THE SEA—SALT WATER—WAVES AND THEIR EFFECTS—UNDER WATER—THE FLOOR OF THE OCEAN. From our childhood the sea has been the companion and playmate of thousands, the seashore their playground. Men have selected it for their professional training and livelihood. Authors write of it, poets apostrophize, scientists lecture upon it, and fathom it, bringing up from its depths many a new fact and illustration for those who cannot study it for themselves. There is nothing like it, nothing more majestic, more beautiful, more life-giving than the ocean—nothing so changeable nor so true. From the days when we could toddle along the beach, picking up the shells, we have wondered at the ocean—What was beyond it? What did it conceal? “What hidest thou in thy treasure-caves and cells, Thou hollow-sounding and mysterious main?” Let us endeavour to find out. The first thing that strikes us is the saltness of the sea. Sea water is salt. Why? One reason is because salts are carried into it by rivers, and besides, it is more beneficial as salt water. But let us look at the facts. We know that the earth contains many “salts,” as we can see by the saline springs. We have already given the chemical constitution of sea water, but it will be useful to repeat the proportions.
Some portions of the sea are not so salt as others, or, in other words, not so dense, and the saltness of the water prevents it being frozen so quickly Fig. 697.—Going out. If there is so much salt in the sea, it may be asked, why does it not continually become greatly saltier by additions. The reason is because tons of fresh water are continually pouring in, and though we can scarcely doubt that the sea is becoming gradually more salt as years pass away, the increase is very slight. On the other hand, evaporation is carrying water into the air and leaving the salt behind it. In seas like the Red Sea, where there is a great deal of evaporation and very little addition of fresh water in comparison, the water is extremely salt and bitter. The Baltic has little salt relatively to some parts of the Mediterranean. Supposing that, as some allege, there are rocks of salt at the bottom of the sea, we must remember that springs of fresh water frequently bubble up to the surface of the ocean. This is a very curious phenomenon, and has been attested by Humboldt. He states that near Cuba these springs arise with considerable force, and the vessels trading on that coast get supplies of fresh water from these ocean springs. There is, or was, a similar uprising in the Gulf of Spezzia, and fresh water crustacea inhabit these localities. These occurrences prevent the sea from becoming too salt by evaporation. When salt water becomes tainted it is very offensive—much more so than fresh water. If, therefore, the ocean were not continually in movement, it would be very injurious. So much for the water of the sea; let us now see what it does. We will glance at the surface ere we plunge into the depths. In childhood, and even in after years, we most of us delight in watching the waves of the sea. What finer sight than that we can obtain on the bold Cornish coast with a westerly wind, when the great Atlantic waves come rolling in and dashing up to the tops of the Tintagel cliffs, wearing and grinding them away; hissing up the sands at New Quay, or thundering on the shores of “Bude and Boss”! Then the wind abates, the sea goes down, the billows become waves, the waves to wavelets grow, less and less, until there is a mere ripple on the surface which is never still. The mighty heaving of the ocean breast is the peculiarity of the sea. Fig. 698.—Sea waves. Yet, again, as we stand to watch the waves, or run from them as they sweep in foam upon the sloping sand, we shall find that they increase or decrease in force, and the level of the water rises or sinks by degrees. The tide is flowing or ebbing as the case may be. So we know the surface has another—a current motion—besides the undulation of the water. The currents of the ocean are very valuable attributes, the Gulf Stream in particular bringing us warmth and, indeed, rain. There are three movements of the ocean—waves, currents, and tides. The waves, perhaps, interest us most, as they come rolling in with irregular force, but all mightily impelled by the wind. We have all noticed the ripples on a puddle; the same action of the wind produces the grandeur of the waves of the ocean. The wave comes rolling in before the wind to break against the rocks or beach, and another forms to break in its place; the higher the waves the more quickly they appear to move. But when the Fig. 699.—The Piroroco on the Amazon. The height of waves is very varied. Observers say that forty-four feet is about the highest-known wave from hollow to crest. Waves of thirty-five feet have been often met with, and off the Irish coast and in the Atlantic sailors tell of waves “as big as houses.” But houses differ in size as do waves. The rate which waves are estimated to travel varies with the wind-propelling force. The average hurricane wave travels at about forty-five miles an hour. But earthquake waves—those set in motion by subaqueous disturbance—have been known to travel at the rate of six hundred feet in a second for thousands of miles across the ocean. Such a one occurred after the earthquake which destroyed the town of Arica in August 1868, and the wave crossed the Pacific to Chetham Islands, 5,520 miles, in fifteen hours and twenty minutes. We have many of us seen the great tidal waves, or “bores,” which at certain seasons rush up our rivers—the Severn, for instance—with great violence, and at times forty feet high. These tidal waves are also experienced in the Ganges, the Amazon, and at Bordeaux, as well as in China and elsewhere. Fig. 700.—Tidal Attraction. It may well be imagined that the tides also affect the land, and the theory of these ocean movements is a very interesting study. We have already referred to it under Astronomy, for the Sun’s and Moon’s attraction is the main cause of the phenomenon, which is so familiar and yet so strange. But the consideration of the tides must be again entered upon here ere we proceed to view the effects of the sea upon the land, and how the physical geographical features alter. Isaac Newton rightly attributed the cause of the tides to the attraction of the moon and sun. Spring tides occur when both luminaries are above the meridian, and the neap, or low tides, happen when the sun and moon are farthest apart. The highest tides are perceived after a new or full moon; the lowest, after she has passed the first or third quarter. In January the spring tide is highest of all, because the earth is nearest to the sun then, and his force of attraction, added to that of the moon, causes a very high tide. With the assistance of the accompanying diagrams we shall be able to make the tidal phenomena clear. Fig. 701.—Tidal Attraction. Suppose the moon to be at M, the point J (the sea) will be nearest to the moon and will be attracted, while the earth will exercise a retarding power to a certain extent. This attraction of the water from its usual level causes a kind of vacuum, into which the surrounding water flows and causes a high tide at H. At the opposite side the earth, not the water, is most attracted, and then the water rushes in to a certain extent to fill the vacancy left by the earth’s movement towards the moon. Another high tide is therefore caused at L, but not so high as the tide upon the opposite side, as the Moon is so much nearer the latter. The tide, then, is only the natural movement of the sea water to fill up the space the earth and other portions of the watery mass have vacated in obedience to lunar and solar attraction, which is, to a certain extent, counterbalanced by the attraction and resistance of the earth. The neap tides are caused by the opposing forces of attraction of the sun and moon. The sun, as it were, pulls one way, the moon the other. The latter (being nearer) having twice the power of the former, causes a tide indeed, but it is a low one. The spring tide occurs when sun and moon together attract the water. The effects of the rise of the tide are sometimes very disastrous, Travellers to France will notice the “dunes,” or sandhills of Calais, as the train winds its way to Boulogne. We find that whenever the shore is flat the shingle and sand are blown inwards and form “dunes,” and the sand is distributed far inland, checking all vegetation, and altering the features of the country. The wearing away of rocks by the water, the continual undermining of them by the waves, and sometimes the disengagement of great blocks weighing many tons—all these effects of the sea tend to alter the appearance of the land. We may observe the denudation in many places along the coast—the caves, holes, and tunnels eaten out by the water. In Norway the “Fiords” are very remarkable. They were formed by the upheaval of the land, and tell us of the glaciers which once filled them up. Thus by ice and water the solid land is ground down and eaten away hourly, daily, and for countless centuries, changing the place of the hard rock into a standing water, and the flintstone into a springing well. Fig. 702.—The Dunes. We must now plunge beneath the waves, never fearing the rough surface; we shall find all smooth and quiet at the bottom of the sea. The Bottom of the Sea. What can we tell about the bottom of the sea to which no man has ... “Serene and safe From tempest and from billow; The storms that high above him chafe Ne’er rock his peaceful pillow”! What can we hope to find at the bottom of the sea we cannot reach? Yes, but we can reach it. By sounding with Brooke’s lead (a cannon ball, as shown in the illustration), we can arrive at a certain knowledge of the composition of the ocean bed. The right-hand figure of the two is the lead when being lowered, and while it is sinking the cord remains tight. So soon as it touches the bottom the weight of the cannon ball divides the line, and the tube is easily drawn up again. It has been well greased, and so in the cavity of the rod some shells and sand are found adhering. These fragments tell us the composition of the bottom of the sea. Fig. 703.—Brooke’s Lead. Here we find tiny shells, just as we find them in chalk, the same formation as that which piled up the cliffs which have risen from, or been discovered, by the sea. By other ingenious contrivances water can be fetched up from the bottom of the ocean, and the temperature can be gauged all along the sounding line. The expedition of the Challenger brought many interesting facts to light. Far down in these solitudes are marine animals,—crustacea, star-fish, seaweeds, and shells,—all of which are carried up by the dredge worked by a steam engine; for the resistance is very great, and the weight supported at the depth of two miles must be considerable, and is equal to four atmospheres. A thermometer has come up crushed even in its iron case, and so the creatures which inhabit and find means to live at the bottom of the sea must be specially fitted by Nature for the locality. The configuration of the ocean bed has given rise to many different opinions. It has been maintained that there are mountains and valleys, hills and dales underneath the water, all clothed with marine vegetation, equal in height and depth to the terrestrial hills and vales. Again it has been declared that the ocean bed is level; but we find raised portions, which we call islands, which may be the tops of mountains, or portions of the The sea-bed, however, is very irregular. We find deep and steep valleys, and high hills, but the picturesque peaks caused by the action of air, frost, and water on earth are not, of course, represented under water. Between the Irish coast and Newfoundland we are told the bed is level for nearly four hundred miles. There is a deep declivity before we reach this plain. The centre of the Atlantic is a plain, and on it the most volcanic islands rise, such as Ascension and the Azores. Between England and Greenland there was at one time a land communication, as we have remarked under Geology, and there are submarine terraces now. [An immense river once ran through Western Europe somewhere about where our islands are.] Fig. 704—The Drag Net. Under the Atlantic we have remains of foraminifera and other tiny animals, with red clay and volcanic remains which must have been of submarine origin. The Pacific shows us tops of mountains as islands (Hawaii Isles), and an enormous range must be hidden beneath the waters. What a change in the physical geography of the earth a slight sinking of the water of the ocean would make; England and the Continent would be united, and many sea-mountains (islands) discovered. The greatest ocean depth is four miles and a half, but in many places a few hundred feet less depth than at present would reveal many changes in the land. Every year since the world has gained its present form the streams and rivers have been pouring water, and carrying mud, stones, and gravel ceaselessly into the ocean. In addition to this, the surface water washes the stones away, animals (corals) build up islands from the depths, and take up space in the ocean. We know that if we put our hand in a basin full of water we displace a quantity of the fluid; so we might imagine that, the sea being already full, every island formed would tend to an overflow Rivers pour in water and material. The sun absorbs the water and prevents overflow; tiny animals make shells from the material. All the causes we have mentioned tend to permit the encroachment of the waters, but volcanic action and even earthquakes act also to neutralize this tendency by upheaving hills and mountains, which prevent the invasion of the sea by its elevation or by land depression. We have seen in our chapters upon Geology how the ocean beds have been upheaved, and remains of marine animals are daily found upon our highest hills. Thus the forces which sometimes cause such destruction in the earth are the means whereby the waters are kept in their places. But for volcanic action the land might all disappear by denudation and continual wear and tear, and be deposited at the bottom of the sea! If it were not for currents, of which many defined ones exist in the ocean, and the never-ceasing flow and ebb of the tides, the sea would soon lose its purity and clearness. Though the water is salt and becoming salter, animalculÆ and all kinds of plant-animals would still increase and multiply; so the decay of animal and vegetable matter would quickly render the ocean a source of pestilence and death to mankind, and be most injurious to animal life generally. But the movement is so ceaseless, and the various fish and mammalia (whales, for instance), by preying upon each other, as other animals on earth do, keep up the balance of production, and the organic matter deposited in the sea is also cleared away. That the constant currents of the sea prevent the formation and growth of seaweed is clearly shown by the great “Sargasso Sea,” or tract of weed (Fucus natans), called the Gulf-weed. This great tract embraces thousands of square miles, and is situated in the very middle of the Atlantic Ocean, where there are but few currents; but surrounding it is the Gulf-Stream, an enormous current of water running at a regular rate of four or five miles an hour. This Gulf-Stream is supposed to be caused by the same laws and influences which determine the trade-winds—namely, a constant rarefaction of the water at the tropical parts of the earth, and a corresponding condensation at the Arctic portions, for warm water is much lighter than cold, and when the waters of the tropical regions become lighter, the heavier waters of the cold regions pressing down more forcibly tend to raise them above their proper level; they therefore flow towards those very parts which have sunk down by their contraction, and a constant current takes place; this current is the Gulf-Stream. It runs from the Gulf of Mexico northwards towards Newfoundland, turning by Iceland towards the British Isles, by France and Spain, onwards to the coasts of Africa and South America, the West Indies, and again to the Gulf of Mexico, although the return current does not go by the name of Gulf-Stream. This great stream of water, warmed by the tropical sun, serves the same two purposes fulfilled by the Fig. 705.—Atoll, or Coral Island. In the foregoing pages you have now seen, and, we hope, gained, some information concerning the sea sufficient, at any rate, to induce you to enter more deeply into the subject than we can at present do. We have learnt how the sea water is composed, and what goes on on the surface. We have discussed waves, and referred to tides and currents, the wearing away and the renewal of land by the sea; we have dived beneath the surface, and found something to interest us at the bottom of the ocean. As we come up again we are surprised to find islands or reefs where none existed when we went down. What has caused this sudden appearance? They may have been slowly raised to the surface by coral insects, or suddenly by volcanic action. Let us consider the coral, which plays a very important part in our Physical Geography, before we proceed to the volcanic island.32 The low-lying islands are those formed by the skeletons of the coral insects, and the Coralline Islands are some of the most wonderful productions of nature. They are only found in warm climates, between the twenty-eighth degrees of north and south latitude, and limestone pure and simple is the chief component of the coral reef, as it is of the mountains erupted from the depths of the sea. “The detritus of corals, echinodermata shells, reticularia, and other living creatures,” says a writer on this subject, “deposit not only Fig. 706.—Gorgonia guttata (natural size). Fig. 707.—Coral (Madrepora brachiata). The coral insect is a zoophyte (Anthozoa), which, as may be seen from the illustrations, assumes curious and elegant forms, and the coral it produces is a limy or calcareous deposit, which is fixed upon a rocky base. As years go on these accretions become greater and greater, and at length rise above the water. When a little distance below it, the reefs form dangerous and frequently unsuspected barriers, upon which ships are wrecked. The red coral is dredged up from the Mediterranean, where there are extensive coral fisheries. This coral is found deep in the water, and never rises to the surface. Formerly there were coral reefs in the European seas, but the changes of temperature stopped their production. The “atolls,” or circular coral reefs with an opening at one side, have been described by Professor Darwin. “Who,” says the great naturalist, “would not be struck with wonder and admiration on catching sight for the first time of this vast ring of coral rock, often many miles in diameter? Sometimes a low green island is seen beyond it, with a shore of dazzling whiteness; outside is the foaming surf of the ocean, and within it a broad expanse of tranquil water, of pale green colour and exquisite purity.” These “atolls” mark the situation of sunken islands, and the extension of them and the barrier reefs would seem to indicate a slow but decided sinking of the bottom of the Indian and other oceans; but the “reefs” tell us that the land to which they are attached has not become depressed, and may have become elevated. We may then conclude that a continual rising and depression of the land is taking place in various oceans, indicating a sinking of the ocean bed in one locality and the result of volcanic activity in another, for no active volcanoes are found in the regions of depression. Fig. 708.—Spicules of Gorgonia (magnified). We must now leave the sea and come to land again, to consider volcanoes and volcanic action there. Volcanoes and Earthquakes. The various phenomena of volcanoes form a subject very difficult to be The majority of the volcanoes are found near, or at no very great distance from, the sea. We may therefore expect to find that water has something to do with the eruptions as it has in the case of the Geysers. But this hypothesis will scarcely hold good in every case, though volcanoes of later ages are limited to regions very different from those in which volcanic action used to be. For instance, in America we have only volcanoes on the Pacific side, and the Andes furnish several. Mexico, Central America, and California possess many volcanoes, and as far north as Alaska we find Mount Elias. There are plenty of extinct volcanoes in Europe, but the Mediterranean produces the active vents; and about the Red Sea and the Caspian, and even in the central chain of Asia, there are volcanoes far from water. The Hawaii isles, on the other hand, are all volcanic, and Australasia furnishes us with remarkable specimens; so altogether the testimony tends to prove that where volcanic remains are apparent the sea had at one time been, or now is, near at hand. Burning mountains have been familiar to us from our childhood in pictures, and by stirring narrative of destruction wrought by them. The volcano is generally a mountain rising to a cone, but Vesuvius presented quite the appearance of a hollow basin at the top, before it suddenly broke forth and buried Herculaneum in ashes. Von Buck visited it in 1799, and declares it had at one time risen, like an island, from the sea. There are about two hundred and seventy volcanoes at present in activity; four in Europe; eleven in Iceland and Jan Meyen’s land; in Asia, ninety-three; in Africa, twenty-six; forty-six in North America and the Aleutian Isles; twenty-seven in Central America and the Antilles; in South America, thirty-one; and twenty-four islands with volcanic tendencies largely developed. There may be many more “resting.” Volcanoes, then, are openings or vents which communicate with the melted rock within the earth, and the conical form of volcanoes is owing to the deposits of volcanic matter as it falls from the opening called the crater. If we let a small spade full of mould run through our hands, or from the spade, it will form a small cone, the heavier particles sliding to the base at a certain slope. Thus the volcano builds its own hill, and inside the crater we find cones from which smoke and steam issue. These cones within the cone are the points of issue of vapour and smoke, miniature volcanoes making up a whole. Fig. 709.—Eruption of Vesuvius, August 26th, 1872. The signs of eruptions are much the same, and usually occur a couple Fig. 710.—Birth of a volcano. New volcanoes are continually in process of formation, and at Santorin for hundreds of years volcanic action has been busy in forming islands. These violent efforts of Nature frequently give rise to earthquakes, which are the most destructive of natural convulsions. The records of late occurrences are fresh in the minds of all readers, and need not be specified. The slow subsidence and gradual upheaval of the land is still going on, but we are frequently startled by the account of a rupture of the ground or the destruction of a portion of a city. The motion of the earthquake is generally in a direct line, and undulating. Sometimes what are termed vertical shocks arise and destroy solidly-built edifices. Mountains have been overturned by earthquake shocks, and trees have been twisted round. Sometimes the ground yawns into enormous fissures. The sea is tossed into great waves and encroaches upon the land, and when the sea recedes the recession of the water is followed by a more terrible invading wave sweeping all before it. Earth tremblings often occur far away from volcanoes, and without any visible connection with volcanic action. There are many aspects of land and water which the student of Fig. 711.—Earthquake fissures. Any elevation rising from a base more than 1,000 feet may fairly be termed a mountain, and solitary mountains are usually volcanic, because eruptive rock does not produce chains of mountains. The origin of mountains is probably due to the contraction and compression of the crust of the earth—not merely the surface, but the whole thickness between us and the supposed molten interior. Mountains did not exist from everlasting, for the very good reason that they are (in most cases) composed of stratified rocks. Stratified rocks are sedimentary rocks, and must have been deposited below water, and hardened long before they were thrust up by pressure. Moreover, we find (as has already been explained) shells and remains of marine animals on the higher summits, which prove to demonstration that these mountains are composed of rocks which were laid down under the sea. Professor Dana was one of the first geologists to advance the theory that contraction and lateral displacement are the causes of the elevation of mountains. A very good illustration of this theory was made by Chamontier, who covered an india-rubber balloon with a thick layer of wax, and when it had hardened sufficiently he pricked a hole in the bladder, which immediately contracted, and the wax at once rose up into tiny similitudes of mountains, showing in a sufficiently clear manner that such protuberances may be Professor Geikie has shown how, by a very simple experiment, the contortion of mountain strata is effected by pressure. A number of cloths or towels placed flat on a table represent the sedimentary rocks. Place a board with a weight on the top, and the towels will remain flattened. But by holding two boards at the sides and pressing them together (the weighted board still remaining), we shall find the towels crumpled and upheaved like the Jurassic strata shown in the illustration (fig. 712). Professor Heim calls the central masses wrinkles of the earth’s crust. So the Alps were pressed up or heaved into the air, the weather—rain, frost, snow, and sunshine—imparting the infinite variety of “Horn,” “Needle,” and “Peak,” so expressively applied in Alpine nomenclature;—the Matterhorn, Wetterhorn, Weisshorn; the Pic du Midi, Aiguille de Dru, Aiguille Verte, and many other mountains in well-trodden Switzerland will occur to the reader at once. Fig. 712.—Anticlinal and synclinal curves of the Jura Mountains. The slopes of mountains—though to the casual observer they may appear very much the same—are very different. We sometimes find a long, easy ascent, —more usually a steepish inclination, perhaps 20°;—in other places, such as on the Matterhorn, an almost perpendicular face. Forty-five degrees rise is very steep, and 53° is the limit of any great mountain’s slope. Cliffs and precipices there are, of course; witness the terrible fall from the Matterhorn to the glacier below—thousands of feet with one tremendous leap from the rock to the ice underneath; but mountain slopes are not precipices. As a rule, we find that one side of a mountain chain is steeper than the opposite one. It is harder to climb up from Italy to Switzerland than to ascend in the opposite direction. The Pyrenees are also steeper on the south side. The Scandinavian mountains likewise are steeper in the west. The Himalaya are steepest towards the sea, so are the Ghauts. We here find a difference between the slopes of the New and Old Worlds. In the former we have the less precipitous mountain slopes towards the east; in the old world they are towards the north, and an inspection of a physical map of the world leads us to the conclusion that the Atlantic and Pacific Oceans are the boundaries of entirely different degrees of slopes. The Pacific and Indian Oceans would appear to border the more precipitous mountain sides; the Atlantic and its connections those less steep. As a rule, we have the most elevated portions of the earth, mountains, and high tablelands, in equatorial regions; and within the torrid zone every terrestrial climate is to be found, owing to the snows of the high mountains Plains are very varied. We have European Heaths and Landes; American Savannahs, Prairies, and Pampas; Asian Steppes, and African Deserts. All of these possess certain features in common, more or less vegetation, and sometimes absolute sterility. Plateaus, or Tablelands, are elevated plains frequently undulating in character. The Plateau of Bolivia is 13,000 feet high, and extends along by the Andes. The tableland of Quito is nearly 10,000 feet high, and borders on the giants Cotopaxi and Chimbarazo. Fig. 713.—The Staubbach (Lauterbrunnen). Rivers and lakes add not only to the wealth of nations by their usefulness, but, by the additional picturesqueness of their appearance, to the beauty of the landscape. The velocity of rivers would be very much increased if it were not for the strong resistance offered by the banks and the stones to the current, and by friction. The Rhine and the Rhone, if thus unimpeded, would flow at a rate considerably over one hundred miles an hour; and our own little stream (the Thames), instead of eddying peacefully and twirling gracefully by Medmenham or Cookham, would rush along at the speed of the train which so often crosses it on its way to the sea. The slopes of river-beds, like the slopes of mountains, vary very considerably, and the inclination of a river varies at different places; in a distance of seven hundred miles the Amazon only falls twelve feet, and the current flows chiefly by impetus already acquired. A slope of one foot in two hundred precludes all navigation, and at still greater inclines rapids and cataracts are formed—the great falls wearing away the river-bed by degrees; so it is calculated that hundreds of years ago Niagara Fall was much farther down the river, and the cataract is slowly moving up stream. In time, as the rock wears away, the height will disappear as the celebrated “Falls,” and will become a rapid within a few miles of the lake. Lakes are derived from river-drainage and springs. Some are very salt, owing to evaporation carrying away so much water, and leaving the accumulated mineral salts. These very salt lakes are likely to dry up, as the supply of water is not equal to the demands of evaporation. Floating islands appear and disappear on many lakes. Derwentwater is one instance. On the uses of lakes and rivers it would be superfluous to dwell. We are more concerned to examine their influence on climate, and in this sense we must also consider mountains. But we will now group all the phenomena of the air and water, and their effect upon climate, under “Meteorology” in the chapters next following. Scene - lake |