fall and actually roll down
to the bottom of the precipice.
In addition to carbonic acid, oxygen, and frost, there are still
other influences at work by which the surface of the earth is made to
crumble. For example, when, during the day, rocks are highly heated by
strong sunshine, and then during night are rapidly cooled by radiation,
the alternate expansion and contraction caused by the extremes of
temperature loosen the particles of the stone, causing them to crumble
away, or even making successive crusts of the stone fall off.
Again, rocks which are at one time well soaked with rain, and at
another time are liable to be dried by the sun’s rays and by wind,
are apt to crumble away. If then it be true, as it is, that a general
wasting of the surface of the land goes on, you may naturally ask why
this should be. Out of the crumbled stones all soil is made, and on the
formation and renewal of the soil we depend for our daily food.
Take up a handful of soil from any field or garden, and look at it
attentively. What is it made of? You see little pieces of crumbling
stone, particles of sand and clay, perhaps a few vegetable fibers; and
the whole soil has a dark color from the decayed remains of plants
and animals diffused through it. Now let us try to learn how these
different materials have been brought together.
Every drop of rain which falls upon the land helps to alter the
surface. You have followed the chemical action of rain when it
dissolves parts of rocks. It is by the constant repetition of the
process, drop after drop, and shower after shower, for years together,
that the rocks become so wasted and worn. But the rain has also a
mechanical action.
Watch what happens when the first pattering drops of a shower begin
to fall upon a smooth surface of sand, such as that of a beach. Each
drop makes a little dint or impression. It thus forces aside the grains
of sand. On sloping ground, where the drops can run together and flow
downward, they are able to push or carry the particles of sand or clay
along. This is called a mechanical action; while the actual solution
of the particles, as you would dissolve sugar or salt, is a chemical
action. Each drop of rain may act in either or both of these ways.
Now you will readily see how it is that rain does so much in the
destruction of rocks. It not only dissolves out some parts of them, and
leaves a crumbling crust on the surface, but it washes away this crust,
and thereby exposes a fresh surface to decay. There is in this way a
continual pushing along of powdered stone over the earth’s surface.
Part of this material accumulates in hollows, and on sloping or level
ground; part is swept into the rivers, and carried away into the sea.
As the mouldering of the surface of the land is always going on, there
is a constant formation of soil. Indeed, if this were not the case,
if after a layer of soil had been formed upon the ground, it were to
remain there unmoved and unrenewed, the plants would by degrees take
out of it all the earthy materials they could, and leave it in a barren
or exhausted state. But some of it is being slowly carried away by
rain, fresh particles from mouldering rocks are being washed over it by
the same agent, while the rock or sub-soil underneath is all the while
decaying into soil. The loose stones, too, are continually crumbling
down and making new earth. And thus, day by day, the soil is slowly
renewed.
Plants, also, help to form and renew the soil. They send their roots
among the grains and joints of the stones, and loosen them. Their
decaying fibers supply most of the carbonic acid by which these stones
are attacked, and furnish also most of the organic matter in the soil.
Even the common worms, which you see when you dig up a spadeful of
earth, are of great service in mixing the soil and bringing what lies
underneath up to the surface.
One part of the rain sinks under the ground, and you have traced its
progress there until it comes to the surface again. You have now to
trace, in a similar way, the other portion of the rainfall which flows
along the surface in brooks and rivers.
You can not readily meet with a better illustration of this subject
than that which is furnished by a gently sloping road during a heavy
shower of rain. Let us suppose that you know such a road, and that
just as the rain is beginning you take up your station at some part
where the road has a well-marked descent. At first you notice that each
of the large heavy drops of rain makes in the dust, or sand, one of
the little dints or rain-prints already described. As the shower gets
heavier these rain-prints are effaced, and the road soon streams with
water. Now mark in what manner the water moves.
Looking at the road more narrowly, you remark that it is full of little
roughnesses—at one place a long rut, at another a projecting stone,
with many more inequalities which your eye could not easily detect
when the road was dry, but which the water at once discloses. Every
little dimple and projection affects the flow of the water. You see
how the raindrops gather together into slender streamlets of running
water which course along the hollows, and how the jutting stones and
pieces of earth seem to turn these streamlets now to one side and now
to another.
Toward the top of the slope only feeble runnels of water are to be
seen. But further down they become fewer in number, and at the same
time larger in size. They unite as they descend; and the larger and
swifter streamlets at the foot of the descent are thus made up of a
great many smaller ones from the higher parts of the slope.
Why does the water run down the sloping road? why do rivers flow? and
why should they always move constantly in the same direction? They do
so for the same reason that a stone falls to the ground when it drops
out of your hand; because they are under the sway of that attraction
toward the center of the earth, to which, as you know, the name of
gravity is given. Every drop of rain falls to the earth because it is
drawn downward by the force of this attraction. When it reaches the
ground it is still, as much as ever, under the same influence; and it
flows downward in the readiest channel it can find. Its fall from the
clouds to the earth is direct and rapid; its descent from the mountains
to the sea, as part of a stream, is often long and slow; but the cause
of the movement is the same in either case. The winding to and fro of
streams, the rush of rapids, the roar of cataracts, the noiseless flow
of the deep sullen currents, are all proofs how paramount is the sway
of the law of gravity over the waters of the globe.
Drawn down in this way by the action of gravity, all that portion of
the rain which does not sink into the earth must at once begin to move
downward along the nearest slopes, and continue flowing until it can
get no further. On the surface of the land there are hollows called
lakes, which arrest part of the flowing water, just as there are
hollows on the road which serve to collect some of the rain. But in
most cases they let the water run out at the lower end as fast as it
runs in at the upper, and therefore do not serve as permanent resting
places for the water. The streams which escape from lakes go on as
before, working their way to the seashore. So that the course of all
streams is a downward one; and the sea is the great reservoir into
which the water of the land is continually pouring.
The brooks and rivers of a country are thus the natural drains, by
which the surplus rainfall, not required by the soil or by springs,
is led back again into the sea. When we consider the great amount of
rain, and the enormous number of brooks in the higher parts of the
country, it seems, at first, hardly possible for all these streams to
reach the sea without overflowing the lower grounds. But this does not
take place; for when two streams unite into one, they do not require a
channel twice as broad as either of their single water-courses. On the
contrary, such an union gives rise to a stream which is not so broad
as either of the two from which it flows. But it becomes swifter and
deeper.
Let us return to the illustration of the roadway in rain. Starting from
the foot of the slope, you found the streamlets of rain getting smaller
and smaller, and when you came to the top there were none at all. If,
however, you were to descend the road on the other side of the ridge,
you would probably meet with other streamlets coursing down-hill in
the opposite direction. At the summit the rain seems to divide, part
flowing off to one side, and part to the other.
In the same way, were you to ascend some river from the sea, you would
watch it becoming narrower as you traced it inland, and branching more
and more into tributary streams, and these again subdividing into
almost endless little brooks. But take any of the branches which unite
to form the main stream, and trace it upward. You come, in the end,
to the first beginnings of a little brook, and going a little further
you reach the summit, down the other side of which all the streams are
flowing to the opposite quarter. The line which separates two sets of
streams in this way is called the water-shed. In England, for example,
one series of rivers flows into the Atlantic, another into the North
Sea. If you trace upon a map a line separating all the upper streams
of the one side from those of the other, that line will mark the
water-shed of the country.
But there is one important point where the illustration of the road
in rain quite fails. It is only when rain is falling, or immediately
after a heavy shower, that the rills are seen upon the road. When the
rain ceases the water begins to dry up, till in a short time the road
becomes once more firm and dusty. But the brooks and rivers do not
cease to flow when the rain ceases to fall. In the heat of summer, when
perhaps there has been no rain for many days together, the rivers still
roll on, smaller usually than they were in winter, but still with ample
flow. What keeps them full? If you remember what you have already been
told about underground water, you will answer that rivers are fed by
springs as well as by rain.
Though the weather may be rainless, the springs continue to give out
their supplies of water, and these keep the rivers going. But if
great drought comes, many of the springs, particularly the shallow
ones, cease to flow, and the rivers fed by them shrink up or get dry
altogether. The great rivers of the globe, such as the Mississippi,
drain such vast territories, that any mere local rain or drought makes
no sensible difference in their mass of water.
In some parts of the world, however, the rivers are larger in summer
and autumn than they are in winter and spring. The Rhine, for instance,
begins to rise as the heat of summer increases, and to fall as the cold
of winter comes on. This happens because the river has its source among
snowy mountains. Snow melts rapidly in summer, and the water which
streams from it finds its way into the brooks and rivers, which are
thereby greatly swollen. In winter, on the other hand, the snow remains
unmelted; the moisture which falls from the air upon the mountains
is chiefly snow; and the cold is such as to freeze the brooks. Hence
the supplies of water at the sources of these rivers are, in winter,
greatly diminished, and the rivers themselves become proportionately
smaller.
[To be continued.]