It has been ascertained that water covers about three times more of
the earth’s surface than the land does. We could not tell that merely
by what we can see from any part of this country, or indeed of any
country. It is because men have sailed round the world, and have
crossed it in many directions, that the proportion of land and water
has come to be known.
Take a school-globe and turn it slowly round on its axis. You see at
a glance how much larger the surface of water is than the surface of
land. But you may notice several other interesting things about the
distribution of land and water.
In the first place you will find that the water is all connected
together into one great mass, which we call the sea. The land, on the
other hand, is much broken up by the way the sea runs into it; and some
parts are cut off from the main mass of land, so as to form islands in
the sea. Britain is one of the pieces of land so cut off.
In the second place, you cannot fail to notice how much more land lies
on the north than on the south of the equator. If you turn the globe so
that your eye shall look straight down on the site of London, you will
find that most of the land on the globe comes into sight; whereas, if
you turn the globe exactly round, and look straight down on the area of
New Zealand, you will see most of the sea. London thus stands about the
centre of the land-hemisphere, midway among the countries of the earth.
And no doubt this central position has not been without its influence
in fostering the progress of British commerce.
In the third place, you will notice that by the way in which the masses
of land are placed, parts of the sea are to some extent separated from
each other. These masses of land are called continents, and the wide
sheets of water between are termed oceans. Picture to yourselves that
the surface of the solid part of the earth is uneven, some portions
rising into broad swellings and ridges, others sinking into wide
hollows and basins. Now, into these hollows the sea has been gathered,
and only those upstanding parts which rise above the level of the sea
form the land.
When you come to examine the water of the sea, you find that it differs
from the water with which you are familiar on the land, inasmuch as
it is salt. It contains something which you do not notice in ordinary
spring or river water. If you take a drop of clear spring water, and
allow it to evaporate from a piece of glass, you will find no trace
left behind. Take, however, a drop of sea water and allow it to
evaporate. You find a little white point or film left behind, and on
placing that film under a microscope you see it to consist of delicate
crystals of common or sea salt. It would not matter from what ocean you
took the drop of water, it would still show the crystals of salt on
being evaporated.
There are some other things beside common salt in sea water. But the
salt is the most abundant, and we need not trouble about the rest at
present. Now, where did all this mineral matter in the sea come from?
The salt of the sea is all derived from the waste of the rocks.
It has already been pointed out how, both underground and on the
surface of the land, water is always dissolving out of the rocks
various mineral substances, of which salt is one. Hence the water of
springs and rivers contains salt, and this is borne away into the
sea. So that all over the world there must be a vast quantity of salt
carried into the ocean every year.
The sea gives off again by evaporation as much water as it receives
from rain and from the rivers of the land. But the salt carried into it
remains behind. If you take some salt water and evaporate it the pure
water disappears, and the salt is left. So it is with the sea. Streams
are every day carrying fresh supplies of salt into the sea. Every day,
too, millions of tons of water are passing from the ocean into vapor
in the atmosphere. The waters of the sea must consequently be getting
salter by degrees. The process, however, is an extremely slow one.
Although sea water has probably been gradually growing in saltness ever
since rivers first flowed into the great sea, it is even now by no
means as salt as it might be. In the Atlantic Ocean, for example, the
total quantity of the different salts amounts only to about three and a
half parts in every hundred parts of water. But in the Dead Sea, which
is extremely salt, the proportion is as much as twenty-four parts in
the hundred of water.
Standing by the shore and watching for a little the surface of the sea,
you notice how restless it is. Even on the calmest summer day, a slight
ripple or a gentle heaving motion will be seen.
Again, if you watch a little longer, you will find that whether the
sea is calm or rough, it does not remain always at the same limit upon
the beach. At one part of the day the edge of the water reaches to
the upper part of the sloping beach; some six hours afterward it has
retired to the lower part. You may watch it falling and rising day by
day, and year by year, with so much regularity that its motion can be
predicted long beforehand. This ebb and flow of the sea forms what are
called tides.
If you cork up an empty bottle and throw it into the sea, it will of
course float. But it will not remain long where it fell. It will begin
to move away, and may travel for a long distance until thrown upon some
shore again. Bottles cast upon mid-ocean have been known to be carried
in this way for many hundreds of miles. This surface-drift of the sea
water corresponds generally with the direction in which the prevalent
winds blow.
But it is not merely the surface water which moves. You have learnt a
little about icebergs; and one fact about them which you must remember
is that, large as they may seem, there is about seven times more of
their mass below water than above it. Now, it sometimes happens that an
iceberg is seen sailing on, even right in the face of a strong wind.
This shows that it is moving, not with the wind, but with a strong
under-current in the sea. In short, the sea is found to be traversed by
many currents, some flowing from cold to warm regions, and others from
warm to cold.
Here, then, are four facts about the sea:—1st, it has a restless
surface, disturbed by ripples and waves; 2ndly, it is constantly
heaving with the ebb and flow of the tides; 3dly, its surface waters
drift with the wind; and 4thly, it possesses currents like the
atmosphere.
For the present it will be enough if we learn something regarding the
first of these facts—the waves of the sea.
Here again you may profitably illustrate by familiar objects what goes
on upon so vast a scale in nature. Take a basin, or a long trough of
water, and blow upon the water at one edge. You throw its surface into
ripples, which, as you will observe, start from the place where your
breath first hits the water, and roll onward until they break in little
wavelets upon the opposite margin of the basin.
What you do in a small way is the same action by which the waves of the
sea are formed. All these disturbances of the smoothness of the sea
are due to disturbances of the air. Wind acts upon the water of the
sea as your breath does on that of the basin. Striking the surface it
throws the water into ripples or undulations, and in continuing to blow
along the surface it gives these additional force, until driven on by a
furious gale they grow into huge billows.
When waves roll in on the land, they break one after another upon the
shore, as your ripples break upon the side of the basin. And they
continue to roll in after the wind has fallen, in the same way that the
ripples in the basin will go on curling for a little after you have
ceased to blow. The surface of the sea, like that of water generally,
is very sensitive. If it is thrown into undulations, it does not become
motionless the moment the cause of disturbance has passed away, but
continues moving in the same way, but in a gradually lessening degree,
until it comes to rest.
The restlessness of the surface of the sea becomes in this way a
reflection of the restlessness of the air. It is the constant moving to
and fro of currents of air, either gentle or violent, which roughens
the sea with waves. When the air for a time is calm above, the sea
sleeps peacefully below; when the sky darkens, and a tempest bursts
forth, the sea is lashed into waves, which roll in and break with
enormous force upon the land.
You have heard, perhaps you have even seen, something of the
destruction which is worked by the waves of the sea. Every year piers
and sea walls are broken down, pieces of the coast are washed away, and
the shores are strewn with the wreck of ships. So that, beside all the
waste which the surface of the land undergoes from rain, and frost,
and streams, there is another form of destruction going on along the
coast-line.
On some parts of the coast-line of the east of England, where the rock
is easily worn away, the sea advances on the land at a rate of two or
three feet every year. Towns and villages which existed a few centuries
ago, have one by one disappeared, and their sites are now a long way
out under the restless waters of the North Sea. On the west coast of
Ireland and Scotland, however, where the rocks are usually hard and
resisting, the rate of waste has been comparatively small.
It would be worth your while the first time you happen to be at the
coast, to ascertain what means the sea takes to waste the land. This
you can easily do by watching what happens on a rocky beach. Get to
some sandy or gravelly part of the beach, over which the waves are
breaking, and keep your eye on the water when it runs back after a wave
has burst. You see all the grains of gravel and sand hurrying down the
slope with the water; and if the gravel happens to be coarse, it makes
a harsh grating noise as its stones rub against each other—a noise
sometimes loud enough to be heard miles away. As the next wave comes
curling along, you will mark that the sand and gravel, after slackening
their downward pace, are caught up by the bottom of the advancing wave
and dragged up the beach again, only to be hurried down once more as
the water retires to allow another wave to do the same work.
By this continual up and down movement of the water, the sand and
stones on the beach are kept grinding against each other, as in a mill.
Consequently they are worn away. The stones become smaller, until they
pass into mere sand, and the sand, growing finer, is swept away out to
sea and laid down at the bottom.
But not only the loose materials on the shore suffer in this way an
incessant wear and tear, the solid rocks underneath, wherever they come
to the surface, are ground down in the same process. When the waves
dash against a cliff they hurl the loose stones forward, and batter the
rocks with them. Here and there in some softer part, as in some crevice
of the cliff, these stones gather together, and when the sea runs high
they are kept whirling and grinding at the base of the cliff till, in
the end, a cave is actually bored by the sea in the solid rock, very
much in the same way as holes are bored by a river in the bed of its
channel. The stones of course are ground to sand in the process, but
their place is supplied by others swept up by the waves. If you enter
one of these sea-caves when the water is low, you will see how smoothed
and polished its sides and roof are, and how well rounded and worn are
the stones lying on its floor.
So far as we know, the bottom of the sea is very much like the surface
of the land. It has heights and hollows, lines of valleys and ranges of
hills. We can not see down to the bottom where the water is very deep,
but we can let down a long line with a weight tied to the end of it,
and find out both how deep the water is, and what is the nature of the
bottom, whether rock or gravel, sand, mud, or shells. This measuring of
the depths of the water is called sounding, and the weight at the end
of the line goes by the name of the sounding-lead.
Soundings have been made over many parts of the sea, and something is
now known about its bottom, though much still remains to be discovered.
The Atlantic Ocean is the best known. In sounding it, before laying
down the telegraphic cable which stretches across under the sea from
this country to America, a depth of 14,500 feet, or two miles and
three-quarters, was reached. But between the Azores and the Bermudas a
sounding has been obtained of seven miles and a half. If you could lift
up the Himalaya mountains, which are the highest on the globe, reaching
a height of 29,000 feet above the sea, and set them down in the deepest
part of the Atlantic, they would not only sink out of sight, but their
tops would actually be about two miles below the surface.
A great part of the wide sea must be one or two miles deep. But it is
not all so deep as that, for even in mid-ocean some parts of its bottom
rise up to the surface and form islands. As a rule it deepens in tracts
furthest from land, and shallows toward the land. Hence those parts of
the sea which run in among islands and promontories are, for the most
part, comparatively shallow.
You may readily enough understand how it is that soundings are made,
though you can see how difficult it must be to work a sounding line
several miles long. Yet men are able not only to measure the depth of
the water, but by means of the instrument called a dredge, to bring up
bucketfuls of whatever may be lying on the sea floor, from even the
deepest parts of the ocean. In this way during the last few years a
great deal of additional knowledge has been gathered as to the nature
of the sea floor, and the kind of plants and animals which live there.
We now know that even in some of the deepest places which have yet
been dredged there is plenty of animal life, such as shells, corals,
star-fishes, and still more humble creatures.
We can not, indeed, examine the sea bottom with anything like the same
minuteness as the surface of the land. Yet a great deal may be learnt
regarding it.
If you put together some of the facts with which we have been dealing
in the foregoing lessons, you may for yourselves make out some of the
most important changes which are in progress on the floor of the sea.
For example, try to think what must become of all the wasted rock which
is every year removed from the surface of the land. It is carried into
the sea by streams, as you have now learnt. But what happens to it
when it gets there? From the time when it was loosened from the sides
of the mountains, hills, or valleys, this decomposed material has been
seeking, like water, to reach a lower level. On reaching the hollows
of the sea bottom it can not descend any further, but must necessarily
accumulate there.
It is evident, then, that between the floor of the sea and the surface
of the land, there must be this great difference: that whereas the land
is undergoing a continual destruction of its surface, from mountain
crest to sea shore, the sea bottom, on the other hand, is constantly
receiving fresh materials on its surface. The one is increased in
proportion as the other is diminished. So that even without knowing
anything regarding what men have found out by means of deep soundings,
you could confidently assert that every year there must be vast
quantities of gravel, sand and mud laid down upon the floor of the sea,
because you know that these materials are worn away from the land.
Again, you have learnt that the restless agitation of the sea is due to
movements of the air, and that the destruction which the sea can effect
on the land is due chiefly to the action of the waves caused by wind.
But this action must be merely a surface one. The influence of the
waves can not reach to the bottom of the deep sea. Consequently that
bottom lies beyond the reach of the various kinds of destruction which
so alter the face of the land. The materials which are derived from the
waste of the land can lie on the sea floor without further disturbance
than they may suffer from the quiet flow of such ocean currents as
touch the bottom.
In what way, then, are the gravel, sand and mud disposed of when they
reach the sea?
As these materials are all brought from the land, they accumulate on
those parts of the sea floor which border the land, rather than at a
distance. We may expect to find banks of sand and gravel in shallow
seas and near land, but not in the middle of the ocean.
You may form some notion, on a small scale, as to how the materials
are arranged on the sea bottom by examining the channel of a river in
a season of drought. At one place, where the current has been strong,
there may be a bank of gravel; at another place, where the currents
of the river have met, you will find, perhaps, a ridge of sand which
they have heaped up; while in those places where the flow of the stream
has been more gentle, the channel may be covered with a layer of fine
silt or mud. You remember that a muddy river may be made to deposit its
mud if it overflows its banks so far as to spread over flat land which
checks its flow.
The more powerful a current of water, the larger will be the stones it
can move along. Hence coarse gravel is not likely to be found over the
bottom of the sea, except near the land, where the waves can sweep it
out into the path of strong sea currents. Sand will be carried further
out, and laid down in great sheets, or in banks. The finer mud and silt
may be borne by currents for hundreds of miles before at last settling
down upon the sea bottom.
In this way, according to the nearness of the land, and the strength of
the ocean currents, the sand, mud, and gravel worn from the land are
spread out in vast sheets and banks over the bottom of the sea.