READINGS IN PHYSICAL SCIENCE.[H]

READINGS IN PHYSICAL SCIENCE.[H]

IV.—THE SEA.

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.